Settled in review

The three questions this design left open have answers, and they are the same answer three times: a test that quietly tolerates drift stops being a test.

Cassette matching is strict by BODY, and every turn is consumed exactly once. The sketch proposed matching a structural envelope plus a normalized prompt hash, with named holes for volatile spans. Still rejected: an edited prompt template is exactly what this feature exists to catch, so a matcher with holes in it would be excused from catching the main case. A replayed call must match a recorded turn byte-for-byte, and an extra call is a failure.

Position was the contract in v1, and it has been dropped, because it assumed something real agents do not provide: a strictly sequential trajectory. goose issues its task call and a session-title call CONCURRENTLY and does not wait for the second. Record sees whichever lands first, replay serves from the cassette instantly and sees the other, and a positional matcher reported a divergence when nothing about the agent had changed. Order between concurrent calls is therefore not asserted - the agent does not guarantee it, so a recording cannot either. A sequential trajectory is unaffected: the earliest unconsumed match wins, so turn K still matches turn K, and its divergence message is unchanged.

This is the "reordering tolerance" the first version of this section deferred with "nothing has [demanded it]". The first third-party agent tried demanded it.

Envelope comparison survived, but as a REPORTING rule rather than a matching one: model, tool names and message roles are compared and reported before any message body, because a byte diff of two 8000-token prompts is unreadable and "you added a tool" is a one-line answer.

Divergence fails at the first bad turn. No searching forward for a turn that fits. Once a trajectory has diverged its later turns say nothing about the system under test, and continuing would report a cascade whose only real cause was the first failure. (Reordering tolerance, which this bullet once deferred, is now part of matching - see above. Failing fast is unaffected: it is about not searching PAST a genuine divergence, not about the order of concurrent calls.)

reply is the final assistant message of the conversation the flow is about. Not the process's stdout, which this document originally suggested. Stdout is whatever a harness chose to print - a banner, a spinner, nothing at all - and it differs per driver.

"The trajectory's last assistant message" was the v1 rule, and it is not enough, because an agent may talk to the model about something other than the task. goose asks it to name the session, in a call with its own system prompt, issued concurrently and not waited for. Its answer is an assistant message, so reply became a coin flip: whichever call landed second won, and record succeeded roughly two times in three.

A side conversation is recognisable by its system prompt, since turns that continue one conversation share one. Turns are grouped by system prompt and the thread with the most turns wins; ties go to the thread carrying the most request text, because the conversation doing the work carries the agent's real system prompt and its tool schemas while a housekeeping call is small. Both halves are order-independent, which is the point.

It is a heuristic, and the limit is worth stating: an agent whose side conversation is BIGGER than its real one would defeat the tie-break. A cassette with a single system prompt - the ordinary case - takes the identical path it always did.

A trajectory whose last turn is a tool call has not replied yet, which is a real state and reads as absent rather than as empty text.