highlight active subagents

api/openapi.yaml

@@ -1409,7 +1409,7 @@ components:
 
     AgentSession:
       type: object
-      required: [id, task_id, owner, vendor, model, vendor_session_id, state, role, created_by, created_at]
+      required: [id, task_id, owner, vendor, model, vendor_session_id, state, role, created_by, created_at, subagents]
       properties:
         id: { type: string }
         task_id: { type: string, description: Empty for a lead session. }
@@ -1426,6 +1426,7 @@ components:
         created_at: { type: string, format: date-time }
         forked_from: { type: string, description: "A fork's first start resumes this vendor session, cut at forked_at, under a new vendor_session_id; absent for other sessions." }
         forked_at: { type: string, description: "With forked_from: the vendor's id of the last thing the fork keeps, a message for Claude Code, a turn for Codex." }
+        subagents: { type: integer, description: "How many of the agent's subagents are running, background ones included after the turn that started them has ended." }
     SessionCreate:
       type: object
       required: [prompt]