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add compatibility language to version header requirement #668

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bhosmer-ant merged 1 commit into from
Jun 6, 2025
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add compatibility language to version header requirement #668

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11 changes: 8 additions & 3 deletions docs/specification/draft/basic/transports.mdx
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Expand Up @@ -243,13 +243,18 @@ If using HTTP, the client **MUST** include the `MCP-Protocol-Version:
<protocol-version>` HTTP header on all subsequent requests to the MCP
server, allowing the MCP server to respond based on the MCP protocol version.

For example: `MCP-Protocol-Version: 2025-03-26`

The protocol version sent by the client **SHOULD** be the one [negotiated during
initialization](/specification/draft/basic/lifecycle#version-negotiation).

For example: `MCP-Protocol-Version: 2025-03-26`
For backwards compatibility, if the server does _not_ receive an `MCP-Protocol-Version`
header, and has no other way to identify the version - for example, by relying on the
protocol version negotiated during initialization - the server **SHOULD** assume protocol
version `2025-03-26`.

If the server receives a request with a missing, invalid, or unsupported
MCP-Protocol-VERSION, it **MUST** respond with `400 Bad Request`.
If the server receives a request with an invalid or unsupported
`MCP-Protocol-Version`, it **MUST** respond with `400 Bad Request`.

### Backwards Compatibility

Expand Down