Debug an error with context

Example input

Python 3.12 script raises KeyError: user_id when reading a JSON record; I expected every record to have that key.

Expected output

Likely cause, minimal fix, explanation, and a test case; label uncertainty.

Copy-ready template

Help debug this issue. Language and version: [details]. Relevant code: [small reproducible snippet]. Exact error: [paste error]. Expected behavior: [behavior]. Actual behavior: [behavior]. What I already tried: [steps]. Explain the likely cause, propose the smallest fix, and provide a focused test. State uncertainty and do not assume unseen files.

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