Phx Challenge Skill for Claude

Challenge Mode Reviews

Rigorous, critical review patterns inspired by Boris Cherny's "Grill me" approach. Push beyond first solutions to ensure quality.

Iron Laws - Never Violate These

• No approval without verification - Don't approve until all concerns addressed
• Assume bugs exist - Look for edge cases, race conditions, missing handlers
• Question everything - Even "obvious" code can hide issues
• Demand proof - Ask for tests, show state transitions, verify behavior

Adversarial Lenses (Apply to ALL Modes)

• "What Would Break This?" — Production failure modes under load, during deploys, with unexpected data
• "Assumption Stress Test" — List every assumption; which are most fragile?
• "Contradictions Finder" — Find contradictions between tests/implementation, docs/behavior, or within the changeset

Challenge Modes

Ecto Challenge ($elixir-phoenix:phx-challenge ecto)

Grill the developer on database changes:

Migration Safety

• Will this migration lock the table in production?
• What happens to existing records without the new field?
• Is the migration reversible?
• Are there any unsafe operations (column removal, type change)?

Query Performance

• Have you introduced any N+1 queries?
• Are there missing indexes for new WHERE clauses?
• Will this query scale with data growth?

Schema Integrity

• Are all constraints enforced at database level?
• What happens during rolling deployment (old code, new schema)?
• Are foreign key cascades correct?

Backward Compatibility

• Will old code work during deployment?
• Are there any breaking changes to the context API?

LiveView Challenge ($elixir-phoenix:phx-challenge liveview)

Prove the LiveView handles all cases:

Event Coverage

• List every handle_event clause and expected socket state
• What happens if socket assigns are missing when event fires?
• Are there race conditions between user events and server pushes?

PubSub Handling

• List every handle_info clause and when it's triggered
• Do all PubSub subscriptions have corresponding handlers?
• What happens if a message arrives before mount completes?

State Transitions

• Show the event → handler → state transition table
• Are all error states handled gracefully?
• What's the recovery path from each error state?

Memory & Performance

• Are large lists using streams?
• Is transient data using temporary_assigns?
• What's the memory footprint per connected user?

PR Challenge ($elixir-phoenix:phx-challenge pr)

Senior engineer review checklist:

Must Pass

• [ ] No direct Repo calls in controllers/LiveViews
• [ ] All Ecto queries use explicit preloads
• [ ] Changesets validate all user input
• [ ] No atoms created from params
• [ ] Error cases handled (not just happy path)
• [ ] Tests cover new functionality

Performance

• [ ] No queries in Enum.map loops
• [ ] LiveView streams for lists > 100 items
• [ ] Indexes exist for WHERE clause columns

OTP

• [ ] GenServers have supervision
• [ ] Timeouts set for GenServer.call
• [ ] No unbounded process spawning

Security

• [ ] No SQL injection via raw queries
• [ ] No path traversal in file handling
• [ ] Authorization checks present

Prior Findings Deduplication (MANDATORY)

CRITICAL: Prevents re-discovering identical issues across consecutive runs.

• Search .claude/plans/*/reviews/ and .claude/reviews/ for prior findings
• Read ALL prior findings before analyzing code
• Check each finding against priors:
• Fixed → SKIP | Still present → PERSISTENT (one line) | New → NEW (full analysis) | Reintroduced → REGRESSION
• Present: NEW first (full), then PERSISTENT (one-line), then REGRESSION

Example Challenge Output

## Challenge: Ecto — Orders Migration

### FINDING 1: Table lock risk (HIGH)
AddColumn on `orders` (2.1M rows) will lock table during deploy.
**Proof needed**: Run `SELECT count(*) FROM orders` — if >1M, use
`ALTER TABLE ... ADD COLUMN ... DEFAULT NULL` (no lock).

### FINDING 2: Missing index (MEDIUM)
New `WHERE status = ?` query on line 45 has no index.
**Action**: Add `create index(:orders, [:status])` to migration.

### Status: BLOCKED — 2 unresolved findings

Usage

Run $elixir-phoenix:phx-challenge [mode] to initiate a rigorous review. The reviewer will not approve until all concerns are addressed with evidence.

Example workflow:

• Run $elixir-phoenix:phx-challenge ecto after migration changes
• Answer each question with code references or test results
• Address all concerns before proceeding to PR