1,238 API errors in two days, and every response said 200 OK

Recorded gRPC-web traffic revealed that an API gateway had returned 1,238 INVALID_ARGUMENT errors over two recording windows while reporting HTTP 200, causing conventional HTTP-status searches and passing unit tests to miss failures. Analysis linked several rejected requests to dashboard behaviors such as invalid regex filters, unsupported legacy service parameters, excessive time ranges, and snapshot queries with incompatible filters, all of which displayed missing data or “No traffic” rather than gateway error messages. The team fixed the affected grid and rate-panel error handling, corrected snapshot filter behavior, and strengthened mocks and regression tests so they reproduce the gateway’s actual rejection responses. A coding agent using recorded traffic identified part of one issue cheaply, though human review found additional correctness concerns not evident in the recording. The account recommends searching gRPC-web recordings for nonzero Grpc-Status headers, validating that new tests fail on original code, preserving captured traffic, and sanitizing data before sharing it with AI tools.

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