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phx:trace

The phx:trace skill builds call trees from Elixir function entry points using mix xref to visualize how data flows through Phoenix applications. Use it when debugging unexpected runtime values, understanding how bugs reach specific code, planning function signature changes, or analyzing incomplete stack traces by extracting both callers and their argument patterns up to a maximum depth of ten levels.

View source Repository: claude-elixir-phoenix

Install in Claude Code

Copy

git clone --depth 1 https://github.com/oliver-kriska/claude-elixir-phoenix /tmp/phx-trace && cp -r /tmp/phx-trace/plugins/elixir-phoenix/skills/call-tracing ~/.claude/skills/phx-trace

Then start a new Claude Code session; the skill loads automatically.

Definition

SKILL.md

# Call Tracing

Build call trees showing how functions are reached from entry points.

## Iron Laws - Never Violate These

1. **Always use `mix xref callers` first** - It's authoritative; grep is fallback only
2. **Stop at entry points** - Controllers, LiveView callbacks, Oban workers, GenServer callbacks
3. **Track visited MFAs** - Prevent infinite loops from circular calls
4. **Extract argument patterns** - Just knowing "who calls" isn't enough; HOW they call matters
5. **Max depth 10** - Deeper trees indicate architectural issues, not useful traces

## When to Build Call Tree (Use Proactively)

| Condition | Why Call Tree Helps |
|-----------|---------------------|
| Unexpected nil/value at runtime | Trace where the value originates |
| Bug can't reproduce locally | See all entry points that reach the code |
| Changing function signature | Find all callers and their argument patterns |
| Incomplete stack trace | Get full path context |
| "Where does X come from?" | Visual answer to data flow question |

## Quick Trace

Run `mix xref callers MyApp.Accounts.update_user/2` to find all callers. Then read the reported locations to see argument patterns.

## Entry Points (Stop Here)

| Pattern | Type |
|---------|------|
| `def mount/3`, `def handle_event/3` | LiveView |
| `def index/2`, `def show/2`, `def create/2` | Controller |
| `def perform(%Oban.Job{})` | Oban Worker |
| `def handle_call/3`, `def handle_cast/2` | GenServer |

## Delegate to call-tracer Agent

For full recursive tree with argument extraction and **parallel category tracing**:

```
Agent(subagent_type: "call-tracer", prompt: "Build call tree for MyApp.Accounts.update_user/2")
```

The call-tracer agent uses **parallel subagents** for each entry point category:

- Controllers subagent (HTTP paths)
- LiveView subagent (WebSocket paths)
- Workers subagent (Background jobs)
- Internal subagent (Cross-context calls)

Each gets fresh 200k context for deep exploration.

## Output Location

`.claude/plans/{slug}/research/call-tree-{function}.md`

## References

For detailed patterns:

- `${CLAUDE_SKILL_DIR}/references/mix-xref-usage.md` - Full mix xref commands and options
- `${CLAUDE_SKILL_DIR}/references/entry-points.md` - All Phoenix/OTP entry point patterns
- `${CLAUDE_SKILL_DIR}/references/argument-extraction.md` - AST parsing for argument patterns