Debug a stack trace

Diagnose an exception from evidence and propose the smallest verifiable fix.

Copy-ready prompt

Diagnose the failure below.

Return: (1) the most likely root cause with evidence from the trace, (2) up to two plausible alternatives and how to rule them out, (3) the minimal repair, and (4) a regression test. Do not invent files or runtime state.

Stack trace:
```
[TRACE]
```
Relevant code:
```
[CODE]
```

Try with app/auto-code

#debugging #stack trace #root cause #tests