Coordinator: Claude On-Platform

An on-platform coordinator is an ordinary interactive session - the platform launched it, you're attached to its terminal, and its harness happens to be Claude Code. Nothing extra makes it a coordinator; the moment it calls one of the six spawn/lifecycle tools, it is one.

What's automatic

Every interactive session already has its own per-session MCP endpoint (/api/mcp/[sessionId]), and the swarm tool family is served on it unconditionally - the same six tools documented in Spawning sub-agents, with no separate install or configuration step:

• No credential to manage. Off-platform, you present a spawn credential on every call. On-platform, the platform mints a short-lived one from your own session's lineage row automatically, for each call - there is nothing to fetch, refresh, or embed.
• No MCP server to register. The tools are already present in tools/list against your session's own endpoint.
• Identity is your session. watch_session/replay_session/get_session_receipt/steer_child resolve who you are and where you sit in the spawn tree directly from your session's own row - there is no separate registration step comparable to the off-platform requester-registration requirement the external facade has for a2a_* calls.

Hearing back from children

Steering a live child (steer_child) works the same way regardless of where the coordinator itself runs - delivery into a busy child depends on the child's harness, not on whether the coordinator is on- or off-platform. See A2A Delivery Tiers for the honest per-harness breakdown.

For asynchronous mailbox traffic (a child completing, or messaging you unprompted), an on-platform coordinator has the same durable floor as every other caller: poll a2a_inbox, and expect a best-effort heartbeat-cadence nudge when you're live - not an instant push. The richer fallback ladder the Claude Code plugin builds (channel push, a background monitor, a Stop-hook drain check) is a client-side construct for the off-platform Claude Code CLI's own hook and plugin system running on your machine; an on-platform session does not wire an equivalent automatically today. If you want that same ladder inside a platform-launched session, you would need to set it up inside that session's own workspace the same way you would locally - this is not something Rensei verifies or documents as a supported path yet.

Related pages

• Spawning sub-agents - the tool vocabulary and authority-ceiling model this page assumes
• Claude, off-platform (plugin) - the richer fallback ladder available outside the platform
• Interactive Sessions - how the session itself gets launched and attached
• A2A Delivery Tiers - what "delivered" means for steer_child and the mailbox