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\n

name: trace\ndescription: \"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.\"\neffort: medium

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Call Tracing

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

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Iron Laws - Never Violate These

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    \n
  1. Always use mix xref callers first - It's authoritative; grep is fallback only
    2. \n
  2. Stop at entry points - Controllers, LiveView callbacks, Oban workers, GenServer callbacks
    3. \n
  3. Track visited MFAs - Prevent infinite loops from circular calls
    4. \n
  4. Extract argument patterns - Just knowing \"who calls\" isn't enough; HOW they call matters
    5. \n
  5. Max depth 10 - Deeper trees indicate architectural issues, not useful traces
    6. \n
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When to Build Call Tree (Use Proactively)

\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
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
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Quick Trace

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Run the caller query first, then inspect another function in the chain as needed:

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mix xref callers MyApp.Accounts.update_user/2\nmix xref callers MyApp.Accounts.get_user/1\n
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Read the reported locations to see argument patterns.

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Entry Points (Stop Here)

\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
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
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Delegate to call-tracer Agent

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For full recursive tree with argument extraction and parallel category tracing:

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Determine the effective maximum nesting depth. Use an explicit positive-integer\nCLAUDE_CODE_MAX_SUBAGENT_SPAWN_DEPTH value first; when it is unset, inspect\nclaude --version (the default is 1 in 2.1.217–2.1.218 and 3 in 2.1.219+).\nIf the version is unavailable, conservatively use 1. At depth 2+, delegate to\nthe orchestrator below. At depth 1, keep orchestration in this main session:\nspawn the applicable controller,\nLiveView, worker, and internal tracing prompts directly, then merge their\nresults. Never spawn an orchestrator that cannot delegate.

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Agent(subagent_type: \"phx:call-tracer\", prompt: \"Build call tree for MyApp.Accounts.update_user/2\")\n
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The call-tracer agent uses parallel subagents for each entry point category:

\n\n

Each gets a fresh context for deep exploration.

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Output Location

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

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References

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For detailed patterns:

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