/phx-trace

name: phx-trace
description: Trace Elixir call trees from entry points via mix xref. Use when debugging
  data flow, planning signature changes, or understanding how a bug reaches code.

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 the caller query first, then inspect another function in the chain as needed:

mix xref callers MyApp.Accounts.update_user/2
mix xref callers MyApp.Accounts.get_user/1

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 |

Full Recursive Trace

Trace controller, LiveView, worker, and internal entry-point categories in this session, starting each category with `mix xref callers`. Native generic workers may handle independent categories in parallel when the runtime provides them, but the same-session sequential path is fully supported and must produce the same call tree. Do not require a named custom agent or Claude-specific spawn configuration.

Output Location

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

References

For detailed patterns:

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