Count sync subagent spawns against descendant limits

src/tools/delegate-task/sync-task.ts

@@ -27,15 +27,23 @@ export async function executeSyncTask(
   const toastManager = getTaskToastManager()
   let taskId: string | undefined
   let syncSessionID: string | undefined
+  let spawnReservation:
+    | Awaited<ReturnType<ExecutorContext["manager"]["reserveSubagentSpawn"]>>
+    | undefined
 
   try {
-    const spawnContext = typeof manager?.assertCanSpawn === "function"
-      ? await manager.assertCanSpawn(parentContext.sessionID)
-      : {
-          rootSessionID: parentContext.sessionID,
-          parentDepth: 0,
-          childDepth: 1,
-        }
+    if (typeof manager?.reserveSubagentSpawn === "function") {
+      spawnReservation = await manager.reserveSubagentSpawn(parentContext.sessionID)
+    }
+
+    const spawnContext = spawnReservation?.spawnContext
+      ?? (typeof manager?.assertCanSpawn === "function"
+        ? await manager.assertCanSpawn(parentContext.sessionID)
+        : {
+            rootSessionID: parentContext.sessionID,
+            parentDepth: 0,
+            childDepth: 1,
+          })
 
     const createSessionResult = await deps.createSyncSession(client, {
       parentSessionID: parentContext.sessionID,
@@ -45,10 +53,12 @@ export async function executeSyncTask(
     })
 
     if (!createSessionResult.ok) {
+      spawnReservation?.rollback()
       return createSessionResult.error
     }
 
     const sessionID = createSessionResult.sessionID
+    spawnReservation?.commit()
     syncSessionID = sessionID
     subagentSessions.add(sessionID)
     syncSubagentSessions.add(sessionID)
@@ -156,6 +166,7 @@ session_id: ${sessionID}
       }
     }
   } catch (error) {
+    spawnReservation?.rollback()
     return formatDetailedError(error, {
       operation: "Execute task",
       args,