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. effort: medium

Call Tracing

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

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

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

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

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

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:

Each gets fresh 200k context for deep exploration.

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

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