Investigate

---
name: investigate
description: "Find the root cause of Elixir/Phoenix bugs — crashes, exceptions, stack traces, compile errors, LiveView that won't update, silent failures. Use when something is broken or misbehaves. --parallel for 4 tracks."
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`.

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**: once Steps 3-4 identify the root cause
with high confidence, present it directly — four subagents (~80K tokens)
re-verifying a finding already made in this context add cost, not evidence.

## 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.migrations`
(lists pending migrations without running them — ask before migrating).

### 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