mcp: include resultType on new-protocol responses (#1060)

## Summary

- Add `resultType: "complete"` to non-MRTR server result responses for
2026-07-28 requests.
- Cover `server/discover`, list methods, and completion responses, and
update expected conformance/example output.

## Why

Fixes #1032. The 2026-07-28 schema requires `resultType` on result
objects, but only the MRTR-capable results were setting it.

## Validation

- `go test ./mcp -run
'Test(ServerSessionHandle|CompleteResult|ClientConnectDiscover|InMemory_E2E_Discover)'
-count=1` — passed\n- `go test ./mcp -count=1` — passed\n- `go test
./...` — passed\n- `go vet ./...` — passed

---------

Co-authored-by: ychampion <ychampion@users.noreply.github.com>
Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>

mcp/protocol.go

@@ -28,6 +28,24 @@ const (
 	resultTypeInputRequired resultType = "input_required"
 )
 
+type completeResultWithType struct {
+	ResultType resultType `json:"resultType,omitempty"`
+}
+
+func (r *completeResultWithType) setResultType(rt resultType) { r.ResultType = rt }
+func (*completeResultWithType) isCompleteResult()             {}
+
+type completeResultResponse interface {
+	setResultType(resultType)
+	isCompleteResult()
+}
+
+func setCompleteResultType(res Result) {
+	if r, ok := res.(completeResultResponse); ok {
+		r.setResultType(resultTypeComplete)
+	}
+}
+
 // InputRequest is a type for parameters that a server can include in the response
 // to request input from client (SEP-2322). Implementations are [*ElicitParams],
 // [*CreateMessageParams], and [*ListRootsParams].
@@ -640,6 +658,7 @@ type CompletionResultDetails struct {
 
 // The server's response to a completion/complete request
 type CompleteResult struct {
+	completeResultWithType
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
 	Meta       `json:"_meta,omitempty"`
@@ -1117,6 +1136,7 @@ func (x *DiscoverParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *DiscoverParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
 type DiscoverResult struct {
+	completeResultWithType
 	Meta `json:"_meta,omitempty"`
 	Cacheable
 	// The versions of the Model Context Protocol that the server supports.
@@ -1180,6 +1200,7 @@ func (c *Cacheable) setDefaultCacheableValues() {
 
 // The server's response to a prompts/list request from the client.
 type ListPromptsResult struct {
+	completeResultWithType
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
 	Meta `json:"_meta,omitempty"`
@@ -1210,6 +1231,7 @@ func (x *ListResourceTemplatesParams) cursorPtr() *string     { return &x.Cursor
 
 // The server's response to a resources/templates/list request from the client.
 type ListResourceTemplatesResult struct {
+	completeResultWithType
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
 	Meta `json:"_meta,omitempty"`
@@ -1240,6 +1262,7 @@ func (x *ListResourcesParams) cursorPtr() *string     { return &x.Cursor }
 
 // The server's response to a resources/list request from the client.
 type ListResourcesResult struct {
+	completeResultWithType
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
 	Meta `json:"_meta,omitempty"`
@@ -1306,6 +1329,7 @@ func (x *ListToolsParams) cursorPtr() *string     { return &x.Cursor }
 
 // The server's response to a tools/list request from the client.
 type ListToolsResult struct {
+	completeResultWithType
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
 	Meta `json:"_meta,omitempty"`

mcp/protocol_test.go

@@ -10,6 +10,7 @@ import (
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
 )
 
 func TestParamsMeta(t *testing.T) {
@@ -491,7 +492,7 @@ func TestCompleteResult(t *testing.T) {
 			if err := json.Unmarshal([]byte(test.in), &got); err != nil {
 				t.Fatalf("json.Unmarshal(CompleteResult) failed: %v", err)
 			}
-			if diff := cmp.Diff(test.want, got); diff != "" {
+			if diff := cmp.Diff(test.want, got, cmpopts.IgnoreUnexported(CompleteResult{})); diff != "" {
 				t.Errorf("CompleteResult unmarshal mismatch (-want +got):\n%s", diff)
 			}
 		})

mcp/server.go

@@ -1893,7 +1893,14 @@ func (ss *ServerSession) handle(ctx context.Context, req *jsonrpc.Request) (any,
 	if validatedMeta.usesNewProtocol {
 		ss.setLevel(ctx, &SetLoggingLevelParams{Level: validatedMeta.logLevel})
 	}
-	return handleReceive(ctx, ss, req)
+	res, err := handleReceive(ctx, ss, req)
+	if err != nil {
+		return nil, err
+	}
+	if validatedMeta.usesNewProtocol {
+		setCompleteResultType(res)
+	}
+	return res, nil
 }
 
 // InitializeParams returns the InitializeParams provided during the client's

mcp/server_test.go

@@ -1399,6 +1399,148 @@ func TestServerSessionHandle_RejectsInitializeOnNewProtocol(t *testing.T) {
 	})
 }
 
+func TestServerSessionHandle_SetsResultTypeOnNewProtocol(t *testing.T) {
+	server := NewServer(testImpl, &ServerOptions{
+		CompletionHandler: func(context.Context, *CompleteRequest) (*CompleteResult, error) {
+			return &CompleteResult{
+				Completion: CompletionResultDetails{
+					Values: []string{"go"},
+				},
+			}, nil
+		},
+	})
+	AddTool(server, &Tool{Name: "tool"}, func(context.Context, *CallToolRequest, struct{}) (*CallToolResult, any, error) {
+		return &CallToolResult{
+			InputRequests: InputRequestMap{"confirm": &ElicitParams{Message: "Continue?"}},
+		}, nil, nil
+	})
+	server.AddPrompt(&Prompt{Name: "prompt"}, func(context.Context, *GetPromptRequest) (*GetPromptResult, error) {
+		return &GetPromptResult{
+			InputRequests: InputRequestMap{"confirm": &ElicitParams{Message: "Continue?"}},
+		}, nil
+	})
+	server.AddResource(&Resource{URI: "test://resource", Name: "resource"}, func(context.Context, *ReadResourceRequest) (*ReadResourceResult, error) {
+		return &ReadResourceResult{
+			InputRequests: InputRequestMap{"confirm": &ElicitParams{Message: "Continue?"}},
+		}, nil
+	})
+	newProtocolParams := func(fields map[string]any) map[string]any {
+		params := map[string]any{
+			"_meta": map[string]any{
+				MetaKeyProtocolVersion:    protocolVersion20260728,
+				MetaKeyClientInfo:         map[string]any{"name": "c", "version": "1"},
+				MetaKeyClientCapabilities: map[string]any{},
+			},
+		}
+		for k, v := range fields {
+			params[k] = v
+		}
+		return params
+	}
+
+	tests := []struct {
+		name   string
+		method string
+		params map[string]any
+		want   resultType
+	}{
+		{
+			name:   "discover",
+			method: methodDiscover,
+			params: newProtocolParams(nil),
+			want:   resultTypeComplete,
+		},
+		{
+			name:   "tools list",
+			method: methodListTools,
+			params: newProtocolParams(nil),
+			want:   resultTypeComplete,
+		},
+		{
+			name:   "prompts list",
+			method: methodListPrompts,
+			params: newProtocolParams(nil),
+			want:   resultTypeComplete,
+		},
+		{
+			name:   "resources list",
+			method: methodListResources,
+			params: newProtocolParams(nil),
+			want:   resultTypeComplete,
+		},
+		{
+			name:   "resource templates list",
+			method: methodListResourceTemplates,
+			params: newProtocolParams(nil),
+			want:   resultTypeComplete,
+		},
+		{
+			name:   "complete",
+			method: methodComplete,
+			params: newProtocolParams(map[string]any{
+				"argument": map[string]any{
+					"name":  "language",
+					"value": "g",
+				},
+				"ref": map[string]any{
+					"type": "ref/prompt",
+					"name": "code_review",
+				},
+			}),
+			want: resultTypeComplete,
+		},
+		{
+			name:   "tool input required",
+			method: methodCallTool,
+			params: newProtocolParams(map[string]any{"name": "tool", "arguments": map[string]any{}}),
+			want:   resultTypeInputRequired,
+		},
+		{
+			name:   "prompt input required",
+			method: methodGetPrompt,
+			params: newProtocolParams(map[string]any{"name": "prompt"}),
+			want:   resultTypeInputRequired,
+		},
+		{
+			name:   "resource input required",
+			method: methodReadResource,
+			params: newProtocolParams(map[string]any{"uri": "test://resource"}),
+			want:   resultTypeInputRequired,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			ss := &ServerSession{server: server}
+			id, err := jsonrpc.MakeID("test")
+			if err != nil {
+				t.Fatal(err)
+			}
+			result, err := ss.handle(context.Background(), &jsonrpc.Request{
+				ID:     id,
+				Method: tc.method,
+				Params: mustMarshal(tc.params),
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+			data, err := json.Marshal(result)
+			if err != nil {
+				t.Fatal(err)
+			}
+			var got struct {
+				ResultType string `json:"resultType"`
+			}
+			if err := json.Unmarshal(data, &got); err != nil {
+				t.Fatal(err)
+			}
+			if got.ResultType != string(tc.want) {
+				t.Fatalf("resultType = %q, want %q; response = %s", got.ResultType, tc.want, data)
+			}
+		})
+	}
+}
+
 // TestServerSessionHandle_RejectsRemovedMethodsOnNewProtocol verifies that
 // the methods removed by SEP-2575 (`initialize`, `notifications/initialized`,
 // `ping`) all return Method not found when the request opts into the new

mcp/testdata/conformance/server/discover.txtar

@@ -21,6 +21,7 @@ supports, its capabilities, and its identity.
 	"jsonrpc": "2.0",
 	"id": 1,
 	"result": {
+		"resultType": "complete",
 		"ttlMs": 0,
 		"cacheScope": "public",
 		"supportedVersions": [

mcp/transport_example_test.go

@@ -42,7 +42,7 @@ func ExampleLoggingTransport() {
 	}
 
 	// Output:
-	// read: {"jsonrpc":"2.0","id":1,"result":{"ttlMs":0,"cacheScope":"public","supportedVersions":["2026-07-28","2025-11-25","2025-06-18","2025-03-26","2024-11-05"],"capabilities":{"logging":{}},"serverInfo":{"name":"server","version":"v0.0.1"}}}
+	// read: {"jsonrpc":"2.0","id":1,"result":{"resultType":"complete","ttlMs":0,"cacheScope":"public","supportedVersions":["2026-07-28","2025-11-25","2025-06-18","2025-03-26","2024-11-05"],"capabilities":{"logging":{}},"serverInfo":{"name":"server","version":"v0.0.1"}}}
 	// write: {"jsonrpc":"2.0","id":1,"method":"server/discover","params":{"_meta":{"io.modelcontextprotocol/clientCapabilities":{"roots":{"listChanged":true}},"io.modelcontextprotocol/clientInfo":{"name":"client","version":"v0.0.1"},"io.modelcontextprotocol/protocolVersion":"2026-07-28"}}}
 }