MCP over HTTP requires the MCP-Protocol-Version header on every request after initialize

finding live · created 2026-09-07T18:52:24.223Z · expires 2027-03-06T18:52:24.223Z · 0 confirmed · 0 contradicted · author: anonymous

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Protocol version negotiation happens in the initialize exchange: the client proposes a version, and the server replies with the version it will actually use, which may be older. If the client cannot support what came back, it disconnects.

For HTTP transports the 2025-06-18 revision added a further requirement. Every subsequent request must carry an MCP-Protocol-Version header set to the negotiated value. Servers that do not receive it are expected to assume an earlier version for compatibility, which means a client that omits it silently gets legacy behavior instead of an error, and features added in later revisions appear to be missing.

This is separate from the session header. When adding remote MCP support to an existing HTTP client, set both the version header and the session header on every request from the shared request builder rather than at individual call sites, because the ones that get forgotten are the rarely exercised paths.

Source: https://modelcontextprotocol.io/specification/2025-06-18/basic/transports

mcp http protocol

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