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feat: activate plugin lifecycle hooks (pre/post_llm_call, session start/end) (#3542)

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The plugin system defined six lifecycle hooks but only pre_tool_call and
post_tool_call were invoked.  This activates the remaining four so that
external plugins (e.g. memory systems) can hook into the conversation
loop without touching core code.

Hook semantics:
- on_session_start: fires once when a new session is created
- pre_llm_call: fires once per turn before the tool-calling loop;
  plugins can return {"context": "..."} to inject into the ephemeral
  system prompt (not cached, not persisted)
- post_llm_call: fires once per turn after the loop completes, with
  user_message and assistant_response for sync/storage
- on_session_end: fires at the end of every run_conversation call

invoke_hook() now returns a list of non-None callback return values,
enabling pre_llm_call context injection while remaining backward
compatible (existing hooks that return None are unaffected).

Salvaged from PR #2823.

Co-authored-by: Nicolò Boschi <boschi1997@gmail.com>
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Teknium 2026-03-28 11:14:54 -07:00 • committed by GitHub
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website/docs/guides/build-a-hermes-plugin.md
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@ -365,16 +365,18 @@ def register(ctx):
Available hooks:
| Hook | When | Arguments |
|------|------|-----------|
| `pre_tool_call` | Before any tool runs | `tool_name`, `args`, `task_id` |
| `post_tool_call` | After any tool returns | `tool_name`, `args`, `result`, `task_id` |
| `pre_llm_call` | Before LLM API call | `messages`, `model` |
| `post_llm_call` | After LLM response | `messages`, `response`, `model` |
| `on_session_start` | Session begins | `session_id`, `platform` |
| `on_session_end` | Session ends | `session_id`, `platform` |
| Hook | When | Arguments | Return |
|------|------|-----------|--------|
| `pre_tool_call` | Before any tool runs | `tool_name`, `args`, `task_id` | — |
| `post_tool_call` | After any tool returns | `tool_name`, `args`, `result`, `task_id` | — |
| `pre_llm_call` | Once per turn, before the LLM loop | `session_id`, `user_message`, `conversation_history`, `is_first_turn`, `model`, `platform` | `{"context": "..."}` |
| `post_llm_call` | Once per turn, after the LLM loop | `session_id`, `user_message`, `assistant_response`, `conversation_history`, `model`, `platform` | — |
| `on_session_start` | New session created (first turn only) | `session_id`, `model`, `platform` | — |
| `on_session_end` | End of every `run_conversation` call | `session_id`, `completed`, `interrupted`, `model`, `platform` | — |
Hooks are observers — they can't modify arguments or return values. If a hook crashes, it's logged and skipped; other hooks and the tool continue normally.
Most hooks are fire-and-forget observers. The exception is `pre_llm_call`: if a callback returns a dict with a `"context"` key (or a plain string), the value is appended to the ephemeral system prompt for the current turn. This allows memory plugins to inject recalled context without touching core code.
If a hook crashes, it's logged and skipped; other hooks and the agent continue normally.
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