Code Review Skill | Agent Skills

Code Review

Overview

Review a change for correctness, security, performance, and maintainability, then produce prioritized, actionable feedback. Optimize signal: gate on what breaks users; comment (don't gate) on the rest.

Review vs. adversarial security audit

Different jobs — don't conflate:

Code review (this skill): holistic, author-empathetic. Reviews the diff and its blast radius against stated intent across four lenses. Assumes good faith; catches the bugs a careful peer catches.
Security audit (/loom-security-audit, /loom-threat-model): attacker mindset, threat model, whole-attack-surface. Assumes hostile input everywhere.

For auth, crypto, payments, deserialization, or anything touching a trust boundary: do the code review AND trigger a security audit. A passing code review is not a security sign-off.

Severity taxonomy

Label every comment. Only the first two gate the merge.

| Severity | Gate? | Meaning | | ------------------ | ----- | --------------------------------------------------------------------------------- | | BLOCKER | ✅ | Security hole, data loss, crash, corruption. Merge is unsafe. | | SILENT_FAILURE | ✅ | Exit 0 but the operation actually failed (sandbox-blocked download, partial fetch, stale cache, swallowed error). Always investigate before merge. | | CRITICAL | ✅ | Logic error / real bug that will bite in normal use. | | MAJOR | ❌ | Maintainability, tech debt, missing test for a real path. Fix soon. | | MINOR / nit | ❌ | Style, naming, micro-optimization. Prefix with nit: so the author can skip it. |

Approve-with-comments discipline: if nothing is BLOCKER/CRITICAL, approve and leave the MAJOR/MINOR notes as non-blocking. Don't hold a PR hostage over nits or personal style. Blocking on taste is the top reviewer anti-pattern — it trains authors to ignore you.

Method

Read intent first. PR description, linked issue, commit messages. Review against what the change claims to do; flag scope creep separately from bugs.
Review the diff AND its blast radius. A hunk is not self-contained. For every changed symbol, rg its callers and callees — a signature change, a new early-return, a changed default, or a widened type ripples outward. Bugs hide at the seams the diff doesn't show.
Four lenses per hunk: correctness → security → performance → maintainability (below).
Missing-tests / missing-error-path pass (separate sweep — easy to skip).
Verdict: approve / approve-with-comments / request-changes, each comment severity-tagged.

Blast-radius checklist

[ ] Callers of every changed function/signature updated (search, don't assume)
[ ] Callees: are new preconditions actually guaranteed by callers?
[ ] Changed default value / enum variant / error type — who relied on the old one?
[ ] Concurrency: new shared state, lock ordering, await points holding a guard?
[ ] Public API / serialized format / DB schema change — back-compat and consumers?
[ ] Tests, docs, and types updated alongside behavior?

Four lenses

Correctness: edge cases (empty, null, zero, negative, overflow, unicode, TZ/DST), off-by-one, error paths, resource cleanup (files/locks/connections on every return incl. early ones), idempotency/retry safety, race conditions. Trace the unhappy path, not just the happy one.
Security: input validation at the boundary, injection (SQL/command/path/SSRF/XSS), authz on every sensitive op, secrets not logged, crypto-grade randomness. (Deep dive → security audit.)
Performance: algorithmic complexity, N+1 queries, unnecessary allocation/clone in hot paths, missing pagination/streaming for unbounded data, O(n)-in-a-loop membership checks (want a set/map). Don't nitpick micro-perf off the hot path.
Maintainability: does the abstraction fit the problem? single responsibility, honest naming, no copy-paste of logic that will drift, no dead/speculative code, comments explain why not what.

Missing tests & error paths

The most common real defect in a passing PR: an untested error path. Ask:

[ ] New branch / early-return / catch with no test exercising it?
[ ] External call (network/db/fs) — what happens on timeout, 500, empty result?
[ ] Does a returned Result/Err/rejected promise get handled, or silently dropped?
[ ] New public function without a test for its failure mode, not just success?

Loom orchestration review

For code produced by loom stages, add these (they catch the "compiles + tests pass but doesn't work" class):

Silent failure (BLOCKER): exit 0 with error/warning on stderr; sandbox blocked a download but stage completed; external dep referenced but not installed/reachable.
Wiring (CRITICAL): feature compiles and tests pass but is never imported / registered / mounted / reachable by a real user. Verify: is the module imported at the entry point? command/route registered? event handler connected? DI binding present? Can a user actually invoke it? (See /loom-wiring-test.)
Dependency reality: package actually installed (not just in manifest); data file actually downloaded (not just referenced); endpoint actually reachable (not just configured).

Domain quick-checklists

Compressed — expand the relevant one only when the diff touches that domain.

Infra/IaC: no hardcoded secrets; least-privilege IAM; state backend secured; resource limits/requests (k8s); non-root, minimal base image, no secrets in layers (Docker); rollback path (CI/CD).
Data pipeline: schema validation; null/dup handling; idempotent + exactly-once where claimed; dead-letter queue; partitioning/batching; storage lifecycle for cost.
ML: seed set for reproducibility; train/val/test split has no leakage; data + model versioned; drift monitoring and rollback in prod; bias/fairness on training data.

Reviewer output format

Group by severity, cite file:line, state issue → impact → fix. Keep it scannable.

# Review: auth/login.py

## BLOCKER
- **L45 SQL injection** — `f"... WHERE email = '{email}'"` interpolates user input.
  Impact: arbitrary SQL. Fix: `cursor.execute("... WHERE email = %s", (email,))`.

## MAJOR
- **L112 unchecked None** — `send_email(user.email, ...)` after `.first()` can `None`-deref
  when the user doesn't exist. Add a guard returning 404.

## nit
- **L23** `GetUser` → `get_user` (snake_case).

## Good
- Clean validation split (L30-40); solid test coverage on the happy path.

Two tiny wrong→right patterns worth internalizing:

# perf: O(n·m) — membership check rebuilds nothing but scans a list each pass
if user.id in active_ids:      # list → O(n) per lookup
# → hoist once:
active = set(active_ids)       # O(1) per lookup
if user.id in active:

Author-empathy phrasing

Same fact, better delivery — critique the code, ask don't command, give the reason:

❌ "This is wrong." → ✅ "This deref crashes when user is None (L112) — guard it?"
❌ "Why didn't you use a set?" → ✅ "A set here makes this O(1) per lookup; worth it on the hot path."
❌ "Bad naming." → ✅ "nit: data is vague — active_orders?"

Acknowledge good work explicitly; it makes the blocking comments land.

Verify before done

[ ] Read the PR intent; scope creep flagged separately from bugs
[ ] Every changed symbol's callers/callees checked (blast radius, not just the hunk)
[ ] All four lenses applied; unhappy paths traced
[ ] Missing-test / error-path sweep done
[ ] Loom: wiring + silent-failure + dependency-reality checks (if orchestrated code)
[ ] Every comment severity-tagged; nits marked nit:; verdict matches (only BLOCKER/CRITICAL gate)
[ ] Security-sensitive surface → security audit triggered, not just reviewed

Agent Skills: Code Review

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