Code debugger with tests

Act as a senior [LANGUAGE] reviewer. Analyze the code and error I provide. First identify the root cause with concrete evidence, then propose the smallest change that fixes it without rewriting unrelated areas. Deliver: 1) diagnosis, 2) complete patch, 3) tests that reproduce the failure and prove the fix, 4) regression risks. Do not invent files or APIs. If essential information is missing, state exactly what you need.

Code: [CODE]
Error: [ERROR]
Environment: [ENVIRONMENT]