/phx-investigate

name: phx-investigate
description: Investigate Elixir/Phoenix bugs root-cause first. Reproduce failures,
  cite evidence, and use optional Amp subagents only when useful.

Investigate Bug

Investigate Elixir/Phoenix bugs root-cause first. Reproduce or establish the failing behavior before recommending a fix, and cite concrete paths and lines.

Usage

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

Treat the text after the skill name as the bug description. `--parallel` asks for independent investigation tracks when native Amp subagent tooling is available; it is an optimization, never a requirement.

Iron Laws

1. **Read the error literally first** — extract the exception, message, failing assertion, and first relevant application frame before theorizing. 2. **Check the obvious before going deep** — compile errors, missing migrations, atom/string mismatches, nil values, stale servers, and changeset errors explain many failures. 3. **Reproduce before proposing a fix** — run the smallest relevant test or controlled command and record its output. If reproduction is impossible, state exactly what evidence establishes the failure instead. 4. **Confirm the root cause with evidence** — distinguish the observed failure, the causal code path, and the proposed correction. 5. **Do not edit while investigating unless the user asks for a fix** — the investigation result is evidence and a recommendation, not an implicit patch.

Workflow

1. Consult Existing Evidence

Search `.claude/solutions/`, recent diffs, tests, logs, and the literal error. Do not block if `.claude/solutions/` does not exist.

2. Capture Runtime Context When Available

Tidewave is optional. If its tools are configured, use them for logs, source locations, safe queries, or hypothesis checks. Otherwise use repository files, `mix` commands, and local logs. Never fail or ask the user to install Tidewave merely to continue an investigation.

3. Run Sanity Checks

Choose focused checks that fit the report, such as:

mix compile --warnings-as-errors
mix test test/path_test.exs --trace

Do not run migrations or other state-changing commands unless they are necessary, safe for the fixture, and authorized by the user.

4. Reproduce Before Fixing

Capture the exact command, failure, and relevant output. Read `references/error-patterns.md`, then inspect only the code needed to trace the failure from entry point to cause.

5. Check the Obvious

Check saved files, atom/string keys, preload state, pattern matches, nil values, return values, server restarts, and changeset errors. For silent LiveView form failures, inspect `{:error, changeset}` and rendered validation errors before JS.

6. Trace and Test the Hypothesis

Use targeted searches, source reads, tests, or non-mutating diagnostics. Only add temporary source diagnostics if the user explicitly authorizes edits, and remove them before reporting. Cite `path:line` evidence for both the failing behavior and the causal code.

If native Amp subagents are available and the bug genuinely spans independent areas, delegate read-only tracks by concern. Otherwise perform the same tracks sequentially in this session. Do not require named custom agents.

7. Report

Use `references/investigation-template.md`. Include:

• reproduction or evidence establishing the failure;
• root cause, not merely the symptom;
• relevant paths and lines;
• confidence and any unverified assumptions;
• the smallest safe fix or next diagnostic step.

Route follow-up work with `phx-quick`, `phx-plan`, or `phx-compound` when appropriate. Do not invoke another skill unless the user asks you to continue.

References

• `references/error-patterns.md` — common errors and checklist
• `references/investigation-template.md` — output format
• `references/debug-commands.md` — debug commands and common fixes

Amp native parallel investigation

For a non-trivial failure with independent reproduction, root-cause, impact, and fix-strategy questions, call `elixir_phoenix_parallel_investigate` once. Its four local child threads are enforced read-only (`Read` and `finder` only). Reconcile their output in this parent thread and verify every claimed evidence path before editing. If the tool is unavailable or a child fails, run only the missing track sequentially. Simple failures should stay sequential.