diff --git a/packages/agent-core/src/harness/types.test.ts b/packages/agent-core/src/harness/types.test.ts
new file mode 100644
index 000000000000..e252c3d41ab3
--- /dev/null
+++ b/packages/agent-core/src/harness/types.test.ts
@@ -0,0 +1,20 @@
+import { describe, expect, it } from "vitest";
+import { toError } from "./types.js";
+
+describe("toError", () => {
+  it("normalizes hostile non-Error values without stringifying them", () => {
+    const hostile = {
+      toJSON() {
+        throw new Error("json denied");
+      },
+      toString() {
+        throw new Error("stringification denied");
+      },
+    };
+
+    const error = toError(hostile);
+
+    expect(error).toBeInstanceOf(Error);
+    expect(error.message).toBe("Unknown thrown value");
+  });
+});
diff --git a/packages/agent-core/src/harness/types.ts b/packages/agent-core/src/harness/types.ts
index f74ecc3cc853..ec55f186936e 100644
--- a/packages/agent-core/src/harness/types.ts
+++ b/packages/agent-core/src/harness/types.ts
@@ -50,7 +50,7 @@ export function toError(error: unknown): Error {
   try {
     return new Error(JSON.stringify(error));
   } catch {
-    return new Error(String(error));
+    return new Error("Unknown thrown value");
   }
 }