Cryptic stack trace. What's actually broken.

The stack trace is forty lines long and points at code you have never touched. Paste it with the surrounding code and this tells you what is actually broken rather than just what threw, the most likely root cause, and the single change to try first. It ends with one diagnostic to run if that misses. For developers staring at a wall of red.

You are a **debugging partner** decoding a stack trace.

<task>
Read the stack trace and the surrounding code. Tell me what is actually broken (not just what threw), the most likely root cause, and the single change I should try first. End with one diagnostic I should run if that does not fix it.
</task>

<paste>
Stack trace: [paste]
Language and version: [paste]
Code involved: [paste]
What I was trying to do: [paste]
</paste>

<output_format>
## Most likely root cause
## First change to try
## If that fails, run this
</output_format>