mcp: fix broken client root capabilities

To address #607, add ClientCapabilities.RootsV2, and populate it when
constructing sending and receiving InitializeParams.

Also generally improve documentation for protocol types related to
capabilities.

Fixes #607

Author: findleyr

mcp/client.go

@@ -128,7 +128,11 @@ type ClientSessionOptions struct {
 
 func (c *Client) capabilities() *ClientCapabilities {
 	caps := &ClientCapabilities{}
+	// Due to an oversight (#607), roots require special handling.
 	caps.Roots.ListChanged = true
+	caps.RootsV2 = &RootsCapabilities{
+		ListChanged: true,
+	}
 	if c.opts.CreateMessageHandler != nil {
 		caps.Sampling = &SamplingCapabilities{}
 	}

mcp/client_test.go

@@ -192,6 +192,7 @@ func TestClientPaginateVariousPageSizes(t *testing.T) {
 }
 
 func TestClientCapabilities(t *testing.T) {
+	canListRoots := RootsCapabilities{ListChanged: true}
 	testCases := []struct {
 		name             string
 		configureClient  func(s *Client)
@@ -202,9 +203,8 @@ func TestClientCapabilities(t *testing.T) {
 			name:            "With initial capabilities",
 			configureClient: func(s *Client) {},
 			wantCapabilities: &ClientCapabilities{
-				Roots: struct {
-					ListChanged bool "json:\"listChanged,omitempty\""
-				}{ListChanged: true},
+				Roots:   canListRoots,
+				RootsV2: &canListRoots,
 			},
 		},
 		{
@@ -216,9 +216,8 @@ func TestClientCapabilities(t *testing.T) {
 				},
 			},
 			wantCapabilities: &ClientCapabilities{
-				Roots: struct {
-					ListChanged bool "json:\"listChanged,omitempty\""
-				}{ListChanged: true},
+				Roots:    canListRoots,
+				RootsV2:  &canListRoots,
 				Sampling: &SamplingCapabilities{},
 			},
 		},
@@ -232,9 +231,8 @@ func TestClientCapabilities(t *testing.T) {
 				},
 			},
 			wantCapabilities: &ClientCapabilities{
-				Roots: struct {
-					ListChanged bool "json:\"listChanged,omitempty\""
-				}{ListChanged: true},
+				Roots:   canListRoots,
+				RootsV2: &canListRoots,
 				Elicitation: &ElicitationCapabilities{
 					Form: &FormElicitationCapabilities{},
 				},
@@ -253,6 +251,7 @@ func TestClientCapabilities(t *testing.T) {
 				Roots: struct {
 					ListChanged bool "json:\"listChanged,omitempty\""
 				}{ListChanged: true},
+				RootsV2: &RootsCapabilities{ListChanged: true},
 				Elicitation: &ElicitationCapabilities{
 					URL: &URLElicitationCapabilities{},
 				},
@@ -271,6 +270,7 @@ func TestClientCapabilities(t *testing.T) {
 				Roots: struct {
 					ListChanged bool "json:\"listChanged,omitempty\""
 				}{ListChanged: true},
+				RootsV2: &RootsCapabilities{ListChanged: true},
 				Elicitation: &ElicitationCapabilities{
 					Form: &FormElicitationCapabilities{},
 					URL:  &URLElicitationCapabilities{},

mcp/protocol.go

@@ -177,23 +177,58 @@ func (x *CancelledParams) isParams()              {}
 func (x *CancelledParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *CancelledParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
+// RootsCapabilities describes a client's support for roots.
+type RootsCapabilities struct {
+	// ListChanged reports whether the client supports notifications for
+	// changes to the roots list.
+	ListChanged bool `json:"listChanged,omitempty"`
+}
+
 // Capabilities a client may support. Known capabilities are defined here, in
 // this schema, but this is not a closed set: any client can define its own,
 // additional capabilities.
 type ClientCapabilities struct {
-	// Experimental, non-standard capabilities that the client supports.
+	// Experimental reports non-standard capabilities that the client supports.
 	Experimental map[string]any `json:"experimental,omitempty"`
-	// Present if the client supports listing roots.
+	// Roots describes the client's support for roots.
+	//
+	// Deprecated: use RootsV2. As described in #607, Roots should have been a
+	// pointer to a RootsCapabilities value. Roots will be continue to be
+	// populated, but any new fields will only be added in the RootsV2 field.
 	Roots struct {
-		// Whether the client supports notifications for changes to the roots list.
+		// ListChanged reports whether the client supports notifications for
+		// changes to the roots list.
 		ListChanged bool `json:"listChanged,omitempty"`
 	} `json:"roots,omitempty"`
-	// Present if the client supports sampling from an LLM.
+	// RootsV2 is present if the client supports roots.
+	RootsV2 *RootsCapabilities `json:"-"`
+	// Sampling is present if the client supports sampling from an LLM.
 	Sampling *SamplingCapabilities `json:"sampling,omitempty"`
-	// Present if the client supports elicitation from the server.
+	// Elicitation is present if the client supports elicitation from the server.
 	Elicitation *ElicitationCapabilities `json:"elicitation,omitempty"`
 }
 
+func (c *ClientCapabilities) toV2() *clientCapabilitiesV2 {
+	return &clientCapabilitiesV2{
+		ClientCapabilities: *c,
+		Roots:              c.RootsV2,
+	}
+}
+
+// clientCapabilitiesV2 is a version of ClientCapabilities that fixes the bug
+// described in #607: Roots should have been a pointer to value type
+// RootsCapabilities.
+type clientCapabilitiesV2 struct {
+	ClientCapabilities
+	Roots *RootsCapabilities `json:"roots,omitempty"`
+}
+
+func (c *clientCapabilitiesV2) toV1() *ClientCapabilities {
+	caps := c.ClientCapabilities
+	caps.RootsV2 = c.Roots
+	return &caps
+}
+
 type CompleteParamsArgument struct {
 	// The name of the argument
 	Name string `json:"name"`
@@ -373,27 +408,53 @@ type GetPromptResult struct {
 
 func (*GetPromptResult) isResult() {}
 
+// InitializeParams is sent by the client to initialize the session.
 type InitializeParams struct {
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
-	Meta         `json:"_meta,omitempty"`
+	Meta `json:"_meta,omitempty"`
+	// Capabilities describes the client's capabilities.
 	Capabilities *ClientCapabilities `json:"capabilities"`
-	ClientInfo   *Implementation     `json:"clientInfo"`
-	// The latest version of the Model Context Protocol that the client supports.
-	// The client may decide to support older versions as well.
+	// ClientInfo provides information about the client.
+	ClientInfo *Implementation `json:"clientInfo"`
+	// ProtocolVersion is the latest version of the Model Context Protocol that
+	// the client supports.
 	ProtocolVersion string `json:"protocolVersion"`
 }
 
+func (p *InitializeParams) toV2() *initializeParamsV2 {
+	return &initializeParamsV2{
+		InitializeParams: *p,
+		Capabilities:     p.Capabilities.toV2(),
+	}
+}
+
+// initializeParamsV2 works around the mistake in #607: Capabilities.Roots
+// should have been a pointer.
+type initializeParamsV2 struct {
+	InitializeParams
+	Capabilities *clientCapabilitiesV2 `json:"capabilities"`
+}
+
+func (p *initializeParamsV2) toV1() *InitializeParams {
+	p1 := p.InitializeParams
+	if p.Capabilities != nil {
+		p1.Capabilities = p.Capabilities.toV1()
+	}
+	return &p1
+}
+
 func (x *InitializeParams) isParams()              {}
 func (x *InitializeParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *InitializeParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
-// After receiving an initialize request from the client, the server sends this
-// response.
+// InitializeResult is sent by the server in response to an initialize request
+// from the client.
 type InitializeResult struct {
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
-	Meta         `json:"_meta,omitempty"`
+	Meta `json:"_meta,omitempty"`
+	// Capabilities describes the server's capabilities.
 	Capabilities *ServerCapabilities `json:"capabilities"`
 	// Instructions describing how to use the server and its features.
 	//
@@ -411,8 +472,8 @@ type InitializeResult struct {
 func (*InitializeResult) isResult() {}
 
 type InitializedParams struct {
-	// This property is reserved by the protocol to allow clients and servers to
-	// attach additional metadata to their responses.
+	// Meta is reserved by the protocol to allow clients and servers to attach
+	// additional metadata to their responses.
 	Meta `json:"_meta,omitempty"`
 }
 
@@ -875,7 +936,10 @@ func (x *RootsListChangedParams) isParams()              {}
 func (x *RootsListChangedParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *RootsListChangedParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
-// SamplingCapabilities describes the capabilities for sampling.
+// TODO: to be consistent with ServerCapabilities, move the capability types
+// below directly above ClientCapabilities.
+
+// SamplingCapabilities describes the client's support for sampling.
 type SamplingCapabilities struct{}
 
 // ElicitationCapabilities describes the capabilities for elicitation.
@@ -1160,50 +1224,52 @@ type Implementation struct {
 	Icons []Icon `json:"icons,omitempty"`
 }
 
-// Present if the server supports argument autocompletion suggestions.
+// CompletionCapabilities describes the server's support for argument autocompletion.
 type CompletionCapabilities struct{}
 
-// Present if the server supports sending log messages to the client.
+// LoggingCapabilities describes the server's support for sending log messages to the client.
 type LoggingCapabilities struct{}
 
-// Present if the server offers any prompt templates.
+// PromptCapabilities describes the server's support for prompts.
 type PromptCapabilities struct {
 	// Whether this server supports notifications for changes to the prompt list.
 	ListChanged bool `json:"listChanged,omitempty"`
 }
 
-// Present if the server offers any resources to read.
+// ResourceCapabilities describes the server's support for resources.
 type ResourceCapabilities struct {
-	// Whether this server supports notifications for changes to the resource list.
+	// ListChanged reports whether the client supports notifications for
+	// changes to the resource list.
 	ListChanged bool `json:"listChanged,omitempty"`
-	// Whether this server supports subscribing to resource updates.
+	// Subscribe reports whether this server supports subscribing to resource
+	// updates.
 	Subscribe bool `json:"subscribe,omitempty"`
 }
 
-// Capabilities that a server may support. Known capabilities are defined here,
-// in this schema, but this is not a closed set: any server can define its own,
-// additional capabilities.
+// ToolCapabilities describes the server's support for tools.
+type ToolCapabilities struct {
+	// ListChanged reports whether the client supports notifications for
+	// changes to the tool list.
+	ListChanged bool `json:"listChanged,omitempty"`
+}
+
+// ServerCapabilities describes capabilities that a server supports.
 type ServerCapabilities struct {
-	// Present if the server supports argument autocompletion suggestions.
-	Completions *CompletionCapabilities `json:"completions,omitempty"`
-	// Experimental, non-standard capabilities that the server supports.
+	// Experimental reports non-standard capabilities that the server supports.
 	Experimental map[string]any `json:"experimental,omitempty"`
-	// Present if the server supports sending log messages to the client.
+	// Completions is present if the server supports argument autocompletion
+	// suggestions.
+	Completions *CompletionCapabilities `json:"completions,omitempty"`
+	// Logging is present if the server supports log messages.
 	Logging *LoggingCapabilities `json:"logging,omitempty"`
-	// Present if the server offers any prompt templates.
+	// Prompts is present if the server supports prompts.
 	Prompts *PromptCapabilities `json:"prompts,omitempty"`
-	// Present if the server offers any resources to read.
+	// Resources is present if the server supports resourcs.
 	Resources *ResourceCapabilities `json:"resources,omitempty"`
-	// Present if the server offers any tools to call.
+	// Tools is present if the supports tools.
 	Tools *ToolCapabilities `json:"tools,omitempty"`
 }
 
-// Present if the server offers any tools to call.
-type ToolCapabilities struct {
-	// Whether this server supports notifications for changes to the tool list.
-	ListChanged bool `json:"listChanged,omitempty"`
-}
-
 const (
 	methodCallTool                  = "tools/call"
 	notificationCancelled           = "notifications/cancelled"

mcp/server.go

@@ -1138,7 +1138,7 @@ func (s *Server) AddReceivingMiddleware(middleware ...Middleware) {
 // curating these method flags.
 var serverMethodInfos = map[string]methodInfo{
 	methodComplete:               newServerMethodInfo(serverMethod((*Server).complete), 0),
-	methodInitialize:             newServerMethodInfo(serverSessionMethod((*ServerSession).initialize), 0),
+	methodInitialize:             initializeMethodInfo(),
 	methodPing:                   newServerMethodInfo(serverSessionMethod((*ServerSession).ping), missingParamsOK),
 	methodListPrompts:            newServerMethodInfo(serverMethod((*Server).listPrompts), missingParamsOK),
 	methodGetPrompt:              newServerMethodInfo(serverMethod((*Server).getPrompt), 0),
@@ -1156,6 +1156,25 @@ var serverMethodInfos = map[string]methodInfo{
 	notificationProgress:         newServerMethodInfo(serverSessionMethod((*ServerSession).callProgressNotificationHandler), notification),
 }
 
+// initializeMethodInfo handles the workaround for #607: we must set
+// params.Capabilities.RootsV2.
+func initializeMethodInfo() methodInfo {
+	info := newServerMethodInfo(serverSessionMethod((*ServerSession).initialize), 0)
+	info.unmarshalParams = func(m json.RawMessage) (Params, error) {
+		var params *initializeParamsV2
+		if m != nil {
+			if err := json.Unmarshal(m, &params); err != nil {
+				return nil, fmt.Errorf("unmarshaling %q into a %T: %w", m, params, err)
+			}
+		}
+		if params == nil {
+			return nil, fmt.Errorf(`missing required "params"`)
+		}
+		return params.toV1(), nil
+	}
+	return info
+}
+
 func (ss *ServerSession) sendingMethodInfos() map[string]methodInfo { return clientMethodInfos }
 
 func (ss *ServerSession) receivingMethodInfos() map[string]methodInfo { return serverMethodInfos }