mcp: reject null titled enum entries in elicitation schemas

mcp/client.go

@@ -1124,6 +1124,9 @@ func validateElicitArrayProperty(propName string, propSchema *jsonschema.Schema)
 }
 
 func validateTitledEnumEntry(entry *jsonschema.Schema) error {
+	if entry == nil {
+		return fmt.Errorf("titled enum entries must be objects")
+	}
 	if entry.Const == nil {
 		return fmt.Errorf("const is required for titled enum entries")
 	}

mcp/mcp_test.go

@@ -1650,6 +1650,34 @@ func TestElicitationSchemaValidation(t *testing.T) {
 			},
 			expectedError: "elicit schema property \"priority\" oneOf has invalid entry: const is required for titled enum entries",
 		},
+		{
+			name: "titled enum with null oneOf entry",
+			schema: &jsonschema.Schema{
+				Type: "object",
+				Properties: map[string]*jsonschema.Schema{
+					"priority": {
+						Type:  "string",
+						OneOf: []*jsonschema.Schema{nil},
+					},
+				},
+			},
+			expectedError: "elicit schema property \"priority\" oneOf has invalid entry: titled enum entries must be objects",
+		},
+		{
+			name: "multi select titled enum with null anyOf entry",
+			schema: &jsonschema.Schema{
+				Type: "object",
+				Properties: map[string]*jsonschema.Schema{
+					"priorities": {
+						Type: "array",
+						Items: &jsonschema.Schema{
+							AnyOf: []*jsonschema.Schema{nil},
+						},
+					},
+				},
+			},
+			expectedError: "elicit schema property \"priorities\" items has invalid entry: titled enum entries must be objects",
+		},
 		{
 			name: "untyped property",
 			schema: &jsonschema.Schema{