diff --git a/Cargo.lock b/Cargo.lock
index daeb63dd7..73735e41f 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -635,6 +635,7 @@ dependencies = [
  "pretty_assertions",
  "prettyplease",
  "testing_macros",
+ "thiserror 2.0.6",
  "tokio",
  "url",
 ]
diff --git a/core/00_infra.js b/core/00_infra.js
index 39e7a3c66..253ea1dde 100644
--- a/core/00_infra.js
+++ b/core/00_infra.js
@@ -102,10 +102,7 @@
         for (const property of new SafeArrayIterator(additionalProperties)) {
           const key = property[0];
           if (!(key in error)) {
-            ObjectDefineProperty(error, key, {
-              value: property[1],
-              writable: false,
-            });
+            error[key] = property[1];
           }
         }
       }
diff --git a/testing/Cargo.toml b/testing/Cargo.toml
index 1b415b2b9..b9cbf6520 100644
--- a/testing/Cargo.toml
+++ b/testing/Cargo.toml
@@ -20,6 +20,7 @@ deno_core.workspace = true
 deno_core.features = ["unsafe_use_unprotected_platform", "snapshot_flags_eager_parse"]
 deno_error.workspace = true
 futures.workspace = true
+thiserror.workspace = true
 tokio.workspace = true
 url.workspace = true
 
diff --git a/testing/checkin/runner/extensions.rs b/testing/checkin/runner/extensions.rs
index ad51fab8b..3bad7976f 100644
--- a/testing/checkin/runner/extensions.rs
+++ b/testing/checkin/runner/extensions.rs
@@ -39,6 +39,7 @@ deno_core::extension!(
     ops_error::op_async_throw_error_lazy,
     ops_error::op_async_throw_error_deferred,
     ops_error::op_error_custom_sync,
+    ops_error::op_error_custom_with_code_sync,
     ops_buffer::op_v8slice_store,
     ops_buffer::op_v8slice_clone,
     ops_worker::op_worker_spawn,
diff --git a/testing/checkin/runner/ops_error.rs b/testing/checkin/runner/ops_error.rs
index 66d57fbcf..3fee67903 100644
--- a/testing/checkin/runner/ops_error.rs
+++ b/testing/checkin/runner/ops_error.rs
@@ -24,3 +24,20 @@ pub fn op_error_custom_sync(
 ) -> Result<(), JsErrorBox> {
   Err(JsErrorBox::new("BadResource", message))
 }
+
+#[derive(Debug, thiserror::Error, deno_error::JsError)]
+#[class(type)]
+#[error("{message}")]
+struct MyError {
+  message: String,
+  #[property]
+  code: u32,
+}
+
+#[op2(fast)]
+pub fn op_error_custom_with_code_sync(
+  #[string] message: String,
+  code: u32,
+) -> Result<(), MyError> {
+  Err(MyError { message, code })
+}
diff --git a/testing/checkin/runtime/error.ts b/testing/checkin/runtime/error.ts
index 5c5b62a29..b109154c4 100644
--- a/testing/checkin/runtime/error.ts
+++ b/testing/checkin/runtime/error.ts
@@ -4,6 +4,7 @@ import {
   op_async_throw_error_eager,
   op_async_throw_error_lazy,
   op_error_custom_sync,
+  op_error_custom_with_code_sync,
 } from "ext:core/ops";
 
 export async function asyncThrow(kind: "lazy" | "eager" | "deferred") {
@@ -18,3 +19,7 @@ export async function asyncThrow(kind: "lazy" | "eager" | "deferred") {
 export function throwCustomError(message: string) {
   op_error_custom_sync(message);
 }
+
+export function throwCustomErrorWithCode(message: string, code: number) {
+  op_error_custom_with_code_sync(message, code);
+}
diff --git a/testing/lib.rs b/testing/lib.rs
index 2e69a26b7..510b27a63 100644
--- a/testing/lib.rs
+++ b/testing/lib.rs
@@ -41,6 +41,7 @@ unit_test!(
   microtask_test,
   ops_async_test,
   ops_buffer_test,
+  ops_error_test,
   resource_test,
   serialize_deserialize_test,
   stats_test,
diff --git a/testing/ops.d.ts b/testing/ops.d.ts
index 74a109ad2..e603d0b03 100644
--- a/testing/ops.d.ts
+++ b/testing/ops.d.ts
@@ -12,6 +12,7 @@ export function op_async_throw_error_lazy(...any: any[]): any;
 export function op_error_context_async(...any: any[]): any;
 export function op_error_context_sync(...any: any[]): any;
 export function op_error_custom_sync(...any: any[]): any;
+export function op_error_custom_with_code_sync(...any: any[]): any;
 
 export function op_worker_await_close(...any: any[]): any;
 export function op_worker_parent(...any: any[]): any;
diff --git a/testing/unit/ops_error_test.ts b/testing/unit/ops_error_test.ts
new file mode 100644
index 000000000..6ac870e7a
--- /dev/null
+++ b/testing/unit/ops_error_test.ts
@@ -0,0 +1,13 @@
+// Copyright 2018-2025 the Deno authors. MIT license.
+import { assertEquals, test } from "checkin:testing";
+import { throwCustomErrorWithCode } from "checkin:error";
+
+test(function additionalPropertyIsWritable() {
+  try {
+    throwCustomErrorWithCode("foo", 1);
+  } catch (e) {
+    assertEquals(e.message, "foo");
+    assertEquals(e.code, 1);
+    e.code = 2;
+  }
+});