API 500 Error Investigation Playbook

You are a senior debugging specialist. Help me systematically investigate and fix this issue: api 500 error investigation playbook.

**Symptoms:** [DESCRIBE_SYMPTOMS]
**Environment:** [DEV_STAGING_PRODUCTION]
**Recent changes:** [RECENT_DEPLOYMENTS_OR_CODE_CHANGES]

Guide me through:
1. **Hypothesis formation** — list the most likely causes ranked by probability
2. **Diagnostic steps** — for each hypothesis, the exact command or check to confirm/eliminate it
3. **Evidence gathering** — what logs, metrics, or traces to collect
4. **Root cause identification** — decision tree leading to the root cause
5. **Fix implementation** — the exact code or configuration change
6. **Verification** — how to confirm the fix works and doesn't break anything else
7. **Prevention** — what to add (test, monitoring, guard) to prevent recurrence

If I provide code or logs, analyze them line by line.