Writing Durable Comments

A comment earns its place by explaining something the code cannot say for itself and that will still be true next year: current behavior, an invariant, a non-obvious "why", an upstream/vendor/host quirk, a security or compat constraint, a regression- or RT-safety guard. It must not encode how the code got here — that provenance belongs in the commit message, planning/, docs/reports/, the changelog, or a commit trailer. This repo will be open-sourced; every comment is read by strangers with no access to our issue tracker or session history.

This skill exists so new code stops adding the exact provenance the repo-wide cleanup is removing. The cleanup backlog + classification rules are parked at planning/2026-07-02-comment-hygiene-PAUSED.md.

Never write these (the "Remove" category)

• Phase / milestone / slice labels. No (Phase 4), Phase N will…, 4f-style sub-phase labels, Tier A Slice 7, Feature 3, plan item 7.2, item 6.12 follow-up. [phaseN] / [phase3-large] Catch2 tags are forbidden. Capital-letter+digit plan tags (C2, D6, M1) are the same kind of breadcrumb — the lint can't safely match them (they collide with MIDI note names and register labels), so it's on you to keep them out of comments.
• Agent / review breadcrumbs. No Codex P1/P2/P0, codex review, [codex-*], "per the reviewer", "sub-PR", "slice N of".
• Process/pipeline provenance. No [coverage], [codecov], [requested] selector tags; no "added for coverage".
• Independent / reference-lineage process notes. The provenance audit trail lives in the Reference-Lineage: independent … commit trailer, never in a source comment. Describe the DAW/host behavior, not how you derived it.
• Bare #1234 as the explanation. A PR/issue number is not a reason. If the issue explains a still-live invariant, state the invariant; a durable [issue-NNN] anchor on a regression test is fine (note: # is reserved in Catch2 tags — use [issue-NNN]).
• Speculative future / roadmap. No "Future v2 …", "will support X later", TODO-for-a-someday-feature. (Real example removed: a Future v2 license-key payloads (post-RSA migration) bullet that was already inaccurate — current crypto is AES-256-GCM.)
• WIP / "temporary" / "hack for now". Either it's the behavior (describe it) or it's a tracked task (put it in planning/).
• Stale claims. A comment describing code that no longer exists is worse than none (real example removed: an orphaned CMAKE_BUILD_TYPE paragraph in a test that no longer checks a build-type field).

Always safe to write (the "Keep" category)

• Upstream / vendor quirks, with a link or version anchor.
• Security constraints, threat-model notes.
• Compatibility constraints and stated non-goals.
• Regression-guard and RT-safety rationale ("no allocation on the audio thread", "denormals flushed here").
• The non-obvious why behind a surprising line.

Rewrite, don't delete, when there's a real invariant

Provenance that encodes a stable fact gets restated as current behavior, stripped of the phase/PR wrapper. Real before → after cases from the cleanup:

Before (process provenance)	After (durable behavior)
Phase 4d adds feedback	feedback needs a previous-block slot
pulp_build_info.hpp (Tier A Slice 7)	Build info
test header macOS plan item 7.2 / 7.1b	header describing the current coverage
MPE header naming a phase	"describes UMP status handling and shared factories"
raw-MIDI sysex case naming a PR	"describes aborted-sysex recovery"

Test names and tags

• No # in Catch2 tags (reserved). Use [issue-NNN] for a durable regression anchor, and behavioral tags for what the test covers: [rt-safety], [parity], [thread]. Never [phaseN], [codecov], [coverage], [requested], [codex-*], "which session shipped it".
• A TEST_CASE name states what is verified, not when/why-in-the-project it was added.

Self-check before you commit

Scan your own diff for the breadcrumbs above — cheaper than a cleanup PR later:

# stale provenance in ADDED lines of the staged/working diff
git diff --cached -U0 | rg '^\+' | \
  rg -n -iE '\b(phase\s?[0-9]|slice [0-9]|tier [a-z] slice|plan item|sub-?pr|codex (p[012]|review)|\[(phase[0-9a-z-]*|codecov|coverage|requested|codex[a-z0-9-]*)\]|future v[0-9]|WIP|clean-?room)\b' \
  && echo "↑ reconsider these — see the code-comments skill" || echo "clean"

# repo-wide evergreen-scope lint (docs/reference/** + skills)
python3 tools/scripts/docs_noise_lint.py --mode=report

Enforcement today: docs_noise_lint.py covers docs/reference/** and .agents/skills/**/SKILL.md (diff-scoped in CI + pre-push). Source-comment enforcement is the author's responsibility until the lint's source scope lands — so run the rg self-check above on any source diff that adds comments.

The rule in one line

Write the comment as a present-tense statement of what the code does, why it's non-obvious, or what external constraint it honors. The narrative of how it was built goes in the commit and planning/.