fix(browser): return JSON-RPC errors for malformed CDP client frames in relay bridge

krissding · claude · krissding · commit bfe6dd375130 · 2026-07-08T14:38:44.000+08:00
Silently dropping malformed JSON or invalid CDP request objects leaves
automation clients waiting for their own timeout. Return standard
JSON-RPC parse error (-32700) and invalid-request error (-32600) at the
CDP client socket boundary, matching the precedent in gateway MCP HTTP.

Co-Authored-By: Claude Sonnet 4.6 &lt;noreply@anthropic.com&gt;
diff --git a/extensions/browser/src/browser/extension-relay/relay-bridge.test.ts b/extensions/browser/src/browser/extension-relay/relay-bridge.test.ts
@@ -368,4 +368,44 @@ describe("ExtensionRelayBridge", () => {
     expect(socket.closed).toBe(true);
     expect(bridge.extensionConnected).toBe(false);
   });
+
+  it("responds with JSON-RPC parse error on malformed JSON from CDP client", () => {
+    const bridge = new ExtensionRelayBridge();
+    const client = new FakeSocket();
+    const cdp = bridge.attachCdpClientSocket(client);
+    cdp.onMessage("{");
+    const frames = client.frames();
+    expect(frames).toHaveLength(1);
+    expect(frames[0]).toMatchObject({
+      id: null,
+      error: { code: -32700, message: "Parse error" },
+    });
+  });
+
+  it("responds with JSON-RPC invalid request error when CDP client frame lacks id or method", () => {
+    const bridge = new ExtensionRelayBridge();
+    const client = new FakeSocket();
+    const cdp = bridge.attachCdpClientSocket(client);
+    // Missing id and method
+    cdp.onMessage(JSON.stringify({ params: {} }));
+    const frames = client.frames();
+    expect(frames).toHaveLength(1);
+    expect(frames[0]).toMatchObject({
+      id: null,
+      error: { code: -32600, message: "Invalid Request" },
+    });
+  });
+
+  it("responds with invalid request error when CDP client frame has id but no method", () => {
+    const bridge = new ExtensionRelayBridge();
+    const client = new FakeSocket();
+    const cdp = bridge.attachCdpClientSocket(client);
+    cdp.onMessage(JSON.stringify({ id: 7 }));
+    const frames = client.frames();
+    expect(frames).toHaveLength(1);
+    expect(frames[0]).toMatchObject({
+      id: 7,
+      error: { code: -32600, message: "Invalid Request" },
+    });
+  });
 });
diff --git a/extensions/browser/src/browser/extension-relay/relay-bridge.ts b/extensions/browser/src/browser/extension-relay/relay-bridge.ts
@@ -74,7 +74,12 @@ type ExtensionIdentity = {
   extensionVersion: string;
 };
 
-function toErrorPayload(id: number, sessionId: string | undefined, message: string, code = -32000) {
+function toErrorPayload(
+  id: number | null,
+  sessionId: string | undefined,
+  message: string,
+  code = -32000,
+) {
   return JSON.stringify({ id, ...(sessionId ? { sessionId } : {}), error: { code, message } });
 }
 
@@ -498,9 +503,18 @@ export class ExtensionRelayBridge {
       try {
         request = JSON.parse(raw) as CdpRequest;
       } catch {
+        client.socket.send(toErrorPayload(null, undefined, "Parse error", -32700));
         return;
       }
       if (typeof request?.id !== "number" || typeof request?.method !== "string") {
+        client.socket.send(
+          toErrorPayload(
+            typeof request?.id === "number" ? request.id : null,
+            undefined,
+            "Invalid Request",
+            -32600,
+          ),
+        );
         return;
       }
       void this.handleCdpRequest(client, request);
