[diegosouzapw]

Summary

• Classifies the 7 remaining empty catch {} blocks in the SSE critical path (open-sse/utils/stream.ts x6, open-sse/utils/streamHandler.ts x1) into two buckets: intentional (our own best-effort cleanup/telemetry — gets a one-line rationale comment, no logging) vs should-log (the caller-supplied onFailure callback — a throw there previously vanished silently, changing control flow with zero trace).
• Adds an "Error handling / empty catch blocks" subsection to CONTRIBUTING.md (near Code Style) codifying that rule for future waves.
• Scope is exactly this SSE critical-path slice, per the plan-file split (executors/services follow in later PRs). Does not touch the other ~140 empty catches elsewhere in the repo; open-sse/executors/cursor.ts is already at zero from fix(stream): add logging to empty catch blocks in stream error handling #8143 and untouched here.

Sites classified

• stream.ts L816 consumeToolFinishTime — intentional (comment)
• stream.ts L1833, L2112 — telemetry/latency recording — intentional (comment)
• stream.ts L1870, L1998 markToolFinish — intentional (comment)
• stream.ts L1946 onFailure(failurePayload) — should log: kept the catch (never break the stream) but added console.debug so a buggy consumer callback is now discoverable
• streamHandler.ts L586 controller.close() after disconnect — intentional (comment; the canonical example the issue cites)

How tested

TDD per Hard Rule #18 — new test file tests/unit/stream-onfailure-callback-logging.test.ts:

• Wrote the failing test first: asserted a console.debug call referencing onFailure is emitted when the caller-supplied callback throws. Failed before the change (empty catch swallowed silently, no log).
• Implemented the fix (kept catch, added console.debug), test now passes.
• Regression guard: onFailure returning normally emits no log and behaves byte-identically — proves the happy path and the 6 comment-only sites are behavior-free.

node --import tsx/esm --test tests/unit/stream-onfailure-callback-logging.test.ts
# 2/2 passing

Also verified: tests/unit/stream-utils.test.ts + tests/unit/claude-empty-stream-error-3685.test.ts still 58/58 green (no regression in adjacent stream-path coverage).

Gates run locally: typecheck:core clean, eslint clean on changed files, check:cycles clean, check-file-size.mjs clean (tightened an unrelated 3-line JSDoc block to 1 line to stay under the frozen stream.ts baseline after adding the log), check-test-discovery.mjs clean. check:complexity-ratchets shows the same pre-existing base-red as the pristine tip (2167/956 vs baseline 2130/951) — unaffected by this change.

Closes #8142