feat(mcptoolset): surface elicitation and tool result _meta

tool/mcptoolset/client.go

@@ -53,10 +53,10 @@ var refreshableErrors = []error{
 }
 
 // newConnectionRefresher creates a new connectionRefresher with the given client and transport.
-// If client is nil, a default MCP client will be created.
-func newConnectionRefresher(client *mcp.Client, transport mcp.Transport) *connectionRefresher {
+// If client is nil, a default MCP client will be created with the given options.
+func newConnectionRefresher(client *mcp.Client, transport mcp.Transport, options *mcp.ClientOptions) *connectionRefresher {
 	if client == nil {
-		client = mcp.NewClient(&mcp.Implementation{Name: "adk-mcp-client", Version: version.Version}, nil)
+		client = mcp.NewClient(&mcp.Implementation{Name: "adk-mcp-client", Version: version.Version}, options)
 	}
 	return &connectionRefresher{
 		client:    client,

tool/mcptoolset/set.go

@@ -16,6 +16,7 @@
 package mcptoolset
 
 import (
+	"context"
 	"fmt"
 
 	"github.com/modelcontextprotocol/go-sdk/mcp"
@@ -47,8 +48,29 @@ import (
 //		},
 //	})
 func New(cfg Config) (tool.Toolset, error) {
+	var clientOptions *mcp.ClientOptions
+	if cfg.ElicitationHandler != nil || cfg.ElicitationCompleteHandler != nil {
+		if cfg.Client != nil {
+			return nil, fmt.Errorf("mcptoolset: ElicitationHandler and ElicitationCompleteHandler cannot be combined with a custom Client; set them in the client's mcp.ClientOptions instead")
+		}
+		clientOptions = &mcp.ClientOptions{
+			ElicitationHandler:         cfg.ElicitationHandler,
+			ElicitationCompleteHandler: cfg.ElicitationCompleteHandler,
+			// The capability inferred from ElicitationHandler alone covers only
+			// form mode; URL mode must be declared explicitly. RootsV2 preserves
+			// the default roots capability, which setting Capabilities would
+			// otherwise disable.
+			Capabilities: &mcp.ClientCapabilities{
+				Elicitation: &mcp.ElicitationCapabilities{
+					Form: &mcp.FormElicitationCapabilities{},
+					URL:  &mcp.URLElicitationCapabilities{},
+				},
+				RootsV2: &mcp.RootCapabilities{ListChanged: true},
+			},
+		}
+	}
 	return &set{
-		mcpClient:                   newConnectionRefresher(cfg.Client, cfg.Transport),
+		mcpClient:                   newConnectionRefresher(cfg.Client, cfg.Transport, clientOptions),
 		toolFilter:                  cfg.ToolFilter,
 		requireConfirmation:         cfg.RequireConfirmation,
 		requireConfirmationProvider: cfg.RequireConfirmationProvider,
@@ -72,6 +94,21 @@ type Config struct {
 	// a Human-in-the-Loop (HITL) confirmation request when a tool is invoked.
 	RequireConfirmation bool
 
+	// ElicitationHandler handles elicitation/create requests from the MCP
+	// server, including URL-mode elicitations that servers use for
+	// out-of-band interactions such as auth challenges. Setting it makes the
+	// client advertise the elicitation capability.
+	// It can only be set when Client is nil; for a custom Client, set the
+	// handler in the client's mcp.ClientOptions instead.
+	ElicitationHandler func(context.Context, *mcp.ElicitRequest) (*mcp.ElicitResult, error)
+
+	// ElicitationCompleteHandler handles notifications/elicitation/complete
+	// notifications, which servers send when an out-of-band (URL-mode)
+	// elicitation has been completed.
+	// It can only be set when Client is nil; for a custom Client, set the
+	// handler in the client's mcp.ClientOptions instead.
+	ElicitationCompleteHandler func(context.Context, *mcp.ElicitationCompleteNotificationRequest)
+
 	// RequireConfirmationProvider allows for dynamic determination of whether
 	// user confirmation is needed. This field is a function called at runtime to decide if
 	// a confirmation request should be sent. The function takes the toolName and tool's input parameters as arguments.

tool/mcptoolset/set_test.go

@@ -16,6 +16,7 @@ package mcptoolset_test
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -796,3 +797,169 @@ func TestNewToolSet_RequireConfirmationProvider_Validation(t *testing.T) {
 		})
 	}
 }
+
+func TestCallToolMetaPassthrough(t *testing.T) {
+	challengeMeta := map[string]any{
+		"example.com/auth": map[string]any{"url": "https://idp.example.com/login"},
+	}
+
+	tests := []struct {
+		name    string
+		handler mcp.ToolHandler
+		want    map[string]any
+	}{
+		{
+			name: "text result with meta",
+			handler: func(ctx context.Context, req *mcp.CallToolRequest) (*mcp.CallToolResult, error) {
+				return &mcp.CallToolResult{
+					Meta:    mcp.Meta(challengeMeta),
+					Content: []mcp.Content{&mcp.TextContent{Text: "login required"}},
+				}, nil
+			},
+			want: map[string]any{
+				"output": "login required",
+				"_meta":  challengeMeta,
+			},
+		},
+		{
+			name: "structured result with meta",
+			handler: func(ctx context.Context, req *mcp.CallToolRequest) (*mcp.CallToolResult, error) {
+				return &mcp.CallToolResult{
+					Meta:              mcp.Meta(challengeMeta),
+					Content:           []mcp.Content{&mcp.TextContent{Text: "login required"}},
+					StructuredContent: map[string]any{"status": "unauthenticated"},
+				}, nil
+			},
+			want: map[string]any{
+				"output": map[string]any{"status": "unauthenticated"},
+				"_meta":  challengeMeta,
+			},
+		},
+		{
+			name: "text result without meta",
+			handler: func(ctx context.Context, req *mcp.CallToolRequest) (*mcp.CallToolResult, error) {
+				return &mcp.CallToolResult{
+					Content: []mcp.Content{&mcp.TextContent{Text: "all good"}},
+				}, nil
+			},
+			want: map[string]any{
+				"output": "all good",
+			},
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			clientTransport, serverTransport := mcp.NewInMemoryTransports()
+
+			server := mcp.NewServer(&mcp.Implementation{Name: "test_server", Version: "v1.0.0"}, nil)
+			server.AddTool(&mcp.Tool{
+				Name:        "gateway_tool",
+				Description: "returns a result with metadata",
+				InputSchema: json.RawMessage(`{"type":"object"}`),
+			}, tc.handler)
+			if _, err := server.Connect(t.Context(), serverTransport, nil); err != nil {
+				t.Fatal(err)
+			}
+
+			ts, err := mcptoolset.New(mcptoolset.Config{Transport: clientTransport})
+			if err != nil {
+				t.Fatalf("Failed to create MCP tool set: %v", err)
+			}
+
+			invCtx := icontext.NewInvocationContext(t.Context(), icontext.InvocationContextParams{})
+			tools, err := ts.Tools(icontext.NewReadonlyContext(invCtx))
+			if err != nil {
+				t.Fatalf("Tools call failed: %v", err)
+			}
+			toolCtx := agent.NewToolContext(invCtx, "", nil, nil)
+
+			fnTool := tools[0].(toolinternal.FunctionTool)
+			result, err := fnTool.Run(toolCtx, map[string]any{})
+			if err != nil {
+				t.Fatalf("Tool call failed: %v", err)
+			}
+
+			if diff := cmp.Diff(tc.want, result); diff != "" {
+				t.Errorf("Tool result mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
+func TestElicitationHandler(t *testing.T) {
+	const loginURL = "https://idp.example.com/login"
+
+	server := mcp.NewServer(&mcp.Implementation{Name: "test_server", Version: "v1.0.0"}, nil)
+	server.AddTool(&mcp.Tool{
+		Name:        "gateway_tool",
+		Description: "elicits a login before responding",
+		InputSchema: json.RawMessage(`{"type":"object"}`),
+	}, func(ctx context.Context, req *mcp.CallToolRequest) (*mcp.CallToolResult, error) {
+		res, err := req.Session.Elicit(ctx, &mcp.ElicitParams{
+			Mode:          "url",
+			Message:       "please log in",
+			URL:           loginURL,
+			ElicitationID: "elicitation-1",
+		})
+		if err != nil {
+			return nil, err
+		}
+		return &mcp.CallToolResult{
+			Content: []mcp.Content{&mcp.TextContent{Text: "elicitation " + res.Action + "ed"}},
+		}, nil
+	})
+
+	clientTransport, serverTransport := mcp.NewInMemoryTransports()
+	if _, err := server.Connect(t.Context(), serverTransport, nil); err != nil {
+		t.Fatal(err)
+	}
+
+	var gotURL string
+	ts, err := mcptoolset.New(mcptoolset.Config{
+		Transport: clientTransport,
+		ElicitationHandler: func(ctx context.Context, req *mcp.ElicitRequest) (*mcp.ElicitResult, error) {
+			gotURL = req.Params.URL
+			return &mcp.ElicitResult{Action: "accept"}, nil
+		},
+	})
+	if err != nil {
+		t.Fatalf("Failed to create MCP tool set: %v", err)
+	}
+
+	invCtx := icontext.NewInvocationContext(t.Context(), icontext.InvocationContextParams{})
+	tools, err := ts.Tools(icontext.NewReadonlyContext(invCtx))
+	if err != nil {
+		t.Fatalf("Tools call failed: %v", err)
+	}
+	toolCtx := agent.NewToolContext(invCtx, "", nil, nil)
+
+	fnTool := tools[0].(toolinternal.FunctionTool)
+	result, err := fnTool.Run(toolCtx, map[string]any{})
+	if err != nil {
+		t.Fatalf("Tool call failed: %v", err)
+	}
+
+	if gotURL != loginURL {
+		t.Errorf("Elicitation handler got URL %q, want %q", gotURL, loginURL)
+	}
+	want := map[string]any{"output": "elicitation accepted"}
+	if diff := cmp.Diff(want, result); diff != "" {
+		t.Errorf("Tool result mismatch (-want +got):\n%s", diff)
+	}
+}
+
+func TestNewRejectsElicitationHandlerWithCustomClient(t *testing.T) {
+	clientTransport, _ := mcp.NewInMemoryTransports()
+
+	_, err := mcptoolset.New(mcptoolset.Config{
+		Client:    mcp.NewClient(&mcp.Implementation{Name: "custom", Version: "v1.0.0"}, nil),
+		Transport: clientTransport,
+		ElicitationHandler: func(ctx context.Context, req *mcp.ElicitRequest) (*mcp.ElicitResult, error) {
+			return &mcp.ElicitResult{Action: "decline"}, nil
+		},
+	})
+	if err == nil {
+		t.Fatal("expected error when combining ElicitationHandler with a custom Client, got nil")
+	}
+}

tool/mcptoolset/tool.go

@@ -147,9 +147,7 @@ func (t *mcpTool) Run(ctx agent.Context, args any) (map[string]any, error) {
 	}
 
 	if res.StructuredContent != nil {
-		return map[string]any{
-			"output": res.StructuredContent,
-		}, nil
+		return functionResponse(res, res.StructuredContent), nil
 	}
 
 	textResponse := strings.Builder{}
@@ -169,9 +167,22 @@ func (t *mcpTool) Run(ctx agent.Context, args any) (map[string]any, error) {
 		return nil, errors.New("no text content in tool response")
 	}
 
-	return map[string]any{
-		"output": textResponse.String(),
-	}, nil
+	return functionResponse(res, textResponse.String()), nil
+}
+
+// functionResponse builds the function response map for a tool result.
+// The result's _meta field is preserved under the "_meta" key, mirroring the
+// raw MCP serialization, so that metadata attached by the server (e.g. auth
+// challenges from MCP gateways) reaches callbacks and the embedding
+// application instead of being silently dropped.
+func functionResponse(res *mcp.CallToolResult, output any) map[string]any {
+	response := map[string]any{
+		"output": output,
+	}
+	if len(res.Meta) > 0 {
+		response["_meta"] = map[string]any(res.Meta)
+	}
+	return response
 }
 
 var (