diff --git a/src/vs/platform/agentHost/common/state/sessionProtocol.ts b/src/vs/platform/agentHost/common/state/sessionProtocol.ts
index ebbad8027ad..9a2380278b2 100644
--- a/src/vs/platform/agentHost/common/state/sessionProtocol.ts
+++ b/src/vs/platform/agentHost/common/state/sessionProtocol.ts
@@ -76,6 +76,7 @@ export function isJsonRpcResponse(msg: IProtocolMessage): msg is IJsonRpcRespons
 
 // ---- JSON-RPC error codes ---------------------------------------------------
 
+export const JSON_RPC_PARSE_ERROR = -32700;
 export const JSON_RPC_INTERNAL_ERROR = -32603;
 
 // ---- Shared data types ------------------------------------------------------
diff --git a/src/vs/platform/agentHost/node/webSocketTransport.ts b/src/vs/platform/agentHost/node/webSocketTransport.ts
index 8c53abdab01..3301d87c568 100644
--- a/src/vs/platform/agentHost/node/webSocketTransport.ts
+++ b/src/vs/platform/agentHost/node/webSocketTransport.ts
@@ -10,7 +10,7 @@ import { Emitter } from '../../../base/common/event.js';
 import { Disposable } from '../../../base/common/lifecycle.js';
 import { connectionTokenQueryName } from '../../../base/common/network.js';
 import { ILogService } from '../../log/common/log.js';
-import type { IProtocolMessage } from '../common/state/sessionProtocol.js';
+import { JSON_RPC_PARSE_ERROR, type IProtocolMessage } from '../common/state/sessionProtocol.js';
 import type { IProtocolServer, IProtocolTransport } from '../common/state/sessionTransport.js';
 import { protocolReplacer, protocolReviver } from '../common/state/jsonSerialization.js';
 
@@ -58,7 +58,7 @@ export class WebSocketProtocolTransport extends Disposable implements IProtocolT
 				const message = JSON.parse(text, protocolReviver) as IProtocolMessage;
 				this._onMessage.fire(message);
 			} catch {
-				// Malformed message — drop. No logger available at transport level.
+				this.send({ jsonrpc: '2.0', id: null!, error: { code: JSON_RPC_PARSE_ERROR, message: 'Parse error' } });
 			}
 		});
 
diff --git a/src/vs/platform/agentHost/test/node/protocolWebSocket.integrationTest.ts b/src/vs/platform/agentHost/test/node/protocolWebSocket.integrationTest.ts
index 0f411e14a7e..367a0369f1a 100644
--- a/src/vs/platform/agentHost/test/node/protocolWebSocket.integrationTest.ts
+++ b/src/vs/platform/agentHost/test/node/protocolWebSocket.integrationTest.ts
@@ -13,6 +13,7 @@ import { protocolReplacer, protocolReviver } from '../../common/state/jsonSerial
 import {
 	isJsonRpcNotification,
 	isJsonRpcResponse,
+	JSON_RPC_PARSE_ERROR,
 	type IActionBroadcastParams,
 	type IFetchTurnsResult,
 	type IJsonRpcErrorResponse,
@@ -140,6 +141,31 @@ class TestProtocolClient {
 		return predicate ? this._notifications.filter(predicate) : [...this._notifications];
 	}
 
+	/** Send a raw string over the WebSocket without JSON serialization. */
+	sendRaw(data: string): void {
+		this._ws.send(data);
+	}
+
+	/** Wait for the next raw message from the server. */
+	waitForRawMessage(timeoutMs = 5000): Promise<unknown> {
+		return new Promise((resolve, reject) => {
+			const timer = setTimeout(() => {
+				cleanup();
+				reject(new Error(`Timeout waiting for raw message (${timeoutMs}ms)`));
+			}, timeoutMs);
+			const onMsg = (data: Buffer | string) => {
+				cleanup();
+				const text = typeof data === 'string' ? data : data.toString('utf-8');
+				resolve(JSON.parse(text));
+			};
+			const cleanup = () => {
+				clearTimeout(timer);
+				this._ws.removeListener('message', onMsg);
+			};
+			this._ws.on('message', onMsg);
+		});
+	}
+
 	close(): void {
 		for (const w of this._notifWaiters) {
 			w.reject(new Error('Client closed'));
@@ -636,4 +662,21 @@ suite('Protocol WebSocket E2E', function () {
 		const state = snapshot.state as ISessionState;
 		assert.strictEqual(state.summary.model, 'new-mock-model');
 	});
+
+	test('malformed JSON message returns parse error', async function () {
+		this.timeout(10_000);
+
+		const raw = new TestProtocolClient(server.port);
+		await raw.connect();
+
+		const responsePromise = raw.waitForRawMessage();
+		raw.sendRaw('this is not valid json{{{');
+
+		const response = await responsePromise as IJsonRpcErrorResponse;
+		assert.strictEqual(response.jsonrpc, '2.0');
+		assert.strictEqual(response.id, null);
+		assert.strictEqual(response.error.code, JSON_RPC_PARSE_ERROR);
+
+		raw.close();
+	});
 });