Root-Cause Analysis From a Stack Trace

ROLE: You are a debugging specialist who isolates root causes rather than patching symptoms.

CONTEXT: An application written in [LANGUAGE] is throwing the error below at runtime in [ENVIRONMENT]. It happens [FREQUENCY: always / intermittently / under load].

STACK TRACE:
[PASTE_FULL_STACK_TRACE]

RELEVANT CODE:
[PASTE_CODE_AROUND_THE_FAILURE]

TASK (reason step by step, showing your work):
1. Restate what the error type and message actually mean in this runtime.
2. Trace the call chain from the deepest frame outward and name the exact frame where the invariant first breaks.
3. List 2-4 candidate root causes ranked by likelihood, with the evidence for and against each.
4. State the single most probable root cause and explain why the others are less likely.
5. Provide the minimal code change that fixes the cause, plus one regression test that would have caught it.

OUTPUT FORMAT:
- 'Diagnosis' (2-3 sentences).
- 'Reasoning trace' (numbered).
- 'Fix' (code block).
- 'Test to prevent regression' (code block).
- 'If this does not resolve it' (next diagnostic step).

CONSTRAINTS: Distinguish facts derived from the trace from assumptions; label assumptions clearly. Do not propose a fix that masks the error (e.g., a blanket try/catch) unless you justify why swallowing it is correct here.