Add a Code Quality Hook

Opik runs all linters through one pipeline: pre-commit locally, and the 🐙 Code Quality workflow (.github/workflows/code_quality.yml) in CI. CI does not run pre-commit wholesale — it derives, per PR, the set of hooks that actually have work using scripts/precommit-detect-hooks.py, then runs one CI job per matched linter. That design makes adding a linter a small, fixed set of edits — but each edit is load-bearing, and skipping one produces a silent gap (the hook runs nowhere, or renders blank in the summary, or provisions the wrong runtime) rather than a loud failure. This skill is the checklist.

The whole recipe is a generalization of two real PRs: actionlint (live in .pre-commit-config.yaml today — grep it as you read) and hadolint (OPIK-6673, PR #7352 — the first Docker-image hook, toolchain: none). Read the actionlint hook alongside this doc; it is the canonical, verifiable example.

The wiring files

Each linter touches these four files. Do all four.

1. .pre-commit-config.yaml — add the hook

Add the upstream hook (repo / rev / id). Pin rev to a tag or SHA — never a floating ref.

An explicit files: regex is mandatory, not optional. This is the single most common miss. The CI matrix detector (precommit-detect-hooks.py) routes work to hooks by path regex, not by pre-commit's types:. Most upstream hooks (actionlint, hadolint) ship a types:-only match with no files:. If you copy them verbatim, the detector cannot route any file to your hook and CI silently never runs it — pre-commit locally still works, so the gap hides until something slips through. The detector guards against this: it raises loudly if a hook has types:/types_or: without files: (see precommit-detect-hooks.py lines ~115). So a missing files: fails the detect step rather than regressing silently — but you still must write the regex.

Write a files: regex that captures exactly the paths the linter should gate. Example (actionlint — workflows only):

- repo: https://github.com/rhysd/actionlint
  rev: v1.7.12
  hooks:
    - id: actionlint
      name: ⚙️ actionlint — github workflows
      files: ^\.github/workflows/.+\.(yml|yaml)$

The name: is what the reader and the CI summary see — give it a clear, emoji-prefixed display name matching the house style of the other hooks. The description keyword you add in step 2 is matched against this name, so pick a name containing a stable, distinctive substring (e.g. actionlint, hadolint).

2. scripts/precommit-hook-descriptions.tsv — add the description row

This TSV is the single source of truth for the per-hook descriptions shown in the Code Quality timing/skipped tables (the CI summary comment). Format: <keyword>\t<description>. The keyword is matched as a substring of the hook display name (name: from step 1).

Miss this and the hook renders with a blank description in the CI summary. Add a row:

hadolint	Lint Dockerfiles

Order matters — most-specific first. Matching is first-substring-wins top-to-bottom, so a more specific keyword must precede any that it contains (the file already does this: ruff-format before ruff). If your keyword is a substring of an existing one, place it above that line.

3. scripts/precommit-detect-hooks.py — toolchain and content-type maps

Two maps in this file may need an entry. Most new hooks need neither (default is toolchain: none, no content-type narrowing) — but decide deliberately:

• TOOLCHAIN_BY_ID — add your hook id here only if the leg's CI job must provision a heavy runtime: java (shells out to mvn), node-fe / node-ts (shells out to npm). Pre-commit's own hooks self-provision in isolated envs and need nothing; a language: golang hook (actionlint) self-builds; a Docker-image hook (hadolint) runs the image — all of these are none. code_quality.yml branches its setup steps on matrix.leg.toolchain, so a wrong value means a job either wastes minutes provisioning an unused runtime or lacks the runtime it needs.

• TYPED_IDS — add your hook id here only if it carries an upstream types: that narrows its files: match to a content suffix, so the detector doesn't emit a leg that would no-op at runtime. Value is the tuple of suffixes the hook actually acts on (e.g. (".py", ".pyi")). Symptom of a missing entry: the CI timing comment reports fewer ran rows than emitted legs ("detect over-emitted a leg"). If your files: regex is already suffix-precise (like actionlint's \.(yml|yaml)$), you don't need TYPED_IDS.

Rule of thumb by hook type:

Hook type	TOOLCHAIN_BY_ID	TYPED_IDS
Docker-image linter (hadolint)	none (omit)	usually omit — make files: suffix-precise
language: golang/self-built (actionlint)	none (omit)	omit if files: is suffix-precise
Python tool (ruff, mypy)	none (omit)	add suffixes if files: is a broad dir regex
Shells out to mvn	java	as needed
Shells out to npm (FE/TS)	node-fe / node-ts	as needed

4. CONTRIBUTING.md — "how to run locally" note

Add a short section alongside the existing GitHub Actions workflows (actionlint) note: what the linter checks, that it runs in the unified 🐙 Code Quality workflow and locally via pre-commit, and that make hooks enables it. If the hook needs a local dependency (a Docker daemon for hadolint, etc.), say so; if pre-commit self-provisions it (actionlint builds from source), say that instead.

Retroactive step — fix the existing violations

Adding a linter to a repo that has never run it will surface pre-existing violations. The gate must be green on day one. Policy, in order of preference:

1. Fix the violation in place — this is the default. Most findings are real and worth fixing.
2. Suppress inline, with a reason, next to the code — only when a fix would be genuinely undesirable. The canonical case: exact-pinning rolling-channel OS packages (e.g. apt-get install foo=1.2.3), which rots as mirrors move — the honest answer is an inline # hadolint ignore=DL3008 with a one-line why. Inline suppression is scoped to that one line and visible in review.
3. Never a global config-file ignore. A repo-wide ignore (a .hadolint.yaml ignored: list, an eslint config-level disable) fails open on every future file — it silently exempts code no one has reviewed. Inline-with-reason keeps the gate strict on everything new.

See the OPIK-6673 discussion for a worked case where some rules genuinely couldn't be honestly fixed and inline-with-reason was the right call.

Verification loop

Before opening the PR, confirm the wiring end-to-end — don't trust that the four edits compose:

1. Detect emits a leg for a target file. Run the detector against a file the hook should gate and confirm your hook id appears in legs with the right toolchain:

   python3 scripts/precommit-detect-hooks.py .pre-commit-config.yaml path/to/target.file
   

   If it lands in skipped instead, your files: regex doesn't match. If it errors about types: without files:, add the files: regex (step 1).

2. Hook passes clean. Run it on the current tree and confirm green (this is also the retroactive check):

   pre-commit run <hook-id> --all-files
   

3. Hook fails on a new violation. Introduce a deliberate violation in a target file and confirm the hook catches it, then revert. A hook that can't fail isn't gating anything.

4. Description resolves. Confirm the summary won't render blank:

   echo "<your hook display name>" | python3 scripts/precommit-hook-desc.py
   

   Expect <name>\t<description> — an empty second column means the keyword row (step 2) is missing or mis-ordered.

Checklist

• .pre-commit-config.yaml: hook added with pinned rev and an explicit files: regex
• scripts/precommit-hook-descriptions.tsv: keyword→description row, most-specific-first ordering
• scripts/precommit-detect-hooks.py: TOOLCHAIN_BY_ID entry iff heavy runtime needed; TYPED_IDS entry iff types:-narrowed and files: isn't suffix-precise
• CONTRIBUTING.md: local "how to run" note added
• Retroactive: existing violations fixed (or inline-suppressed-with-reason); gate green on day one
• Verified: detect emits the leg, hook passes clean, hook fails on a new violation, description resolves non-blank