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fix: if MCP-Protocol-Version header is not provided, assume 2025-03-26 #855

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fix: if MCP-Protocol-Version header is not provided, assume 2025-03-26 #855

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10 changes: 5 additions & 5 deletions docs/specification/2025-06-18/basic/transports.mdx
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Expand Up @@ -250,14 +250,14 @@ For example: `MCP-Protocol-Version: 2025-06-18`
The protocol version sent by the client **SHOULD** be the one [negotiated during
initialization](/specification/2025-06-18/basic/lifecycle#version-negotiation).

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 `2025-06-18`.

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

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 **MAY** assume protocol
version `2025-03-26`.

### Backwards Compatibility

Clients and servers can maintain backwards compatibility with the deprecated [HTTP+SSE
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