diff --git a/src/agents/transport-stream-shared.test.ts b/src/agents/transport-stream-shared.test.ts
index bd7ed3cb76d..b11270a63a2 100644
--- a/src/agents/transport-stream-shared.test.ts
+++ b/src/agents/transport-stream-shared.test.ts
@@ -2,6 +2,7 @@
 // final/error stream termination helpers used by provider transports.
 import { describe, expect, it, vi } from "vitest";
 import {
+  assignTransportErrorDetails,
   failTransportStream,
   finalizeTransportStream,
   mergeTransportHeaders,
@@ -100,4 +101,17 @@ describe("transport stream shared helpers", () => {
     });
     expect(end).toHaveBeenCalledTimes(1);
   });
+
+  it("does not throw while recording non-JSON transport rejections", () => {
+    const circular: Record<string, unknown> = {};
+    circular.self = circular;
+
+    for (const error of [1n, circular]) {
+      const output: { stopReason: string; errorMessage?: string } = { stopReason: "stop" };
+
+      expect(() => assignTransportErrorDetails(output, error)).not.toThrow();
+      expect(output.stopReason).toBe("error");
+      expect(output.errorMessage).toBeTruthy();
+    }
+  });
 });
diff --git a/src/agents/transport-stream-shared.ts b/src/agents/transport-stream-shared.ts
index 965b9bfbbca..097fc5d0861 100644
--- a/src/agents/transport-stream-shared.ts
+++ b/src/agents/transport-stream-shared.ts
@@ -178,6 +178,21 @@ function stringifyErrorBody(value: unknown): string | undefined {
   }
 }
 
+function stringifyTransportErrorMessage(value: unknown): string | undefined {
+  if (value instanceof Error) {
+    return value.message;
+  }
+  const encoded = stringifyErrorBody(value);
+  if (encoded !== undefined) {
+    return encoded;
+  }
+  try {
+    return String(value);
+  } catch {
+    return undefined;
+  }
+}
+
 function normalizeTransportErrorBody(value: unknown): string | undefined {
   const text = stringifyErrorBody(value);
   if (!text?.trim()) {
@@ -216,7 +231,7 @@ export function assignTransportErrorDetails(
   signal?: AbortSignal,
 ): void {
   output.stopReason = signal?.aborted ? "aborted" : "error";
-  output.errorMessage = error instanceof Error ? error.message : JSON.stringify(error);
+  output.errorMessage = stringifyTransportErrorMessage(error);
   Object.assign(output, extractTransportErrorDetails(error));
 }