/trace

name: 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."
effort: medium

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 |

Delegate to call-tracer Agent

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

Determine the effective maximum nesting depth. Use an explicit positive-integer `CLAUDE_CODE_MAX_SUBAGENT_SPAWN_DEPTH` value first; when it is unset, inspect `claude --version` (the default is 1 in 2.1.217–2.1.218 and 3 in 2.1.219+). If the version is unavailable, conservatively use 1. At depth 2+, delegate to the orchestrator below. At depth 1, keep orchestration in this main session: spawn the applicable controller, LiveView, worker, and internal tracing prompts directly, then merge their results. Never spawn an orchestrator that cannot delegate.

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

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 a fresh 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