Problem

Plugins that hold per-turn external resources have no way to know when the user interrupts the run with /stop. The existing on_session_finalize hook covers /new (slow path) but is not fired by:

• /stop — see gateway/run.py::_interrupt_and_clear_session (called from _handle_stop_command)
• The /new fast-path inside _dispatch_message (running-agent guard at gateway/run.py:6122) — interrupts the agent before _handle_reset_command runs

For these paths the plugin's outbound work just sits there until it times out, while the agent has already moved on.

Concrete use case

hermes-livekit v0.3.0 lets clients connected over a LiveKit data channel register tools that the agent can invoke (notifications, browser actions, robotic-arm controls, etc.). When the LLM picks one of those tools, the plugin's proxy coroutine awaits a result future with a 30s timeout:

return await asyncio.wait_for(future, timeout=TOOL_CALL_TIMEOUT)

If the user runs /stop mid-call, the agent loop's interrupt flag flips but the proxy keeps waiting — the client stays in "we owe a result" mode for the full timeout window, and the agent:tool-call-cancelled notification we'd want to publish to the client never goes out.

The same shape applies to any plugin that proxies a tool call to an external service (MCP-style outbound RPC, remote-fn-call gateways, etc.).

Proposed solution

Add an agent_loop_stopped plugin hook fired from _interrupt_and_clear_session right after running_agent.interrupt(). Kwargs:

• session_key (str) — which session was interrupted
• platform (str) — source.platform.value
• reason (str) — interrupt_reason (e.g. \"user_stop\", \"reset\")
• invalidation_reason (str) — finer-grained tag (\"stop_command\", \"new_command\", \"stop_command_pending\", \"stop_command_handler\")

Dispatch wrapped in try/except so a misbehaving plugin can't block the interrupt. Symmetric with how on_session_finalize is fired today.

Why a new hook, not a reused one

• on_session_finalize is per-session; fires only when the session is finalized. /stop doesn't finalize the session, and the /new fast-path interrupts before finalize runs.
• subagent_stop fires per-delegation, not per-turn.
• The async gateway hooks (session:end, session:reset) cover /new only, and are file-based hooks, not plugin-registered ones.

No existing hook gives plugins a "the current turn just got abandoned, drop your per-turn external work" signal.

PR

PR: #27208 PR adds the dispatch, registers agent_loop_stopped in VALID_HOOKS with docstring comments, and includes a 3-case pytest covering hook presence, dispatch-on-interrupt, and fault tolerance when a handler raises.

Tested on

• macOS 25.4.1 (Darwin)
• Python 3.11
• Hermes v0.14.0 / commit 3b39096 (current main as of 2026-05-16)