diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index 7d69fa57e..d6df74d4b 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -4,7 +4,7 @@
 
 ### Fixed
 
-- Fixed Amazon Bedrock stream error handler crashing with `TypeError: undefined is not an object (evaluating 'baseMessage.includes')` when a non-`Error` value is thrown and `JSON.stringify` returns `undefined` ([#5539](https://github.com/can1357/oh-my-pi/issues/5539)).
+- Fixed Amazon Bedrock stream error handling for non-`Error` values that `JSON.stringify` cannot serialize ([#5539](https://github.com/can1357/oh-my-pi/issues/5539)).
 
 ## [16.5.2] - 2026-07-14
 
diff --git a/packages/ai/src/providers/amazon-bedrock.ts b/packages/ai/src/providers/amazon-bedrock.ts
index 93692e8e6..6a3cf1f02 100644
--- a/packages/ai/src/providers/amazon-bedrock.ts
+++ b/packages/ai/src/providers/amazon-bedrock.ts
@@ -510,7 +510,12 @@ export const streamBedrock: StreamFunction<"bedrock-converse-stream"> = (
 			for (const block of output.content) {
 				if (block.type === "toolCall") clearStreamingPartialJson(block);
 			}
-			const baseMessage = error instanceof Error ? error.message : (JSON.stringify(error) ?? String(error));
+			let baseMessage: string;
+			try {
+				baseMessage = error instanceof Error ? error.message : (JSON.stringify(error) ?? String(error));
+			} catch {
+				baseMessage = String(error);
+			}
 			// Enrich error with thinking block diagnostics for signature-related failures
 			let diagnostics = "";
 			if (baseMessage.includes("signature") || baseMessage.includes("thinking")) {
diff --git a/packages/ai/test/bedrock-inference-profile.test.ts b/packages/ai/test/bedrock-inference-profile.test.ts
index 75092e7f4..08314bb84 100644
--- a/packages/ai/test/bedrock-inference-profile.test.ts
+++ b/packages/ai/test/bedrock-inference-profile.test.ts
@@ -150,17 +150,19 @@ describe("Bedrock cross-region inference-profile geo routing", () => {
 });
 
 describe("Bedrock error handling", () => {
-	// Regression (#5539): a non-`Error` thrown inside the stream body where
-	// `JSON.stringify` returns `undefined` (e.g. `undefined`, a function, a
-	// circular object) must not crash the catch block via `baseMessage.includes(...)`.
-	// The stream must close cleanly with an error result instead of an unhandled
-	// `TypeError: undefined is not an object (evaluating 'baseMessage.includes')`.
-	test("surfaces a stream error when a non-Error value is thrown", async () => {
+	const circular: Record<string, unknown> = {};
+	circular.self = circular;
+
+	test.each([
+		["undefined", undefined],
+		["BigInt", 1n],
+		["circular object", circular],
+	])("surfaces a stream error when %s is thrown", async (_name, thrown) => {
 		const result = await streamBedrock(profileModel, userContext(), {
 			bearerToken: "test-token",
 			maxTokens: 16,
 			onPayload: () => {
-				throw undefined;
+				throw thrown;
 			},
 		}).result();