phx-investigate

name	phx:investigate
description	Investigate bugs and errors in Elixir/Phoenix — root-cause analysis for crashes, exceptions, stack traces, test failures. Use --parallel for deep 4-track investigation.
effort	high
argument-hint	<bug description> [--parallel]

Investigate Bug

Investigate bugs using the Ralph Wiggum approach: check the obvious, read errors literally.

Usage

/phx:investigate Users can't log in after password reset
/phx:investigate FunctionClauseError in UserController.show
/phx:investigate Complex auth bug --parallel

Arguments

$ARGUMENTS = Bug description or error message. Add --parallel for deep 4-track investigation.

Mode Selection

Use parallel mode (spawn deep-bug-investigator) when: bug mentions 3+ modules, spans multiple contexts, is intermittent or involves concurrency, or user says --parallel/deep.

Otherwise: Run the sequential workflow below.

Avoid confirmatory subagents: Do NOT spawn parallel subagents to "verify" findings you already identified in the main context. If Step 3-4 already identified the root cause with high confidence, present it directly — don't spend ~80K tokens on 4 subagents to confirm what's already obvious (confirmed waste: session c135330a).

Iron Laws

1. Read the error message literally first — Most bugs tell you exactly what's wrong; resist the urge to theorize before reading what the system is saying
2. Check the obvious before going deep — Compile errors, missing migrations, atom/string mismatches explain 80% of bugs; exhausting the Ralph Wiggum checklist saves hours
3. Check changeset errors before UI debugging — Silent form saves are almost always {:error, changeset} with validation failures, not viewport or JS issues
4. Consult compound docs before investigating fresh — A previously solved problem saves the entire investigation cycle; always search .claude/solutions/ first
5. NEVER guess at a fix before reproducing — Reproduce first, then identify root cause, then fix. Skipping steps causes wrong fixes
6. DO NOT apply a fix without confirming root cause — Verify your hypothesis with evidence (logs, tests, IO.inspect) before changing code

Investigation Workflow

Step 0: Consult Compound Docs

Search .claude/solutions/ for relevant keywords using Grep.

If matching solution exists, present it and ask: "Apply this fix, or investigate fresh?"

Step 0a: Runtime Auto-Capture (Tidewave -- PRIMARY when available)

If Tidewave MCP is detected, start here instead of asking the user to paste errors. Auto-capture runtime context:

1. mcp__tidewave__get_logs level: :error -- capture recent errors
2. Parse stacktraces, correlate with source via mcp__tidewave__get_source_location
3. For data bugs: mcp__tidewave__execute_sql_query to inspect state
4. For logic bugs: mcp__tidewave__project_eval to test hypotheses
5. For UI bugs: mcp__tidewave__get_source_location with component name

Present pre-populated context to the user:

Auto-captured from runtime:

• Error: {parsed error from logs}
• Location: {file:line from get_source_location}

Investigating this. Correct if wrong.

This eliminates copy-pasting errors between app and agent. If Tidewave NOT available: Fall through to Step 1.

Step 1: Sanity Checks

Run mix compile --warnings-as-errors 2>&1 | head -50, then mix ecto.migrate.

Step 2: Reproduce

Run mix test test/path_test.exs --trace. Then read the last 200 lines of log/dev.log and search for "error" or "exception" patterns.

Step 3: Read Error LITERALLY

Parse the error message — check ${CLAUDE_SKILL_DIR}/references/error-patterns.md.

Step 4: Check the Obvious (Ralph Wiggum Checklist)

File saved? Atom vs string? Data preloaded? Pattern match correct? Nil? Return value? Server restarted?

LiveView form saves silently failing? Check changeset errors FIRST — not viewport, click mechanics, or JS. A missing hidden_input for a required embedded field causes {:error, changeset} with no visible UI feedback.

Step 5: IO.inspect / Tidewave project_eval

Step 6: Identify Root Cause

Find what's actually happening vs what should happen.

Step 7: Hand Off

Present root cause + evidence. Then route by fix size:

• Small, contained fix → offer to apply directly or via /phx:quick
• Multi-file or risky fix → suggest /phx:plan {root cause summary} so the fix gets task structure and review
• Non-obvious root cause → after the fix lands, suggest /phx:compound

Autonomous Iteration

Use /ralph-loop:ralph-loop for autonomous debugging with clear completion criteria and --max-iterations.

References

• ${CLAUDE_SKILL_DIR}/references/error-patterns.md — Common errors and checklist
• ${CLAUDE_SKILL_DIR}/references/investigation-template.md — Output format
• ${CLAUDE_SKILL_DIR}/references/debug-commands.md — Debug commands and common fixes