mcp: move complete resultType into protocol types

Author: ychampion

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].
@@ -637,6 +655,7 @@ 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"`
@@ -1114,6 +1133,7 @@ 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.
@@ -1177,6 +1197,7 @@ 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"`
@@ -1207,6 +1228,7 @@ 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"`
@@ -1237,6 +1259,7 @@ 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"`
@@ -1303,6 +1326,7 @@ 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

@@ -1906,43 +1906,11 @@ func (ss *ServerSession) handle(ctx context.Context, req *jsonrpc.Request) (any,
 		return nil, err
 	}
 	if validatedMeta.usesNewProtocol {
-		return withCompleteResultType(res), nil
+		setCompleteResultType(res)
 	}
 	return res, nil
 }
 
-func withCompleteResultType(res Result) any {
-	switch res.(type) {
-	case *CompleteResult, *DiscoverResult, *ListPromptsResult, *ListResourceTemplatesResult, *ListResourcesResult, *ListToolsResult:
-		return completeResult{Result: res}
-	default:
-		return res
-	}
-}
-
-type completeResult struct {
-	Result
-}
-
-func (r completeResult) MarshalJSON() ([]byte, error) {
-	data, err := json.Marshal(r.Result)
-	if err != nil {
-		return nil, err
-	}
-	data = bytes.TrimSpace(data)
-	if len(data) == 2 && data[0] == '{' && data[1] == '}' {
-		return []byte(`{"resultType":"complete"}`), nil
-	}
-	if len(data) < 2 || data[0] != '{' || data[len(data)-1] != '}' {
-		return data, nil
-	}
-	out := make([]byte, 0, len(data)+len(`,"resultType":"complete"`))
-	out = append(out, data[:len(data)-1]...)
-	out = append(out, `,"resultType":"complete"`...)
-	out = append(out, '}')
-	return out, nil
-}
-
 // InitializeParams returns the InitializeParams provided during the client's
 // initial connection.
 func (ss *ServerSession) InitializeParams() *InitializeParams {

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": [
@@ -36,7 +37,6 @@ supports, its capabilities, and its identity.
 		"serverInfo": {
 			"name": "testServer",
 			"version": "v1.0.0"
-		},
-		"resultType": "complete"
+		}
 	}
 }

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"},"resultType":"complete"}}
+	// 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"}}}
 }