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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")

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