Writing Code Comments Agent Skill

Writing code comments

Run this before adding or editing any comment. The default is no comment. Good code with clear names carries most of its meaning on its own; a comment earns its place only when it tells a reader something the code cannot.

The gate: one question

Before writing a comment, answer:

> What does this tell a future reader that the code itself doesn't?

If the answer is "it restates what the code does", delete it. Rename the variable or extract a function instead.

A comment worth keeping answers a _why_ the code can't:

• ✅ # ATOMIC_REQUESTS is off, so wrap the two writes that must commit together
• ✅ // Stripe sends the amount in cents; the rest of our system uses dollars
• ✅ # Kept in sync with the enum in migrations/0042; update both

Delete these

Narration that restates the code

• ❌ # increment the counter above counter += 1
• ❌ // loop over users above for user of users
• ❌ # return the result above return result

If a block needs narration to be followed, the fix is smaller functions and better names, not a comment.

Change history and chat context

Never record how the code got here. That belongs in the commit message and PR description, where it's attached to the diff and searchable. In the source it's noise that goes stale immediately.

• ❌ # previously used a set here, switched to a list for ordering
• ❌ // per PR #1234 / # as discussed / # changed because the old way broke
• ❌ # AI: generated this helper / // agent: refactored
• ❌ # TODO(2024-01): remove after migration left in long after the migration

Commented-out code

Delete it; the version history has it if it's needed again. Commented-out code is ambiguous to the next reader, who can't tell whether it's a note, a rollback plan, or an accident.

Redundant docstrings and type restatements

• ❌ A docstring that repeats the function name in prose: """Gets the user by id.""" on get_user_by_id
• ❌ # type: string on an already-typed field
• ❌ Python test doc comments (the repo convention is none; the test name says it)

Keep these

• A why that isn't obvious from the code: a workaround, a performance trade-off, a spec quirk, an ordering constraint.
• A warning about a consequence that lives elsewhere: "changing this breaks the cache key", "callers rely on this being sorted".
• A pointer to context a reader can't reconstruct from the repo: a link to the spec, ticket, or the reason a surprising value was chosen.

Style

Write comments the way you'd write technical documentation: explicit and precise. State the reasoning so the reader does not have to infer it. Length is not a target in either direction: don't clip a comment to look terse, and don't pad it to look thorough. Say what needs saying and stop.

• Be explicit and technical. State the cause and effect. Name the actual conditions, values, and consequences. A reader should not have to reconstruct your reasoning from a hint.
• Let length follow the content. One line is fine when one line covers it; use more when the reasoning needs more. Neither brevity nor length is the goal.
• No em-dash. The tell to avoid is the clipped two-part phrase joined by a dash, like # do the thing — it's faster. Use a real connective instead ("because", "so that", "which means", "to avoid").
• Explain why, not what. The what is in the code; the why usually is not.
• Preserve existing comments when moving or refactoring code, unless the change makes them wrong. Don't drop an existing why just because you're relocating the function.
• Match the surrounding density. Don't add a comment to every line of a file that had none; don't strip a well-commented module bare.

The fix for the em-dash is the connective, not more words. A short comment is fine once the dash is gone:

• ❌ # batch here — avoids N+1
• ✅ # batch here to avoid an N+1 against posthog_organizationmembership

When you're tempted to comment

Try, in order: (1) a better name, (2) a smaller function, (3) a type. Reach for a comment only when none of those can carry the meaning.