phx:trace

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 mix xref callers MyApp.Accounts.update_user/2 to find all callers. Then 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:

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:

• Controllers subagent (HTTP paths)
• LiveView subagent (WebSocket paths)
• Workers subagent (Background jobs)
• Internal subagent (Cross-context calls)

Each gets fresh 200k 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