gstack/docs/skill-gates.md

Skill gates: opt-in enforcement hooks

gstack's advisory skills say what a careful agent does; a gate makes one rule hold. Each gate is a Claude Code hook that is off unless you turn it on, never changes the skill's text, and leaves a countable marker for every decision it makes (GST-1461). They follow the contract hook in skill-routing-and-contracts.md: the same switch order, the same per-session record, the same fail-open rule.

Gate	Switch	What it holds
/investigate	investigate_gate	After /investigate loads, changes to tracked source wait until the session states a root cause
/plan-ceo-review	ceo_review_gate	After /plan-ceo-review loads, the final stop (of the session or of a review subagent) waits until the review's output file ranks the top risks

Turn a gate on

The switch resolves like the contract hook's, and the first one set wins:

1. The ./setup flag: --investigate-gate / --no-investigate-gate (also --investigate-gate=on|off), or --ceo-review-gate / --no-ceo-review-gate (also --ceo-review-gate=on|off). A flag is saved to config.
2. The environment: GSTACK_INVESTIGATE_GATE=on|off or GSTACK_CEO_REVIEW_GATE=on|off. Not saved.
3. Config: gstack-config set investigate_gate on|off or gstack-config set ceo_review_gate on|off.
4. The default, SWITCH_DEFAULT in lib/skill-route.ts: off for both.

Setup registers a gate's shims in ~/.claude/settings.json when its switch is on (four for /investigate; Stop and SubagentStop for /plan-ceo-review) and removes them when it is off. The hooks also read the switch on every call, so gstack-config set investigate_gate off takes effect on the next tool call or stop.

Headless runs

For claude -p or an eval harness, skip settings.json:

gstack-skill-gates settings --investigate --ceo-review > gates.json
claude -p "<task>" --settings gates.json

Name one gate or both. The JSON registers their hooks from this checkout and sets GSTACK_INVESTIGATE_GATE=on and GSTACK_CEO_REVIEW_GATE=on through its env block. Under the Agent SDK, pass the same JSON as settings or include user in settingSources after running setup; with neither, or with --bare, no hook loads and no fired marker appears. Check every session with gstack-skill-gates report --session <id> --expect armed (exit codes).

The /investigate gate

Once a session loads /investigate (the Skill tool, a typed /investigate, or a Read of its installed SKILL.md), the gate arms on the next Edit, Write, MultiEdit, NotebookEdit or Bash call. From then until the session states a root cause:

• Edit, Write, MultiEdit and NotebookEdit on a tracked source file are denied before they run.
• Bash can't be judged from its command line (sed -i, a redirect, tee, python -c, perl -pi, node -e, git apply, a heredoc, a build step that rewrites a file). So the gate compares the tree instead. When it arms it pins a baseline: the repository root, HEAD, its refs and stash list, and the content of every tracked file that differs from HEAD. Every later Edit/Write/Bash hook call, every PostToolUse and PostToolUseFailure on Bash, and Stop compare the tree with that baseline under a session lock. A tracked source file whose content changed is put back and the call or stop is blocked with the files named. Because every check uses the same baseline, a write from a background command or a parallel call is caught at the next check whichever call made it.

A change is not a fix write, and is folded into the baseline, when the file's new content is its blob in HEAD (git checkout -- f, git stash), in a commit that existed before the baseline (git checkout <branch>, git bisect), in a commit a remote-tracking ref holds (git pull, git merge origin/main), or in a stash entry that existed before (git stash pop). A commit made after the baseline that touches a gated file is undone with git reset --soft to its newest ancestor that existed before (or that a remote-tracking ref holds), so a fix committed on another branch moves that branch back to its own tip, including a branch behind the baseline HEAD (git checkout main && <fix> && git commit from a feature branch moves main back, and the feature branch is untouched). An amend or rebase of commits from before the baseline is undone to that branch's tip when the gate armed: the baseline HEAD on the same branch, or the other branch's own recorded tip. A branch created after the baseline goes back to that newest older ancestor. The commit stays in the reflog, those index entries are reset, the files are put back, and the block names the commit's other changes still staged. A commit pushed to a remote counts as upstream, so a fix pushed in the same command is not undone.

What counts as source. A file tracked by git that is not a test path (gstack-diff-scope's test rule plus fixtures/), a doc (.md, .mdx, .rst, .txt, .adoc), a lockfile (bun.lock, package-lock.json, yarn.lock, pnpm-lock.yaml, Cargo.lock, go.sum, uv.lock, poetry.lock, Gemfile.lock and the like), a linguist-generated path (read from the baseline commit's attributes, so a .gitattributes written after the gate armed exempts nothing), or a tracked symlink. New untracked files are never blocked, so a repro script, a new failing test and temporary logging in a scratch file all work. Changes to excluded files are recorded as observed, never reverted.

What releases it. A line in the assistant's own reply, after at least one Read, Grep, Glob or Bash call since the load:

Root cause: <what is actually wrong, with file:line>

That is the Root cause: label of the skill's DEBUG REPORT. A placeholder ([what was actually wrong], TBD) or a hedge (unknown, unclear, still investigating, see below, n/a) does not count, and neither does the text inside a tool result or a file. The block text names the form, and the rejected value when there was one. After the release the gate does nothing for the rest of that /investigate run; a new load arms it again.

Restoring is careful. The gate never writes through a symlink: it refuses a parent that is not a real directory inside the repository and replaces the path with a rename, so a tracked file swapped for a link to ~/.bashrc comes back as the file and the link's target is untouched. Files clean at the baseline come back through git cat-file --filters (line endings and LFS smudging intact); files dirty at the baseline come back from a raw copy in the session directory, keyed by the sha256 of its bytes. A file clean at the baseline keeps its current permissions (the tree's mode only when the file is gone); a file dirty at the baseline gets its baseline mode back. Each restored file's index entry goes back to the baseline's, including a change that was only staged, so a rejected edit that was git added is not left staged and staged work is not dropped. Before a file is put back, the content it replaced is copied into the session directory and the block names that copy, so an edit made in an editor or another session at the same time is never lost. A path that cannot be put back (it is now a directory, the disk is read-only) gives GATE_RESTORE_FAILED.

Known gaps. Paths marked assume-unchanged or skip-worktree, untracked files that shadow a tracked module, and ignored directories are outside the check. While the gate is armed it also puts back an edit you make to tracked source in your editor, since it cannot tell who changed the file.

The /plan-ceo-review gate

Once a session loads /plan-ceo-review (the Skill tool, a typed /plan-ceo-review, or a Read of its installed SKILL.md), its final stop is held until the review's output ranks the top risks. Only a final stop counts, by the contract hook's rule (isFinalTurn in lib/skill-contract.ts): a STATUS line other than NEEDS_CONTEXT, or a turn that does not end waiting on the user. A stop that asks the user a question (a closing ?, AskUserQuestion, STATUS: NEEDS_CONTEXT) in an interactive session is never blocked; in a session no human answers (gstack-session-kind says headless or unattended) it counts as final.

What meets it. One output file, modified after the load (its mtime is later than the load record's transcript timestamp), with a markdown heading whose text contains Findings or Top risks (any level, any case), followed inside that section by a numbered list that starts at 1 and has at least one item that is not a placeholder ([risk], TBD, ...). A bullet list, a list starting at 2, a list under the next same-level heading, or a section of placeholders does not count:

## Findings

1. Payment retries can double-charge when the provider times out.
2. No rollback path for the new orders schema.

Which files count, in order, stopping at the first one that meets it:

1. GSTACK_CEO_REVIEW_OUTPUT, when set (relative paths resolve against the session's cwd).
2. Files the run wrote with Write, Edit or MultiEdit.
3. Markdown files in the git repository at the session's cwd that differ from HEAD or are untracked and not ignored.
4. $GSTACK_STATE_ROOT/projects/<slug>/ceo-plans/*.md, with the slug from bin/gstack-slug.

Bash commands are never parsed: a review written with a heredoc or tee is found by the repository and ceo-plans scans.

The block. Its reason starts with the blocked via=stop marker and says exactly what to write and where: a ## Findings section ranking the top risks as a numbered list, in GSTACK_CEO_REVIEW_OUTPUT, else the last markdown file the run wrote, else the last plan file it read (never a file in a skills/ directory, judged from the repository root so a project that itself sits under a skills/ folder still counts, such as the review's own methodology sections; such a file never releases the gate either), plus each file checked and why it fell short. It never names the off switch.

The cap. A run is blocked at most 3 times. The next final stop without ranked risks goes through with an exhausted marker and exhausted_at in the record. stop_hook_active (Claude Code sets it once a Stop hook has blocked the turn) does not skip the check; the cap ends the repeats. After released or exhausted the gate does nothing for the rest of that run; a new load starts a new run.

Subagents. The SubagentStop shim reads the payload's agent_transcript_path (Claude Code 2.1 sends it with agent_id) and gates the subagent only when that subagent's own transcript loaded /plan-ceo-review, so an Explore or test subagent finishing in a session that ran the review is never touched. Its records are sidechain records, parsed as such. A subagent always ends on a final turn (no human answers it), and its run is counted under agent in the record. A payload without the path, or with an empty one, is left alone.

Markers

Every decision is one marker line:

GSTACK_GATE <hook> <event> session=<session id> [key=value ...]

Event	When	Fields
fired	the gate armed for a skill run (once per run; for /plan-ceo-review, its first final stop)	skill, on (the hook event), root (the repository folder name, /investigate only)
blocked	each refusal	skill, `via=edit
released	the root cause was stated, or an output file ranks the top risks (once per run)	skill, cause (first 12 hex of the cause's sha256) or file (the output file's name), blocked
exhausted	/plan-ceo-review only: a fourth final stop after 3 blocks, which goes through	skill, blocked
observed	an excluded file changed	skill, files
skipped	the gate could not arm (once per session and reason)	skill, `reason=no-git
error	a hook error; the call or stop went through	code, why

Where each one lands:

• Session record (the count source): $GSTACK_STATE_ROOT/skill-gates/<session id>/record.json, mode 0600, written atomically under the session lock. Per gate and skill run it holds fired_at, released_at, cause or file, blocked, observed, exhausted_at, agent (a subagent's run), plus the gate's errors and skipped reasons. gstack-skill-gates report reads these.
• Trace log: $GSTACK_STATE_ROOT/hooks/investigate-gate.log and hooks/ceo-review-gate.log, one line per marker in the contract hook's shape (<time> session=<id> event=<event> key=value ...). It rotates at 1 MiB, so count from the record.
• Transcript: a block puts the marker on the first line of its reason, which Claude Code shows the model (a denied tool's tool_result, or a hook_blocking_error attachment for PostToolUse, Stop and SubagentStop). fired, released, exhausted and error ride systemMessage, which the transcript records as a hook_system_message attachment and the model never sees, so counting them does not change the session.

The session directory also holds the baseline and the raw copies of files that were dirty when the gate armed. Session directories untouched for 7 days are deleted the next time a gate arms.

Errors

A gate never blocks because it broke. On any error it writes an error marker with a code to the record, the trace log, stderr and the transcript (systemMessage), and exits 0 with no decision, so the call or stop goes through.

Code	Cause	Fix
GATE_BAD_PAYLOAD	the hook input is not a JSON object with session_id and transcript_path, or is over 1 MiB	a Claude Code version change; update gstack
GATE_TRANSCRIPT	the transcript is missing, over 32 MiB, or not a shape the parser knows, or the /plan-ceo-review load record has no timestamp	see skill-routing-and-contracts.md
GATE_TIMEOUT	the hook ran past its 5 second deadline (GSTACK_SKILL_GATE_DEADLINE_MS changes it)	a very large dirty tree or a slow disk
GATE_GIT	a git command failed or HEAD stopped resolving	run the named git command in the repository
GATE_STATE	the session record or baseline could not be read or written	make $GSTACK_STATE_ROOT writable, or delete skill-gates/<session id>/
GATE_LOCKED	another hook held the session lock for over 3 seconds	transient; a lock older than 30 seconds is taken over
GATE_SLUG	/plan-ceo-review: bin/gstack-slug failed, so the ceo-plans directory could not be checked	run gstack-slug in the repository; GSTACK_PROJECT_SLUG pins it
GATE_RESTORE_FAILED	a changed file could not be put back	the block lists restored and failed files; run git diff HEAD -- <files> and fix them by hand
GATE_UNEXPECTED	any other throw	report it with the trace log line

Exit codes

gstack-skill-gates report [--session <id>] [--expect armed] [--json] lists each session's gate counts: runs, fired, blocked, released, exhausted, observed and errors.

Exit	Code	Meaning
0		done
1	SKILL_GATES_USAGE	a bad argument
2	GATE_NOT_ARMED	--expect armed and a session has no fired marker: the hooks never loaded (see Headless runs) or the skill never ran
3	GATE_ERRORED	a session recorded a hook error, so some calls or stops went through unchecked

Headless smoke

bun scripts/skill-gates-smoke.ts runs the real Claude Code CLI against a local stand-in for the Anthropic Messages API that plays two scripted sessions. The first loads /investigate, reads the file, tries an Edit, tries sed -i, states the root cause and edits. The second loads /plan-ceo-review, writes plan.md with no Findings and ends its turn (the Stop is blocked), then appends a ranked Findings section through a Bash heredoc and ends again (released). No model is called and nothing is spent. It fetches the pinned CLI (CLAUDE_CODE_VERSION in the script, verified at 2.1.318) with npm into a temp directory at run time and never stores it; --claude <path> uses one you have. It checks the record, the trace log, the transcript attachments, that systemMessage markers never reach a request, that the sed -i write was put back, and that the CEO review was released by plan.md. Run it after a Claude Code upgrade.