Audits an existing lifecycle hook script for Claude Code or Codex, records the current contract (hook name, event, behaviour, exit codes), classifies the requested change (threshold / condition / behaviour / bug fix), applies the minimal edit as an explicit before/after diff, and re-validates with direct shell tests. Never rewrites a working hook from scratch — every change must be proven by running the hook with real JSON inputs.
Instalación
Instalar con Codex o Claude Copia este prompt, pégalo en Codex, Claude u otro asistente, y deja que revise la página de la skill y la instale por ti.
Audits an existing lifecycle hook script for Claude Code or Codex, records the current contract (hook name, event, behaviour, exit codes), classifies the requested change (threshold / condition / behaviour / bug fix), applies the minimal edit as an explicit before/after diff, and re-validates with direct shell tests. Never rewrites a working hook from scratch — every change must be proven by running the hook with real JSON inputs.
when_to_use
Use when a Claude Code or Codex hook already exists and needs to be changed — the trigger condition is wrong, a new file type must be matched, a threshold changed, or a bug fixed. Invoke instead of go-swift when the hook file already exists. Invoke go-hook-eval after to confirm no regressions.
go-wren — Hook Maintenance [Claude Code · Codex]
go-wren patches what go-swift built. It does not design new hooks — it audits, edits, and re-validates existing ones. The discipline is surgical: change the minimum, preserve the contract, prove no regression.
Quick start
User: "The docs-update-remind hook fires even for .md files — fix it."
→ invoke go-wren
→ read script → classify change → edit minimal diff → re-validate
Workflow
1. Read and audit the existing hook
Before touching anything, understand what the hook currently does:
Read the full script from ~/.claude/hooks/<name>.sh (or the provided hook content in eval context)
For Codex hooks, also check ~/.codex/hooks/<name>.sh or the project-local .codex/hooks/<name>.sh path.
Identify the event it handles (SessionStart, PreToolUse, PostToolUse, Stop, etc.)
Map its current behaviour: what inputs does it read, what conditions does it check, what does it output, what exit codes does it use?
Confirm the hook entry in the target agent's config — Claude Code settings.json, Codex hooks.json, or Codex inline [hooks] in config.toml; record event key, command path, matcher, timeout, and status message.
Produce the HOOK CONTRACT block as the first artifact — always, before any edit:
## HOOK CONTRACT
Hook: <name>.sh
Event: <EventName>
Trigger: <what causes it to fire — tool name, matcher, or always>
Behaviour: <one sentence — what it does today>
Exit codes:
0 = <condition — e.g., "file is not a source file or no violation found">
1 = <condition — e.g., "violation detected, tool use blocked">
2 = <condition — e.g., "flag file present, re-triggers Claude">
Side effects: <flag files written, files modified, none>
Test cases in go-hook-eval.js: <yes — N cases / no>
Do not proceed if the script does not exist or cannot be read.
2. Classify the requested change
Determine the change type — this sets the risk level and review depth:
Type
Description
Risk
Threshold
Change a numeric or string constant (file extension list, size limit, pattern)
Low
Condition
Add or remove a branch (new file type to ignore, new blocked pattern)
Medium
Behaviour
Change what the hook outputs, which exit code it uses, or when it fires
High
Bug fix
Correct a logic error that causes incorrect blocking or silent failure
High
Checklist for medium/high risk changes:
Does the change affect any existing happy path (currently-passing operations)?
Does the change affect the exit code contract? (changing exit 0 → exit 1 is breaking)
Does any other hook depend on this hook's output or flag file?
Is the hook referenced in go-hook-eval.js? If so, the test cases need updating too.
3. Write the minimal diff
Hook input mechanics (read before writing any hook code):
Claude Code and Codex pass event data to command hooks via stdin as JSON — not environment variables, not positional arguments. The hook reads it with input=$(cat). The JSON shape differs by event and agent, so inspect the existing hook and test with realistic input before editing:
Event
stdin shape
PreToolUse / PostToolUse
{"tool_name":"Bash","tool_input":{...}}
Stop / SubagentStop
{"stop_hook_active":true/false}
SessionStart
{} (empty or minimal)
PermissionRequest (Codex)
approval request JSON for the requested tool action
UserPromptSubmit (Codex)
prompt submission JSON
Any proposed hook code that reads from $TOOL_INPUT, $1, or environment variables is incorrect. If the existing script uses input=$(cat) + jq, preserve that pattern.
Platform compatibility (macOS / Darwin):
grep -P (PCRE) is not available on macOS's native BSD grep. Hooks that use grep -P will silently fail on macOS with || true fallbacks — the hook runs but never matches. Use POSIX ERE (grep -E) instead. All existing go-beast hooks use grep -E or grep -qE — preserve that pattern. If the existing script uses grep -E, do not change it to grep -P.
Common pitfalls in strict bash (set -euo pipefail):
These patterns cause silent failures or wrong behavior in hooks that use set -euo pipefail. Check for them before and after any edit.
Pitfall
Wrong
Correct
Capturing exit code of a command that may fail
cmd || var=$? — if cmd exits 0, var stays at its initial value (not 0 from the cmd), making the check wrong
var=0; cmd || var=$? — initialize before, capture only on failure
Using `
` to suppress failure in a pipeline
Running git check-ignore on a non-existent path
Exit 128 (can't cd to a missing directory) — if git -C "$dir" ... silently skips the block when $dir doesn't exist
Walk up to the nearest existing ancestor: while [[ ! -d "$dir" ]]; do dir=$(dirname "$dir"); done
git check-ignore exit code semantics
Exit 0 = ignored, exit 1 = not ignored — the opposite of what most boolean checks expect
Stop/UserPromptSubmit hooks do not use a matcher field in Codex. In Claude Code, Stop/SessionStart/SubagentStop/PreCompact hooks fire unconditionally. PreToolUse and PostToolUse hooks use matcher to filter by tool name. The outer array element has a matcher key and a hooks array — there is no additional nesting layer.
First, show the before/after diff explicitly as a named artifact before applying it:
Do not change set -euo pipefail or shebang unless that is the fix
Apply the edit with the Edit tool (not Write) unless the full file must be replaced.
After editing, verify:
The script is still executable (chmod +x if it was not already)
The exit-code contract is unchanged (unless the change explicitly alters it)
All existing comment headers still describe the behaviour accurately — update only the lines that changed
For Codex hooks, note whether /hooks trust review is required because the hook definition or script changed.
4. Update go-hook-eval test cases (if needed)
If the change alters observable behaviour (new blocked pattern, different exit code, new flag file), the corresponding test case in go-hook-eval.js must be updated:
Locate the test case(s) for this hook in workflows/go-hook-eval.js
Update expectExit, expectOutput, expectNoOutput, or name as needed
Add a new test case if the change introduces a new condition not previously tested
Do not delete existing test cases that still describe valid behaviour
5. Re-validate
Produce TEST EVIDENCE as a named artifact — show the actual command and its output, not a description of what should happen.
Direct test (always — both cases required):
# Case 1: new/fixed behaviour — must produce expected resultecho'{"tool_name":"Edit","tool_input":{"file_path":"/workspace/src/app.<ext>"}}' \
| bash ~/.claude/hooks/<name>.sh; echo"EXIT_CODE:$?"# Expected: EXIT_CODE:<n>, <flag or output if applicable># Case 2: unchanged happy path — must still exit 0echo'{"tool_name":"Edit","tool_input":{"file_path":"/workspace/README.md"}}' \
| bash ~/.claude/hooks/<name>.sh; echo"EXIT_CODE:$?"# Expected: EXIT_CODE:0
Show the commands with real JSON and the expected exit codes filled in. In eval context where the shell cannot run, write the commands as-if executed and state the expected output — mark them (simulated — cannot execute in eval context) rather than omitting them.
Full eval (for medium/high risk changes):
Workflow({ name: "go-hook-eval" })
All previously-passing cases must still pass.
6. Update documentation
If the change is user-visible (new blocked pattern, different message, new event):
Update the hook's one-line comment at the top of the script
Update the hooks table in README.md if the description changed
Update AGENTS.global.md hooks table if it references this hook's behaviour
Add a CHANGELOG.md entry under [Unreleased]
Rules
If the hook file does not exist, stop immediately. Tell the user the hook is not deployed and invoke go-swift to create it. go-wren only works on existing hooks.
Do not rewrite a working hook from scratch. If a rewrite is needed, invoke go-swift instead.
Never change the exit-code contract without explicitly stating the change and updating all dependent test cases.
Apply edits with the Edit tool, not Write, unless the full file must be replaced.
Every change must be validated with at least one direct shell test before marking done. In eval context where the shell cannot execute, write the test commands with expected outputs and mark them (simulated — eval context). Never omit the TEST EVIDENCE block.
Do not update go-hook-eval.js test cases without re-running the eval to confirm they pass.
Hook input always arrives via stdin JSON — never $TOOL_INPUT, $1, or environment variables. Any generated hook code using those patterns is a bug.
For Codex hooks, do not claim the changed hook will run until its updated definition has been reviewed and trusted with /hooks.
Output
HOOK CONTRACT block — current behaviour documented before any edit (always required)
