Phx Investigate Agent Skill

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

$elixir-phoenix:phx-investigate Users can't log in after password reset
$elixir-phoenix:phx-investigate FunctionClauseError in UserController.show
$elixir-phoenix: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 Codex subagent tooling is

available; it is an optimization, never a requirement.

Iron Laws

• Read the error literally first — extract the exception, message, failing

assertion, and first relevant application frame before theorizing.

• Check the obvious before going deep — compile errors, missing migrations,

atom/string mismatches, nil values, stale servers, and changeset errors explain

many failures.

• 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.

• Confirm the root cause with evidence — distinguish the observed failure,

the causal code path, and the proposed correction.

• 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 Codex 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 $elixir-phoenix:phx-quick, $elixir-phoenix:phx-plan, or $elixir-phoenix: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