mcp/server: expose InitializeParams to ServerSession (#336)

This CL enables the server session to see what capabilities the client
has by introducing the InitializeParams() function.

This CL also adds a test to ensure ClientCapabilities is accurate.

Fixes #141

mcp/client.go

@@ -105,6 +105,15 @@ func (e unsupportedProtocolVersionError) Error() string {
 // ClientSessionOptions is reserved for future use.
 type ClientSessionOptions struct{}
 
+func (c *Client) capabilities() *ClientCapabilities {
+	caps := &ClientCapabilities{}
+	caps.Roots.ListChanged = true
+	if c.opts.CreateMessageHandler != nil {
+		caps.Sampling = &SamplingCapabilities{}
+	}
+	return caps
+}
+
 // Connect begins an MCP session by connecting to a server over the given
 // transport, and initializing the session.
 //
@@ -118,16 +127,10 @@ func (c *Client) Connect(ctx context.Context, t Transport, _ *ClientSessionOptio
 		return nil, err
 	}
 
-	caps := &ClientCapabilities{}
-	caps.Roots.ListChanged = true
-	if c.opts.CreateMessageHandler != nil {
-		caps.Sampling = &SamplingCapabilities{}
-	}
-
 	params := &InitializeParams{
 		ProtocolVersion: latestProtocolVersion,
 		ClientInfo:      c.impl,
-		Capabilities:    caps,
+		Capabilities:    c.capabilities(),
 	}
 	req := &ClientRequest[*InitializeParams]{Session: cs, Params: params}
 	res, err := handleSend[*InitializeResult](ctx, methodInitialize, req)

mcp/client_test.go

@@ -190,3 +190,48 @@ func TestClientPaginateVariousPageSizes(t *testing.T) {
 		})
 	}
 }
+
+func TestClientCapabilities(t *testing.T) {
+	testCases := []struct {
+		name             string
+		configureClient  func(s *Client)
+		clientOpts       ClientOptions
+		wantCapabilities *ClientCapabilities
+	}{
+		{
+			name:            "With initial capabilities",
+			configureClient: func(s *Client) {},
+			wantCapabilities: &ClientCapabilities{
+				Roots: struct {
+					ListChanged bool "json:\"listChanged,omitempty\""
+				}{ListChanged: true},
+			},
+		},
+		{
+			name:            "With sampling",
+			configureClient: func(s *Client) {},
+			clientOpts: ClientOptions{
+				CreateMessageHandler: func(context.Context, *ClientRequest[*CreateMessageParams]) (*CreateMessageResult, error) {
+					return nil, nil
+				},
+			},
+			wantCapabilities: &ClientCapabilities{
+				Roots: struct {
+					ListChanged bool "json:\"listChanged,omitempty\""
+				}{ListChanged: true},
+				Sampling: &SamplingCapabilities{},
+			},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			client := NewClient(testImpl, &tc.clientOpts)
+			tc.configureClient(client)
+			gotCapabilities := client.capabilities()
+			if diff := cmp.Diff(tc.wantCapabilities, gotCapabilities); diff != "" {
+				t.Errorf("capabilities() mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}

mcp/server.go

@@ -849,6 +849,8 @@ func (ss *ServerSession) handle(ctx context.Context, req *jsonrpc.Request) (any,
 	return handleReceive(ctx, ss, req)
 }
 
+func (ss *ServerSession) InitializeParams() *InitializeParams { return ss.state.InitializeParams }
+
 func (ss *ServerSession) initialize(ctx context.Context, params *InitializeParams) (*InitializeResult, error) {
 	if params == nil {
 		return nil, fmt.Errorf("%w: \"params\" must be be provided", jsonrpc2.ErrInvalidParams)