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A server configured with SupportedProtocolVersions containing only versions >= 2026-07-28 should not silently serve legacy clients that use the initialize/initialized handshake protocol. Previously, such clients would receive a successful (if confusing) response: stateful servers would accept any post-initialize method call because ephemeral session setup bypassed the version check, and stateless servers would always appear initialized due to ephemeral connect opts synthesizing fake InitializeParams. Fix the stateful path by detecting a 2026-only server configuration in the uninitialized branch of handle() and returning CodeUnsupportedProtocolVersion before falling through to the generic "method invalid during initialization" error. Fix the stateless path by checking the legacy flag in serveStateless() immediately after ephemeralConnectOpts(), before any method dispatch. Both paths include an UnsupportedProtocolVersionData payload listing the server's supported versions so clients can adapt. Fixes modelcontextprotocol#1313
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Fixes #1313.
Note: this PR does not change initialize behavior on a 2026-only server, it still succeeds with protocolVersion20251125 per the existing "server with no handshake version" test. We can discuss if we want to reject initialize with -32022 as well.
A server configured with
SupportedProtocolVersionscontaining only versions>= 2026-07-28should reject legacy clients that use theinitialize/initializedhandshake protocol with a clearCodeUnsupportedProtocolVersionerror, rather than silently serving them in a degraded state.What was broken
Two paths allowed legacy requests to slip through on a 2026-only server:
Stateful, uninitialized:
handle()reached the "method invalid during initialization" error path for any non-initializecall, but returned a generic error instead ofCodeUnsupportedProtocolVersion. The caller had no signal that the server will never support the legacy protocol.Stateless:
serveStateless()callsephemeralConnectOpts(), which synthesizes fakeInitializeParamsfor every request — making every session appear already initialized. This caused all legacy stateless calls to dispatch successfully and produce results, with no rejection at all.Fix
server.go— stateful uninitialized path:In the
handle()branch for uninitialized legacy calls, detect when the server has no legacy-supporting version configured and returnCodeUnsupportedProtocolVersionwith anUnsupportedProtocolVersionDatapayload before falling through to the generic error.streamable.go— stateless path:In
serveStateless(), afterephemeralConnectOpts(), check whether the server has any legacy-supporting version configured. If not, write aCodeUnsupportedProtocolVersionerror response and return.Behavior
initialize: allowed —negotiatedVersionreturnsprotocolVersion20251125as a courtesy per existing semantics inshared.go.initializecall (stateful, uninitialized): rejected withCodeUnsupportedProtocolVersion.CodeUnsupportedProtocolVersionand HTTP 400.SupportedProtocolVersionsare unaffected.Tests
TestServerSupportedProtocolVersions_InitializeLegacy: verifiesinitializesucceeds on a 2026-only server and returnsprotocolVersion20251125.TestServerSupportedProtocolVersions_LegacyDispatch: verifies a legacytools/listcall without_metais rejected withCodeUnsupportedProtocolVersionon a 2026-only stateful server.TestStreamableStateless_RejectsLegacyOn2026OnlyServer: verifies a legacy POST to a stateless 2026-only server returns HTTP 400 withCodeUnsupportedProtocolVersion.