When a response, stream, or batch item can fail independently, explicitly validate it and keep that failure from corrupting unrelated results or silently becoming a "pass." Two rules apply broadly: 1. Validate before trusting: never treat an unconfirmed or malformed response as success. Check HTTP status, required fields, and type before using a value (e.g. reject a non-200 or missing-`total_score` response before scoring, reject an empty `classification` unless the provider explicitly confirms `noLabelsDetected`, reject a streamed tool call that never received its arguments). Skipping this check lets `not-x` assertions invert an error into a false pass. 2. Isolate and tag failures at their source: wrap per-item processing (rendering a single template var, serializing one span, exporting one batch) in its own try/catch so one bad item doesn't abort the whole operation — but only suppress the failure when it's truly inconsequential (e.g. an unreferenced template value), and always re-validate types before calling methods like `.trim()` rather than assuming the shape of upstream/SDK data. When an internal grading/execution error does occur, tag it distinctly (e.g. `metadata.graderError`) so downstream code reports it as an infrastructure ERROR rather than an ordinary assertion failure, and so it is never auto-passed by negation. Example: ```ts // Bad: assumes success, lets an error become a pass/fail const resp = await matchesScore(...); return finalize(resp, assertion, inverse); // Good: validate first, so negation can't invert a failed call into a pass if (resp.status !== 200 || typeof resp.total_score !== 'number') { return { pass: false, score: 0, reason: `Scoring failed: ${resp.status}`, metadata: { graderError: true } }; } return finalize(resp, assertion, inverse); ``` Apply this pattern whenever code consumes an external response, a streamed event, or a batch of independent items: confirm it actually succeeded, keep one failure from taking down others, and mark true infrastructure failures so they aren't mistaken for a normal negative result.