> ## Documentation Index
> Fetch the complete documentation index at: https://docs.usenexio.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Engine versions

> The honest configuration and schema freeze boundary for released engines.

An engine version is a numbered `major.minor` release. A released exact version
identifies immutable configuration bytes plus immutable request and response
schema bytes and hashes.

The version does not freeze the executable artifact, model output, external
data, provider behavior, wording, latency, or other nondeterministic behavior.
Nexio keeps runtime changes compatible by testing every deploy candidate against
every active supported contract.

## Pins

| Pin     | Meaning                                                                                         |
| ------- | ----------------------------------------------------------------------------------------------- |
| `N.M`   | Resolve one exact released configuration and schema pair.                                       |
| `N.x`   | Resolve the latest released minor in major N only when the engine policy permits auto-tracking. |
| `draft` | Resolve mutable unpublished config. Sandbox only.                                               |

An `exact_required` engine rejects omission and `N.x`. Legacy-policy public
intake resolves an omitted pin to the latest release. Bare major and three-part
semantic versions are rejected for every engine.

Resolution occurs at submission. The concrete version and config hash are
stamped on the run, and a later publish cannot move an in-flight run. Polling
and webhook `data.run` return the resolved `engine_version` when present.

## Discovery

`GET /api/v1/engines/{engine_slug}/versions` returns exact versions,
changelogs, release timestamps, and available request and response schema
hashes. Schema bodies live in the immutable
[supported-version registry](/contracts/supported-versions/index.json).

See [Version discovery and exact pinning](/guides/upgrading-engine-versions) for
the deliberate upgrade workflow.