Scripts are written once
and read at 3am.

A shell scripting language for the era when machines write the scripts and humans only get to review them: readable by default, structurally immune to injection, and auditable before a single process starts.

A grammar descended from HyperTalk, an interpreter rather than a login shell.

What you get

Everything below follows from one decision: a frost program is a parse tree, never a string. Nothing is interpolated, nothing is re-parsed, and no value can become syntax. That single property is what lets a script be checked as a contract instead of trusted as a guess.

Capability	What it buys you
No interpolation, no eval	Injection is unrepresentable, not mitigated. Hostile text stays text wherever it came from: a filename, an issue title, a web page an agent just read.
--explain	A capability manifest before anything runs: every program spawned, file read or written, secret released. Approving capabilities takes seconds; deriving them by reading code takes minutes, and that is where mistakes happen.
--policy	Business rules checked against the tree. Not just may it use curl, but how many times, for how long, and whether it cleans up after itself. And every rule explains itself when it fires. Violations exit 3 and the script never starts.
Sealed secrets	A value from the role-gated keystore cannot be printed by accident. The seal survives concatenation, comparison is constant-time, and --explain names every place a secret is released to a program.
Modules	An import declares a capability ceiling, so reading the entry file gives a sound upper bound on the whole program. A shared module that later grows a network call breaks the build at the import site rather than quietly widening someone's manifest.
--sandbox	The kernel holds the boundary while the script runs, so a path the analyser could not resolve is confined anyway. Fails closed: where the boundary cannot be enforced, frost refuses to run rather than warning and continuing.
the run id	One identity per execution, supplied by the pipeline or generated. It reaches the recording, the trace and every child process, so a log line three layers down joins back to the run that caused it. Also an idempotency key, and a scratch path that cannot collide.
--record / --replay	Snapshot testing for shell scripts. A recording is a fixture you can commit; replay spawns no process, writes no file, and reports a divergence rather than a stack trace. Secret values are never written down.
Records and JSON	the "name" of the "user" of report: API responses without a second language in the file. Shelling out to jq handed the auditor a string it could not see into; a record is part of the tree.
the error output	Why a command failed, not just that it did, without sh -c "... 2>&1", which is the one construct the auditor flags and the spec forbids.
Declared record shapes	with fields "status", "number" makes a mistyped field a --check failure instead of a silent empty, and verifies the payload at the line that parsed it.
--events	NDJSON for Splunk, New Relic, Datadog or a collector. Every command timed, every effect reported, secrets redacted. The finish event says which approved capabilities went unused, which is a signal a shell cannot produce and which drives tightening an approval before it is abused.
Dead code	Unreachable statements, handlers nobody calls, values computed and dropped. Harmless individually, and together the clearest sign a generated script contains more than anyone intended.
frost mcp	The review tools over Model Context Protocol, stdio JSON-RPC, no dependencies. It cannot run a script and reads no files, by design: frost exists because the decision to run sits with a person, and a server that executes on request moves it back to the machine.
frost context	What a model should read before writing frost: the forms, the reserved words taken from the parser, and the constructs it deliberately lacks. Every snippet in it is parsed by the test suite, so it cannot teach a form that does not work.
Volume limits	require at most 10 megabytes of output and --max-output 10MB. A deadline says nothing about a command that answers instantly with a gigabyte. The child is killed at the ceiling rather than measured after the fact, because a limit that notices afterwards prevents nothing.
Nested interpreters	xargs sh -c, env sh -c, sudo sh -c, find -exec, ssh host "...". The escape check used to fire only when the interpreter was the program name, so every indirect form reported nothing at all. A manifest may overstate; understating is what makes it worse than none.
frost diff	frost diff old.frost new.frost compares two versions by what they can do, not by their text. Three rearranged lines can be a widening and thirty can be a rename, so a review that reads the text diff is reading the wrong artefact.
Output masking	A program handed a credential often echoes it back. frost finds the plaintext in what a child wrote and re-seals it, so it redacts wherever the script prints it. Exact-match only: a mask that guesses at shapes fails in both directions and gets trusted for the one it fails at quietly.
Repair report	A refusal names the narrowest policy change that would clear it, and states what else that change would allow. It is never a patch, and under --automated it declines to answer: an agent handed the exact edit that clears its own refusal has been handed the instructions for widening its own bounds.
Environment rules	forbid reading the environment "AWS_*" and require reading only the environment "PATH". Setting had a rule and reading did not, which was the wrong way round.
--deadline	A budget for the whole run, honoured with cleanup and exiting 124. A loop doing arithmetic has no capabilities, so the manifest called it clean; an unbounded loop is now a finding, and a policy can impose the budget centrally.
Site policy	/etc/frost/policy.d/*.policy applies to every run on the host, whether or not anyone passed --policy. Site rules add to a project's and can only narrow them, and every policy applied is named by digest in the manifest and the recording.
--automated	An unattended run refuses --approve and --ignore-approval. A repair loop that can approve is one that approves its own capability escalation.
Signed approvals	--sign-with binds an approval to a named approver and a commit; require an approval signed by "..." names who a host trusts. Verification never degrades: without the cipher, an unverifiable signature is refused.
--approve	Records what a script does today, then binds by default: a regeneration that does more is refused without any flag. A content hash fires on every edit, so it cannot be used on a script an agent rewrites. This fires only when the script gained a capability.
SARIF and an Action	--check --sarif feeds GitHub code scanning, so a refusal appears on the diff line in front of the person merging rather than in a log nobody opens. action.yml wires check, explain, policy and approvals into six lines of workflow.
--policy-from	Writes a starter policy describing what a script already does. The policy engine was the most useful thing here and the least used, because the first step was a blank file.
--json / --repair	Every diagnostic as structured data with the edit attached, so the model that wrote the script can repair it without a human in the loop.

The lifecycle

Each stage is optional, and each one narrows what the next has to trust:

frost --check     s.frost      does it parse, and are the names real?
frost --explain   s.frost      what could it possibly do?
frost --policy p  s.frost      is that allowed here?
frost --sandbox   s.frost      hold the boundary while it runs
frost --record r  s.frost      write down what it actually did
frost s.frost                 refuse it if it gained a capability
frost --events e  s.frost     tell a monitoring system what happened

The first three cost about a millisecond and all happen before a single process starts. The fourth is enforced by the operating system. The fifth turns a run into a fixture.

Where the rest is documented

• LANGUAGE.md: the full reference and grammar.
• MODEL-SPEC.md: a prompt-sized spec, built to be pasted into a system prompt so a model writes correct frost first time.
• CHANGELOG.md: what changed and why.
• CONTRIBUTING.md: how the pieces fit together.
• PLATFORM.md: for the team that operates the machines: what frost enforces, what it cannot, and what actually makes any of it mandatory. The honest answer to that last one lives mostly outside frost.
• Try it in the browser, the scratchpad, and below it frost itself compiled to WebAssembly. The same Python the command line runs, so --explain, --policy, --repair and an approval comparison answer exactly as they would in a terminal. Nothing to install.
• Reference and a visual audit report. (These are committed as docs.html, audit.html and play.html too, but GitHub shows a committed page as source rather than rendering it, so the links above are the ones to follow.)

#!/usr/bin/env frost

put the number of lines in file "access.log" into request count
put "processing" && request count && "requests"

pipe
    run "grep" with "ERROR", "access.log"
    run "awk" with "{print $1}"
    run "sort"
    run "uniq" with "-c"
end pipe

repeat for each line in it as tally
    put the second word of tally && "failed" && the first word of tally && "times"
end repeat

Three things it fixes

Injection is unrepresentable, not mitigated. There is no interpolation and no eval. Arguments are a list handed to execve, never re-parsed.

$ cat hostile.frost
put "notes.txt; rm -rf *" into evil name
run "touch" with evil name

$ frost hostile.frost && ls
'notes.txt; rm -rf *'   keep_me.txt   precious.db

$ bash -c "touch $EVIL" && ls
                        # empty. everything is gone.

Failure stops the script. No set -e to forget. run aborts on non-zero exit; try to run opts out and is greppable in review.

Pipes fail if any stage fails. cat missing.log | wc -l reports success in bash. In frost the first failing stage wins, and there is no way to turn that off.

Why

Shell syntax was optimised for a cost that no longer dominates. cut -d: -f1 is terse because a human typed it a thousand times on a serial terminal. When a model writes the script and a human reads it once: at 3am, while production is down: the scarce resource is comprehension at review time, not keystrokes.

frost inverts the optimisation. It is not a login shell; it is an interpreter you point at a file. Because nobody types it at a prompt, nothing in the design has to be short.

The speed objection does not apply, though not for the reason you might expect. python tools/benchmark.py measures it rather than asserting it:

release.frost           78 lines    parse 664us   audit 290us
one fork+exec of true                            1992us   (the floor)
one fork+exec of git --version                  12767us   (a real command)

Parsing an 80-line script and deriving its entire capability manifest costs about a millisecond. So does starting a process, the two are the same order of magnitude, and which one wins depends entirely on the machine. fork/ exec of true is roughly 0.7ms on Linux and 2.4ms on macOS; git --version is 1.2ms on Linux and 12ms on macOS. Parsing varies far less.

The tempting claim, you can parse verbose syntax faster than you can spawn a process, is therefore true on macOS and false on Linux. It was in this README until CI on Linux disproved it.

What actually holds is the thing the design relies on: parsing is paid once, spawning is paid per command. A script that runs ten commands spends ten process spawns against one parse, so the parse is a rounding error no matter which platform it runs on. Verbosity costs nothing at this scale because the front end is a fixed cost, not because it wins a race.

Side by side

#!/bin/bash
set -euo pipefail
IFS=$'\n\t'

LOG="${1:?usage: report LOGFILE}"
[[ -f "$LOG" ]] || { echo "no log at $LOG" >&2; exit 1; }

n=$(wc -l < "$LOG")
errs=$(awk '$4 ~ /^5/' "$LOG" | wc -l)

echo "requests: $n"
echo "errors:   $errs"
(( errs > 2 )) && { echo "ALERT" >&2; exit 1; }

#!/usr/bin/env frost

put item 1 of the arguments into log path
if log path is empty then
    put "usage: report <logfile>" into standard error
    quit with status 2
end if

if not file (log path) exists then
    put "no log at" && log path into standard error
    quit with status 1
end if

put the number of lines in file (log path) into request count

put 0 into error count
repeat for each line in file (log path) as this request
    if the fourth word of this request starts with "5" then
        add 1 to error count
    end if
end repeat

put "requests:" && request count
put "errors:  " && error count

if error count is greater than 2 then
    put "ALERT" into standard error
    quit with status 1
end if

Longer, and that is the trade. Nothing in the second version needs explaining to someone who has never seen frost: no set -euo pipefail, no IFS incantation, no ${1:?}, no $4 ~ /^5/, no >&2. The safety that bash gets from three lines of ceremony, frost gets from having no other mode.

Patterns and timeouts

Globs for filenames, regex when you actually need it, and neither pretending to be the other:

if filename is like "*.tmp" then delete file filename

if request matches "^(\S+) (\w+) (\d+)$" then
    put match 1 into client address
    put the last match into status code
end if

put every match of "\d+" in request into numbers
replace "(\d+)-(\d+)-(\d+)" with "\3/\2/\1" in date text

Capture groups use the same chunk grammar as everything else: match 1, the last match, the number of matches, the whole match.

Any command can carry a deadline, with a required unit:

run "curl" with "--silent", endpoint within 30 seconds

try to pipe within 1 minute
    run "find" with "/", "-name", "*.log"
    run "xargs" with "wc", "-l"
end pipe

if the result is 124 then put "took too long" into standard error

Timed-out children are killed and reaped: no orphans, no wedged script.

Cleanup that actually runs

Abort-on-failure is the headline default, which makes the way out matter as much as the way through. ensure registers a block when execution reaches it, and it runs when the script ends: normally, on error, on quit, or on Ctrl-C most recent first:

put "held" into file (lock path)

ensure
    delete file (lock path)
end ensure

run "make" with "deploy"

The lock is released whether make succeeds or not. A failure inside a cleanup block is reported but never replaces the error that ended the script.

Lists, without a second grammar

A plural chunk noun with no index is the whole set, so splitting is the grammar from the previous section read the other way:

put the words of headline into terms
put the lines of report into rows
put item 1 of (passwd entry split by ":")     -- cut -d: -f1
put the sorted (the unique terms) joined by ", "

A list keeps its elements separate, which comma-delimited text cannot, an element may contain a comma and stay one element. the sorted X compares numerically when every element is a number, because sorting 10 before 9 is never what a counter meant.

Text and numbers come with the article, so they cost nothing from the name vocabulary, sorted count is still a perfectly good variable:

if the lowercase target is "production" then put "shipping for real"
put the trimmed reply into answer
put the sum of the words of counts

And a handler can be used inside an expression, so composing two of them no longer needs three statements and a temporary:

to double with n
    return n * 2
end double

put the double of 5 + the double of 10        -- 30

An unknown name there is caught when the script is checked, not when the line happens to run.

Structured data, without a second grammar

Every real script eventually calls an API and reads a field out of the answer. Until now that meant run "jq" with ".status": a second language inside a file whose entire argument is that it needs only one, and a string the auditor could not see into. --explain could tell you the script ran jq. It could never tell you what for.

run "curl" with "-fsS", "https://api.example.com/build" within 30 seconds
put the json of it into build

if the "status" of build is not "green" then
    put "build" && the "number" of build && "failed" into standard error
    put the "name" of the "author" of build && "was last to push"
    quit with status 1
end if

Objects become records, arrays become the lists frost already has, and numbers stay numbers: so item 1 of, repeat for each and + 1 all keep working. A missing key is empty, like word 99 of, and a field of empty is empty, so an optional field needs no guard. A field of text is an error, because that means the value is not the shape the script thinks it is.

Parsing a secret seals every field it produces, and serialising redacts field by field rather than all at once, a record you cannot print at all is a record people work around.

put the json of the secret file "credentials.json" into config
put "connecting as" && the "user" of config
connecting as «secret credentials.json»

Knowing why it failed

the error output sits beside it and the result: what the last command wrote to standard error, what it wrote to standard output, and how it exited.

try to run "curl" with "-fsS", url within 30 seconds
if the result is not 0 then
    put "curl failed:" && the error output into standard error
    quit with status 1
end if

The alternative was run "sh" with "-c", "... 2>&1", which reintroduces the shell frost exists to remove and which the auditor flags on sight. Wanting to know why something failed is completely ordinary, and it should not require defeating the language's main guarantee to get it.

A clock that replays

put "started at" && the current timestamp
wait 5 seconds

Both are recorded. --replay serves back the reading that was recorded rather than reading the clock again, a fixture whose timestamps move on every replay is a diff generator, not a fixture: and it does not sleep, so replaying a script that backs off for thirty seconds costs nothing. A script that waits says so in --explain.

Secrets that cannot be logged by accident

The failure worth designing against is not a malicious script. It is put "connecting as" && token in a generated script, running in CI, writing a credential into a log that is retained for a year. That mistake is made by being ordinary, so the fix has to be structural.

A secret is a sealed value. It refuses to become text, and every printing path in the language goes through one conversion: so put, joining, --trace and error messages all redact without knowing secrets exist:

put the secret "db password" into password
put "connecting as" && user && "with" && password

connecting as deploy with «secret db password»

Only the secret spans redact; the rest of the line survives, because a mechanism that destroys your logs is one people route around. The seal is contagious, so "postgres://user:" & password & "@host" is still sealed and still works when it reaches a program.

Streams redact, boundaries release. Printing is the accidental path and is closed. A program's arguments, its standard input, its environment and a file write are deliberate, so they get the real value, and --explain names every place it happens:

Reads these secrets:
  db password  at line 4  (from the keystore)

Lets a secret leave the process:
  on the standard input of psql  at line 9

Values live in a keystore, and each one names the roles that may read it:

frost keystore set prod.keystore "db password" --roles deploy,admin
frost --keystore prod.keystore --role deploy release.frost

If the role cannot open a secret the script names, frost exits 3 and nothing runs. The secret names and the role grants are stored in plaintext, because that is the part a reviewer needs; only the values are encrypted. Roles hold X25519 keypairs, so storing a secret and granting a role need no passphrase, only reading does.

It does not stop a script handing a secret to a program it is allowed to run; nothing at this layer can. And once the plaintext reaches another program, frost cannot follow it. The manifest reports the release rather than pretending otherwise.

Knowing what it did, not just what it could do

--explain answers what a script can do. What it did needed watching.

frost --record run.json deploy.frost      # run it, write down everything
frost --replay run.json deploy.frost      # run it again, spawn nothing

Replay performs nothing at all: no process, no write, no delete, and serves the recorded answers back. So a recording is a fixture: change the script, replay it, and a refactor meant to preserve behaviour either did or did not.

DIVERGED at deploy.frost:3
    the recording ran: echo two
    this run wants:  echo CHANGED

Reformatting replays clean, because matching is on the identity of the effect rather than on line numbers. Secret values are never written down, only their names: and any revealed plaintext is scrubbed from everything recorded, so the fixture is safe to commit.

Telling a monitoring system what happened

frost --events run.ndjson deploy.frost      # or - for standard error

One JSON object per line, flushed as things happen. NDJSON is what Splunk's HTTP collector, New Relic's log API, Datadog, Vector and Fluent Bit all ingest without a translator, and a line-oriented file survives a run that is killed halfway, which a single JSON document does not.

{"event": "run.start",      "declares": {"programs": ["curl"], "hosts": ["x.example"]}}
{"event": "command.start",  "program": "curl", "argv": ["curl", "-fsS", "..."]}
{"event": "command.finish", "program": "curl", "status": 0, "seconds": 0.412}
{"event": "run.finish",     "status": 0, "commands": 3, "waited_seconds": 2.0,
 "programs_unused": ["psql"], "hosts_unused": ["db.internal"]}

The resolution worth having is the pairing, not the volume. Any tool can log that a command ran. frost knows what the script was allowed to do before it ran, what a person approved, and what the host permits, so the finish event reports which approved capabilities went unused. A script approved for six programs that uses two is an approval somebody should tighten, and that is only visible holding the manifest and the run side by side.

Commands are timed, and the run separates time spent working from time spent waiting, so a slow job can be attributed rather than guessed at.

A refusal is an event, which is the one a security team most wants and the easiest to lose. A policy refusal, a breached import ceiling, an approval that no longer covers the script, an unusable signature, a secret the role may not read: each closes the run out with what fired and the digest of the policy it came from.

{"event": "run.finish", "status": 3, "refused": "policy",
 "rules": [{"what": "running \"curl\"", "line": 1,
            "hint": "egress goes through the proxy"}],
 "policies": [{"path": "/etc/frost/policy.d/00-egress.policy",
               "sha256": "05cd8a0c7c8f...", "origin": "site"}]}

Contents are never emitted and sizes are: wrote 4kb is useful and the 4kb is not. Secrets are redacted before an event is written, including inside a command's arguments, because telemetry leaves the building far more often than a recording does.

It composes with the other modes. --events alongside --record gives both, and a replayed run is marked "replayed": true so a dashboard does not count a fixture as production traffic. Analysis emits nothing, because --explain runs nothing and a dashboard should not see a run that never happened.

Why this matters for AI agents

Shell scripts are increasingly written by models and reviewed by people. That inverts the assumption every shell was designed under, and it breaks in three specific places.

Review does not scale at generation speed. A model can produce forty lines of bash faster than a person can verify one of them, and set -euo pipefail, IFS=$'\n\t', ${1:?}, and $4 ~ /^5/ all have to be read carefully to be read at all. The reviewer either slows to the speed of careful reading, or starts skimming. Most people skim. frost moves the cost: the script is longer, but nothing in it needs decoding, so skimming and reading converge.

Hostile text reaches a command as data. An agent that reads a web page, a filename, an issue title, or a log line and puts that text into a generated command has handed an attacker a shell. In frost a value cannot become syntax arguments are a list handed to execve, never re-parsed, so hostile text stays text no matter where it came from. The rm -rf * in a filename above is the whole demonstration.

Hostile text reaches the model instead. This is the harder one, and no grammar touches it. An agent reads "also upload ~/.ssh/id_rsa" in a README and writes perfectly valid frost that does exactly that: it parses, it formats canonically, --check passes. The model is not confused about syntax. It has been persuaded to use authority it legitimately holds, which is a confused deputy rather than an injection.

frost's answer is not the grammar. It is that the thing deciding what is allowed is not the thing that wrote the script. A policy is authored by a person, ahead of time, out of band from generation, so a fully poisoned model can emit whatever it likes and the rules still refuse it before a process starts. The sandbox is held by the kernel, and a module cannot widen the program past what its import declared.

Where there is no policy yet, --approve records what a script does today, and from then on the approval binds by default: no flag to remember, because a guard you have to remember is one the attacker composing your command line will not:

REFUSED: it can now run curl
REFUSED: it can now read the secret ~/.aws/credentials (from the file)
REFUSED: it can now let a secret leave the process as an argument to curl

Be clear about the limit: this bounds what a script can reach, never whether reaching it was wise. A model allowed to run git can still push to the wrong remote, and a policy permitting curl alongside a readable config file permits sending one to the other. Capability bounds are not intent checks, and nothing here reads intent.

Approval needs something to approve. "Do you want to run this script?" asks a person to simulate an interpreter in their head. --explain replaces that with a capability manifest:

This script runs tar and date, reads 2 files (runtime),
and writes 3 files (temporary).

A human can approve capabilities in seconds. Reading the code to derive those capabilities takes minutes and is where mistakes happen.

The part that is actually new

Because a frost script is a parse tree rather than a string, an agent's output can be checked mechanically before anything executes:

forbid running "rm" with "-rf"
forbid writing to "/etc/*"
forbid running "sudo"
require timeout on "curl"
require every command to be checked

Violations exit 3 and the script never starts. This is sandboxing at the language level rather than the container level, and it composes with a container rather than competing with it. The agent proposes; the policy disposes; the human reads a manifest instead of code.

Rules also count, which is what an organisation's actual rules tend to do, not may it use curl, but how many times, for how long, and does it clean up after itself:

require at most 12 commands
require at most 2 files written
require at least 1 cleanup
forbid more than 2 runs of "curl"
forbid any files deleted
require timeout on "*" between 1 and 120 seconds

Units are reconciled, so a policy written in seconds catches a script written in minutes. A limit that is exceeded points at the line that crossed it. A deadline computed at runtime is refused rather than assumed acceptable, on the same principle as the manifest: say what is unknowable, do not guess it.

Rules say why. A rule's trailing comment is its hint, and frost prints it when the rule fires, so a refusal explains what to do instead rather than just saying no:

REFUSED: running "sudo"
  deploy.frost:1  run "sudo" with "systemctl", "restart", "api"
  why: the deploy role already has the permissions it needs

No new syntax; policy authors already write that comment.

Modules that cannot hide anything

Sharing handlers across scripts is the feature most likely to break the one invariant frost depends on. That the tree you audit is the program you run, and the audit sees all of it. So the goal is not safe modules; it is modules that cannot put capability outside the manifest.

use "lib/db.frost" for the connect, the migrate which may run "psql"

A module is declarations only: handler definitions and imports, nothing that runs when it is imported. The path is a literal the parser insists on, and it resolves relative to the importing file with no search path, nothing above the entry script's directory, and no registry. Imports name exactly what they bring in, so a collision is an error rather than one module silently replacing another's handler.

--explain audits the whole closure and says where each capability came from:

lib/db.frost   (imported by deploy.frost:1)
  Runs these programs:
    psql  at line 2

And the which may clause is the part that makes single-file review survive multi-file code. A module defaults to no capabilities at all. If it does more than its import declared, the program is refused before anything runs:

REFUSED: lib/sneaky.frost may not run curl
  The import at deploy.frost:2 allows: nothing but compute.

A ceiling bounds the whole subtree an import pulls in, not just the file it names, a module allowed to run psql cannot import a second one that runs curl. So a reviewer who reads only the entry file has a sound upper bound on the whole program, and a shared module that later grows a network call breaks the build at the import site rather than quietly widening someone's manifest. frost --lock and --frozen pin the bytes.

Closing the loop with the thing that wrote it

Every refusal above is a sentence for a person. --json is the same information as data, with the edit attached wherever frost already knew it:

{"code": "missing-then", "line": 2, "column": 20,
 "message": "expected 'then' but found end of line",
 "repairs": [{"kind": "replace-line", "line": 2,
              "text": "if error count is 0 then", "confidence": "high"}]}

frost --repair --write applies the high-confidence ones and repeats until nothing certain is left: fixing one error reveals the next, so a single pass would give up on any script with two mistakes. A pass is kept only if it made progress, which is what makes it safe to run unattended:

repaired deploy.frost (3 change(s))
  line 2: an 'if' condition is closed by 'then'
  line 3: run takes a program and a list of arguments, never a command line
  line 5: a global is written 'the global <name>'

That is the loop: generate, check, repair, re-check, with a policy deciding what is acceptable and a manifest a human approves at the end.

Boundaries the kernel holds

Everything above reasons about the text of a script, and is careful to say when it cannot know something. Once the script runs, an unknowable path is a real path. So the boundary is declared once and held at runtime:

sandbox may run "git", "make"
sandbox may read "*"
sandbox may write "build/*"

frost --policy prod.policy --sandbox deploy.frost

sh: /tmp/anywhere-else.txt: Operation not permitted

Child processes are confined by the operating system, sandbox-exec on macOS, bubblewrap on Linux, so a path the analyser could not resolve is confined anyway. frost's own file operations are checked by frost, which is a weaker guarantee, and the docs keep the two apart rather than blurring them.

Two things it deliberately will not do. Per-host network rules are refused, not faked: macOS filters on addresses and a Linux namespace is all-or-nothing, so sandbox may reach "api.github.com" is a parse error and sandbox may reach the network means exactly what it says. And if a boundary is declared but cannot be enforced here, frost refuses to run rather than warning and continuing, including when the backend is present but a live self-test shows it not actually confining.

That self-test runs two controls, not one. A forbidden write must be refused and a permitted write must succeed. Checking only the first is the trap the feature is most likely to ship with: a sandbox that fails to start blocks the forbidden write too, so every "is it blocked?" assertion passes and the thing reports itself healthy while confining nothing. Both backends were caught by the second control, Linux dying on a network namespace it was not allowed to enter, macOS naming an unresolved /tmp path the kernel never matches.

Built-in checks catch the classics with no policy at all, curl … | sh is reported as downloaded code piped into a shell, and a script that reads ~/.ssh/id_rsa and then makes a network call is flagged as secrets read, then the network is contacted, the shape of data theft. Both are facts about the tree, not pattern matches on the text, and both hold even when the sensitive path is assembled at runtime from a variable and a string fragment.

What this does not do

Analysis covers literals. If a script builds a program name or path at runtime, frost reports it as built at runtime rather than guessing, the manifest tells you that something is unknowable, not what it is. A determined script can still put itself out of reach that way, and a policy that permits a command permits its consequences. This narrows the blast radius and makes review tractable; it is not a sandbox and does not claim to be.

The feature that carries the language

Chunk expressions. One uniform grammar replaces cut, awk '{print $3}', sed -n '7p', head, and tail:

put the third word of line 7 of file "access.log"
put the last item of csv row
put words 2 to 4 of headline
put the number of lines in report

Text addressing is most of what shell scripting actually is, and this is the best notation anyone has shipped for it. It was HyperTalk's, and it deserves another run.

A script you can check before you run it

Before you run it, you can read exactly what it is allowed to do. Bash cannot do this.

Because frost is parsed rather than string-substituted, a script's capabilities are visible in the tree. --explain prints them:

Runs these programs:
  rm  at line 8   (no timeout)
  curl  at line 12  (1 allowed to fail, no timeout)

Writes these files:
  /etc/cleanup.state  at line 11

And --policy enforces rules before anything is spawned:

forbid running "rm" with "-rf"
forbid writing to "/etc/*"
require timeout on "curl"
require every command to be checked

REFUSED: running "rm" with "-rf"
  cleanup.frost:8   run "rm" with "-rf", scratch folder

2 rule violation(s); the script was not run.

rm -rf "$DIR" in bash is a string until it executes, so there is nothing to inspect first. In frost the program and its arguments are separate nodes, which is what makes a script checkable as a contract instead of trusted as a guess.

Built-in checks

Beyond a policy file, every script gets a standing set of checks. On a script that looks like routine cleanup:

Findings:
  [DANGER ] line 12  Recursive forced delete
  [DANGER ] line 15  Writes to a system location (/etc/cleanup.state)
  [DANGER ] line 18  Permissive or recursive permission change
  [caution] line 21  No timeout on curl
  [caution] line 21  Failure ignored (curl)
  [DANGER ] line 26  Downloaded code piped into a shell

Verdict: dangerous

The audit report, or open audit.html locally, shows four scripts, a fake "dotfile backup" that exfiltrates your keys and a cleanup script that quietly does four dangerous things, both refused, alongside a health check and a log analyzer that pass.

Install

Requires Python 3.10+. No dependencies.

pip install frostlang
frost --version

Or with Homebrew:

brew install keithadler/frost/frost

The keystore is the one optional extra, because it needs a real cipher:

pip install "frostlang[keystore]"

From a checkout instead, which is what you want if you are changing frost:

git clone https://github.com/keithadler/frost.git && cd frost
ln -s "$PWD/frost" /usr/local/bin/frost
frost examples/hello.frost

Coexists with zsh: you are adding an interpreter, not replacing your shell. Make scripts executable with a shebang and run them directly:

chmod +x report.frost
./report.frost access.log

Try it in 30 seconds

No dependencies beyond Python 3.10. Paste this whole block:

git clone https://github.com/keithadler/frost.git && cd frost

cat > hello.frost <<'END'
run "date" with "+%A"
put "Today is" && it

put the number of lines in file "/etc/hosts" into host lines
put "Your hosts file has" && host lines && "lines"
END

./frost hello.frost

Today is Monday
Your hosts file has 4 lines

Now ask what that script is allowed to do, without running it:

./frost --explain hello.frost

This script runs date, and reads 1 file (system).

Runs these programs:
  date  at line 1  (no timeout)

Reads these files:
  /etc/hosts  at line 4

Verdict: clean

Then try to break it. This filename is a shell injection payload:

cat > risky.frost <<'END'
put "notes.txt; rm -rf *" into filename
run "touch" with filename
run "ls" with "-1"
put it
END

./frost risky.frost

hello.frost
keep_me.txt
notes.txt; rm -rf *
risky.frost

A file was created with that literal name. Nothing was deleted. The same two lines in bash would have emptied the directory.

Reviewing

frost --explain deploy.frost      # everything it can do, before it runs
frost --brief scripts/            # one line each, for a log with forty
frost --check --strict deploy.frost   # exits 1 on a dangerous verdict
frost diff HEAD~1 deploy.frost    # did this rewrite widen what it can do?
cat buffer.frost | frost --check -    # a buffer that is not on disk yet
frost --watch deploy.frost        # re-review on every save

A frost.policy beside the script, or above it, is found and applied. The path used is printed, and --no-policy opts out.

Starting a project

frost init

Writes main.frost and frost.policy beside it, and prints the three commands worth running next. The policy is generated from the script, so the two agree before you have typed anything: every later edit is measured against something that was true once.

It will not overwrite either file.

In a pipeline

As a pre-commit hook:

repos:
  - repo: https://github.com/keithadler/frost
    rev: v0.10.0
    hooks:
      - id: frost-check

frost-check runs --check --strict, which exits 1 on a dangerous verdict. frost-explain, frost-policy and frost-format are there too.

In a container, for CI without a Python setup step:

docker run --rm -v "$PWD:/work" ghcr.io/keithadler/frost:latest \
  --check --strict deploy.frost

It reviews scripts. It is a poor place to run one, deliberately: the image has none of the tools a real script calls, and an image built for reviewing should not become the place things execute.

Giving it to an agent

Two commands, for the two halves of the loop.

frost context prints what a model needs in order to write frost: the statement forms, the reserved words taken from the parser itself, and the constructs frost deliberately lacks, since the mistakes a model makes are ${x}, backticks and an invented let. It is a few thousand characters, which is the point: LANGUAGE.md argues a case across thousands of lines and is the wrong document to paste into a context window. The same text is committed as MODEL-CONTEXT.md, and every snippet in it is parsed by the test suite, so it cannot teach a form that does not work.

frost mcp serves the review tools over Model Context Protocol on stdio: frost_check, frost_explain, frost_policy, frost_diff and frost_grammar. Add it to Claude Code with:

claude mcp add frost -- frost mcp

or by hand, in claude_desktop_config.json or any other MCP client:

{
  "mcpServers": {
    "frost": {
      "command": "frost",
      "args": ["mcp"]
    }
  }
}

It cannot run a script, and it reads no files. That is the design rather than a gap. frost exists because a machine writes the script and a person decides whether it runs; a server that executes on request moves the decision back to the machine, and a tool that took a file path would let whatever holds the other end of the pipe read anything the process can reach. A refusal comes back with the rule that fired and a refusal to draft the widening, because an agent handed the exact edit that clears its own refusal has been handed the instructions for widening its own bounds.

Tooling

Scratchpad: the fastest way to understand chunk expressions:

frost --try

frost> the third word of line 2 of it
/api/login
frost> the number of lines in it
7
frost> every match of "\d+" in the first line of it
5 items: 10, 0, 0, 1, 200

The scratchpad is the same thing in a browser, with the text editable and a set of worked examples. Its evaluator is a second implementation, and tools/verify_chunks.py runs 1,288 expressions through both it and the real interpreter on every build, the page is not written if they disagree.

Formatter: canonical layout, comments preserved:

frost --format script.frost          # print
frost --format --write script.frost  # rewrite in place

It refuses to format a script that does not parse, is idempotent, and produces an identical parse tree, so it cannot quietly change meaning. The examples in this repo are its style reference, and a test fails if any of them drifts.

Editor support: editors/ holds a TextMate grammar and a VS Code manifest, generated from the parser so the highlighted keywords cannot drift from the real ones, with indent rules taken from the formatter so typing does not fight --format.

For code-generating models: MODEL-SPEC.md is a compact reference sized for a system prompt. Point a model at it and it will emit frost instead of bash; then --explain and --policy check the result before it runs.

Usage

frost script.frost [args...]     run a script
frost --check script.frost       parse only, report errors
frost --ast script.frost         dump the syntax tree
frost --trace script.frost       print each statement as it runs
frost --trace-to-file F s.frost  write that trace to a file instead
frost --run-id ID s.frost        name this execution (else FROST_RUN_ID)
frost --automated s.frost        unattended: refuse anything that widens
frost --events F s.frost         one JSON object per event (- for stderr)
frost --events-format otel s.frost   OTLP/JSON traces instead of NDJSON
frost --deadline N s.frost       stop the whole run after N seconds
frost --enforce-hosts s.frost    check a command's real destination at spawn
frost --egress-rules squid s.frost   the allow-list, as proxy configuration
frost --new-approver-key F       a signing key for approvals
frost --approve --sign-with F s.frost   an approval somebody is accountable for
frost --check --sarif s.frost    findings for code scanning on a pull request
frost --policy-from s.frost      a starter policy describing what it does
frost --explain --against F s.frost   what changed since it was approved
frost --exit-codes [--json]      what each exit status means
frost --completion bash|zsh      a completion script
frost --explain script.frost     describe what it can do, without running it
frost --check --json s.frost     diagnostics as JSON, with repairs
frost --repair [--write] s.frost apply the repairs frost is sure about
frost --lock s.frost             record the sha256 of every module
frost --frozen s.frost           refuse to run if a module changed
frost --approve s.frost          record what it may do, in <script>.approved
frost --as-approved s.frost      insist an approval exists, and match it
frost --ignore-approval s.frost  run despite <script>.approved
frost --record run.json s.frost  run it and write down everything it did
frost --replay run.json s.frost  run it against a recording, spawning nothing
frost --policy p --sandbox s.frost   hold the declared boundary at runtime
frost --explain --json s.frost   the manifest, as JSON
frost --policy rules.policy s.frost   enforce rules, then run if it passes
frost --try [subject.txt]        scratchpad for chunk expressions
frost --keystore F --role R s.frost   run with access to secrets
frost keystore init|set|get|list|grant|revoke|roles
frost --format [--write] s.frost canonical layout
frost --version

frost's own options end at the script path; everything after it belongs to the script, so frost report.frost --check passes --check to the script.

Exit status is the contract:

  0  ran, or answered a question
  1  the script failed, or --explain judged it dangerous
  2  could not read or parse it, or the arguments were wrong
  3  a policy refused it; nothing was run
  4  a replay diverged from its recording
130  interrupted

Layout

frost                 executable entry point
frostlang/
    lexer.py          tokeniser
    parser.py         recursive-descent parser
    ast.py            node definitions
    interp.py         tree-walking evaluator
    audit.py          capability manifest, danger checks, policy engine
    program_audit.py  the same, across an imported closure
    modules.py        import resolution, ceilings, lockfile
    baseline.py       what a script was approved to do
    runid.py          one identity per execution
    site.py           policy the host brings, and its provenance
    telemetry.py      events for a monitoring system
    egress.py         the allow-list, as proxy configuration
    signing.py        approvals bound to an approver and a commit
    sarif.py          findings for code review tools
    scaffold.py       a starter policy from a manifest
    browser.py        the analysis surface with no OS underneath
    sandbox.py        capability boundaries the kernel holds
    sealed.py         values that cannot be printed by accident
    keystore.py       role-gated envelope encryption
    keystore_cli.py   the frost-keys command
    journal.py        record and replay
    diagnostics.py    structured findings and repair payloads
    structured.py     records, and the bridge to JSON
    formatter.py      canonical layout
    repl.py           the --try scratchpad
    cli.py            driver and error reporting
examples/             runnable scripts
tests/                2267 tests, python3 -m pytest tests/ -q
    gen.py            generates valid frost, for the property tests
    golden/           recorded --explain output for every example
LANGUAGE.md           full reference and grammar
docs.html             browsable docs (tools/build_docs.py)
audit.html            visual audit report (tools/build_audit.py)
play.html             live scratchpad (tools/build_play.py)
MODEL-SPEC.md         prompt-sized reference (tools/build_model_spec.py)
PLATFORM.md           running frost as a platform control
action.yml            the GitHub Action, tested against this repo
web/chunks.js         browser evaluator, verified against frostlang/
canary_browser.py     boots play.html in Chromium and checks its answers
build_site.py         assembles what GitHub Pages publishes
action.yml            the GitHub Action
editors/              syntax highlighting

Status

Version 0.10.0. The language runs, the examples are real, and 2267 tests cover lexing, parsing, chunk semantics, pattern matching, timeouts, process execution, pipe failure, static analysis, policy enforcement, and the injection property.

The gaps this file used to list are closed. In the order they were listed:

• Handlers write globals with put 99 into the global total, and global is reserved so the near-miss is an error rather than a local of that name.
• run "sort" reading names puts text on a program's standard input.
• ensure ... end ensure releases what a script took, whether it finished, failed, quit, or was interrupted.
• Lists are real: the words of X, split by, joined by, the sorted X, and put "c" after names appends an element rather than concatenating.
• String and number functions: the uppercase X, the trimmed X, the rounded X, the sum of X.
• run "make" in folder build path, and put "clang" into the environment variable "CC".
• run "make" showing output hands the terminal to the child, for long builds and interactive programs.
• Handlers are callable in expressions: the double of 5.
• Modules: use "lib/db.frost" for the connect which may run "psql", declarations only, with a capability ceiling at the import site and --lock/--frozen to pin the bytes.
• Streaming input: repeat for each line in the standard input consumes lines as they arrive, so a frost filter works against a producer that never ends.
• Quantitative policy: counts, ranges and limits, with units reconciled and a hint on every rule.
• Secrets: a role-gated keystore, sealed values, and redaction that survives concatenation.
• Runtime confinement: --sandbox, held by sandbox-exec on macOS and bubblewrap on Linux, verified confining in CI on both.
• Record and replay: --record writes a committable fixture; --replay spawns nothing.
• Telemetry: --events writes NDJSON for a monitoring system, with every command timed, every refusal reported, and secrets redacted.
• A datacenter's own rules: /etc/frost/policy.d/*, applied to every run, composing so they can only narrow, and named by digest in the manifest and the recording.
• Approvals somebody signed: Ed25519, bound to an approver and a commit, verified against keys a policy names.
• An automation guard: --automated refuses anything that would widen what a script may do.
• Records and JSON: the json of it, the "status" of report, nested fields, the json text of: with sealing preserved in both directions.
• Captured standard error: the error output, beside it and the result.
• A clock and wait: both recorded, so a replay is still deterministic and still fast.

Remaining, honestly:

• --explain reads what is derivable, not what is computed. A name whose definitions are all literals is followed through, including when they differ a branch picking one of two hosts is reported as two hosts. A host is read out of a joined URL when the literal closes the authority, so "https://api.github.com/repos/" & repo reaches api.github.com. A value genuinely assembled at runtime is still reported as unknowable rather than guessed, which is why --sandbox exists: the kernel confines what the analyser could not resolve.
• A declared shape is one level deep. with fields "a", "b" checks the top level of a record. A nested shape has to be declared by pulling the inner record out into its own name first.
• OTLP is a batch format, so a killed run leaves no trace. --events-format otel writes its document when the run ends. NDJSON is the default because it streams, and a run killed halfway still leaves every line up to the moment it died.
• No compile-to-bash mode for machines without frost installed. The interpreter is a tree walker; a bytecode pass would be straightforward if process spawn ever stopped dominating the runtime, which it will not.
• Per-host rules are checked in two places and enforced in a third. The static check reads the script; --enforce-hosts reads a command's real arguments in the moment before it spawns, which is the only place a computed URL can be judged; and --egress-rules squid writes the allow-list as configuration for the proxy that actually holds it. frost holds the first two, and a program that ignores its arguments is stopped only by the third. forbid reaching "*.telemetry.example" and require reaching only "api.github.com" are policy rules, checked against the text before anything runs, and an unknowable destination fails them closed. The sandbox is still all-or-nothing: sandbox may reach "api.github.com" remains a parse error, because macOS filters on addresses and a Linux namespace has no middle setting. Two different guarantees, and the one the kernel holds is the weaker of the two here.

The name

Robert Frost was born in San Francisco in 1874, and is remembered for poems a person can read on first pass and not exhaust on the tenth. Plain surface, real weight underneath, nothing you have to decode before you are allowed to judge it.

That is the trick this language is after. A script written by a machine and read by a person at 3am has to be legible immediately and still hold up when somebody reads it properly the next morning. Terseness was a virtue when a human typed every character into a serial terminal. It stopped being one when the writing got cheap and the reading became the whole job.

Making machine-written code reachable by the humans accountable for it is the entire point. The poet is a better model for that than the shell ever was.

License

MIT. See LICENSE. Do what you like with it; keep the copyright notice.

Version 0.7.0

frost is a scripting language for the jobs shell scripts do, with a grammar descended from HyperTalk. It exists because the economics changed: scripts are now written quickly and read carefully, often by someone who did not write them and is reading at 3am while something is broken. Terseness was the right optimisation for a serial terminal. It is the wrong one for a code review.

frost is not a login shell. It is an interpreter you point at a file:

#!/usr/bin/env frost

That choice is deliberate. Because nobody types frost at a prompt forty times a day, there is no pressure anywhere in the design to shorten anything.

---

---

1. Design rules

Four rules explain nearly every decision in the language.

Values never become syntax. There is no string interpolation and no eval. Arguments to a program are a list, passed directly to execve. A variable holding notes.txt; rm -rf * produces a file with that literal name. Injection is not mitigated in frost; it is unrepresentable.

Failure stops the script. run aborts on a non-zero exit unless you wrote try to run. There is no set -e to remember, because there is no other mode.

Pipes fail if any stage fails. Not just the last one. bash's default here has probably caused more silent data loss than any other decision in shell design.

The keyword vocabulary is small and closed. This is what makes error count a legal variable name, and it is also the discipline that keeps frost from pretending to understand English. put the first line of X into Y works; grab the first line from X does not, and says so clearly.

---

2. Lexical structure

Line-oriented. One statement per line. Newlines are significant.

Comments run from -- or # to end of line.

-- This is a comment.
# So is this.

Line continuation. A trailing backslash folds the next line in. An argument list may also wrap after a comma without one.

run "rsync" with "--archive", "--verbose", \
    source folder, destination folder

Case-insensitive keywords and names. Put, put, and PUT are the same word. Names are normalised to lowercase internally.

Shebang. A first line beginning #! is ignored.

String literals use double quotes and support \n, \t, \", \\.

---

3. Values

frost has four value types.

Type	Examples	Notes
Text	"hello", empty	empty is the empty string
Number	42, 3.14, -7	Integers stay integers
Truth	true, false
List	the arguments	Produced by the language, not written literally

Conversion is automatic and predictable. "10" is greater than "9" is true, because both sides look like numbers, so both are compared as numbers. If either side does not look like a number, both are compared as text.

Truthiness: false, 0, "", "false", and "no" are false. Everything else is true.

---

4. Variables and names

A name is one or more ordinary words separated by spaces.

put 0 into error count
put "release-1.4" into tag name
put the current folder into working directory

This is the reason the keyword list is short. A word is available for use in a name unless it is structurally load-bearing. Chunk nouns (line, word, item, character) are not reserved, line count is a perfectly good variable, because frost recognises chunk expressions by context rather than by reserving the noun.

Assignment is put ... into .... Reading a name that was never assigned is an error, not an empty string:

Error at report.frost:12
    12 |     put total cost
       there is no variable named 'total cost'
       hint: assign it first with:  put ... into total cost

You can also build text onto an existing variable:

put "world" into message
put "hello " before message      -- "hello world"
put "!" after message            -- "hello world!"

---

5. Chunk expressions

This is the centre of the language. Chunk expressions replace cut, awk '{print $3}', sed -n '7p', head, and tail with one uniform grammar that reads the same everywhere.

Four chunk nouns: character, word, line, item.

• word splits on whitespace
• line splits on newlines
• item splits on commas, trimming surrounding spaces
• character splits into single characters

By ordinal

put the first word of headline
put the third line of report
put the last item of csv row
put the middle word of phrase
put any line of quotes file        -- picks one at random

Ordinals run first through tenth, plus last, middle, and any.

By number

put word 3 of headline
put the line 7 of report
put word -1 of headline            -- negative counts from the end

By range

put words 2 to 4 of headline
put lines 10 to 20 of report
put characters 1 to 3 of name      -- "fro"

A range is rejoined with the natural separator for its kind: words with spaces, lines with newlines, items with ", ".

Counting

put the number of lines in file "access.log"
put the number of words in this sentence
put the length of name             -- characters, or list elements

Nesting

Chunk expressions compose, and this is where they pay for themselves:

put the third word of line 7 of file "access.log"

The bash equivalent is sed -n '7p' access.log | awk '{print $3}', which requires knowing two tool dialects. The frost version requires knowing English.

Records and JSON

A record is an ordered mapping from text keys to values, and JSON is how one usually arrives. Objects become records, arrays become the lists frost already has, and the scalars map the obvious way:

run "curl" with "-fsS", "https://api.example.com/build" within 30 seconds
put the json of it into build

put the "status" of build
put the "name" of the "author" of build
put item 1 of the "tags" of build
put the "attempt" of build + 1

This exists because the alternative was run "jq" with ".status", which puts a second language in a file whose whole argument is that it needs only one, and hands the auditor a string it cannot see into. --explain could say the script runs jq; it could never say what jq was asked for.

Build one field at a time. The first assignment creates the record, so no declaration is needed:

put "green" into the "status" of summary
put 2 into the "failures" of summary
put the json text of summary into file "report.json"

the keys of R and the values of R are lists. A record printed with put prints as JSON, because a record that printed as its type name would send you straight back to jq to look at your own data.

Missing keys. Asking for a key a record does not have yields empty, exactly as word 99 of does, and a field of empty is empty, so the "name" of the "user" of build is safe on a payload with no user. Asking for a field of text is an error: that means the value is not the shape the script thinks it is, and empty would hide the bug while it is still cheap to find.

Secrets survive the round trip. Parsing a sealed value seals every field it produces, and serialising redacts field by field rather than all at once:

put the json of the secret file "credentials.json" into config
put "connecting as" && the "user" of config
-- connecting as «secret credentials.json»

Declaring the shape of a record

A missing key is empty, which is right for an optional field and lethal for a typo. the "staus" of build reads as empty, the comparison against it quietly goes the wrong way, and nothing anywhere says that staus was never a field.

So say what shape you expect:

run "curl" with "-fsS", url within 30 seconds
put the json of it into build with fields "status", "number", "author"

Two things follow from that one line.

The names become checkable. the "staus" of build is now a mistake --check catches, with the near miss suggested as a repair, before a single process starts.

The payload is verified where it arrives. If the response does not carry number, the script stops at the line that parsed it and says what was there instead, rather than failing somewhere further down wearing a different disguise.

Only names that were declared are checked, and only against a literal key. The declaration is your claim about the payload, so checking against it is checking the script against itself. Inferring a shape from whatever JSON happened to arrive during development would reject correct scripts, so frost does not do it. A record with no declaration is not second-guessed at all.

A name reassigned without a claim loses its shape, and a shape declared inside a block does not leak out of it. Field names must be written out: one built at runtime could not be checked, which is the whole point of declaring it.

Standard error

the error output is what the last command wrote to its standard error, the same way it is what it wrote to standard output and the result is its exit status:

try to run "curl" with "-fsS", url within 30 seconds
if the result is not 0 then
    put "curl failed:" && the error output into standard error
    quit with status 1
end if

Without this the only way to see why something failed was run "sh" with "-c", "... 2>&1": which reintroduces the shell frost exists to avoid, and which the auditor flags. Needing an error message should not require defeating the language's main guarantee.

A command's standard error is still written through to the terminal as it happens, so a failure is never silent whether or not anything reads it. In a pipe, the error output is the last stage's; the earlier stages write straight to the terminal.

Time and waiting

put the current date          -- 2026-08-10
put the current time          -- 14:23:05
put the current timestamp     -- 2026-08-10T14:23:05Z   (UTC)
put the current seconds       -- epoch seconds, for measuring a duration

wait 3 seconds
wait 500 milliseconds

The unit is required, for the same reason within requires one: a bare 3 means seconds to one reader and milliseconds to another.

Both are recorded. --replay serves back the clock reading that was recorded rather than reading the clock again, a fixture whose timestamps move on every replay is a diff generator, not a fixture: and it does not actually sleep, so replaying a script that backs off for thirty seconds is no slower than replaying anything else.

A script that waits says so in --explain, under Waits:. It is not a capability, since it touches nothing, but a reviewer approving a CI job wants to know it sleeps for ten minutes.

The identity of a run

put "starting" && the run id
run "curl" with "-H", ("Idempotency-Key: " & the run id), url within 30 seconds
put "build/" & the run id & "/out.txt" into scratch

A script run by an agent or a pipeline is never asked "what happened" in the abstract. It is asked what that run did: the one in the incident, the one whose fixture is on disk, the one an API saw a duplicate request from. So each execution has an id.

--run-id ID sets it, otherwise FROST_RUN_ID, otherwise frost generates a UUID. An id from outside always wins, because joining frost's record to the pipeline's is the whole point and a job id is more useful than anything frost could invent.

It reaches four places. The recording carries it at the top level, so a fixture can be joined to an audit log without being parsed. The trace opens with it. Every child process inherits it as FROST_RUN_ID. And the script can read it, which is what makes it usable as an idempotency key or as a scratch path that cannot collide with a concurrent run.

A replay reports the id of the run it is replaying, not a fresh one, for the same reason it serves the recorded clock: a fixture that changed on every replay would not be a fixture.

Ids are checked rather than trusted: letters, digits, dot, colon, dash and underscore, up to 128 characters. The value ends up in log lines, in child environments and in any path a script builds from it, so a newline in it would forge a log entry and a slash would move a file. That is the ordinary shape of trusting text from somewhere else, which is the thing this language exists to refuse.

Watching a run, and auditing one

Two different questions, and it is worth reaching for the right one.

What did it do? --record run.json writes down every effect: each command with its arguments, standard input, output and exit status, every file read or written, every environment variable, every clock reading. Secrets are never recorded. That is the auditable artefact, and it is a replayable fixture as well.

The recording is written however the run ends. A run that failed, was interrupted or wedged is exactly the one somebody needs to read afterwards, and for a while frost was throwing precisely those away.

What did it execute? --trace prints each statement as it runs, and --trace-to-file FILE puts that somewhere you can read later:

[frost]    1  put 0 into error count
[frost]    2  repeat for each line in the standard input as row
[frost]    3      if row contains "ERROR" then add 1 to error count

A recording cannot answer this. It holds effects, so a script that took the wrong branch and therefore did nothing produces an empty recording and no explanation. The trace shows the condition being evaluated and the branch not taken.

The trace is flushed line by line, because the run worth tracing is often the one that never finishes and a buffered trace of a wedged script is an empty file. It prints source text and never runtime values, so a credential cannot reach it.

Rules about what a script may run

forbid running "sudo"
require running only "git", "npm", "docker-*"
require at most 0 shell escapes

The deny form names what you fear. The allow form names what you trust, and it is the one that holds: a deny-list cannot be completed, because it is a list of the programs somebody thought of.

A program named at runtime is refused by the allow form rather than assumed to be on it. Assuming it passes is what makes an allow-list decorative, and it is the same answer require reaching only gives to a destination built at runtime.

require at most 0 shell escapes covers what forbid running "sh" cannot. That rule matches the program frost spawns, which for xargs sh -c is xargs, so it never fires on the indirect form. The count sees both, and both the count and the finding come from one detector so they cannot disagree.

Rules about where a script reaches

forbid reaching "*.telemetry.example"   -- no third-party reporting from here
require reaching only "api.github.com", "*.internal"

Checked against the text, before anything runs. That is possible because the analyser reads a host out of a joined URL when the literal closes the authority, "https://api.github.com/repos/" & repo reaches api.github.com, and nothing after the slash can move it, and because a name whose definitions are all literals contributes every one of them, so a branch choosing between two hosts is two known hosts rather than an unknown one.

A destination that genuinely cannot be read fails both rules. "Cannot be shown to be allowed" is not "is allowed", and an allow-list that quietly passed the one case nobody can check would be worth nothing.

Checking the destination that only exists at runtime

An allow-list refuses a destination it cannot read, which is right and makes the rule unusable for any URL built at runtime: a script that fetches a URL from its input can never satisfy require reaching only, so people delete the allow-list rather than the dynamic URL.

--enforce-hosts moves the check to where the value is concrete:

frost --enforce-hosts --policy prod.policy fetch.frost

The host is read out of the command's real arguments in the moment before it spawns, and a destination the policy does not allow is refused there. With the flag on, a statically unknowable destination is no longer refused up front, because it will be judged when it exists. A network command whose destination frost still cannot read at that point is refused, exactly as the static check does: cannot be shown to be on the list is not the same as is.

frost holds this, not the kernel. A program that ignores its arguments and dials out on its own is untouched by it. It closes the gap between a policy that could only refuse dynamic URLs and one that can permit the right ones; it does not become a boundary by doing so.

Writing the rules for the thing that can enforce it

frost --egress-rules squid --policy prod.policy > frost.acl

The allow-list a policy states, as configuration for a forward proxy. Both come from one file, so the list review reads and the list that actually holds cannot drift, which is the failure that makes host policies decorative.

A hostname allow-list is not something a packet filter can express: nftables and iptables match addresses, resolutions change, one address serves many names. Emitting an nftables ruleset from hostnames would produce a file that looks like enforcement and is wrong the first time a CDN rotates. A proxy sees the name, which is the layer where the rule can be stated exactly. --egress-rules list gives one host per line for whatever configuration already exists.

This is not the sandbox. sandbox may reach "api.github.com" is still a parse error: macOS filters on addresses rather than names, and a Linux network namespace has no middle setting, so a per-host boundary is not something the kernel here can hold. A policy bounds what the text can reach; the sandbox bounds what the process can reach, all or nothing. Keeping those apart matters, because the second is the stronger guarantee and the first is the more precise one, and pretending either is the other would be a lie in whichever direction someone relied on it.

Code that cannot run, and code nobody uses

A script written by a machine has a shape. Invented helpers, a branch after a return, a value computed and dropped: each is harmless on its own, and together they are the clearest sign that what is on the page is not what anybody intended.

[caution] line 9   Code after the script has already stopped
[note   ] line 1   The handler 'helper' is never called
[note   ] line 5   'error count' is set and never read

Three separate questions, all decidable from the text. Unreachable: statements after a quit, return, exit repeat or next repeat in the same block, reported once per block rather than once per line, because a run of ten dead lines is one mistake. Never called: a handler used nowhere in the program: across an import, not within one file, since defined here and called there is the normal shape of a module. Never read: a name assigned and never used, where reading it by any route counts, including add 1 to n, which reads the value before writing it.

These are notes rather than dangers, except the unreachable one. Dead code is a smell, not a hazard, and a verdict that shouted about an unused variable would be a verdict people stop reading. A policy can still refuse it: require at most 0 dead code.

Rules about the environment

forbid reading the environment "AWS_*"       -- credentials come from the keystore
require reading only the environment "PATH", "HOME", "CI"

Setting a variable already had a rule and reading one did not, which is the wrong way round: what a script takes from the environment is where the credentials are. Both forms exist now, and they are separate from forbid setting, because a policy that looked like it covered reads while only covering writes would be worse than one that plainly does not.

A variable named at runtime fails an allow-list closed, on the same rule as everywhere else: cannot be shown to be allowed is not allowed.

Loops that cannot end, and runs that will not

within 30 seconds bounds one command, and a policy can bound how many commands there are. Neither touches a loop doing arithmetic, which spawns nothing, reads nothing and writes nothing. It has no capabilities at all, so the manifest had nothing to report and reported it approvingly:

This script does nothing observable.

Verdict: clean

That script never terminates. The cheapest way for a generated script to wedge a runner was the one thing frost called harmless.

A loop with no way out is now a finding. repeat forever, repeat while true and repeat until false are checked for anything in the body that could end them: an exit repeat, a quit, a return.

[DANGER ] line 2  A loop that cannot end (repeat forever)

Presence counts, not reachability. An exit repeat behind a condition that never fires still counts, which understates the problem and never overstates it. That is the right way round for a check which would otherwise flag working code and be switched off. A condition that is not literally true is left alone, because repeat while n is less than 10 may well terminate and guessing is how a check earns a reputation for crying wolf.

A budget bounds the whole run.

frost --deadline 300 deploy.frost

require the run to finish within 5 minutes

The run stops when the budget is spent, exiting 124, what a shell reports for a timeout, and what frost already returns when a single command runs too long. The same answer to the same question at a different scale.

It is raised rather than killed, so ensure blocks still run. A deadline that skipped cleanup would leave exactly the mess it was meant to bound. The tightest budget wins, so a flag cannot widen what a site policy imposed.

Telling a monitoring system what happened

frost --events run.ndjson deploy.frost      # or - for standard error

One JSON object per line, flushed as things happen. NDJSON is what Splunk's HTTP collector, New Relic's log API, Datadog, Vector and Fluent Bit all ingest without a translator, and a line-oriented file survives a run that is killed halfway, which a single JSON document does not.

Every event shares an envelope: ts, run, script, seq, event. A collector routes on event and groups on run without knowing anything else about frost.

{"event": "run.start",      "declares": {"programs": ["curl"], "hosts": ["x.example"]}}
{"event": "command.start",  "program": "curl", "argv": ["curl", "-fsS", "..."]}
{"event": "command.finish", "program": "curl", "status": 0, "seconds": 0.412}
{"event": "run.finish",     "status": 0, "commands": 3, "waited_seconds": 2.0,
 "programs_unused": ["psql"], "hosts_unused": ["db.internal"]}

The resolution worth having is the pairing. Any tool can log that a command ran. frost knows what the script was allowed to do before it ran, what a person approved, and what the host permits, so the finish event can say which approved capabilities went unused. A script approved for six programs that uses two is an approval that should be tightened, and that is only visible holding the manifest and the run side by side.

Commands are timed, and the run separates time spent working from time spent waiting, so a slow job can be attributed rather than guessed at.

Contents are never emitted, sizes are: "wrote 4kb" is useful and the 4kb is not. Secrets are redacted before an event is written, including in a command's arguments, because telemetry leaves the building more often than a recording does.

Traces, where a dashboard wants them.

frost --events run.json --events-format otel deploy.frost

OTLP/JSON, which New Relic and Datadog read natively: a root span for the run and a child span per command, so a slow job renders as a flame graph rather than a table. The instrumentation already existed, because a command has a start, an end and a status, which is a span with the labels changed.

NDJSON stays the default and the trade-off is worth knowing rather than discovering. OTLP is a batch format, so the document is written when the run ends: a run killed hard leaves nothing, where NDJSON would have left every line up to the moment it died.

Trace ids are derived from the run id, so a replay of a recording produces the trace id of the run it replays. Same reason the clock is recorded: a fixture whose identity moves is not one.

A refusal is an attribute on the root span rather than an error status by itself, because a monitoring system that cannot tell a refused run from a broken one will page for the wrong one.

A refusal is an event. The one a security team most wants, and the one that is easiest to lose: a policy refusal, a breached import ceiling, an approval that no longer covers the script, an unusable signature, a secret the role may not read. Each closes the run out with what fired and which policy it came from, so a dashboard counting starts against finishes does not drift every time a policy does its job.

{"event": "run.finish", "status": 3, "refused": "policy",
 "rules": [{"what": "running \"curl\"", "line": 1,
            "hint": "egress goes through the proxy"}],
 "policies": [{"path": "/etc/frost/policy.d/00-egress.policy",
               "sha256": "05cd8a0c7c8f...", "origin": "site"}]}

Analysis produces no run events. --explain runs nothing, so there is no run to report on and a dashboard should not see one.

It composes with the other modes. --events alongside --record gives both, and a replayed run is marked "replayed": true so a dashboard does not count a fixture as production traffic.

Policy the machine brings

Rules in /etc/frost/policy.d/*.policy apply to every run on that host, whether or not anyone passed --policy. A policy beside the script is controlled by whoever writes the script, which is right for a project's own rules and useless as a datacenter control: the thing being constrained should not be holding the constraint.

Site rules are added to the project's, never replaced by them. That the result can only get stricter is not a special case, it falls out of how policy works: every rule is checked independently and all must pass, so two require reaching only lists compose as an intersection and there is no syntax that removes a rule.

There is deliberately no variable meaning "use this policy instead". A knob that relaxes a host rule is a bypass with a friendly name, and the first thing anyone does with a failing build is look for one. FROST_SITE_POLICY_DIR only adds a directory, for a container or a test without a writable /etc.

A site policy that is present and unreadable is a refusal, not a shrug.

Every policy that was applied is named by path and digest in --explain and in any recording:

Governed by:
  /etc/frost/policy.d/00-egress.policy  05cd8a0c7c8f  (site)
  deploy.policy                         9f2a1b40c7de  (project)

Without that, an audit can show a policy existed and never that this run was subject to it, which is a claim about a control rather than a control.

Runs nobody is watching

frost --automated deploy.frost        # or FROST_AUTOMATED=1

An automated run refuses --approve and --ignore-approval. A loop that can approve is a loop that approves its own capability escalation, and the failure is mundane rather than exotic: an agent hits REFUSED: it can now reach telemetry.example, and the most helpful-looking next step in its search is to re-approve. Everything else works normally, so a repair loop can fix syntax and can never fix its way past the gate.

Approvals somebody is accountable for

An unsigned approval says that something was approved. It does not say who approved it or what they were looking at, and anything that can write the file can grant itself the approval.

frost --new-approver-key ~/.frost/keys/alice
frost --approve --sign-with ~/.frost/keys/alice --approver alice \
      --commit $GITHUB_SHA deploy.frost

require an approval signed by "kA1b2c...", "kZ9y8x..."

The signature covers the capability set, the script, the commit, and the approver's own name and key, so it cannot be lifted onto somebody else. An approval signed by a key the policy does not name is refused, and so is an unsigned one.

Making signatures needs the keystore extra. Verifying never degrades: if a policy demands signed approvals and the library is missing, the answer is a refusal. An unverifiable signature is not a valid one.

The commit is recorded and not checked here, because frost cannot know which revision is being deployed, only which one the approver said they read. Comparing it against the checkout belongs in the pipeline that did the checkout.

Approving what a script may do

--frozen asks whether a script is byte-identical to the one that was reviewed. That is the right question for a vendored module and the wrong one for a script a model regenerates: every regeneration trips it, so you re-lock every time, and re-locking every time means the check has stopped telling you anything.

--approve asks the question that survives regeneration, did it get more powerful?

frost --approve deploy.frost

That writes deploy.frost.approved, a record of every capability the script has today: the programs it runs, the hosts it reaches, the files it reads, writes and deletes, the environment it touches, the secrets it reads and where they are released, and how many names it builds at runtime. No line numbers, so moving a comment does not move the baseline.

Once that file exists, it binds by default. No flag is needed:

frost deploy.frost

runs it only if nothing widened:

REFUSED: it can now run curl
REFUSED: it can now read the secret ~/.aws/credentials (from the file)
REFUSED: it can now let a secret leave the process as an argument to curl

3 capability change(s) since deploy.frost.approved; it was not run.
  Read what changed, then re-approve with --approve.

Binding by default is the difference between a guard and a suggestion. While it was opt-in, a poisoned agent did not have to defeat it. It just left the flag off, and in most agent loops the agent is the thing composing the command line. Skipping it now takes --ignore-approval: still possible, but a choice someone made and a reviewer can see, rather than what happens when nobody types anything. Deleting the approval file works too, and shows up in a diff.

--as-approved keeps a stronger meaning for CI: not honour an approval if there is one but there had better be one, refusing with exit 2 when none exists. --check, --explain and --format are never blocked. They are how you review the change, and blocking them would push people straight to --ignore-approval.

A capability that disappears is not reported and never refuses. The asymmetry is the point: a script that stops touching the network needs no ceremony, and one that starts needs a person. Re-approving prints what changed, so the moment of approval has something in it to read.

The baseline covers imported modules too, a capability that arrives through an import is still a capability, and measuring only the entry file would make "add an import" the way around this.

What it is for. Not injection: frost already stops a value becoming syntax. This is for the case where an agent reads something hostile and writes perfectly valid frost obeying it. The script parses, formats canonically and passes --check; the model was never confused about syntax, it was persuaded to use authority it legitimately holds. A policy file answers that properly, by being written by a person ahead of time: but a policy has to be written, and a baseline needs no rules at all. It compares against the reviewer's own past judgement instead of a security model they had to author.

Destinations are recorded, not just program names. Without that, curl https://api.github.com and curl https://telemetry.example are the same capability: and a persuaded model does not need a new program, only a new destination. The host is taken from literal arguments only: a scheme, or an scp-style user@host:path. A bare example.com is indistinguishable from a filename and is not guessed at, and a network command whose destination is not a literal is recorded as unknowable rather than omitted.

What it is not. A capability bound is not an intent check. A script allowed to run git can still push to the wrong remote, and a baseline that already includes curl and a readable config file will not object to one being sent to the other. This bounds what a script can reach, never whether reaching it was wise.

Streaming input

repeat for each line in the standard input consumes lines as they arrive rather than reading the whole stream first, so a filter works against a producer that never ends:

repeat for each line in the standard input as row
    if row contains "ERROR" then put the uppercase row
    if the number of characters in row is 0 then exit repeat
end repeat

exit repeat gets out of an unbounded stream and leaves the rest in the pipe for whoever reads next. Asking for the standard input as a value still reads what remains, all at once.

Folders, padding, durations and sorting by a key

if "build" is a folder then put "already there"

put "[" & the padded name to 14 & "]"          -- left aligned
put "[" & the padded count to 6 on the left & "]"   -- right aligned, for numbers

put "took" && the duration of 3725             -- 1 hour 2 minutes 5 seconds

put the sorted (the lines of it) by the second word of each

the padded X to N never truncates: a width is a minimum, and silently cutting a value to make a column line up loses the thing the column was for.

the sorted X by KEY evaluates KEY once per item with each bound to that item, and orders numerically when every key is a number, so a column of counts does not put 10 before 9. each means nothing outside a sort key and says so.

the duration of N is for reports and reads as a person would say it. It is not the formatter used in timeout messages, which says "90 seconds" because that is what the author wrote.

Out of range

Asking for a chunk that does not exist yields empty text, not an error. This matches HyperTalk and avoids a class of defensive length checks.

put word 99 of "short phrase"      -- empty, not a crash

---

5a. Lists

A plural chunk noun with no index is the whole set, as a list. Splitting is therefore not a new feature; it is the grammar already in section 5, read the other way.

put the words of "alpha beta gamma" into names
put the lines of report into rows
put the items of "a,b,c" into fields
put the characters of "abc" into letters

A list keeps its elements separate, which comma-delimited text cannot: an element may itself contain a comma.

put the empty list into people
put "Smith, John" after people
put "Doe, Jane" after people
put the number of items in people        -- 2
put item 1 of people                     -- Smith, John

after appends and before prepends when the target is a list; on text they still join, as in section 7.

Any delimiter

The chunk nouns cover whitespace, newlines and commas. For anything else:

put "root:x:0:0:root:/root:/bin/bash" into entry
put item 1 of (entry split by ":")       -- root

and back again:

put the words of "a b c" joined by ", "  -- a, b, c

split by "" is an error rather than a silent character split, because the characters of X already says that and says it more clearly.

Ordering

put the sorted names into ordered
put the reversed names into backwards
put the unique names into distinct

the sorted X compares numerically when every element is a number, and alphabetically otherwise. Sorting 10 before 9 is never what a counter meant. None of the three modifies the original.

---

5b. Functions

Text, applied with the article:

Expression	Result
the uppercase X	X in capitals
the lowercase X	X in lower case
the trimmed X	X without surrounding whitespace

Numbers:

Expression	Result
the rounded X	nearest whole number
the absolute X	magnitude, without the sign
the sum of X	total of a list of numbers
the largest of X / the smallest of X	extremes
the average of X	mean

if the lowercase target is "production" then
    put "deploying to production"
end if

put the sum of the words of "1 2 3 4"    -- 10

An aggregate of an empty list is an error, not zero: the sum of nothing is a question with no answer, and returning one would hide the empty list.

These are recognised only after the, so they cost nothing from the name vocabulary. sorted count and average total remain perfectly good variable names.

---

6. Operators

Text

Operator	Meaning	Example	Result
&	join	"a" & "b"	ab
&&	join with a space	"a" && "b"	a b

Arithmetic

+, -, *, /, ^, with conventional precedence. Parentheses group. Division by zero is an error.

Comparison

Both word forms and symbols are accepted. The word forms are preferred in scripts; the symbols exist so that dense arithmetic conditions stay readable.

Word form	Symbol
is	=
is not	!=
is greater than	>
is less than	<
is at least	>=
is at most	<=

Additional text comparisons, which have no symbol form:

if name contains "test" then put "looks like a test"
if path starts with "/usr" then put "system path"
if filename ends with ".tmp" then delete file filename
if answer is empty then quit with status 1
if branch name is in allowed branches then put "allowed"
if filename is like "*.tmp" then put "temporary"
if line matches "^\d+$" then put "all digits"

The article is optional on ordinal chunks, so first word of line and the first word of line are the same expression. Prefer the article in scripts; it reads better in a sentence.

Logic

and, or, not. Short-circuiting.

---

6a. Patterns

Two forms, because the two jobs are different.

Globs, is like

For filename-shaped matching, where regex is overkill and unreadable:

if filename is like "*.tmp" then delete file filename
if name is not like "test_*" then put "not a test"
if log is like "log-????-??.txt" then put "dated log"

* matches any run of characters, ? matches one, [abc] matches a set. Case-sensitive.

Regular expressions, matches

For everything else. frost does not pretend regex is readable; it makes it explicit instead, so a reader knows to slow down at exactly the places that deserve it.

if request matches "^(\S+) (\w+) (\d+)$" then
    put match 1 into client address
    put match 2 into method
    put the last match into status code
end if

A successful matches records its capture groups, which are then addressed with the same chunk grammar as everything else:

Expression	Meaning
match 1, match 2	Capture group by number
the first match, the last match	Capture group by ordinal
the number of matches	How many groups captured
the whole match	The entire matched text

Groups persist until the next matches. A failed match clears them, so the number of matches is 0 after a miss rather than stale. A group that did not participate in the match reads as empty, never as missing.

Patterns are searched, not anchored. Use ^ and $ when you mean the whole subject. Syntax is standard regular expressions; an invalid pattern is a clear runtime error naming the problem.

Finding every occurrence

put every match of "\d+" in request into numbers
put the number of items in numbers
put item 1 of numbers

Returns a list of the matched text, not of groups.

Replacing

put "2026-08-09" into date text
replace "(\d+)-(\d+)-(\d+)" with "\3/\2/\1" in date text
-- date text is now "09/08/2026"

replace edits a variable in place and replaces every occurrence. Backreferences are \1, \2, and so on.

A note on escapes

\n, \t, \r, \0, \" and \\ are translated in string literals. Every other backslash is preserved, so "\d+" reaches the pattern engine intact and needs no doubling.

---

7. Statements

put

put EXPRESSION                          -- writes to standard output
put EXPRESSION into NAME
put EXPRESSION into standard error
put EXPRESSION into standard output
put EXPRESSION into file "path.txt"     -- overwrites
put EXPRESSION after file "log.txt"     -- appends
put EXPRESSION before NAME              -- prepends to a variable

if

if COND then
    ...
else if COND then
    ...
else
    ...
end if

A single-line form exists for guards:

if attempts is 0 then quit with status 1

repeat

Five forms, all closed by end repeat.

repeat 3 times
    ...
end repeat

repeat with counter from 1 to 10
    ...
end repeat

repeat with counter from 10 to 1 by -1
    ...
end repeat

repeat for each line in file "hosts.txt" as this host
    ...
end repeat

repeat while queue depth is greater than 0
    ...
end repeat

repeat until file "ready.flag" exists
    ...
end repeat

repeat forever
    ...
end repeat

exit repeat leaves the loop. next repeat starts the next pass.

Arithmetic statements

add 1 to error count
subtract 5 from budget
multiply 2 into scale factor
divide 3 into portion size

quit

quit                    -- status 0
quit with status 1

---

8. Running programs

run "git"
run "git" with "status", "--short"
run "cp" with source path, destination path

The program name comes first. Every argument is a separate expression in the with list. The list is passed straight to the operating system. Nothing is re-parsed, so nothing needs quoting or escaping:

put "my report (final).pdf" into filename
run "cp" with filename, "/backup"

That works. There is no shell in the middle to be confused by the spaces or the parentheses.

Writing a whole command line as one string is a syntax error, caught at parse time with a suggested fix:

Syntax error at build.frost:4
     4 | run "ls -la"
       run takes a program name, not a command line
       hint: did you mean:  run "ls" with "-la"

Timeouts

Any run may carry a deadline:

run "curl" with "--silent", endpoint within 30 seconds
run "make" with "test" within 10 minutes
try to run "ping" with "-c", "1", host within 500 milliseconds

Units are required: within 5 is a syntax error, because a bare number could mean seconds or milliseconds and the reader should not have to guess. Accepted units: milliseconds (or ms), seconds, minutes, hours, singular or plural.

When the deadline passes the child is killed. A plain run then aborts the script; a try to run sets the result to 124, the same status GNU timeout uses, and leaves any partial output in it.

Pipes take a deadline for the whole chain, not per stage:

try to pipe within 1 minute
    run "find" with "/", "-name", "*.log"
    run "xargs" with "wc", "-l"
end pipe

if the result is 124 then
    put "search took too long" into standard error
end if

Putting within on an individual stage is a syntax error that says so. When a pipe times out, every stage is killed and reaped, no orphans.

Output

run captures standard output into it, with the trailing newline removed. Standard error passes straight through to the terminal. The exit code lands in the result.

run "git" with "rev-parse", "--short", "HEAD"
put "building revision" && it

To show a program's output, put it:

run "df" with "-h"
put it

Input

reading places text on a program's standard input. It is the heredoc, without a quoting dialect.

run "sort" reading names
run "wc" with "-l" reading log text

The text is data all the way down, exactly as arguments are: it is written to the child's stdin and never re-parsed as anything.

Where a program runs

run "make" with "all" in folder build path

The folder applies to that command only, so a reader need not track a mutable working directory while reading the rest of the script. To move the script itself, assign to the current folder: see section 11.

Watching a program work

run captures output, which means a ten-minute build shows nothing until it finishes and an interactive program cannot work at all. showing output hands the terminal to the child instead:

run "make" with "all" showing output within 30 minutes

Nothing is captured, so it is empty afterwards rather than stale. the result still reports the exit status, and failure still aborts.

These clauses may appear in any order, and each may appear once.

---

9. Pipes

A pipe is a block, not a connective. English has no graceful N-ary connective for this, so frost does not invent one, position carries the meaning.

pipe
    run "cat" with "access.log"
    run "grep" with "ERROR"
    run "sort"
    run "uniq" with "-c"
end pipe

put the number of lines in it into distinct errors

Advantages over a | b | c | d that matter in review: it reads top to bottom, it survives ten stages without becoming a wall, a diff shows exactly which stage changed, and you can comment one stage.

Every stage must be a run. A pipe needs at least two stages.

Pipes fail if any stage fails, and the result reports the first failure. This is set -o pipefail with no way to turn it off.

try to pipe
    run "cat" with "missing.log"
    run "wc" with "-l"
end pipe

put the result       -- 1, not 0

---

10. Failure handling

Plain run and pipe mean this must succeed. If it does not, the script stops and reports the line:

Error at deploy.frost:18
    18 | run "make" with "test"
       'make' failed with status 2
       hint: if this failure is expected, write 'try to run ...' and check 'the result'

try to means I will handle this myself, and is the only case where checking the result is meaningful:

try to run "ping" with "-c", "1", host name
if the result is not 0 then
    put host name && "is unreachable" into standard error
    quit with status 1
end if

The safe path is shorter to write than the risky one, and the risky one announces itself in the source. A reviewer looking for unchecked failures greps for try to.

---

10a. Cleanup

Failure aborts the script. That is the right default, and on its own it leaks every lock file and temporary directory the script had taken.

ensure registers a block when execution reaches it. Registered blocks run when the script ends: normally, on error, on quit, or on interrupt, most recent first, so cleanup unwinds in the reverse of the order things were acquired.

put "/tmp/deploy.lock" into lock path
put "held" into file (lock path)

ensure
    delete file (lock path)
end ensure

run "make" with "deploy"

The lock is released whether make succeeds or not.

A block that is never reached is never registered, so a lock taken inside a branch is only released if it was actually taken. A failure inside a cleanup block is reported on standard error and does not stop the other blocks, and it never replaces the error that ended the script. That error is the one the reader needs first.

---

11. Files

Reading a file is an expression:

put file "config.txt" into settings
put the first line of file "VERSION"

If the path is in a variable, parenthesise it. This is how frost tells file "x" from a variable named file path:

put item 1 of the arguments into log path
put the number of lines in file (log path)

Writing and appending are put targets:

put report text into file "report.txt"
put "another line" after file "report.txt"

Testing and removing:

if file "lock.pid" exists then
    quit with status 1
end if

delete file "lock.pid"

Missing files are a clear error, never silent empty text.

Relative paths resolve against the working directory. ~ expands.

The working folder and the environment

the current folder and the environment variable "NAME" are readable, and they are writable by the same put ... into that writes everything else. There is no separate set keyword.

put "/tmp/build" into the current folder
put "clang" into the environment variable "CC"
put ":/opt/bin" after the environment variable "PATH"

The environment is the script's own copy. A frost script cannot quietly rewrite the environment of whatever ran it, but every program it starts inherits the changes. --explain lists both, and a policy can forbid them, because otherwise setting PATH would be a way around every rule about which programs may run.

---

12. Handlers

A handler is a named block of statements.

to warn about with subject, detail
    put "WARNING:" && subject & ": " & detail into standard error
end warn about

warn about with "disk", "92% full"

The handler name may be several words. end must repeat the name, a rule that costs one word and makes long scripts navigable.

Handlers may return a value, which arrives in it:

to short revision
    run "git" with "rev-parse", "--short", "HEAD"
    return it
end short revision

short revision
put "at revision" && it

Parameters are local. Handlers do not see the caller's variables, and variables assigned inside a handler do not leak out. Argument count is checked at the call.

Reaching a global

A handler can read a global directly, but a plain put inside one creates a local, even when a global of that name exists. To write through, say so:

put 0 into error total

to record with status code
    if status code is not "200" then
        add 1 to the global error total
    end if
end record

the global NAME works for put, before, after, the arithmetic statements, and replace. It also reads past a local of the same name. Writing it at the point of the write, rather than declaring it at the top of the handler, means a reader never has to carry the declaration in their head.

global is a reserved word, so put 5 into global total is an error rather than a local named global total.

Calling a handler in an expression

A handler used with the ... of ... returns its value into the expression:

to double with n
    return n * 2
end double

put the double of 5 + the double of 10       -- 30

An argument binds tightly, exactly as a chunk source does: the double of n - 1 is (the double of n) - 1. Parenthesise when you mean otherwise.

A handler taking no arguments is called without of:

to short revision
    run "git" with "rev-parse", "--short", "HEAD"
    return it
end short revision

put "at revision" && the short revision

The expression form does not touch it; the statement form still lands there. An expression buried inside another expression must not quietly replace the last command's output.

A built-in property always wins: a handler named length does not shadow the length of X. An unknown name is reported when the script is parsed, not when the line runs, so --check catches a typo in a branch that rarely executes.

---

12a. Modules

Everything frost is worth using for rests on one invariant: the tree you audit is the program you run, and the audit sees all of it. A module system is the feature most likely to break that. If a module could contribute a run that --explain does not print, frost would be worse than bash, bash never claimed to have audited anything.

So the design goal is not safe modules. It is modules that cannot put capability outside the manifest, and every rule below is chosen for that.

use "lib/db.frost" for the connect, the migrate which may run "psql"
use "lib/text.frost" for the shout

A module is declarations only

A module file may contain handler definitions and use, and nothing else. A top-level statement in one is refused:

Module error at deploy.frost
       a module may only define handlers, and lib/bad.frost has a statement
       that would run when it is imported
       hint: move it into a handler.

Import-time side effects are the most abused feature of every module system ever shipped: Python's __init__.py, npm's postinstall: and they turn use into run this file. Refusing them means use can never do anything, which is what makes it safe to read the whole graph before deciding anything.

The path is written out in full

use (module name) is a syntax error, not a runtime one. A computed import is eval wearing a hat: it would put the import graph out of reach of the static analysis that every other guarantee in this document depends on.

Resolution is relative, and bounded

A path resolves relative to the file that imports it. There is no search path, no environment variable, no absolute paths, nothing above the entry script's own directory, no network and no registry. One path string resolves to one file, the same way on every machine.

The boundary is the entry script's directory, not the repository root, so a script in tools/ cannot reach a sibling lib/. That is restrictive on purpose: the directory a reviewer opens is the directory the program lives in. Vendoring is the feature: if a module has to live with the script, then the review that covered the repository covered the module.

Imports are explicit, and the graph is a DAG

for the connect, the migrate names exactly what arrives. Only those names are in scope; the module's other handlers are not. Two imports bringing in the same name is an error, as is an import shadowing a local handler, with a single flat table one would silently replace the other, which is a hijack rather than a hygiene problem.

Cycles are refused rather than resolved, and the whole closure is read exactly once: resolve, read, hash, parse, audit and run all come from the same bytes.

Names resolve in the file that defines the code

A handler defined in a module calls its own file's handlers, whether or not the entry script imported them:

-- lib/a.frost
to inner
    return "the module's own"
end inner

to outer
    return the inner
end outer

use "lib/a.frost" for the outer
to inner
    return "the entry script's"
end inner

put the outer          -- the module's own
put the inner          -- the entry script's

The manifest covers the closure

--explain audits every file, attributes each capability to the file it came from, and names the import it arrived through:

lib/db.frost   (imported by deploy.frost:1)
  Runs these programs:
    psql  at line 2  (no timeout)

A module's handlers are audited whether or not anything calls them, which is the sound direction. An unresolvable module fails closed, exit 2 and no manifest at all, because a manifest with a hole in it is the one output that would actively mislead a reviewer.

The ceiling at the import site

This is the part that makes single-file review survive multi-file code. A module defaults to no capabilities: pure computation, chunk expressions and the string and number functions. Widen it explicitly:

use "lib/db.frost" for the connect which may run "psql", "pg_dump"
use "lib/net.frost" for the fetch which may run "curl" and write "/tmp/*"

If the module does more than its import allows, the program is refused before anything runs:

REFUSED: lib/sneaky.frost may not run curl
  The module runs it, but the import does not allow it. The import at
  deploy.frost:2 allows: nothing but compute.

Two things fall out. A reviewer who reads only the entry file has a sound upper bound on what the entire program can do. And a shared module that later grows a network call breaks the build at the import site instead of quietly widening somebody's manifest, which is the supply-chain shape you become exposed to the moment modules can be shared at all.

The vocabulary is the policy language's, pointed inward: run, read, write, delete, set, read secret and change folder, each taking globs. A capability built at runtime always exceeds a ceiling, because a limit that cannot be checked is not a limit.

Pinning what you run

Modules open a window between the audit and the run that a single file never had. The closure being read once closes most of it; a lockfile closes the rest:

frost --lock deploy.frost      # records the sha256 of every file
frost --frozen deploy.frost    # refuses to run if any of them changed

REFUSED: lib/text.frost has changed since the lockfile was written

the program does not match its lockfile; it was not run.

Deliberately absent

No conditional imports, no re-export, no module-level state, no version solving, no namespacing beyond the file path. Every one of those exists to make a large dependency tree tractable, and a large dependency tree is the thing this review model cannot survive. Modules here are for sharing five handlers across four scripts in one repository.

---

13. Special values

Expression	Meaning
it	Output of the last run, pipe, or handler return
the result	Exit status of the last run or pipe
the arguments	Command-line arguments, as a list
the environment variable "NAME"	Environment lookup; empty if unset
the current folder	Working directory; writable
the standard input	Everything piped into the script, read once
the global NAME	A global, from inside a handler
the empty list	A list with no items
empty	The empty string

put item 1 of the arguments into target
put the number of items in the arguments into argument count
put the environment variable "HOME" into home folder

---

13a. Secrets

The failure this exists to prevent is not a malicious script. It is

put "connecting as" && token

in a generated script, running in CI, writing a credential into a log that is retained for a year and readable by everyone in the organisation. That mistake is made by being ordinary, not by being careless, so the fix is structural rather than a rule to remember, the same argument frost makes about injection.

Sealed values

Three expressions produce a sealed value:

put the secret "db password" into password          -- from the keystore
put the secret environment variable "GITHUB_TOKEN" into token
put the secret file "~/.ssh/id_rsa" into key

A sealed value refuses to become text. Every printing path in the language goes through one conversion, so put, joining, --trace, error messages and the scratchpad all redact without knowing secrets exist:

put "connecting as" && user && "with" && password

connecting as deploy with «secret db password»

Only the secret spans redact. If the whole line disappeared, people would route around the seal to keep their logs readable, and a mechanism people route around protects nothing.

The seal is contagious

A connection string built from a password is a password:

put "postgres://user:" & password & "@host/db" into url
put url                             -- postgres://user:«secret db password»@host/db

The same holds for chunks, split by, the transformations, and a value returned from a handler. Anything derived from a secret is a secret.

Where the plaintext is released

Where	What happens
put, put into standard error, --trace, errors	redacted
a released value echoed back by a program	re-sealed, so it redacts
a program's arguments	released
reading <secret>	released
put ... into the environment variable "N"	released
put ... into file "..."	released

The rule: streams redact, boundaries release. Printing is the accidental path and is closed. Handing a value to a program is a deliberate act, so it works, and --explain reports every place it happens.

Comparisons and measurements see through the seal, because the alternative is answering a different question:

if password is empty then quit with status 1
if token starts with "ghp_" then put "looks like a github token"
put the length of password

Equality on a sealed value is a constant-time comparison, so it cannot be turned into an oracle that recovers the value a character at a time.

When a program prints it back

A program handed a credential often echoes it. psql names the connection string in an error, a deploy tool prints the flags it was given, a health check reports the URL it called. That output used to come back into the script as ordinary text: sealed on the way out, plain on the way in, and printed by the next put that touched it.

frost now finds the released plaintext in what the child wrote and re-seals it in place:

put the secret "db_password" into pw
run "psql" with "--password", pw          -- released, and reported
put it                                     -- the password redacts

The value is preserved, not deleted. An earlier version replaced the plaintext with a marker, which closed the leak and broke every script that used the surrounding text: the error message you needed to read was still there, with a hole where the detail was. Re-sealing keeps the text intact and redacts it at the point of printing, which is where the leak actually happens.

Three limits, stated plainly.

Exact match only. It re-seals a secret frost itself released. It does not detect things that look like credentials. A mask that guesses at shapes fails in both directions, and the direction that matters is the quiet one: it gets trusted for the case it misses.

A derivation is not tracked through the child. If a program base64s the value before printing it, the encoded form is not recognised. Nothing at this layer could recognise it.

Inside a pipe, only the last stage's streams pass through frost. The intermediate stages are connected to each other directly, which is what makes a pipe a pipe, and frost never sees what crosses them.

The keystore

A keystore is a file holding encrypted values, each labelled with the roles that may read it. A script runs as exactly one role.

frost keystore init prod.keystore --role deploy
frost keystore add-role prod.keystore admin
frost keystore set prod.keystore "db password" --roles deploy,admin
frost keystore list prod.keystore
frost --keystore prod.keystore --role deploy release.frost

The names of the secrets, the roles, and who may read what are all stored in plaintext, because that is the part a reviewer needs. Only the values are encrypted.

Each role has an X25519 keypair whose private half is encrypted under a passphrase with scrypt; each value has a random data key sealed to every authorised role's public key, with AES-256-GCM throughout. The consequence worth knowing is that storing a secret and granting a role need no passphrase, only reading does. Somebody can add a credential for a role whose passphrase they do not have, which is the usual case.

This needs the cryptography package: pip install "frostlang[keystore]". The interpreter and everything else still have no dependencies, and the two environment-and-file forms of the secret ... work without it.

Refused before it runs

Which secrets a script asks for is a capability, so it appears in the manifest and is checked before anything executes:

$ frost --explain release.frost

Reads these secrets:
  db password  at line 4  (from the keystore)

Lets a secret leave the process:
  on the standard input of psql  at line 9

If the role cannot open a secret the script names, frost exits 3 and nothing runs, the same contract as --policy. A policy can also refuse outright:

forbid reading secret "prod/*"
require at most 2 secrets read
forbid any secret releases

What this does not do

It does not stop a script handing a secret to a program it was already allowed to run. Nothing at this layer can.

Once the plaintext reaches another program, frost cannot follow it into that program. What it can do is recognise the value coming back out, and it does: see below. The manifest still reports the release rather than pretending the seal survives the boundary.

Passing a secret as a command-line argument is reported as a caution for a second reason: arguments are visible to every other process on the machine while the command runs. reading <secret> is the better form and is not flagged.

The keystore is not a secret manager. There is no rotation, no expiry, no audit trail of reads and no network service. If you run Vault or SSM, use those. This exists for the many projects that run neither and keep credentials in a .env that nobody encrypts.

---

13b. Talking to the thing that wrote the script

Every error in this document is a sentence with a line number and usually a hint, which is the right output for a person reading at 3am. It is the wrong output for a model, which has to parse the English and guess an edit.

--json gives the same information as data:

frost --check --json report.frost

{
  "schema": 1,
  "ok": false,
  "exit": 2,
  "diagnostics": [
    {
      "severity": "error",
      "code": "missing-then",
      "message": "expected 'then' but found end of line",
      "line": 2, "column": 20,
      "source": "if error count is 0",
      "hint": "an 'if' condition is followed by 'then'",
      "repairs": [
        {"kind": "replace-line", "line": 2,
         "text": "if error count is 0 then", "confidence": "high",
         "why": "an 'if' condition is closed by 'then'"}
      ]
    }
  ]
}

It works with --check, --explain, --policy, and on a runtime failure, so one flag covers every way a script can be refused.

Repairs

A repair is an edit, not advice. Most come from information the front end already had, several hints in this document literally contain the corrected line, so handing it over as data costs nothing and saves a round trip.

Confidence	Meaning
high	a mechanical rewrite; the parser knew the answer
likely	the fix is right, a detail is inferred: which unit a timeout meant, where a missing end repeat goes
guess	a name that looks close to one that exists

frost --repair --write report.frost

applies high repairs only, and repeats until nothing is left that it is sure about: a recursive-descent parser stops at the first error, so fixing one reveals the next, and a single pass would give up on any script with two mistakes.

A pass is kept only if it made progress: the script now parses, or the first error moved strictly later. That is what makes the loop safe to run unattended. An error with no mechanical fix gets no repair at all, because a wrong repair costs a round trip and teaches the wrong grammar.

---

13c. Recording and replaying a run

What a script can do is knowable before it runs. What it did was not knowable at all, you ran it and watched.

frost --record run.json deploy.frost      # run it, write down everything
frost --replay run.json deploy.frost      # run it again, spawn nothing

A recording holds every command with its arguments, standard input, output and exit status; every file read and its contents; every environment variable read; and whatever was piped in. Replay performs nothing: no process is spawned, no file is written, nothing is deleted. The recorded answers are served back in order.

That makes a recording a fixture. Change the script, replay it, and every command it would run is compared against what it ran before, so a refactor meant to preserve behaviour either did or did not, and you find out without a database or a network.

A difference is reported rather than raised:

DIVERGED at deploy.frost:3
    the recording ran: echo two
    this run wants:  echo CHANGED

Matching is on the identity of the effect, which program with which arguments, which path: not on line numbers, so reformatting or adding comments replays clean. Exit status is 4 for a divergence, distinct from a policy refusal.

Secret values are never recorded, only their names, and any plaintext the run revealed is scrubbed from everything written down, including command arguments, which is where the first version of this leaked. So a recording is safe to commit, which is the point: a fixture you cannot check in is not a fixture. The scrubbing is exact-match, so a program that transforms a secret before printing it will defeat it; the manifest already reports that the secret was released there.

---

14. Grammar

EBNF. { } is zero or more, [ ] optional, | alternation.

program      = { statement NEWLINE } ;

statement    = put | run | try | pipe | if | repeat | quit | ensure
             | handler | return | arith | loopctl | delete | call ;

put          = "put" expression [ ("into" | "before" | "after") target ] ;
target       = "standard" ("output" | "error")
             | "file" expression
             | "the" "global" identifier
             | "the" "environment" "variable" primary
             | "the" "current" "folder"
             | identifier ;

run          = "run" expression [ "with" arglist ] { runclause } ;
runclause    = timeout
             | "reading" expression
             | "in" "folder" expression
             | "showing" "output" ;
timeout      = "within" expression timeunit ;
timeunit     = "millisecond" | "milliseconds" | "ms"
             | "second" | "seconds" | "minute" | "minutes"
             | "hour" | "hours" ;
arglist      = expression { "," expression } ;
try          = "try" "to" ( run | pipe ) ;

pipe         = "pipe" { pipeclause } NEWLINE run NEWLINE run { NEWLINE run }
               NEWLINE "end" "pipe" ;
pipeclause   = timeout | "reading" expression | "in" "folder" expression ;

ensure       = "ensure" NEWLINE block "end" "ensure" ;

if           = "if" expression "then"
               ( statement
               | NEWLINE block [ "else" ( if | NEWLINE block ) ]
                 "end" "if" ) ;

repeat       = "repeat" repeatspec NEWLINE block "end" "repeat" ;
repeatspec   = expression "times"
             | "with" identifier "from" expression "to" expression
               [ ("by" | "step") expression ]
             | "for" "each" chunknoun "in" expression "as" identifier
             | ("while" | "until") expression
             | "forever" ;
loopctl      = ("exit" | "next") "repeat" ;

quit         = "quit" [ "with" "status" expression ] ;
handler      = "to" identifier [ "with" identifier { "," identifier } ]
               NEWLINE block "end" identifier ;
return       = "return" [ expression ] ;
arith        = "add" expression "to" assigntarget
             | "subtract" expression "from" assigntarget
             | ("multiply" | "divide") expression "into" assigntarget ;
assigntarget = [ "the" "global" ] identifier ;
delete       = "delete" "file" expression ;
replace      = "replace" expression "with" expression "in" assigntarget ;
call         = identifier [ "with" arglist ] ;

block        = { statement NEWLINE } ;

(* expressions, loosest binding first *)
expression   = or ;
or           = and { "or" and } ;
and          = notexpr { "and" notexpr } ;
notexpr      = "not" notexpr | comparison ;
comparison   = concat [ compop concat | "exists" ] ;
compop       = "is" [ "not" ] [ "greater" "than" | "less" "than"
                              | "at" "least" | "at" "most" | "in" | "empty" ]
             | "contains" | "matches" | "is" "like"
             | "starts" "with" | "ends" "with"
             | "=" | "!=" | "<" | ">" | "<=" | ">=" ;
concat       = postfix { ("&" | "&&") postfix } ;
postfix      = additive { ("split" | "joined") "by" additive } ;
additive     = multiplicative { ("+" | "-") multiplicative } ;
multiplicative = unary { ("*" | "/" | "^") unary } ;
unary        = [ "-" ] primary ;

primary      = NUMBER | STRING | "it" | "empty" | "true" | "false"
             | "(" expression ")"
             | "file" ( STRING | "(" expression ")" )
             | "every" "match" "of" primary "in" primary
             | thephrase
             | chunk
             | identifier ;

thephrase    = "the" ( "result" | "arguments" | "current" "folder"
                     | "standard" "input" | "empty" "list"
                     | "secret" primary
                     | "secret" "environment" "variable" primary
                     | "secret" "file" primary
                     | "global" identifier
                     | "whole" "match" | "matches"
                     | "environment" "variable" primary
                     | "length" "of" primary
                     | "number" "of" chunknoun ("in" | "of") primary
                     | transform [ "of" ] unary
                     | aggregate "of" unary
                     | chunkplural "of" primary
                     | ordinal chunknoun "of" primary
                     | chunknoun index [ "to" index ] "of" primary
                     | identifier [ "of" unary { "," unary } ] ) ;

transform    = "uppercase" | "lowercase" | "trimmed" | "sorted"
             | "reversed" | "unique" | "rounded" | "absolute" ;
aggregate    = "sum" | "largest" | "smallest" | "average" ;

(* The last thephrase alternative is a handler called in an expression.
   Every form above it wins the name, so a handler cannot shadow a
   built-in property. An unknown name is rejected after parsing. *)

chunk        = chunknoun index [ "to" index ] "of" primary ;
index        = ordinal | [ "-" ] NUMBER | additive ;
ordinal      = "first" | "second" | ... | "tenth"
             | "last" | "middle" | "any" ;
chunknoun    = "character" | "char" | "word" | "line" | "item" | "match"
             | plural forms ;
(* for chunknoun "match" the `of X` tail is implicit: the last match *)

identifier   = WORD { WORD }   (* words not in the reserved set *) ;

Reserved words

These cannot appear inside a name:

add        after      and        are        as         at
before     by         contains   delete     divide     each
else       empty      end        ends       ensure     every
exists     exit       false      for        forever    from
global     greater    if         in         into       is
it         joined     least      less       like       matches
may        most       multiply   next       not        of
or         pipe       put        quit       reading    repeat
replace    return     run        showing    split      standard
starts     step       subtract   than       the        then
times      to         true       try        until      use
which      while      whole      with       within

Notably absent: line, word, item, character, match, file, status, error, output, count, name, path, result. These are recognised by position, so they remain available for names like line count, error count, match count, file path, and exit status.

---

14a. Reading a script before you run it

A frost script is parsed, not string-substituted, so everything it can do is visible in the tree. Two tools use that.

--explain

A capability manifest: what the script touches, without running it.

$ frost --explain cleanup.frost

Runs these programs:
  chmod  at line 13  (no timeout)
  curl  at line 12  (1 allowed to fail, no timeout)
  find  at line 4   (no timeout)
  rm  at line 8   (no timeout)

Writes these files:
  /etc/cleanup.state  at line 11

Can exit with:
  status 1

Only literals are reported. A program name or path assembled at runtime is listed as built at runtime and counted in a closing note, because a manifest that quietly omits things is worse than no manifest.

--explain also runs a set of built-in checks that need no policy file at all. They fire on every script:

Severity	Examples
danger	rm -rf, delete with a wildcard, sudo, chmod 777, dd, writing or deleting under /etc /usr /System, reading ~/.ssh .env *.pem, a shell escape via sh -c, a network fetch piped into an interpreter, and secrets read followed by a network call (the exfiltration pattern)
caution	recursive delete, a network command with no timeout, a try to run whose result is never examined, a program name built at runtime
note	any command that reaches the network, with the host named where it is a literal

The verdict is clean, caution, dangerous, or blocked, and --explain exits non-zero on dangerous so it can gate a commit hook.

--policy

Rules checked before a single process is spawned. If any rule is broken the script does not run, and frost exits with status 3.

forbid running "rm" with "-rf"
forbid running "sudo"
forbid writing to "/etc/*"
forbid deleting "/*"

warn running "curl"

require timeout on "curl"
require every command to be checked

$ frost --policy production.policy cleanup.frost

REFUSED: running "rm" with "-rf"
  cleanup.frost:8   run "rm" with "-rf", scratch folder
REFUSED: writing to /etc/cleanup.state
  cleanup.frost:11  put "cleaned" into file "/etc/cleanup.state"
warning: running "curl"
  cleanup.frost:12  try to run "curl" with "--silent", metrics url

2 rule violation(s); the script was not run.

Subjects are globs, so forbid writing to "/etc/*" covers the whole tree. forbid running "rm" with "-rf" matches the program and an argument, so ordinary rm still works, the rule targets the dangerous combination rather than banning a useful tool.

require every command to be checked understands the difference between an ignored failure and a handled one: a try to run whose result is examined in the next statement or two passes, while one whose failure is silently dropped does not.

The capabilities added since, setting an environment variable and changing the working folder: have rules of their own, because otherwise they would be a way around the rest of the policy:

forbid setting "PATH"
forbid setting "LD_*"
forbid changing folder

Saying why

A rule's trailing comment is its hint, and frost prints it when the rule fires. A refusal that says only no leaves the reader to guess what to do instead:

forbid running "sudo"          -- the deploy role already has the permissions it needs
require timeout on "curl"      -- an unbounded fetch wedges the whole pipeline
require at least 1 cleanup     -- every job must release its lock, even when it fails

REFUSED: running "sudo"
  deploy.frost:1  run "sudo" with "systemctl", "restart", "api"
  why: the deploy role already has the permissions it needs

There is no new syntax: policy authors already write that comment, so every policy that already exists gains the explanation for free. A comment on its own line is a section header and is not attached to any rule.

Counting rules

The rules above ask whether something appears at all. Business rules usually ask how much, so the same vocabulary counts:

require at most 12 commands
require at least 1 cleanup
require between 1 and 5 files written
forbid more than 2 runs of "curl"
forbid any files deleted

forbid more than N and require at most N are the same rule said two ways, as are forbid fewer than N and require at least N; use whichever reads better for the noun. forbid any X is a limit of zero. warn may replace forbid or require to report without blocking.

The countable nouns, singular or plural:

Noun	Counts
commands	every program the script can run
network commands	those that reach the internet
runs of "curl"	commands whose program matches a glob
files read, files written, files deleted	file access by path
environment reads, environment writes	environment variables touched
folder changes	assignments to the current folder
cleanups	ensure blocks
unchecked commands	try to run whose result is never examined
commands without a timeout	commands with no deadline
runtime names	program names or paths built at runtime
handlers	handler definitions
pipes	pipe stages

A count that is exceeded reports the line of the occurrence that crossed the limit, not the whole script. A count that falls short has no line to point at, so it is reported against the file.

The sandbox boundary

Every rule above is a statement about the text of a script, and every one of them is honest about its limits: a path built at runtime is reported as unknowable rather than guessed. That honesty is also the gap. Once the script runs, an unknowable path is a real path.

A boundary closes it. Declared in the same file, but allow-shaped, because a deny-list cannot become a sandbox, forbid writing to "/etc/*" says nothing about what writing is permitted:

sandbox may run "git", "make"
sandbox may read "*"
sandbox may write "build/*", "/tmp/frost-*"
sandbox may reach the network

frost --policy prod.policy --sandbox deploy.frost

A path the analyser could not resolve is still confined, because the confinement never needed it resolved.

Two enforcers, and the difference is real

Child processes are confined by the operating system: sandbox-exec on macOS, bubblewrap on Linux. Once a program runs inside one, frost is not in the loop: the kernel refuses the write. That holds even if the program is hostile, even if frost has a bug.

frost's own file operations are confined by frost. put X into file (path) never becomes a child process, so the check is a check in the interpreter, enforced by the same code being trusted to run the script at all. A weaker claim, and named differently here for that reason.

What it cannot do

Per-host network rules. The obvious thing to want is may reach api.github.com and nothing else. macOS's sandbox language filters on addresses, not names; a Linux namespace gives you a network or no network. A hostname allow-list needs a proxy, which is a different program. So network is all-or-nothing, sandbox may reach the network says exactly that, and

sandbox may reach "api.github.com"

is refused when the policy is read rather than accepted and quietly under-enforced. A boundary that does not hold is worse than no boundary, because somebody relies on it.

Platforms with no backend. If a boundary is declared and cannot be enforced here, frost refuses to run. It does not warn and continue. Before each run it also executes a real confined command that tries to write outside its boundary, and refuses if that write succeeds: present is not the same as working.

Anything a permitted program then does. A sandbox that may run git may run every git subcommand. Confinement bounds the blast radius; it does not read intent.

Bounded timeouts

require timeout on "curl" asks only that a deadline exists. A deadline of six hours satisfies it, and so does one of a millisecond, the first hangs the script, the second kills healthy work. So the bound can be given:

require timeout on "curl" of at most 30 seconds
require timeout on "*" of at least 1 second
require timeout on "*" between 1 and 120 seconds

Units are reconciled between the rule and the script: a policy written in seconds catches a script written in minutes, because within 2 minutes is already 2 * 60 in the tree and folds to 120 before the comparison.

A timeout computed at runtime cannot be checked ahead of time, and is refused rather than assumed acceptable, the same rule the manifest follows for a program name built at runtime.

Sensitive-path detection works on literal fragments, so a path assembled at runtime, file (home & "/.ssh/id_rsa"): is still recognised even though no whole-string literal ever appears in the source.

This is the part a traditional shell cannot offer. rm -rf "$DIR" is a string until the moment it executes, so there is nothing to inspect beforehand. In frost the program and its arguments are separate nodes in a tree, which is what makes a script checkable as a contract rather than trusted as a guess.

---

14aa. Bounding how much a run moves

A deadline bounds how long a command may take. It says nothing about a command that answers instantly with a gigabyte, and that is the cheaper mistake to make: cat of the wrong file, yes with no head, a log query with no date range. None of them looks like anything in a manifest.

require at most 10 megabytes of output
require at most 2 megabytes from one command
require at most 500 kilobytes written to files

frost --max-output 10MB --max-one-command 2MB --max-written 500kb deploy.frost

A policy and a flag both apply and the tighter of the two wins, so a flag can narrow a site limit and never widen one. Sizes are decimal: a megabyte is 1,000,000 bytes, because that is what the word means and a policy that says one thing and means another misleads exactly the person working out which limit fired.

The ceiling for any single command is whatever is left of the run budget. That is what makes the run budget real: a per-command limit on its own bounds nothing, since a script can stay under it and repeat.

The child is killed at the ceiling, not measured after it. frost reads the output as it arrives and stops the process when it passes the limit. A limit applied to captured output would report the overrun accurately and prevent none of it, which is the shape of a number that exists to be quoted rather than to hold. A file write is checked before the write happens, for the same reason: noticing afterwards means the disk is already full.

A run that passes a limit exits 125, distinct from the 124 a deadline uses, because a repair loop that cannot tell the two apart will raise the wrong one.

Two holes, worth naming:

showing output is not counted. That form connects the child straight to the terminal, so the bytes never pass through frost and no limit here can see them.

Inside a pipe, only the last stage is counted. The intermediate stages are connected to each other directly, which is what makes a pipe a pipe.

The bytes are counted whether or not a limit is set, and reported in the finish event as bytes_returned and bytes_written. The question people ask before setting a ceiling is what the run normally moves, and answering it with a zero because nobody had set a ceiling yet would make the number useless exactly where it is wanted.

---

14ab. Interpreters reached through another program

run "sh" with "-c", text is the explicit escape hatch, and it is reported as a danger. The same escape written a slightly different way used to be reported as nothing:

run "xargs" with "-I", "{}", "sh", "-c", "echo {}"
run "env" with "FOO=1", "bash", "-c", "id"
run "sudo" with "sh", "-c", "id"
run "timeout" with "5", "python3", "-c", "print(1)"
run "find" with ".", "-exec", "rm", "{}", ";"
run "ssh" with "host", "rm -rf /tmp/x"

Every one of those reaches an interpreter, and every one of them has an innocent program name. The check now looks at the arguments as well, and each case says what actually happens rather than sharing one blanket sentence:

A launcher naming an interpreter, then -c. xargs, env, sudo, nice, timeout, nohup, setsid, docker, kubectl and others whose job is to run another program. The text after -c is re-parsed as a command line, exactly as it would be in the direct form.

find -exec and friends. No shell is involved: find execs the program directly. The problem is different and still real, so it is reported as a hidden command rather than a shell escape. It runs once per match, its arguments come from the filesystem rather than from the script, and the program it runs appears nowhere in the manifest.

ssh with a remote command. Everything after the destination is handed to a shell on another machine. None of frost's guarantees apply over there, and saying so is more use than pretending the boundary is not there.

Innocent lookalikes are left alone, because a check that fires on grep -c sh teaches people to ignore it and costs more than the cases it catches. ssh -p 22 host is not a remote command, and run "echo" with "sh" is not an escape.

This list is a floor, not a proof. awk can call system(), make runs a shell for every recipe line, and a program nobody here has heard of may take a --command flag. What the check does is catch the spellings that actually get written and stay quiet where it is unsure, and what the manifest must never do is imply it has looked everywhere.

One limit worth stating: the finding names the escape, but forbid running "sh" matches the program frost spawns, which for xargs sh -c is xargs. The rule does not fire. Refusing the launcher too is the policy author's decision, and the manifest now gives them the reason to make it.

---

14b. frost diff

Two versions of a script, compared by what they can do.

frost diff old.frost new.frost

wider:    it can now run curl
wider:    it can now reach telemetry.example

It exits 3 when the second version is wider than the first, 0 when it is the same or narrower, so it drops into a pre-merge hook without any parsing.

A text diff answers the wrong question. Three rearranged lines can be a widening and thirty can be a rename, and a reviewer reading a regenerated script has no way to tell which they are looking at without reading all of it. This compares the two manifests instead: what the new version can do that the old one could not, and what it no longer does.

What it does not compare is behaviour. Two scripts that run the same commands and reach the same hosts are identical to this, whatever else changed between them. It answers "did the blast radius grow", which is what a reviewer is actually asking of a script a machine rewrote.

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14c. When a rule refuses

A refusal names the rule. The next question is always what would have to change, and answering it by hand means finding the rule and working out what a change to it would mean everywhere else.

What would have to change, if this should be allowed:

  reaching telemetry.example, which is not in the allow-list
    change:  require reaching only "api.github.com", "telemetry.example"
    effect:  widen the allow-list to include telemetry.example
    allows:  every connection to telemetry.example, from any script

None of this is applied. A policy change permits more than the script
that prompted it, so it belongs to whoever owns the policy.

The allows: line is the point. Relaxing forbid running "curl" does not permit this script's curl; it permits every curl on that host, in every script, from then on. A "minimal delta" framing hides exactly that, so every suggestion states its reach.

Where a rule can be tightened rather than removed, the tightening is offered first: require reaching only "a", "b" is a smaller change than deleting the rule, and a reviewer shown only the deletion will take the deletion.

Two things it deliberately will not do.

It never writes to a policy file. The output is a report. Applying it is a decision about every script the policy covers, and it belongs to whoever owns that policy.

Under --automated it declines to answer at all. An agent that hits a refusal and is handed the exact edit that clears it has been handed the instructions for widening its own bounds, which is the failure --automated exists to prevent everywhere else. A machine that cannot approve should not be drafting its own permission slip either.

A subject that does not exist until the script runs gets an honest non-answer rather than an invented one, because no allow-list entry can cover a host assembled at runtime.

---

15. Deliberate omissions

Things frost does not have, and why.

String interpolation. The entire injection surface of shell comes from values being re-read as syntax after substitution. Use & and &&.

A shell escape / eval. Same reason. If you need shell behaviour, run the shell explicitly: run "sh" with "-c", script text. That line is greppable in review, which is the point.

Globbing in run arguments. run "rm" with "*.tmp" passes a literal asterisk. Expansion happens where you can see it, loop over the output of find, or hand the pattern to a program that expands it.

Background jobs, job control, &, fg, bg. These belong to an interactive shell. frost is not one.

A login shell mode. Deliberate. Terminal modes, line editing, process groups, and prompt expansion are most of the work in a shell and none of it is this idea. Keep zsh; add frost.

Terse aliases for anything. There is no -v flag syntax, no punctuation variables, no abbreviations. If a construct is worth having, it is worth spelling.