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An exact N.M pin freezes the released engine configuration plus the request and response schemas identified by that release. It does not freeze nondeterministic model output, external source contents, provider behavior, latency, or the executable artifact. Nexio validates the current runtime against every supported released contract before deployment.

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Pin policies

The fixture and launch-engine integration use exact_required. Do not use N.x for those engines. Bare major and three-part semantic versions are always rejected.

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Discover versions and immutable schemas

  1. Call GET /api/v1/engines/{engine_slug}/versions with an engines:read scoped key bound to the engine.
  2. Select an exact version and record its request_schema_hash, response_schema_hash, released_at, and changelog.
  3. Resolve each hash through the published supported-version registry.
  4. Download the content-addressed schema path from its immutable manifest and verify that the file SHA-256 equals the manifest hash.
The version API intentionally returns hashes and release metadata, not schema bodies. The checked-in content-addressed files are the immutable launch contract.

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Deliberate upgrade flow

1

Freeze the current baseline

Record the exact version and both schema hashes currently used in live requests.
2

Review the target

Read its changelog and compare the immutable request and response schemas.
3

Exercise sandbox outcomes

Submit the target exact version in a canonical sandbox. For the fixture engine, run completed, degraded, and failed test scenarios and verify polling plus webhook handling.
4

Update the integration

Change request generation and response parsing deliberately. Deploy the exact target pin, never an N.x shortcut for an exact-required engine.
5

Reconcile and retain evidence

Store the resolved exact version and schema hashes with your release record.
Rollback means redeploying the previously reviewed exact version. A rollback does not make execution byte-identical to an earlier date because behavior and external sources are outside the freeze boundary.