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mcp: reject discover when transport lacks version #1108

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28 changes: 18 additions & 10 deletions mcp/server.go
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Original file line number Diff line number Diff line change
Expand Up @@ -1863,16 +1863,24 @@ func (ss *ServerSession) handle(ctx context.Context, req *jsonrpc.Request) (any,
return nil, perRequestErr
}

if validatedMeta.usesNewProtocol &&
!slices.Contains(supportedProtocolVersions, validatedMeta.initializeParams.ProtocolVersion) {
data, _ := json.Marshal(UnsupportedProtocolVersionData{
Supported: supportedProtocolVersions,
Requested: validatedMeta.initializeParams.ProtocolVersion,
})
return nil, &jsonrpc.Error{
Code: CodeUnsupportedProtocolVersion,
Message: "unsupported protocol version",
Data: data,
if validatedMeta.usesNewProtocol {
requested := validatedMeta.initializeParams.ProtocolVersion
ss.mu.Lock()
versions := ss.supportedVersions
ss.mu.Unlock()
if versions == nil {
versions = supportedProtocolVersions
}
if !slices.Contains(versions, requested) {
data, _ := json.Marshal(UnsupportedProtocolVersionData{
Supported: versions,
Requested: requested,
})
return nil, &jsonrpc.Error{
Code: CodeUnsupportedProtocolVersion,
Message: "unsupported protocol version",
Data: data,
}
}
}

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43 changes: 25 additions & 18 deletions mcp/streamable_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -3603,13 +3603,11 @@ func TestStreamableClientUnsupportedVersionFallback(t *testing.T) {
}
}

// TestStreamableStateful_AcceptsDiscover verifies that a stateful HTTP server
// accepts a server/discover probe carrying MCP-Protocol-Version: 2026-07-28
// (and the matching _meta.protocolVersion), instead of rejecting it with the
// "stateless required" 400. The SEP-2575 client flow has the client probing
// the server with the new protocol version to learn which versions are
// supported.
func TestStreamableStateful_AcceptsDiscover(t *testing.T) {
// TestStreamableStateful_DiscoverRejectsUnsupportedVersion verifies that a
// stateful HTTP server rejects a server/discover probe carrying
// MCP-Protocol-Version: 2026-07-28 when that version is not supported by the
// transport, per SEP-2575 version negotiation.
func TestStreamableStateful_DiscoverRejectsUnsupportedVersion(t *testing.T) {

server := NewServer(testImpl, nil)
handler := NewStreamableHTTPHandler(func(*http.Request) *Server { return server }, nil)
Expand Down Expand Up @@ -3646,8 +3644,8 @@ func TestStreamableStateful_AcceptsDiscover(t *testing.T) {
}
defer resp.Body.Close()
respBody, _ := io.ReadAll(resp.Body)
if resp.StatusCode != http.StatusOK {
t.Fatalf("status = %d, want 200; body = %s", resp.StatusCode, respBody)
if resp.StatusCode != http.StatusBadRequest {
t.Fatalf("status = %d, want 400; body = %s", resp.StatusCode, respBody)
}

// Parse the JSON-RPC response. The body may arrive as a bare JSON object
Expand All @@ -3667,18 +3665,27 @@ func TestStreamableStateful_AcceptsDiscover(t *testing.T) {
if err := json.Unmarshal(jsonPayload, &rpcResp); err != nil {
t.Fatalf("unmarshal response %q: %v", respBody, err)
}
if rpcResp.Error != nil {
t.Fatalf("discover returned error: %+v (body = %s)", rpcResp.Error, respBody)
if rpcResp.Error == nil {
t.Fatalf("discover should return UnsupportedProtocolVersionError; body = %s", respBody)
}
if rpcResp.Error.Code != CodeUnsupportedProtocolVersion {
t.Fatalf("discover error code = %d, want %d; error = %+v",
rpcResp.Error.Code, CodeUnsupportedProtocolVersion, rpcResp.Error)
}
var data UnsupportedProtocolVersionData
if err := json.Unmarshal(rpcResp.Error.Data, &data); err != nil {
t.Fatalf("unmarshal error data: %v", err)
}
if rpcResp.Result == nil {
t.Fatalf("discover returned no result; body = %s", respBody)
if data.Requested != protocolVersion20260728 {
t.Errorf("UnsupportedProtocolVersionData.Requested = %q, want %q",
data.Requested, protocolVersion20260728)
}
if slices.Contains(rpcResp.Result.SupportedVersions, protocolVersion20260728) {
t.Errorf("DiscoverResult.SupportedVersions = %v, must not include %q on a stateful transport",
rpcResp.Result.SupportedVersions, protocolVersion20260728)
if slices.Contains(data.Supported, protocolVersion20260728) {
t.Errorf("UnsupportedProtocolVersionData.Supported = %v, must not include %q on a stateful transport",
data.Supported, protocolVersion20260728)
}
if len(rpcResp.Result.SupportedVersions) == 0 {
t.Errorf("DiscoverResult.SupportedVersions is empty; want at least one legacy version")
if len(data.Supported) == 0 {
t.Errorf("UnsupportedProtocolVersionData.Supported is empty; want at least one legacy version")
}
}

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