mcp: validation only for accept action (#766)

Fix: #758

- validate schema only for accept action
- add unit test

mcp/client.go

@@ -590,7 +590,7 @@ func (c *Client) elicit(ctx context.Context, req *ElicitRequest) (*ElicitResult,
 			return nil, err
 		}
 		// Validate elicitation result content against requested schema.
-		if schema != nil && res.Content != nil {
+		if res.Action == "accept" && schema != nil && res.Content != nil {
 			resolved, err := schema.Resolve(nil)
 			if err != nil {
 				return nil, &jsonrpc.Error{Code: jsonrpc.CodeInvalidParams, Message: fmt.Sprintf("failed to resolve requested schema: %v", err)}

mcp/elicitation_test.go

@@ -201,3 +201,71 @@ func TestElicitationCompleteNotification(t *testing.T) {
 		t.Fatal("timed out waiting for elicitation complete notification")
 	}
 }
+
+func TestElicitationNoValidationWithoutAccept(t *testing.T) {
+	ctx := context.Background()
+
+	schema := &jsonschema.Schema{
+		Type: "object",
+		Properties: map[string]*jsonschema.Schema{
+			"test": {Type: "string"},
+		},
+		Required: []string{"test"},
+	}
+
+	testCases := []struct {
+		name       string
+		action     string
+		content    map[string]any
+		wantAction string
+	}{
+		{
+			name:       "cancel action",
+			action:     "cancel",
+			content:    nil, // Empty content should be ignored
+			wantAction: "cancel",
+		},
+		{
+			name:       "decline action",
+			action:     "decline",
+			content:    map[string]any{}, // Empty content should be ignored
+			wantAction: "decline",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			ct, st := NewInMemoryTransports()
+			s := NewServer(testImpl, nil)
+			ss, err := s.Connect(ctx, st, nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer ss.Close()
+
+			c := NewClient(testImpl, &ClientOptions{
+				ElicitationHandler: func(context.Context, *ElicitRequest) (*ElicitResult, error) {
+					return &ElicitResult{Action: tc.action, Content: tc.content}, nil
+				},
+			})
+			cs, err := c.Connect(ctx, ct, nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer cs.Close()
+
+			res, err := ss.Elicit(ctx, &ElicitParams{
+				Message:         "Test bug",
+				RequestedSchema: schema,
+			})
+
+			if err != nil {
+				t.Fatalf("Elicit failed: %v", err)
+			}
+
+			if res.Action != tc.wantAction {
+				t.Errorf("Expected action %q, got %q", tc.wantAction, res.Action)
+			}
+		})
+	}
+}

mcp/server.go

@@ -1218,6 +1218,10 @@ func (ss *ServerSession) Elicit(ctx context.Context, params *ElicitParams) (*Eli
 		return nil, err
 	}
 
+	if res.Action != "accept" {
+		return res, nil
+	}
+
 	if params.RequestedSchema == nil {
 		return res, nil
 	}