diff --git a/packages/coding-agent/src/experimental/radius-relay.ts b/packages/coding-agent/src/experimental/radius-relay.ts
index 8daf59974..660428a81 100644
--- a/packages/coding-agent/src/experimental/radius-relay.ts
+++ b/packages/coding-agent/src/experimental/radius-relay.ts
@@ -18,6 +18,11 @@ const HOST_RETRY_MAX_MS = 30_000;
 const MISSING_AUTH_RETRY_MS = 30_000;
 const CLIENT_RETRY_INITIAL_MS = 1_000;
 const CLIENT_RETRY_MAX_MS = 30_000;
+// Undici implements the browser WebSocket API, which permits callers to send
+// only code 1000 or application codes from 3000 through 4999. RFC protocol
+// codes such as 1002 and 1011 may be received but cannot be passed to close().
+const LOCAL_PROTOCOL_ERROR_CLOSE_CODE = 4000;
+const LOCAL_TRANSPORT_ERROR_CLOSE_CODE = 4001;
 
 interface RadiusRelayMessageEvent extends Event {
 	readonly data: unknown;
@@ -193,7 +198,7 @@ export class RadiusRelayHost {
 				try {
 					this.#handleHostMessage(event.data);
 				} catch (error) {
-					socket.close(1002, "Radius relay protocol error");
+					closeWebSocket(socket, LOCAL_PROTOCOL_ERROR_CLOSE_CODE, "Radius relay protocol error");
 					finish(toError(error));
 				}
 			};
@@ -216,9 +221,11 @@ export class RadiusRelayHost {
 			const control = parseHostControlMessage(value);
 			switch (control.type) {
 				case "ping":
-					void this.#sendControl({ version: 1, type: "pong" }).catch((error: unknown) =>
-						this.#socket?.close(1011, errorMessage(error)),
-					);
+					void this.#sendControl({ version: 1, type: "pong" }).catch(() => {
+						if (this.#socket !== undefined) {
+							closeWebSocket(this.#socket, LOCAL_TRANSPORT_ERROR_CLOSE_CODE, "Radius relay send failed");
+						}
+					});
 					return;
 				case "pong":
 					return;
@@ -458,7 +465,7 @@ class RadiusClientByteTransport implements ByteTransport {
 
 	#fail(error: Error): void {
 		if (!this.#markClosed()) return;
-		this.#socket.close(1011, "Radius relay transport error");
+		closeWebSocket(this.#socket, LOCAL_TRANSPORT_ERROR_CLOSE_CODE, "Radius relay transport error");
 		this.#handlers.onError(error);
 	}
 
diff --git a/packages/coding-agent/test/experimental-radius-relay.test.ts b/packages/coding-agent/test/experimental-radius-relay.test.ts
index f47cbe8c6..7cf7bf7cd 100644
--- a/packages/coding-agent/test/experimental-radius-relay.test.ts
+++ b/packages/coding-agent/test/experimental-radius-relay.test.ts
@@ -45,6 +45,9 @@ class FakeWebSocket {
 	}
 
 	close(code = 1000, reason = ""): void {
+		if (code !== 1000 && (code < 3000 || code > 4999)) {
+			throw new DOMException("Invalid close code", "InvalidAccessError");
+		}
 		if (this.readyState === 3) return;
 		this.readyState = 3;
 		this.emit("close", { code, reason });
@@ -69,6 +72,12 @@ class FakeWebSocket {
 		this.emit("error", { error, message: error.message });
 	}
 
+	abnormalClose(error: Error): void {
+		this.readyState = 3;
+		this.emit("error", { error, message: error.message });
+		this.emit("close", { code: 1006, reason: "" });
+	}
+
 	private emit(type: string, event: unknown): void {
 		for (const listener of [...(this.listeners.get(type) ?? [])]) listener(event);
 	}
@@ -260,6 +269,26 @@ describe("experimental Radius relay", () => {
 		expect(onError).not.toHaveBeenCalled();
 	});
 
+	test("reports established abnormal closures so the client can reconnect", async () => {
+		const webSockets = socketFactory();
+		const onClose = vi.fn();
+		const onError = vi.fn();
+		const transportPromise = createRadiusClientTransportFactory({
+			serverId,
+			auth: new RadiusRelayAuthResolver({ type: "token", token: "secret" }),
+			webSocketFactory: webSockets.factory,
+		})({ onData: vi.fn(), onClose, onError });
+		await vi.waitFor(() => expect(webSockets.sockets).toHaveLength(1));
+		const { socket, options } = webSockets.sockets[0]!;
+		socket.open(options.protocol);
+		await transportPromise;
+
+		expect(() => socket.abnormalClose(new Error("network lost"))).not.toThrow();
+		expect(onError).toHaveBeenCalledOnce();
+		expect(onError).toHaveBeenCalledWith(expect.objectContaining({ message: "network lost" }));
+		expect(onClose).not.toHaveBeenCalled();
+	});
+
 	test("reports a useful error when Undici omits WebSocket failure details", async () => {
 		const webSockets = socketFactory();
 		const opening = createRadiusClientTransportFactory({