diff --git a/extensions/qa-channel/src/bus-client.test.ts b/extensions/qa-channel/src/bus-client.test.ts
new file mode 100644
index 00000000000..2c7237c83cf
--- /dev/null
+++ b/extensions/qa-channel/src/bus-client.test.ts
@@ -0,0 +1,56 @@
+import { createServer } from "node:http";
+import { afterEach, describe, expect, it } from "vitest";
+import { pollQaBus } from "./bus-client.js";
+
+async function startJsonServer(handler: () => { statusCode?: number; body: string }) {
+  const server = createServer((_req, res) => {
+    const response = handler();
+    res.writeHead(response.statusCode ?? 200, {
+      "content-type": "application/json; charset=utf-8",
+    });
+    res.end(response.body);
+  });
+
+  await new Promise<void>((resolve, reject) => {
+    server.once("error", reject);
+    server.listen(0, "127.0.0.1", () => resolve());
+  });
+
+  const address = server.address();
+  if (!address || typeof address === "string") {
+    throw new Error("test server failed to bind");
+  }
+
+  return {
+    baseUrl: `http://127.0.0.1:${address.port}`,
+    async stop() {
+      await new Promise<void>((resolve, reject) => {
+        server.close((error) => (error ? reject(error) : resolve()));
+      });
+    },
+  };
+}
+
+describe("qa-bus client", () => {
+  const stops: Array<() => Promise<void>> = [];
+
+  afterEach(async () => {
+    await Promise.all(stops.splice(0).map((stop) => stop()));
+  });
+
+  it("rejects malformed JSON responses instead of throwing from the stream callback", async () => {
+    const server = await startJsonServer(() => ({
+      body: '{"cursor":1,"events":[',
+    }));
+    stops.push(server.stop);
+
+    await expect(
+      pollQaBus({
+        baseUrl: server.baseUrl,
+        accountId: "acct-a",
+        cursor: 0,
+        timeoutMs: 0,
+      }),
+    ).rejects.toThrow(SyntaxError);
+  });
+});
diff --git a/extensions/qa-channel/src/bus-client.ts b/extensions/qa-channel/src/bus-client.ts
index 7229e749c97..3c7c29b2126 100644
--- a/extensions/qa-channel/src/bus-client.ts
+++ b/extensions/qa-channel/src/bus-client.ts
@@ -69,7 +69,13 @@ async function postJson<T>(
         });
         response.on("end", () => {
           const text = Buffer.concat(chunks).toString("utf8");
-          const parsed = text ? (JSON.parse(text) as T | { error?: string }) : ({} as T);
+          let parsed: T | { error?: string };
+          try {
+            parsed = text ? (JSON.parse(text) as T | { error?: string }) : ({} as T);
+          } catch (error) {
+            reject(error);
+            return;
+          }
           if ((response.statusCode ?? 500) < 200 || (response.statusCode ?? 500) >= 300) {
             const error =
               typeof parsed === "object" && parsed && "error" in parsed ? parsed.error : undefined;
diff --git a/extensions/qa-lab/src/bus-server.ts b/extensions/qa-lab/src/bus-server.ts
index 6a35d864890..f4fd7da3296 100644
--- a/extensions/qa-lab/src/bus-server.ts
+++ b/extensions/qa-lab/src/bus-server.ts
@@ -140,7 +140,11 @@ export async function handleQaBusRequest(params: {
           return true;
         }
         try {
-          await params.state.waitForCursorAdvance(input.cursor ?? 0, timeoutMs);
+          await params.state.waitForCursorAdvance(input.cursor ?? 0, timeoutMs, (snapshot) => {
+            return snapshot.events.some(
+              (event) => event.accountId === input.accountId && event.cursor > (input.cursor ?? 0),
+            );
+          });
         } catch {
           // timeout ok for long-poll
         }
diff --git a/extensions/qa-lab/src/bus-state.test.ts b/extensions/qa-lab/src/bus-state.test.ts
index 0060cc350d0..e0c4a065a8b 100644
--- a/extensions/qa-lab/src/bus-state.test.ts
+++ b/extensions/qa-lab/src/bus-state.test.ts
@@ -92,6 +92,35 @@ describe("qa-bus state", () => {
     ).rejects.toThrow("qa-bus wait timeout");
   });
 
+  it("keeps account-scoped cursor waits blocked on unrelated account traffic", async () => {
+    const state = createQaBusState();
+    const pending = state.waitForCursorAdvance(0, 500, (snapshot) => {
+      return snapshot.events.some((event) => event.accountId === "acct-a" && event.cursor > 0);
+    });
+
+    state.addInboundMessage({
+      accountId: "acct-b",
+      conversation: { id: "other", kind: "direct" },
+      senderId: "acct-b-user",
+      text: "unrelated",
+    });
+
+    const beforeMatch = await Promise.race([
+      pending.then(() => "resolved"),
+      new Promise((resolve) => setTimeout(() => resolve("still-waiting"), 20)),
+    ]);
+    expect(beforeMatch).toBe("still-waiting");
+
+    state.addInboundMessage({
+      accountId: "acct-a",
+      conversation: { id: "target", kind: "direct" },
+      senderId: "acct-a-user",
+      text: "matched",
+    });
+
+    await expect(pending).resolves.toBeUndefined();
+  });
+
   it("preserves inline attachments and lets search match attachment metadata", () => {
     const state = createQaBusState();
 
diff --git a/extensions/qa-lab/src/bus-state.ts b/extensions/qa-lab/src/bus-state.ts
index 3ed8724fe69..f8c867631fb 100644
--- a/extensions/qa-lab/src/bus-state.ts
+++ b/extensions/qa-lab/src/bus-state.ts
@@ -23,6 +23,7 @@ import type {
   QaBusReadMessageInput,
   QaBusReactToMessageInput,
   QaBusSearchMessagesInput,
+  QaBusStateSnapshot,
   QaBusThread,
   QaBusWaitForInput,
 } from "./runtime-api.js";
@@ -282,8 +283,12 @@ export function createQaBusState() {
     async waitFor(input: QaBusWaitForInput) {
       return await waiters.waitFor(input);
     },
-    async waitForCursorAdvance(afterCursor: number, timeoutMs: number) {
-      return await waiters.waitForCursorAdvance(afterCursor, timeoutMs);
+    async waitForCursorAdvance(
+      afterCursor: number,
+      timeoutMs: number,
+      shouldResolve?: (snapshot: QaBusStateSnapshot) => boolean,
+    ) {
+      return await waiters.waitForCursorAdvance(afterCursor, timeoutMs, shouldResolve);
     },
   };
 }
diff --git a/extensions/qa-lab/src/bus-waiters.ts b/extensions/qa-lab/src/bus-waiters.ts
index 7a37222aa07..68174276c08 100644
--- a/extensions/qa-lab/src/bus-waiters.ts
+++ b/extensions/qa-lab/src/bus-waiters.ts
@@ -22,6 +22,7 @@ type CursorWaiter = {
   reject: (error: Error) => void;
   timer: NodeJS.Timeout;
   afterCursor: number;
+  shouldResolve?: (snapshot: QaBusStateSnapshot) => boolean;
 };
 
 function createQaBusMatcher(
@@ -79,6 +80,9 @@ export function createQaBusWaiterStore(getSnapshot: () => QaBusStateSnapshot) {
         if (snapshot.cursor <= waiter.afterCursor) {
           continue;
         }
+        if (waiter.shouldResolve && !waiter.shouldResolve(snapshot)) {
+          continue;
+        }
         clearTimeout(waiter.timer);
         cursorWaiters.delete(waiter);
         waiter.resolve();
@@ -104,8 +108,13 @@ export function createQaBusWaiterStore(getSnapshot: () => QaBusStateSnapshot) {
         waiters.add(waiter);
       });
     },
-    async waitForCursorAdvance(afterCursor: number, timeoutMs: number) {
-      if (getSnapshot().cursor > afterCursor) {
+    async waitForCursorAdvance(
+      afterCursor: number,
+      timeoutMs: number,
+      shouldResolve?: (snapshot: QaBusStateSnapshot) => boolean,
+    ) {
+      const snapshot = getSnapshot();
+      if (snapshot.cursor > afterCursor && (!shouldResolve || shouldResolve(snapshot))) {
         return;
       }
       return await new Promise<void>((resolve, reject) => {
@@ -113,6 +122,7 @@ export function createQaBusWaiterStore(getSnapshot: () => QaBusStateSnapshot) {
           resolve,
           reject,
           afterCursor,
+          shouldResolve,
           timer: setTimeout(() => {
             cursorWaiters.delete(waiter);
             reject(new Error(`qa-bus wait timeout after ${timeoutMs}ms`));