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.
Before spawning it, determine the effective maximum nesting depth. Use an
explicit positive-integer CLAUDE_CODE_MAX_SUBAGENT_SPAWN_DEPTH value first;
when it is unset, inspect claude --version (the default is 1 in 2.1.217–2.1.218
and 3 in 2.1.219+). If the version is unavailable, conservatively use 1. At
depth 3+, use the deep-bug-investigator orchestrator. At depth 1 or 2, keep
orchestration in this main session: spawn the four
focused tracks (reproduction, root cause, impact, fix strategy) directly in
one parallel batch, wait for all four, then synthesize their evidence. Never
spawn an orchestrator that cannot delegate.
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
- 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
- Check the obvious before going deep — Compile errors, missing migrations, atom/string mismatches explain 80% of bugs; exhausting the Ralph Wiggum checklist saves hours
- Check changeset errors before UI debugging — Silent form saves are almost always
{:error, changeset}with validation failures, not viewport or JS issues - Consult compound docs before investigating fresh — A previously solved problem saves the entire investigation cycle; always search
.claude/solutions/first - NEVER guess at a fix before reproducing — Reproduce first, then identify root cause, then fix. Skipping steps causes wrong fixes
- 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:
mcp__tidewave__get_logs level: :error-- capture recent errors- Parse stacktraces, correlate with source via
mcp__tidewave__get_source_location - For data bugs:
mcp__tidewave__execute_sql_queryto inspect state - For logic bugs:
mcp__tidewave__project_evalto test hypotheses - For UI bugs:
mcp__tidewave__get_source_locationwith 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
