fix(server): keep browser automation available without a tab

apps/server/src/mcp/toolkits/preview/tools.ts

@@ -52,7 +52,7 @@ const readonlyBrowserTool = <T extends Tool.Any>(tool: T): T =>
 
 const PreviewStatusTool = Tool.make("preview_status", {
   description:
-    "Report preview tab state: owner, pending dialog, URL, title, loading, viewport, and size. `tabs` lists this thread's tabs. `profiles` lists logins for `preview_open`. Read any listed tab. Act only on your tab, or an unclaimed tab with no human in control.",
+    "Report tab state: owner, dialog, URL, title, loading and viewport. No tab (`tabId:null`): try `preview_open`. `tabs`: thread tabs; `profiles`: logins. Read any tab; act on yours or unclaimed tabs without human control.",
   parameters: PreviewAutomationTabTargetInput,
   success: PreviewAutomationStatus,
   failure: PreviewToolFailure,

apps/server/src/preview/ServerBrowser.test.ts

@@ -2035,6 +2035,39 @@ it.live("agents read a human's tab with no arguments and act on it only while no
   ).pipe(Effect.provide(layer)),
 );
 
+it.live("browser automation stays available without an open tab", () =>
+  Effect.scoped(
+    Effect.gen(function* () {
+      const { broker, tabId } = yield* ready;
+      const emptyScope = {
+        ...scope,
+        thread: { ...scope.thread, threadId: ThreadId.make("empty-browser-thread") },
+      };
+      const empty = yield* broker.invoke<PreviewAutomationStatus>({
+        scope: emptyScope,
+        operation: "status",
+        input: {},
+      });
+      expect(empty).toMatchObject({ available: true, visible: false, tabId: null });
+      yield* broker.invoke({ scope, tabId, operation: "close", input: {} });
+      const closed = yield* broker.invoke<PreviewAutomationStatus>({
+        scope,
+        operation: "status",
+        input: {},
+      });
+      expect(closed).toMatchObject({ available: true, visible: false, tabId: null });
+      expect(contexts).toHaveLength(1);
+      const reopened = yield* broker.invoke<PreviewAutomationStatus>({
+        scope,
+        operation: "open",
+        input: { show: false },
+      });
+      expect(reopened.available).toBe(true);
+      expect(reopened.tabId).not.toBeNull();
+    }),
+  ).pipe(Effect.provide(layer)),
+);
+
 it.live("a session's own tab stays its default while other sessions' tabs are listed", () =>
   Effect.scoped(
     Effect.gen(function* () {

apps/server/src/preview/ServerBrowser.ts

@@ -1577,7 +1577,7 @@ const make = Effect.gen(function* () {
   ): Promise<PreviewAutomationStatus> => {
     if (!tab) {
       return {
-        available: false,
+        available: true,
         visible: false,
         tabId: null,
         url: null,

apps/server/src/provider/agentContextBudget.json

@@ -1,5 +1,5 @@
 {
   "claudeSystemAppendBytes": 5133,
   "codexAdditionalContextBytes": 6863,
-  "mcpToolListJsonBytes": 47498
+  "mcpToolListJsonBytes": 47461
 }