fix(tui): gate background shortcut by capability (#32837)

packages/tui/src/routes/session/index.tsx

@@ -206,15 +206,17 @@ export function Session() {
   })
   const messages = createMemo(() => sync.data.message[route.sessionID] ?? [])
   const foregroundTasks = createMemo(() =>
-    messages().flatMap((message) =>
-      (sync.data.part[message.id] ?? []).filter(
-        (part): part is ToolPart =>
-          part.type === "tool" &&
-          part.tool === "task" &&
-          part.state.status === "running" &&
-          part.state.metadata?.background !== true,
-      ),
-    ),
+    sync.data.capabilities.experimentalBackgroundSubagents
+      ? messages().flatMap((message) =>
+          (sync.data.part[message.id] ?? []).filter(
+            (part): part is ToolPart =>
+              part.type === "tool" &&
+              part.tool === "task" &&
+              part.state.status === "running" &&
+              part.state.metadata?.background !== true,
+          ),
+        )
+      : [],
   )
   const userMessageIDs = createMemo(
     () =>
@@ -1510,13 +1512,16 @@ function AssistantMessage(props: { message: AssistantMessage; parts: Part[]; las
             {childShortcut()}
             <span style={{ fg: theme.textMuted }}> view subagents</span>
             <Show
-              when={props.parts.some(
-                (x) =>
-                  x.type === "tool" &&
-                  x.tool === "task" &&
-                  x.state.status === "running" &&
-                  x.state.metadata?.background !== true,
-              )}
+              when={
+                sync.data.capabilities.experimentalBackgroundSubagents &&
+                props.parts.some(
+                  (x) =>
+                    x.type === "tool" &&
+                    x.tool === "task" &&
+                    x.state.status === "running" &&
+                    x.state.metadata?.background !== true,
+                )
+              }
             >
               <span style={{ fg: theme.textMuted }}> · </span>
               {backgroundShortcut()}