diff --git a/packages/core/src/session/parts.ts b/packages/core/src/session/parts.ts
index db11811..67375ab 100644
--- a/packages/core/src/session/parts.ts
+++ b/packages/core/src/session/parts.ts
@@ -30,7 +30,7 @@ export const ToolPartSchema = PartBaseSchema.extend({
   type: z.literal('tool'),
   toolCallId: z.string(),
   toolName: z.string(),
-  args: z.record(z.string(), z.unknown()),
+  args: z.record(z.string(), z.unknown()).default({}),
   status: ToolStatusSchema,
   result: z.unknown().optional(),
   error: z.string().optional(),