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

# Versioning

The Model Context Protocol uses string-based version identifiers following the format
`YYYY-MM-DD`, to indicate the last date backwards incompatible changes were made.

<Info>
  The protocol version will *not* be incremented when the
  protocol is updated, as long as the changes maintain backwards compatibility. This allows
  for incremental improvements while preserving interoperability.
</Info>

## Revisions

Revisions may be marked as:

* **Draft**: in-progress specifications, not yet ready for consumption.
* **Current**: the current protocol version, which is ready for use and may continue to
  receive backwards compatible changes.
* **Final**: past, complete specifications that will not be changed.

The **current** protocol version is [**2025-11-25**](/specification/2025-11-25/).

## Feature States

Individual features of the specification may additionally be marked as
**Deprecated** under the
[feature lifecycle and deprecation policy](/community/feature-lifecycle):
the feature remains part of the specification, but is scheduled for removal.
Deprecated features document a migration path (or state that none is required)
and remain in the specification for at least twelve months, or at least
ninety days under the policy's
[expedited-removal exception](/community/feature-lifecycle#expedited-removal),
before they become eligible for removal, after which they may be **Removed**
in a future revision.

Features that are currently Deprecated are listed in the
[deprecated features registry](/specification/draft/deprecated).

## Negotiation

Every request declares the protocol version it is using via the
`io.modelcontextprotocol/protocolVersion` key in its
[`_meta`](/specification/draft/basic/index#meta) field, and the server accepts or
rejects each request independently. On Streamable HTTP, the same value is also carried
in the
[`MCP-Protocol-Version` header](/specification/draft/basic/transports/streamable-http#protocol-version-header).
Clients and servers **MAY** support multiple protocol versions simultaneously.

If the server does not support the requested version, it responds with an
[`UnsupportedProtocolVersionError`](/specification/draft/basic/versioning#protocol-version-negotiation)
listing the versions it does support. The client can then retry the request with a
mutually supported version, or surface an error to the user if none exists.

Clients that want to select a version up front can call
[`server/discover`](/specification/draft/server/discover), a mandatory RPC that
returns the server's supported protocol versions, capabilities, and identity in a
single request. Calling it is optional: a client is free to send any request directly
and handle a version error if one comes back.

For interoperability with servers and clients that implement the
handshake-based protocol revisions (`2025-11-25` and earlier), see
[Backward Compatibility](/specification/draft/basic/versioning#backward-compatibility-with-initialization-based-versions).