phx:trace
The phx:trace skill builds call trees from Elixir function entry points using mix xref to visualize how data flows through Phoenix applications. Use it when debugging unexpected runtime values, understanding how bugs reach specific code, planning function signature changes, or analyzing incomplete stack traces by extracting both callers and their argument patterns up to a maximum depth of ten levels.
git clone --depth 1 https://github.com/oliver-kriska/claude-elixir-phoenix /tmp/phx-trace && cp -r /tmp/phx-trace/plugins/elixir-phoenix/skills/call-tracing ~/.claude/skills/phx-traceSKILL.md
# 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 patternsAsh Framework — resources, actions, policies, aggregates, calculations,
Compatibility alias for the Elixir/Phoenix plugin's LiveView assigns audit. Invoke explicitly with /lv:assigns.
Inspect LiveView socket assigns for memory bloat — missing temporary_assigns, unused assigns, unbounded lists needing streams, memory estimates. Use when LiveView memory grows or you need to add temporary_assigns.
Project health audit and health check — architecture, performance, tests, dependencies, code quality. Use when assessing overall project health, before releases, or after refactors.
Analyze Phoenix context boundaries and module coupling via mix xref. Use when checking cross-context calls, validating dependencies, before splitting modules, or reviewing architecture.
Brainstorm Elixir/Phoenix features — explore ideas, compare approaches, gather requirements. Use when vague idea, not sure how to approach, or want to discuss before plan.
Interactive briefing of a plan file — explains reasoning, schema decisions, component choices. Use when developers need to understand a plan before approving.
Summarize and review what changed while you were away. Use after a weekend, vacation, or flight to check missed PRs, git commits, Linear tickets, and meetings — one prioritized brief, not a firehose.