Transports changedspecification/2025-11-25/basic/transports
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5050> [!NOTE]
5151>
5252> This replaces the [HTTP+SSE
53> transport](/specification/2024-11-05/basic/transports#http-with-sse) from
53> transport](/specification/2024-11-05/basic/transports.md#http-with-sse) from
5454> protocol version 2024-11-05. See the [backwards compatibility](#backwards-compatibility)
5555> guide below.
5656
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186186### Session Management
187187
188188An MCP "session" consists of logically related interactions between a client and a
189server, beginning with the [initialization phase](/specification/2025-11-25/basic/lifecycle). To support
189server, beginning with the [initialization phase](/specification/2025-11-25/basic/lifecycle.md). To support
190190servers which want to establish stateful sessions:
191191
1921921. A server using the Streamable HTTP transport **MAY** assign a session ID at
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196196 securely generated UUID, a JWT, or a cryptographic hash).
197197 * The session ID **MUST** only contain visible ASCII characters (ranging from 0x21 to
198198 0x7E).
199 * The client **MUST** handle the session ID in a secure manner, see [Session Hijacking mitigations](/specification/2025-11-25/basic/security_best_practices#session-hijacking) for more details.
199 * The client **MUST** handle the session ID in a secure manner, see [Session Hijacking mitigations](/specification/2025-11-25/basic/security_best_practices.md#session-hijacking) for more details.
2002002. If an `MCP-Session-Id` is returned by the server during initialization, clients using
201201 the Streamable HTTP transport **MUST** include it in the `MCP-Session-Id` header on
202202 all of their subsequent HTTP requests.
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262262For example: `MCP-Protocol-Version: 2025-11-25`
263263
264264The protocol version sent by the client **SHOULD** be the one [negotiated during
265initialization](/specification/2025-11-25/basic/lifecycle#version-negotiation).
265initialization](/specification/2025-11-25/basic/lifecycle.md#version-negotiation).
266266
267267For backwards compatibility, if the server does *not* receive an `MCP-Protocol-Version`
268268header, and has no other way to identify the version - for example, by relying on the
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275275### Backwards Compatibility
276276
277277Clients and servers can maintain backwards compatibility with the deprecated [HTTP+SSE
278transport](/specification/2024-11-05/basic/transports#http-with-sse) (from
278transport](/specification/2024-11-05/basic/transports.md#http-with-sse) (from
279279protocol version 2024-11-05) as follows:
280280
281281**Servers** wanting to support older clients should:
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