diff --git a/packages/ai/src/api/anthropic-messages.ts b/packages/ai/src/api/anthropic-messages.ts
index a6fd502bd..ba25b59a3 100644
--- a/packages/ai/src/api/anthropic-messages.ts
+++ b/packages/ai/src/api/anthropic-messages.ts
@@ -390,9 +390,10 @@ function getThreadErrorCode(error: InstanceType<typeof Anthropic.APIError>): str
 }
 
 /**
- * Sends a thread request. A rejection (never a started stream) is resent once in full: as a new
- * thread when the old one can't be continued, otherwise without `thread`, which also turns threads
- * off for the session. Returns the response and the thread request that produced it, if any.
+ * Sends a thread request. A rejection about the thread (never a started stream) is resent once in
+ * full: as a new thread when the old one can't be continued, or without `thread` when the endpoint
+ * can't use threads, which also turns threads off for the session. Other errors are passed on.
+ * Returns the response and the thread request that produced it, if any.
  */
 async function sendThreadRequest(
 	thread: PreparedThreadRequest,
@@ -404,17 +405,18 @@ async function sendThreadRequest(
 	try {
 		return { response: await send(thread.body), thread };
 	} catch (error) {
-		// Rate limits and auth errors are not about the thread; strict tool errors go to the
-		// strict tool fallback in the caller.
-		if (
-			signal?.aborted ||
-			!(error instanceof Anthropic.APIError) ||
-			(error.status !== 400 && error.status !== 404) ||
-			isStrictToolLimitErrorMessage(error.message)
-		) {
-			throw error;
-		}
+		if (signal?.aborted || !(error instanceof Anthropic.APIError)) throw error;
 		const errorCode = getThreadErrorCode(error);
+		const startNewThread = errorCode !== undefined && NEW_THREAD_ERRORS.has(errorCode);
+		// The endpoint can't use threads: it can't thread this request, or it doesn't know the
+		// `thread` field and rejects it without a code ("thread: Extra inputs are not permitted"),
+		// e.g. with an `anthropic-beta` header that lacks the threads beta.
+		const threadsUnsupported =
+			errorCode === "thread_unsupported_request" ||
+			(errorCode === undefined && error.status === 400 && error.message.includes("thread"));
+		// Any other error (prompt too long, unknown model, strict tool limits, ...) is not about the
+		// thread, and resending would only fail again.
+		if (!startNewThread && !threadsUnsupported) throw error;
 		// Recorded before the resend, so a failed resend keeps it.
 		appendAssistantMessageDiagnostic(output, {
 			type: "anthropic_thread_fallback",
@@ -422,12 +424,11 @@ async function sendThreadRequest(
 			error: { message: error.message },
 			details: { reason: errorCode ?? String(error.status) },
 		});
-		if (errorCode !== undefined && NEW_THREAD_ERRORS.has(errorCode)) {
+		if (startNewThread) {
 			const created = createThreadRequest(thread.sessionId, params);
 			return { response: await send(created.body), thread: created };
 		}
-		// The endpoint rejects something about threads for this session. With the threads beta, a
-		// request without `thread` is a normal request.
+		// With the threads beta, a request without `thread` is a normal request.
 		const response = await send(params);
 		setThreadState(thread.sessionId, "off");
 		return { response, thread: undefined };
diff --git a/packages/ai/test/anthropic-message-threads.test.ts b/packages/ai/test/anthropic-message-threads.test.ts
index d9bc67f98..569caf298 100644
--- a/packages/ai/test/anthropic-message-threads.test.ts
+++ b/packages/ai/test/anthropic-message-threads.test.ts
@@ -382,14 +382,14 @@ describe("Anthropic message threads", () => {
 	});
 
 	it.each([
-		["thread_unsupported_request", "thread_unsupported_request"],
-		["a 400 without error code", undefined],
-	])("turns threads off for the session after %s on continue", async (_name, errorCode) => {
+		["thread_unsupported_request", "thread_unsupported_request", "rejected"],
+		["a 400 without error code about the thread field", undefined, "thread: Extra inputs are not permitted"],
+	])("turns threads off for the session after %s on continue", async (_name, errorCode, message) => {
 		const h = harness();
 		const messages = [user("first")];
 		await say(h, messages);
 		messages.push(user("second"));
-		h.queue.push(() => errorResponse(400, "rejected", errorCode));
+		h.queue.push(() => errorResponse(400, message, errorCode));
 		const result = await say(h, messages);
 
 		expect(result.stopReason).toBe("stop");
@@ -411,7 +411,7 @@ describe("Anthropic message threads", () => {
 		await say(h, messages);
 		messages.push(user("second"));
 		h.queue.push(
-			() => errorResponse(400, "rejected"),
+			() => errorResponse(400, "thread: Extra inputs are not permitted"),
 			() => errorResponse(400, "Real problem"),
 		);
 		const failed = await turn(h, messages);
@@ -423,6 +423,26 @@ describe("Anthropic message threads", () => {
 		expect(threadOf(h.requests[3])).toEqual({ type: "create" });
 	});
 
+	it.each([
+		[400, "prompt is too long: 250000 tokens > 200000 maximum"],
+		[404, "model: claude-test"],
+	])("passes a %s that is not about the thread on without resending", async (status, message) => {
+		const h = harness();
+		const messages = [user("first")];
+		await say(h, messages);
+		messages.push(user("second"));
+		h.queue.push(() => errorResponse(status, message));
+		const failed = await turn(h, messages);
+		expect(failed.stopReason).toBe("error");
+		expect(failed.errorMessage).toContain(message);
+		expect(h.requests).toHaveLength(2);
+		expect(failed.diagnostics).toBeUndefined();
+
+		// Threads stay on: after a fix such as compaction, the next request starts a new thread.
+		await turn(h, messages);
+		expect(threadOf(h.requests[2])).toEqual({ type: "create" });
+	});
+
 	it("leaves strict tool rejections to the strict tool fallback", async () => {
 		const model: Model<"anthropic-messages"> = { ...baseModel, compat: { supportsStrictTools: true } };
 		const strictTool: Tool = { ...tool("lookup"), constrainedSampling: { type: "json_schema", strict: "prefer" } };