Author a Claude Code Hook

Create a hook that matches the existing ~40 hooks in hooks/scripts/, wire it in claude/settings.json, add an eval.

Phase 1 — Understand the trigger

Clarify with the user (use grilling if ambiguous):

• Event: PreToolUse (can block/allow), PostToolUse (inject context after), SessionStart, Stop, UserPromptSubmit, SubagentStart/SubagentStop, PreCompact.
• Matcher: tool regex (Edit|Write, Bash, Read) or "" for all.
• Effect: inject additionalContext (most common), or gate a tool (exit 2 = deny, with reason on stderr).
• Dedup need: should it re-announce every call, or back off (NOTIFY_EVERY pattern)?

Read 1–2 nearby hooks of the same event type before writing (dir_context_refs.py for PreToolUse+context, impact_surface_hint.py for PostToolUse+context, protect_files.py for gating, start_context.py for SessionStart).

Phase 2 — Write the script

Follow references/conventions.md exactly: shebang #!/usr/bin/python3, utils import via sys.path, get_hooks_logger("Name"), read stdin with a MAX_STDIN cap, parse JSON defensively (exit 0 on any failure), use get_by_key for payload access, resolve project via CLAUDE_PROJECT_DIR env, wrap main() in try/except logger.exception; sys.exit(0). A hook must never crash the tool call — every failure path is sys.exit(0).

Output shape for context injection:

json.dumps({"hookSpecificOutput": {
    "hookEventName": "PreToolUse",  # match the actual event
    "additionalContext": json.dumps({...})
}})

Save to hooks/scripts/<snake_name>.py, chmod +x.

Phase 3 — Wire settings.json

Add the hook command under the right event/matcher block in claude/settings.json, using $CLAUDE_PROJECT_DIR/.claude/hooks/scripts/<name>.py (that is the deployed path — .claude/ symlinks to the repo). Place it near sibling hooks of the same purpose. Keep the log_hooks.py entry last in its block.

Give the user the exact JSON snippet to paste, showing the full event block with the new entry in place.

Phase 4 — Eval

Add hooks/evals/<name>.json (mode: stdout for context hooks, exit_code for gating hooks) with cases covering: the happy path (expect_output: true + expect_pattern), non-matching tool (no output), missing file_path/field (no output), malformed JSON (raw_stdin), empty stdin. Run python3 hooks/evals/eval.py hooks/evals/<name>.json and confirm all pass.

Delivery

Report: hook path, the settings.json snippet (verbatim), eval result. One or two lines on what it does. Do not paste the full script body into chat.

Reference files

• references/conventions.md — the exact boilerplate skeleton, utils helpers available, output shapes for each event, the dedup/back-off pattern, gating (exit 2) pattern, and the "never crash the tool call" rule.

---

Traceability: recurring across 16+ session prompts ("criar um hook que…", "comando para colar no settings", "fazer um hook equivalente em python") and 29 fix(hooks) commits (Jul–Aug 2026) fixing the same classes of mistake — wrong env var for project root, missing dedup, matcher not covering AskUserQuestion, non-atomic cache writes, silent exceptions. Sources: $CLAUDE_CONFIG_DIR/projects/-home-ronnas-develop-personal-aipp/*.jsonl, git log.