Call Tracing

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

Iron Laws - Never Violate These

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

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

Find all callers

mix xref callers MyApp.Accounts.update_user/2

Output: lib/web/controllers/user_controller.ex:45: MyApp.Accounts.update_user/2

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:

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.

Output Location

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

References

For detailed patterns: