session tests: auxiliary captions race the Codex redaction fixture request counter #1614

What happened

The full Spark acceptance run for PR #1607 revision d44b8b7de failed TestCodexAccessTokenSplitAcrossTextDeltasNeverReachesAnyCompletedSink with backend calls = 3, although the fixture expected exactly two. A controlled local-fake-backend reproduction recorded conversation → caption → conversation: the third request was legitimate auxiliary narration while a tool was running.

Replication

In the named test in internal/session/codex_client_test.go, keep the manual tool running beyond the caption timer (500 ms). The fake backend receives a caption request in addition to the initial conversation response and terminal follow-up. Its single global counter treats that auxiliary request as a conversation step, so timing and machine load change the fixture behavior.

This requires no external provider or credentials. The reproduction used a one-second diagnostic delay only to establish the sequence; the permanent regression should synchronize on channels rather than sleep.

Where

The fake backend in TestCodexAccessTokenSplitAcrossTextDeltasNeverReachesAnyCompletedSink; auxiliary work is launched from the session tool-loop caption path.

The fix

Make the fixture distinguish caption requests from conversation requests. Intentionally interleave a caption using a bounded channel handshake and answer it independently. Keep the exact two-conversation-request assertion and every existing redaction assertion across completed sinks.

Acceptance

• The fake-provider session workflow deterministically exercises a caption between the two conversation requests and retains all credential-redaction checks.
• The focused test and full changed-package acceptance pass on Spark, including execution records and exact revision.
• No model feature is disabled and no test exclusion or relaxed security assertion is introduced.

Found while validating PR #1607. Spark fleet job 001235 reproduced the old failure with safe request classifications; no real provider calls were used.