/phx-trace

name: phx-trace
description: Use when debugging how a value or request reaches Elixir code, finding
  who calls a function, or planning a signature change. Builds the call tree with
  mix xref callers instead of reading files one by one.

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