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Drift covers a successful response whose shape changed against a baseline. It says nothing about a provider answering a request differently from the sandbox built from its own spec: a 422 on a duplicate name the spec never mentions, a 5xx where the sandbox succeeds, a success code the spec does not declare. Divergence covers that. For every exchange the agent records, after redaction, it rebuilds the request and runs it through an ephemeral fork of the linked sandbox, seeded with what the agent already knows: the traffic overlay and the rules. Then it compares the sandbox’s answer with the provider’s.
That alert came from a second create carrying a reference the provider had already seen. The spec declares 201 for the create and nothing about references being unique, so the sandbox said 201 and the provider said 422.

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What is compared

The status class, the exact status, the error code field and the reason field, in that order. The first difference that can be established decides the category: A fork that answers 401, 403, 404 or 405 is not comparable: the rebuilt request did not reach the operation, usually because it reads a resource the fork never stored. Those are skipped and counted, never scored.

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Redaction is never a match

The agent compares the redacted recording, because that is all it keeps. When the fields the comparison needs were redacted, the result is unverifiable and no event is emitted. Two cases:
  • The provider and the sandbox answered the same 4xx, and the error code or reason the provider sent was dropped before it reached disk.
  • The sandbox refused the rebuilt request because a field the redactor dropped was required. The request the fork saw is not the request your app sent.
/healthz counts both outcomes alongside the comparisons made:

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No warmup

Divergence needs only the spec and the sandbox, so it fires from the first request. One alert per fingerprint, on first occurrence, the way declared drift alerts. The fingerprint covers the upstream, method, template, category, the status pair and the request field names that were present. Field names only. A value never reaches a fingerprint, an event or an alert.

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Listing and reproducing

reproduce arms the provider’s real status and body on the route, so the refusal happens on your laptop. It does not yet teach the sandbox the rule itself; a rule answering 422 when the reference already exists does that today, and automatic promotion from a divergence is next. A spec_drift divergence where the provider accepted what the sandbox refuses has nothing to arm, and reproduce says so.

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The event

kind is behaviour_divergence, before is the sandbox’s status, after the provider’s, field the request field names, and category one of the three above. See the drift event schema.

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Cost

The fork is built once per upstream and reused for a minute, then rebuilt. Each recorded exchange costs one in-process request against it. All of it runs on the observe path, behind the same panic isolation as the rest of the agent, after the response has already been served. The data plane is untouched. See Data plane safety.