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mcp: allow restricting the protocol versions a server supports (#1199)

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Fixes #945
This commit is contained in:
Guglielmo Colombo 2026-08-28 16:06:10 +02:00 • committed by GitHub
commit 21c18c6229
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1 changed files with 397 additions and 35 deletions
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30
mcp/shared.go
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@ -63,18 +63,38 @@ var supportedProtocolVersions = []string{
protocolVersion20241105,
}
// SupportedProtocolVersions returns the protocol versions supported by this
// version of the SDK, newest first.
//
// Use it to discover the values accepted by
// [ServerOptions.SupportedProtocolVersions]. The returned slice is a copy:
// modifying it does not change what the SDK supports.
func SupportedProtocolVersions() []string { return slices.Clone(supportedProtocolVersions) }
// negotiatedVersion returns the effective protocol version to use, given a
// client version.
func negotiatedVersion(clientVersion string) string {
// In general, prefer to use the clientVersion, but if we don't support the
// client's version, use the latest version.
// client version and the versions the server supports, newest first.
func negotiatedVersion(clientVersion string, supported []string) string {
// In general, prefer to use the clientVersion, but if not supported
// by the server, use the latest version the server supports.
//
// Cap the supported versions at the legacy protocolVersion20251125, as this
// method is used by the initialize method which is deprecated in
// version protocolVersion20260728.
if slices.Contains(supportedProtocolVersions, clientVersion) && clientVersion < protocolVersion20260728 {
if slices.Contains(supported, clientVersion) && clientVersion < protocolVersion20260728 {
return clientVersion
}
for _, v := range supported {
if v < protocolVersion20260728 {
return v
}
}
// If the server was enforced to support only protocolVersion20260728 and
// later, the initialize function should not be called.
// If a client calls the initialize function, return the protocolVersion20251125.
// As according to spec (https://modelcontextprotocol.io/specification/2025-11-25/basic/lifecycle#version-negotiation):
// "If the server supports the requested protocol version, it MUST respond with the same version."
// "Otherwise, the server MUST respond with another protocol version it supports."
// "This SHOULD be the latest version supported by the server."
return protocolVersion20251125
}