mcp: emit UnsupportedProtocolVersionError for stateful servers rejecting SEP-2575 requests

mcp/streamable.go

@@ -1514,10 +1514,25 @@ func (c *streamableServerConn) servePOST(w http.ResponseWriter, req *http.Reques
 				// rejection as it should learn about the supported protocols from the
 				// DiscoverResult response.
 				if !c.stateless && jreq.Method != methodDiscover {
-					http.Error(w, fmt.Sprintf(
-						"Bad Request: protocol version %q is only supported on stateless HTTP servers (set StreamableHTTPOptions.Stateless = true)",
-						protocolVersion),
-						http.StatusBadRequest)
+					// This server has chosen not to serve >= 2026-07-28 requests
+					// unless configured as stateless (see StreamableHTTPOptions.Stateless).
+					c.logger.Warn("rejecting request for unsupported protocol version; set StreamableHTTPOptions.Stateless = true to serve it",
+						"protocolVersion", protocolVersion)
+					legacyVersions := make([]string, 0, len(supportedProtocolVersions))
+					for _, v := range supportedProtocolVersions {
+						if v < protocolVersion20260728 {
+							legacyVersions = append(legacyVersions, v)
+						}
+					}
+					data, _ := json.Marshal(UnsupportedProtocolVersionData{
+						Supported: legacyVersions,
+						Requested: protocolVersion,
+					})
+					writeJSONRPCError(w, http.StatusBadRequest, jreq.ID, &jsonrpc.Error{
+						Code:    CodeUnsupportedProtocolVersion,
+						Message: fmt.Sprintf("protocol version %q is not supported by this server", protocolVersion),
+						Data:    data,
+					})
 					return
 				}
 				if headerVersion == "" {

mcp/streamable_test.go

@@ -3504,8 +3504,30 @@ func TestStreamableStateful_RejectsNewProtocol(t *testing.T) {
 	if resp.StatusCode != http.StatusBadRequest {
 		t.Fatalf("status = %d, want 400; body = %s", resp.StatusCode, respBody)
 	}
-	if !strings.Contains(string(respBody), "stateless") {
-		t.Errorf("body = %q, want a message mentioning 'stateless'", respBody)
+	msg, err := jsonrpc2.DecodeMessage(respBody)
+	if err != nil {
+		t.Fatalf("failed to decode message: %v; body = %s", err, respBody)
+	}
+	wireResp, ok := msg.(*jsonrpc2.Response)
+	if !ok {
+		t.Fatalf("expected *jsonrpc2.Response, got %T", msg)
+	}
+	var wireErr *jsonrpc2.WireError
+	if !errors.As(wireResp.Error, &wireErr) {
+		t.Fatalf("expected *jsonrpc2.WireError, got %T", wireResp.Error)
+	}
+	if wireErr.Code != CodeUnsupportedProtocolVersion {
+		t.Errorf("error code = %d, want %d", wireErr.Code, CodeUnsupportedProtocolVersion)
+	}
+	var data UnsupportedProtocolVersionData
+	if err := json.Unmarshal(wireErr.Data, &data); err != nil {
+		t.Fatalf("error.Data is not UnsupportedProtocolVersionData: %v", err)
+	}
+	if data.Requested != protocolVersion20260728 {
+		t.Errorf("data.Requested = %q, want %q", data.Requested, protocolVersion20260728)
+	}
+	if slices.Contains(data.Supported, protocolVersion20260728) {
+		t.Errorf("data.Supported = %v, must not contain %q for a stateful server", data.Supported, protocolVersion20260728)
 	}
 }