diff --git a/types/function-calling.ts b/types/function-calling.ts
index 02bdb5fce..53302b004 100644
--- a/types/function-calling.ts
+++ b/types/function-calling.ts
@@ -135,7 +135,7 @@ export interface Schema {
  */
 export interface FunctionDeclarationSchema {
   /** The type of the parameter. */
-  type: SchemaType;
+  type: SchemaType | `${SchemaType}`;
   /** The format of the parameter. */
   properties: { [k: string]: FunctionDeclarationSchemaProperty };
   /** Optional. Description of the parameter. */