fix: default missing function call arguments

Co-authored-by: ndycode <ndycode@users.noreply.github.com>

Author: ndycode

lib/request/request-transformer.ts

@@ -741,12 +741,21 @@ export function filterInput(
 		})
 		.map((item) => {
 			// Strip IDs from all items (Codex API stateless mode)
+			let normalizedItem = item;
 			if (item.id) {
 				const { id: _omit, ...itemWithoutId } = item;
 				void _omit;
-				return itemWithoutId as InputItem;
+				normalizedItem = itemWithoutId as InputItem;
 			}
-			return item;
+
+			if (
+				normalizedItem.type === "function_call" &&
+				(normalizedItem.arguments === undefined || normalizedItem.arguments === null)
+			) {
+				return { ...normalizedItem, arguments: "{}" } as InputItem;
+			}
+
+			return normalizedItem;
 		});
 }
 

test/request-transformer.test.ts

@@ -547,6 +547,20 @@ describe('Request Transformer Module', () => {
 			expect(result![1]).not.toHaveProperty('id');
 		});
 
+		it('should default missing function_call arguments to an empty JSON object', () => {
+			const input: InputItem[] = [
+				{ type: 'function_call', role: 'assistant', call_id: 'call_missing', name: 'read_file' },
+				{ type: 'function_call', role: 'assistant', call_id: 'call_null', name: 'list_files', arguments: null },
+				{ type: 'function_call', role: 'assistant', call_id: 'call_present', name: 'write_file', arguments: '{"path":"README.md"}' },
+			];
+
+			const result = filterInput(input);
+
+			expect(result![0].arguments).toBe('{}');
+			expect(result![1].arguments).toBe('{}');
+			expect(result![2].arguments).toBe('{"path":"README.md"}');
+		});
+
 		it('should return undefined for undefined input', () => {
 			expect(filterInput(undefined)).toBeUndefined();
 		});