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Mesh versions its API by date, not by a number in the URL. The newest version is 2026-09; if you send nothing you get 2026-08. You select one with a header:
The /v1 in the path is not a version. It is a stable namespace and it is not going to change — /v2 is not planned. Everything about the shape of a request or response is governed by the dated version instead.

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Why a version header

Additive changes — a new field, a new optional parameter, a new endpoint — are shipped continuously and do not need a version. You get them without doing anything. A dated version exists for the changes that are not safe: renaming a field, removing one, or changing what a value means. Rather than break your integration or freeze the API forever, those changes land in a new dated version, and requests pinned to an older one keep receiving the older shape. Pinning is what makes that a guarantee rather than a coincidence. A pinned request states which shape your code can parse, so a change to the current shape cannot reach it.

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If you send no header

You are served the baseline — the oldest supported version, which today is 2026-08. That is a deliberate choice, and worth understanding because it inverts what people usually expect from a default:
Publishing a new version never moves existing traffic. Because the baseline is the oldest supported version rather than the newest, an integration that sends no header keeps getting the same shapes after a new version ships. You move only by pinning the new version yourself.
So there is no urgency to pin — but until you do, your code has not stated what it can parse, and the guarantee above does not apply to it.

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Which versions are served

GET /v1/api-versions lists them, oldest first. It takes your rsk_ API key.
baseline marks what an unpinned request gets; latest marks the newest. They are now two different entries — latest is deliberately not the default, so shipping 2026-09 did not move anyone. capabilities lists behaviour a version adds, and it is cumulative: everything introduced at or before that label. Unlike a change to a response shape, a capability is something you can only use once you pin for it — see What each version added.
Worth wiring into your own tests: assert the version your code pins still appears here, and that its status is not sunset. That turns “our pin went stale” into a failing test instead of a 400 in production.

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Pinning a whole API key

Rather than sending the header on every request, you can pin the key itself — useful when the calling code is not yours to change, or when a whole integration should sit on one version. Set it when creating a key, or on an existing one — key management takes an admin key (mak_...), not the rsk_ key being pinned:
Send "api_version": null to clear it. In the dashboard the same setting is on the key’s API Version field under API Keys. Resolution order, highest first:
  1. The X-Mesh-Version header on the request — always wins.
  2. The key’s stored pin.
  3. The baseline.
A key’s pin is resolved when the key is created, from your organisation’s default at that moment — so a key created without an explicit version may show a concrete one afterwards rather than staying empty. Changing your org default later does not move existing keys.
An unsupported version stored on a key is treated differently from an unsupported header: the request falls back to the baseline rather than failing. A typo in your header should be loud; a version we retired out from under a stored pin should not break your traffic.

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Unsupported versions are rejected, not downgraded

If you send a version Mesh does not serve, the request fails:
400, with the versions that are served, and no fallback.
This is intentional, and it is the reason pinning is trustworthy. If a typo or a stale constant quietly fell back to some other version, you would believe you were pinned when you were not, and find out from a shape mismatch in production instead. A 400 on the first call is the better failure.For the same reason, an empty value (X-Mesh-Version: with nothing after it) is treated as a malformed pin, not as “no pin”. If your client builds the header conditionally, omit it entirely when you have no version to send rather than sending it blank.
Validation runs before authentication, so a bad version is rejected with 400 even if the key is also wrong.

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Reading the version that served a request

Every response carries X-Mesh-Version with the version that served it — including error responses, and including requests that sent no header at all. So you can confirm what you actually got rather than assume it, and you can see which version a stored key pin resolved to without looking the key up.
It is in the CORS expose-headers allowlist, so browser JavaScript can read it too.

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Format

A version label is YYYY-MM — four digits, a hyphen, two digits, e.g. 2026-08. Anything else is rejected with the same 400 invalid_api_version. Pass it as a literal string. Do not compute it from the current date: the label names a contract, not today’s month, and there is no version for most months.

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Which version to pin

Pin the version you developed and tested against, and change it deliberately. Concretely: hold the label in one constant in your codebase, send it on every request, and treat bumping it as a change with its own review — because a new version exists precisely when some shape is different, so moving to it means reading what changed.
Do not read the version from configuration that varies by environment. A staging deployment pinned to one version and production pinned to another means the shape your tests exercise is not the shape production parses, which defeats the point.

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What is not covered by a version

  • Model availability and pricing. Models are added, retired, and repriced continuously. Pin a version and you still get the current catalogue — see Models.
  • Rate limits and quotas. Governed by your plan, not the contract. See Rate limits.
  • Provider behaviour. A model’s output quality or latency is the provider’s, and no version freezes it.
  • Bug fixes. A response that did not match its documented shape gets corrected in place, in every version.

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Realtime

The realtime WebSocket API (/v1/realtime) negotiates separately and does not read this header. Sending it on the handshake has no effect.

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Knowing something is going away, without reading this page

When a version or an endpoint is scheduled for retirement, its responses start carrying standard headers — so your monitoring can find out before a human does:
The two dates are in different formats — a unix timestamp and an HTTP-date. If you parse one and reuse the parser on the other you will silently get nothing. That is a property of the RFCs, not of Mesh.
Sunset may be absent while Deprecation is present: “deprecated, retirement date to be announced” is the normal state at announcement time. Absence means undecided, not imminent. These arrive on every response for an affected route, including errors — a 429 from a deprecated endpoint is still deprecated, and an error response is often the only one a client logs. Nothing is deprecated today, so nothing currently emits them. If you read these from browser JavaScript, note they are already in the CORS Access-Control-Expose-Headers allowlist along with X-Mesh-Version.

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What each version added

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Poll a video task by your request id
A video generation task can be fetched with the request_id Mesh returned for the request that created it, as well as with the provider task id. Video task bodies also carry a request_id field. Capability: video.poll_by_request_id. See Video generation.
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The baseline contract
The contract as it stood when dated versions were introduced. This is what you are served if you send no header and have set no pin.

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Support window

Every version GET /v1/api-versions lists is served. Both 2026-08 and 2026-09 are ga, nothing is scheduled for retirement, and no pin can go stale today. A version is never withdrawn without notice. When one is scheduled for retirement it starts returning Deprecation and Sunset response headers, so your client can detect it without reading this page. If you need the retirement policy’s exact numbers before committing to a pin, contact support.