diff --git a/changelog.d/8780-class-instance-object-prototype-read.md b/changelog.d/8780-class-instance-object-prototype-read.md
new file mode 100644
index 0000000000..87a3f5c314
--- /dev/null
+++ b/changelog.d/8780-class-instance-object-prototype-read.md
@@ -0,0 +1,4 @@
+Fixed inherited `Object.prototype` property reads on declared class instances.
+Classes without their own `toString` or `valueOf` now resolve the default
+methods instead of reporting the properties as present while reading them as
+`undefined`, restoring ordinary string coercion such as `String(new C())`.
diff --git a/crates/perry-runtime/src/object/field_get_set/accessors.rs b/crates/perry-runtime/src/object/field_get_set/accessors.rs
index de75ce90db..5f5059da14 100644
--- a/crates/perry-runtime/src/object/field_get_set/accessors.rs
+++ b/crates/perry-runtime/src/object/field_get_set/accessors.rs
@@ -240,7 +240,16 @@ pub(crate) unsafe fn ordinary_object_prototype_property_value(
         return None;
     }
     let class_id = (*obj).class_id;
-    if class_id != 0 && !is_anon_shape_class_id(class_id) {
+    // Declared ES class instances have a registered class id, but still end
+    // their implicit prototype chain at Object.prototype.  Their class
+    // methods/accessors have already been consulted before this fallback, so
+    // a miss must remain eligible for Object.prototype (including user-added
+    // properties).  Keep excluding unregistered native/synthetic class ids:
+    // those object kinds resolve their own intrinsic prototype chains.
+    if class_id != 0
+        && !is_anon_shape_class_id(class_id)
+        && !super::super::class_registry::is_class_id_registered(class_id)
+    {
         return None;
     }
     default_object_prototype_property_value(obj as usize, key)