diff --git a/codex-rs/tool-api/src/bundle.rs b/codex-rs/tool-api/src/bundle.rs
index ba24e1e2dd3c..9c03184c6dd4 100644
--- a/codex-rs/tool-api/src/bundle.rs
+++ b/codex-rs/tool-api/src/bundle.rs
@@ -72,8 +72,56 @@ pub trait ToolExecutor<C>: Send + Sync {
     /// Returns whether the call may mutate user state.
     ///
     /// Hosts can use this conservative signal for serialization or approval
-    /// policy. Context-free read tools should keep the default.
+    /// policy. Read-only tools should override this default.
     fn is_mutating<'a>(&'a self, _call: &'a ToolCall<C>) -> BoolFuture<'a> {
-        Box::pin(async { false })
+        Box::pin(async { true })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::sync::Arc;
+    use std::task::Context;
+    use std::task::Poll;
+    use std::task::Wake;
+    use std::task::Waker;
+
+    use super::*;
+    use crate::JsonToolOutput;
+    use crate::ToolInput;
+
+    struct DefaultMutatingExecutor;
+
+    impl ToolExecutor<()> for DefaultMutatingExecutor {
+        fn execute<'a>(&'a self, _call: ToolCall<()>) -> ToolFuture<'a> {
+            Box::pin(async {
+                Ok(Box::new(JsonToolOutput::new(serde_json::json!(null))) as Box<dyn ToolOutput>)
+            })
+        }
+    }
+
+    struct NoopWaker;
+
+    impl Wake for NoopWaker {
+        fn wake(self: Arc<Self>) {}
+    }
+
+    #[test]
+    fn contributed_tools_default_to_mutating() {
+        let call = ToolCall {
+            context: (),
+            call_id: "call-default-mutating".to_string(),
+            input: ToolInput::Function {
+                arguments: "{}".to_string(),
+            },
+        };
+        let mut future = DefaultMutatingExecutor.is_mutating(&call);
+        let waker = Waker::from(Arc::new(NoopWaker));
+        let mut context = Context::from_waker(&waker);
+
+        assert!(matches!(
+            future.as_mut().poll(&mut context),
+            Poll::Ready(true)
+        ));
     }
 }