/phx:trace
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.
Invoke in v3
Compare runtime support →/phx:trace Amp skill: invoke → phx-trace Codex $elixir-phoenix:phx-trace Pi /skill:phx-trace OpenCode skill tool → name: "phx-trace" Claude Code includes the full custom-agent and hook layer. Generated runtimes load the same skill knowledge with runtime-native invocation.
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Call Tracing
Build call trees showing how functions are reached from entry points.
Iron Laws - Never Violate These
- Always use
mix xref callersfirst - 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
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/1Read 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 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