mcp: update return meta in DiscoverResult (SEP-2575) (#1097)

This PR implements the latest changes introduced by
https://github.com/modelcontextprotocol/modelcontextprotocol/pull/3002

mcp/client.go

@@ -443,11 +443,15 @@ func (c *Client) discover(ctx context.Context, cs *ClientSession) (*InitializeRe
 		}
 	}
 
+	var serverInfo *Implementation
+	if v, ok := decodeMetaValue[*Implementation](res.GetMeta(), MetaKeyServerInfo); ok {
+		serverInfo = v
+	}
 	return &InitializeResult{
 		Capabilities:    res.Capabilities,
 		Instructions:    res.Instructions,
 		ProtocolVersion: negotiated,
-		ServerInfo:      res.ServerInfo,
+		ServerInfo:      serverInfo,
 	}, nil
 }
 
@@ -509,9 +513,12 @@ func (cs *ClientSession) usesNewProtocol() bool {
 	return res != nil && res.ProtocolVersion >= protocolVersion20260728
 }
 
-// injectRequestMeta populates the SEP-2575 per-request `_meta` triple
-// (protocolVersion, clientInfo, clientCapabilities) on the given outgoing
-// request params. Keys already present in params.Meta are not overwritten.
+// injectRequestMeta populates the SEP-2575 per-request `_meta` fields
+// (protocolVersion, optional clientInfo, clientCapabilities) on the given
+// outgoing request params. Keys already present in params.Meta are not
+// overwritten. Per PR modelcontextprotocol/modelcontextprotocol#3002
+// clientInfo is SHOULD (not MUST), and is omitted when the client has no
+// [Implementation] configured.
 func injectRequestMeta[T any, P interface {
 	*T
 	Params
@@ -527,7 +534,7 @@ func injectRequestMeta[T any, P interface {
 	if _, ok := m[MetaKeyProtocolVersion]; !ok {
 		m[MetaKeyProtocolVersion] = res.ProtocolVersion
 	}
-	if _, ok := m[MetaKeyClientInfo]; !ok {
+	if _, ok := m[MetaKeyClientInfo]; !ok && cs.client.impl != nil {
 		m[MetaKeyClientInfo] = cs.client.impl
 	}
 	if _, ok := m[MetaKeyClientCapabilities]; !ok {

mcp/client_test.go

@@ -662,11 +662,13 @@ func TestClientConnectDiscover(t *testing.T) {
 			name: "discover success skips initialize",
 			discoverHandler: func() (Result, error) {
 				return &DiscoverResult{
+					Meta: Meta{
+						MetaKeyServerInfo: &Implementation{Name: "discoverServer", Version: "v1.0.0"},
+					},
 					SupportedVersions: []string{protocolVersion20260728},
 					Capabilities: &ServerCapabilities{
 						Tools: &ToolCapabilities{ListChanged: true},
 					},
-					ServerInfo: &Implementation{Name: "discoverServer", Version: "v1.0.0"},
 				}, nil
 			},
 			wantInitialize: false,
@@ -695,9 +697,11 @@ func TestClientConnectDiscover(t *testing.T) {
 			name: "no overlapping supported version falls back to initialize",
 			discoverHandler: func() (Result, error) {
 				return &DiscoverResult{
+					Meta: Meta{
+						MetaKeyServerInfo: &Implementation{Name: "discoverServer", Version: "v1.0.0"},
+					},
 					SupportedVersions: []string{otherVersionsOnly},
 					Capabilities:      &ServerCapabilities{},
-					ServerInfo:        &Implementation{Name: "discoverServer", Version: "v1.0.0"},
 				}, nil
 			},
 			wantInitialize: true,
@@ -885,9 +889,11 @@ func TestInMemory_E2E_DiscoverFallback_NoOverlap(t *testing.T) {
 		return func(ctx context.Context, method string, req Request) (Result, error) {
 			if method == methodDiscover {
 				return &DiscoverResult{
+					Meta: Meta{
+						MetaKeyServerInfo: &Implementation{Name: "vpre-like-server", Version: "v1"},
+					},
 					SupportedVersions: []string{protocolVersion20251125},
 					Capabilities:      &ServerCapabilities{},
-					ServerInfo:        &Implementation{Name: "vpre-like-server", Version: "v1"},
 				}, nil
 			}
 			return next(ctx, method, req)

mcp/content_nil_test.go

@@ -224,4 +224,12 @@ func TestContentUnmarshalNilWithInvalidContent(t *testing.T) {
 	}
 }
 
-var ctrCmpOpts = []cmp.Option{cmpopts.IgnoreUnexported(mcp.CallToolResult{}, mcp.GetPromptResult{}, mcp.ReadResourceResult{})}
+var ctrCmpOpts = []cmp.Option{
+	cmpopts.IgnoreUnexported(mcp.CallToolResult{}, mcp.GetPromptResult{}, mcp.ReadResourceResult{}),
+	// Server responses under the >= 2026-07-28 protocol carry an auto-populated
+	// [mcp.MetaKeyServerInfo] entry; tests that compare result bodies against
+	// hand-crafted expected values should ignore it.
+	cmpopts.IgnoreFields(mcp.CallToolResult{}, "Meta"),
+	cmpopts.IgnoreFields(mcp.GetPromptResult{}, "Meta"),
+	cmpopts.IgnoreFields(mcp.ReadResourceResult{}, "Meta"),
+}

mcp/mcp_test.go

@@ -2475,7 +2475,15 @@ func TestSetErrorPreservesContent(t *testing.T) {
 	}
 }
 
-var ctrCmpOpts = []cmp.Option{cmpopts.IgnoreUnexported(CallToolResult{}, GetPromptResult{}, ReadResourceResult{})}
+var ctrCmpOpts = []cmp.Option{
+	cmpopts.IgnoreUnexported(CallToolResult{}, GetPromptResult{}, ReadResourceResult{}),
+	// Server responses under the >= 2026-07-28 protocol carry an auto-populated
+	// [MetaKeyServerInfo] entry; tests that compare result bodies against
+	// hand-crafted expected values should ignore it.
+	cmpopts.IgnoreFields(CallToolResult{}, "Meta"),
+	cmpopts.IgnoreFields(GetPromptResult{}, "Meta"),
+	cmpopts.IgnoreFields(ReadResourceResult{}, "Meta"),
+}
 
 // runSubscriptionsListenTest exercises the SEP-2575 auto-listen flow end-to-end
 // against the supplied transport pair. It captures every notification and the

mcp/protocol.go

@@ -1143,8 +1143,6 @@ type DiscoverResult struct {
 	SupportedVersions []string `json:"supportedVersions"`
 	// The server's capabilities.
 	Capabilities *ServerCapabilities `json:"capabilities"`
-	// Information about the server implementation.
-	ServerInfo *Implementation `json:"serverInfo"`
 	// Instructions describing how to use the server and its features.
 	Instructions string `json:"instructions,omitempty"`
 }
@@ -2365,6 +2363,8 @@ const (
 	MetaKeyProtocolVersion = "io.modelcontextprotocol/protocolVersion"
 	// MetaKeyClientInfo carries the client's [Implementation].
 	MetaKeyClientInfo = "io.modelcontextprotocol/clientInfo"
+	// MetaKeyServerInfo carries the server's [Implementation] on responses.
+	MetaKeyServerInfo = "io.modelcontextprotocol/serverInfo"
 	// MetaKeyClientCapabilities carries the client's [ClientCapabilities].
 	MetaKeyClientCapabilities = "io.modelcontextprotocol/clientCapabilities"
 	// MetaKeyLogLevel identifies the desired log level for the request.

mcp/server.go

@@ -888,17 +888,21 @@ func (s *Server) discover(_ context.Context, req *ServerRequest[*DiscoverParams]
 	if versions == nil {
 		versions = slices.Clone(supportedProtocolVersions)
 	}
+	// Read the request-scoped identity/capabilities before acquiring the
+	// session lock: these accessors may fall back to Session.InitializeParams
+	// (which also locks Session.mu), so calling them from inside updateState
+	// would self-deadlock.
+	init := &InitializeParams{
+		ProtocolVersion: req.ProtocolVersion(),
+		Capabilities:    req.ClientCapabilities(),
+		ClientInfo:      req.ClientInfo(),
+	}
 	req.Session.updateState(func(state *ServerSessionState) {
-		state.InitializeParams = &InitializeParams{
-			ProtocolVersion: req.ProtocolVersion(),
-			Capabilities:    req.ClientCapabilities(),
-			ClientInfo:      req.ClientInfo(),
-		}
+		state.InitializeParams = init
 	})
 	res := &DiscoverResult{
 		SupportedVersions: versions,
 		Capabilities:      s.capabilities(),
-		ServerInfo:        s.impl,
 		Instructions:      s.opts.Instructions,
 	}
 	res.setDefaultCacheableValues()
@@ -1924,10 +1928,32 @@ func (ss *ServerSession) handle(ctx context.Context, req *jsonrpc.Request) (any,
 	}
 	if validatedMeta.usesNewProtocol {
 		setCompleteResultType(res)
+		annotateServerInfo(res, ss.server.impl)
 	}
 	return res, nil
 }
 
+// annotateServerInfo sets [MetaKeyServerInfo] on the result's `_meta` unless
+// it is already present. Per SEP-2575, servers should identify themselves in
+// each result's `_meta`.
+func annotateServerInfo(res Result, impl *Implementation) {
+	if res == nil || impl == nil {
+		return
+	}
+	if _, isEmpty := res.(*emptyResult); isEmpty {
+		return
+	}
+	m := res.GetMeta()
+	if m == nil {
+		m = map[string]any{}
+	}
+	if _, ok := m[MetaKeyServerInfo]; ok {
+		return
+	}
+	m[MetaKeyServerInfo] = impl
+	res.SetMeta(m)
+}
+
 // InitializeParams returns the InitializeParams provided during the client's
 // initial connection.
 func (ss *ServerSession) InitializeParams() *InitializeParams {

mcp/shared.go

@@ -535,9 +535,10 @@ type validatedMeta struct {
 // the >= 2026-07-28 protocol via the `_meta` field.
 // If the request has no _meta, or no protocolVersion in _meta, it returns a non-nil
 // validatedMeta with usesNewProtocol set to false, and a nil error.
-// If the request has a protocolVersion in _meta it validates the presence of clientInfo
-// and clientCapabilities in _meta. If either is missing or invalid, it returns nil and
-// a non-nil error. Otherwise, it returns usesNewProtocol set to true and the populated
+// If the request has a protocolVersion in _meta it validates the presence of
+// clientCapabilities in _meta. If it is missing or invalid, it returns nil and
+// a non-nil error. clientInfo is optional; if present but invalid, an error is
+// returned. Otherwise, it returns usesNewProtocol set to true and the populated
 // initializeParams.
 func validateRequestMeta(req *jsonrpc.Request) (*validatedMeta, error) {
 	meta := extractRequestMeta(req.Params)
@@ -548,11 +549,15 @@ func validateRequestMeta(req *jsonrpc.Request) (*validatedMeta, error) {
 	if !ok || protocolVersion < protocolVersion20260728 {
 		return &validatedMeta{usesNewProtocol: false, initializeParams: nil}, nil
 	}
-	clientInfo, ok := decodeMetaValue[*Implementation](meta, MetaKeyClientInfo)
-	if !ok {
-		return nil, &jsonrpc.Error{
-			Code:    jsonrpc.CodeInvalidParams,
-			Message: fmt.Sprintf("missing or invalid _meta field %q", MetaKeyClientInfo),
+	var clientInfo *Implementation
+	if _, present := meta[MetaKeyClientInfo]; present {
+		var ok bool
+		clientInfo, ok = decodeMetaValue[*Implementation](meta, MetaKeyClientInfo)
+		if !ok {
+			return nil, &jsonrpc.Error{
+				Code:    jsonrpc.CodeInvalidParams,
+				Message: fmt.Sprintf("invalid _meta field %q", MetaKeyClientInfo),
+			}
 		}
 	}
 	capabilities, ok := decodeMetaValue[*clientCapabilitiesV2](meta, MetaKeyClientCapabilities)

mcp/shared_test.go

@@ -67,6 +67,19 @@ func TestValidateRequestMeta(t *testing.T) {
 				},
 				"name": "x",
 			},
+			wantUsesNew: true,
+		},
+		{
+			name:   "new protocol invalid clientInfo",
+			method: methodCallTool,
+			params: map[string]any{
+				"_meta": map[string]any{
+					MetaKeyProtocolVersion:    protocolVersion20260728,
+					MetaKeyClientInfo:         "not an object",
+					MetaKeyClientCapabilities: map[string]any{},
+				},
+				"name": "x",
+			},
 			wantUsesNew:     false,
 			wantErrContains: MetaKeyClientInfo,
 		},

mcp/streamable_client_test.go

@@ -1265,11 +1265,13 @@ func TestStreamableClientOAuth_RetrieveError(t *testing.T) {
 // discoverResult is the canned successful DiscoverResult returned by
 // fakeStreamableServer setups in the tests below.
 var discoverResult = &DiscoverResult{
+	Meta: Meta{
+		MetaKeyServerInfo: &Implementation{Name: "discoverServer", Version: "v1.0.0"},
+	},
 	SupportedVersions: []string{protocolVersion20260728},
 	Capabilities: &ServerCapabilities{
 		Tools: &ToolCapabilities{ListChanged: true},
 	},
-	ServerInfo:   &Implementation{Name: "discoverServer", Version: "v1.0.0"},
 	Instructions: "test discover",
 }
 

mcp/streamable_test.go

@@ -794,6 +794,15 @@ func completeCallToolResult() *CallToolResult {
 	return r
 }
 
+// completeCallToolResultWithServerInfo returns a completeCallToolResult
+// annotated with the auto-populated [MetaKeyServerInfo] entry that servers
+// add to every new-protocol response.
+func completeCallToolResultWithServerInfo(impl *Implementation) *CallToolResult {
+	r := completeCallToolResult()
+	r.Meta = Meta{MetaKeyServerInfo: impl}
+	return r
+}
+
 func resp(id int64, result any, err error) *jsonrpc.Response {
 	return &jsonrpc.Response{
 		ID:     jsonrpc2.Int64ID(id),
@@ -1958,6 +1967,7 @@ func TestStreamableMcpHeaderValidation(t *testing.T) {
 		MetaKeyClientInfo:         map[string]any{"name": "testClient", "version": "v1.0.0"},
 		MetaKeyClientCapabilities: map[string]any{},
 	}
+	serverImpl := &Implementation{Name: "testServer", Version: "v1.0.0"}
 
 	testStreamableHandler(t, handler, []streamableRequest{
 		{
@@ -1969,7 +1979,7 @@ func TestStreamableMcpHeaderValidation(t *testing.T) {
 			},
 			messages:       []jsonrpc.Message{req(2, "tools/call", &CallToolParams{Meta: testMeta, Name: "my-tool"})},
 			wantStatusCode: http.StatusOK,
-			wantMessages:   []jsonrpc.Message{resp(2, completeCallToolResult(), nil)},
+			wantMessages:   []jsonrpc.Message{resp(2, completeCallToolResultWithServerInfo(serverImpl), nil)},
 		},
 		{
 			method: "POST",
@@ -2013,7 +2023,7 @@ func TestStreamableMcpHeaderValidation(t *testing.T) {
 			},
 			messages:       []jsonrpc.Message{req(6, "tools/call", &CallToolParams{Meta: testMeta, Name: "my-tool"})},
 			wantStatusCode: http.StatusOK,
-			wantMessages:   []jsonrpc.Message{resp(6, completeCallToolResult(), nil)},
+			wantMessages:   []jsonrpc.Message{resp(6, completeCallToolResultWithServerInfo(serverImpl), nil)},
 		},
 		{
 			method: "POST",
@@ -2029,7 +2039,7 @@ func TestStreamableMcpHeaderValidation(t *testing.T) {
 				Arguments: map[string]any{"region": "us-west1", "query": "SELECT 1"},
 			})},
 			wantStatusCode: http.StatusOK,
-			wantMessages:   []jsonrpc.Message{resp(7, completeCallToolResult(), nil)},
+			wantMessages:   []jsonrpc.Message{resp(7, completeCallToolResultWithServerInfo(serverImpl), nil)},
 		},
 		{
 			method: "POST",

mcp/testdata/conformance/server/discover.txtar

@@ -1,7 +1,7 @@
-The client sends a discover request
-carrying the required per-request _meta triple (protocolVersion, clientInfo,
-clientCapabilities), and the server replies with the protocol versions it
-supports, its capabilities, and its identity.
+The client sends a discover request carrying the per-request _meta
+(protocolVersion, clientInfo, clientCapabilities), and the server
+replies with the protocol versions it supports, its capabilities, and its
+identity via `_meta.io.modelcontextprotocol/serverInfo`.
 
 -- client --
 {
@@ -22,6 +22,12 @@ supports, its capabilities, and its identity.
 	"id": 1,
 	"result": {
 		"resultType": "complete",
+		"_meta": {
+			"io.modelcontextprotocol/serverInfo": {
+				"name": "testServer",
+				"version": "v1.0.0"
+			}
+		},
 		"ttlMs": 0,
 		"cacheScope": "public",
 		"supportedVersions": [
@@ -33,10 +39,6 @@ supports, its capabilities, and its identity.
 		],
 		"capabilities": {
 			"logging": {}
-		},
-		"serverInfo": {
-			"name": "testServer",
-			"version": "v1.0.0"
 		}
 	}
 }

mcp/testdata/conformance/server/mrtr.txtar

@@ -45,6 +45,12 @@ confirmThenGreet
 	"jsonrpc": "2.0",
 	"id": 1,
 	"result": {
+		"_meta": {
+			"io.modelcontextprotocol/serverInfo": {
+				"name": "testServer",
+				"version": "v1.0.0"
+			}
+		},
 		"content": null,
 		"requestState": "step=1",
 		"resultType": "input_required",
@@ -71,6 +77,12 @@ confirmThenGreet
 	"jsonrpc": "2.0",
 	"id": 2,
 	"result": {
+		"_meta": {
+			"io.modelcontextprotocol/serverInfo": {
+				"name": "testServer",
+				"version": "v1.0.0"
+			}
+		},
 		"content": [
 			{
 				"type": "text",

mcp/transport_example_test.go

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