fix(preview): a slow tool-icon status read no longer drops the browser host

After each action the MCP handler reads status with a 500ms budget to
draw the tool's site icon. When the server browser was still settling,
for example after preview_resize, that read timed out and the broker
disconnected the host, so the agent's next call failed with
NoAvailableHost until it reconnected. Background reads that cannot
change the current tab now time out without disconnecting.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: juliusmarminge

apps/server/src/mcp/PreviewAutomationBroker.test.ts

@@ -1529,3 +1529,41 @@ it.effect("keeps a host that responds with an operation timeout", () =>
     }),
   ),
 );
+
+it.effect("keeps the host connected when a background status read times out", () =>
+  Effect.scoped(
+    Effect.gen(function* () {
+      const broker = yield* makeBroker;
+      const requests = requestsFrom(yield* broker.connect(makeHost()));
+      // A busy host answers its actions but not the metadata read behind them.
+      yield* Stream.runForEach(requests, (request) =>
+        request.operation === "status"
+          ? Effect.void
+          : broker.respond({
+              clientId: "client-1",
+              connectionId: request.connectionId,
+              requestId: request.requestId,
+              ok: true,
+              result: { operation: request.operation },
+            }),
+      ).pipe(Effect.forkScoped);
+      yield* Effect.yieldNow;
+
+      const status = yield* broker
+        .invoke<void>({
+          scope,
+          operation: "status",
+          input: {},
+          timeoutMs: 500,
+          updateCurrentTab: false,
+        })
+        .pipe(Effect.flip, Effect.forkScoped);
+      yield* TestClock.adjust(500);
+      expect(yield* Fiber.join(status)).toMatchObject({ _tag: "PreviewAutomationTimeoutError" });
+
+      expect(yield* broker.invoke({ scope, operation: "snapshot", input: {} })).toEqual({
+        operation: "snapshot",
+      });
+    }),
+  ),
+);

apps/server/src/mcp/PreviewAutomationBroker.ts

@@ -648,7 +648,11 @@ export const make = Effect.gen(function* PreviewAutomationBrokerMake() {
           Effect.gen(function* () {
             // An unanswered request invalidates this connection. Do not replay
             // actions: the client may have applied them before becoming unreachable.
-            yield* disconnect(connection.clientId, connection.queue, true);
+            // A background metadata read has a short budget and changes nothing,
+            // so a slow one must not cut the host off from the agent's next call.
+            if (input.updateCurrentTab !== false) {
+              yield* disconnect(connection.clientId, connection.queue, true);
+            }
             return yield* new PreviewAutomationTimeoutError(requestContext);
           }),
         onSome: (value) => Effect.succeed(value as A),