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Teknium 455bf2e853 feat: activate plugin lifecycle hooks (pre/post_llm_call, session start/end) (#3542 )

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>

2026-03-28 11:14:54 -07:00

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Plugins

Hermes has a plugin system for adding custom tools, hooks, slash commands, and integrations without modifying core code.

→ Build a Hermes Plugin — step-by-step guide with a complete working example.

Quick overview

Drop a directory into ~/.hermes/plugins/ with a plugin.yaml and Python code:

~/.hermes/plugins/my-plugin/
├── plugin.yaml      # manifest
├── __init__.py      # register() — wires schemas to handlers
├── schemas.py       # tool schemas (what the LLM sees)
└── tools.py         # tool handlers (what runs when called)

Start Hermes — your tools appear alongside built-in tools. The model can call them immediately.

Project-local plugins under ./.hermes/plugins/ are disabled by default. Enable them only for trusted repositories by setting HERMES_ENABLE_PROJECT_PLUGINS=true before starting Hermes.

What plugins can do

Capability	How
Add tools	`ctx.register_tool(name, schema, handler)`
Add hooks	`ctx.register_hook("post_tool_call", callback)`
Add slash commands	`ctx.register_command("mycommand", handler)`
Ship data files	`Path(__file__).parent / "data" / "file.yaml"`
Bundle skills	Copy `skill.md` to `~/.hermes/skills/` at load time
Gate on env vars	`requires_env: [API_KEY]` in plugin.yaml
Distribute via pip	`[project.entry-points."hermes_agent.plugins"]`

Plugin discovery

Source	Path	Use case
User	`~/.hermes/plugins/`	Personal plugins
Project	`.hermes/plugins/`	Project-specific plugins (requires `HERMES_ENABLE_PROJECT_PLUGINS=true`)
pip	`hermes_agent.plugins` entry_points	Distributed packages

Available hooks

Plugins can register callbacks for these lifecycle events. See the Event Hooks page for full details, callback signatures, and examples.

Hook	Fires when
`pre_tool_call`	Before any tool executes
`post_tool_call`	After any tool returns
`pre_llm_call`	Once per turn, before the LLM loop — can return `{"context": "..."}` to inject into the system prompt
`post_llm_call`	Once per turn, after the LLM loop completes
`on_session_start`	New session created (first turn only)
`on_session_end`	End of every `run_conversation` call

Slash commands

Plugins can register slash commands that work in both CLI and messaging platforms:

def register(ctx):
    ctx.register_command(
        name="greet",
        handler=lambda args: f"Hello, {args or 'world'}!",
        description="Greet someone",
        args_hint="[name]",
        aliases=("hi",),
    )

The handler receives the argument string (everything after /greet) and returns a string to display. Registered commands automatically appear in /help, tab autocomplete, Telegram bot menu, and Slack subcommand mapping.

Parameter	Description
`name`	Command name without slash
`handler`	Callable that takes `args: str` and returns `str
`description`	Shown in `/help`
`args_hint`	Usage hint, e.g. `"[name]"`
`aliases`	Tuple of alternative names
`cli_only`	Only available in CLI
`gateway_only`	Only available in messaging platforms
`gateway_config_gate`	Config dotpath (e.g. `"display.my_option"`). When set on a `cli_only` command, the command becomes available in the gateway if the config value is truthy.

Managing plugins

/plugins              # list loaded plugins in a session
hermes config set display.show_cost true  # show cost in status bar

See the full guide for handler contracts, schema format, hook behavior, error handling, and common mistakes.

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