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.

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

Investigation Workflow

Step 0: Consult Compound Docs

grep -rl "KEYWORD" .claude/solutions/ 2>/dev/null

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

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

Step 2: Reproduce

mix test test/path_test.exs --trace
tail -200 log/dev.log | grep -A 5 -i "error\|exception"

Step 3: Read Error LITERALLY

Parse the error message — check 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

What's actually happening vs what should happen.

Autonomous Iteration

For autonomous debugging, use /ralph-loop:ralph-loop with clear completion criteria and --max-iterations.

References

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