diff --git a/src/__tests__/testing.test.ts b/src/__tests__/testing.test.ts
index f73f3ed..52eb47e 100644
--- a/src/__tests__/testing.test.ts
+++ b/src/__tests__/testing.test.ts
@@ -3,9 +3,11 @@
 // everything unconfigured.
 
 import * as Effect from "effect/Effect";
+import * as Exit from "effect/Exit";
 import { describe, expect, it } from "vitest";
 import { makeWorkflowClient } from "../client.js";
 import { decodeWorkflowResult, makeFakeTemporalClient, simulateAlreadyStarted } from "../testing.js";
+import { wireCodecsFor } from "../wire.js";
 import { Demo } from "./fixtures/demo.js";
 
 const payload = { requestId: "fake-1", mode: "approve" } as const;
@@ -85,4 +87,10 @@ describe("decodeWorkflowResult", () => {
       }),
     ).toThrow(/did not succeed/);
   });
+
+  it("preserves native defect details in the thrown message", () => {
+    const wire = wireCodecsFor(Demo).encodeExit(Exit.die(new Error("boom")));
+
+    expect(() => decodeWorkflowResult(Demo, wire)).toThrow(/boom/);
+  });
 });
diff --git a/src/testing.ts b/src/testing.ts
index 3377632..8ffa80a 100644
--- a/src/testing.ts
+++ b/src/testing.ts
@@ -14,6 +14,7 @@
  * @since 0.1.0
  */
 
+import * as Cause from "effect/Cause";
 import * as Effect from "effect/Effect";
 import * as Exit from "effect/Exit";
 import type * as Schema from "effect/Schema";
@@ -256,7 +257,7 @@ export const decodeWorkflowResult = <W extends Workflow.Any>(
   // SAFETY: the exit was decoded through the workflow's own success schema,
   // so the success value is SuccessOf<W> — the codec seam types it unknown.
   if (Exit.isSuccess(exit)) return exit.value as SuccessOf<W>;
-  throw new Error(`workflow "${workflow._tag}" did not succeed: ${JSON.stringify(exit.cause)}`);
+  throw new Error(`workflow "${workflow._tag}" did not succeed: ${Cause.pretty(exit.cause)}`);
 };
 
 /**