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oliver-kriska / phx-trace

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.

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SKILL.md
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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)

ConditionWhy Call Tree Helps
Unexpected nil/value at runtimeTrace where the value originates
Bug can't reproduce locallySee all entry points that reach the code
Changing function signatureFind all callers and their argument patterns
Incomplete stack traceGet 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)

PatternType
def mount/3, def handle_event/3LiveView
def index/2, def show/2, def create/2Controller
def perform(%Oban.Job{})Oban Worker
def handle_call/3, def handle_cast/2GenServer

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