diff --git a/src/agent/mod.rs b/src/agent/mod.rs
index 554af06..f2f8376 100644
--- a/src/agent/mod.rs
+++ b/src/agent/mod.rs
@@ -485,8 +485,16 @@ impl Agent {
         ui_tx: &UiSender,
         ds: &mut DispatchState,
     ) {
-        let args: serde_json::Value =
-            serde_json::from_str(&call.function.arguments).unwrap_or_default();
+        let args: serde_json::Value = match serde_json::from_str(&call.function.arguments) {
+            Ok(v) => v,
+            Err(e) => {
+                let err = format!("Error: malformed tool call arguments: {e}");
+                let _ = ui_tx.send(UiMessage::Activity(format!("rejected: {} (bad args)", call.function.name)));
+                let mut me = agent.lock().await;
+                me.apply_tool_result(call, err, ui_tx, ds);
+                return;
+            }
+        };
 
         let args_summary = summarize_args(&call.function.name, &args);
         let _ = ui_tx.send(UiMessage::Activity(format!("calling: {}", call.function.name)));