hermes-agent/website/docs/user-guide/features/plugins.md

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11	Plugins	Plugins	Extend Hermes with custom tools, hooks, and integrations via the plugin system

Plugins

Hermes has a plugin system for adding custom tools, hooks, 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.

Minimal working example

Here is a complete plugin that adds a hello_world tool and logs every tool call via a hook.

~/.hermes/plugins/hello-world/plugin.yaml

name: hello-world
version: "1.0"
description: A minimal example plugin

~/.hermes/plugins/hello-world/__init__.py

"""Minimal Hermes plugin — registers a tool and a hook."""


def register(ctx):
    # --- Tool: hello_world ---
    schema = {
        "name": "hello_world",
        "description": "Returns a friendly greeting for the given name.",
        "parameters": {
            "type": "object",
            "properties": {
                "name": {
                    "type": "string",
                    "description": "Name to greet",
                }
            },
            "required": ["name"],
        },
    }

    def handle_hello(params):
        name = params.get("name", "World")
        return f"Hello, {name}! 👋  (from the hello-world plugin)"

    ctx.register_tool("hello_world", schema, handle_hello)

    # --- Hook: log every tool call ---
    def on_tool_call(tool_name, params, result):
        print(f"[hello-world] tool called: {tool_name}")

    ctx.register_hook("post_tool_call", on_tool_call)

Drop both files into ~/.hermes/plugins/hello-world/, restart Hermes, and the model can immediately call hello_world. The hook prints a log line after every tool invocation.

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 CLI commands	ctx.register_cli_command(name, help, setup_fn, handler_fn) — adds hermes <plugin> <subcommand>
Inject messages	ctx.inject_message(content, role="user") — see Injecting Messages
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 — prompted during hermes plugins install
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 context into the user message
post_llm_call	Once per turn, after the LLM loop (successful turns only)
on_session_start	New session created (first turn only)
on_session_end	End of every run_conversation call + CLI exit handler

Managing plugins

hermes plugins                  # interactive toggle UI — enable/disable with checkboxes
hermes plugins list             # table view with enabled/disabled status
hermes plugins install user/repo  # install from Git
hermes plugins update my-plugin   # pull latest
hermes plugins remove my-plugin   # uninstall
hermes plugins enable my-plugin   # re-enable a disabled plugin
hermes plugins disable my-plugin  # disable without removing

Running hermes plugins with no arguments launches an interactive curses checklist (same UI as hermes tools) where you can toggle plugins on/off with arrow keys and space.

Disabled plugins remain installed but are skipped during loading. The disabled list is stored in config.yaml under plugins.disabled:

plugins:
  disabled:
    - my-noisy-plugin

In a running session, /plugins shows which plugins are currently loaded.

Injecting Messages

Plugins can inject messages into the active conversation using ctx.inject_message():

ctx.inject_message("New data arrived from the webhook", role="user")

Signature: ctx.inject_message(content: str, role: str = "user") -> bool

How it works:

• If the agent is idle (waiting for user input), the message is queued as the next input and starts a new turn.
• If the agent is mid-turn (actively running), the message interrupts the current operation — the same as a user typing a new message and pressing Enter.
• For non-"user" roles, the content is prefixed with [role] (e.g. [system] ...).
• Returns True if the message was queued successfully, False if no CLI reference is available (e.g. in gateway mode).

This enables plugins like remote control viewers, messaging bridges, or webhook receivers to feed messages into the conversation from external sources.

:::note inject_message is only available in CLI mode. In gateway mode, there is no CLI reference and the method returns False. :::

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