Hermes のプラグインを作る

my-portable-plugin/
├── plugin.json
├── skills/
│   └── summarize/
│       ├── SKILL.md
│       └── references/
└── mcp.json

hermes plugins install owner/repository --no-enable
hermes plugins list
hermes plugins enable <plugin-name>

mkdir -p ~/.hermes/plugins/calculator
cd ~/.hermes/plugins/calculator

hermes plugins doctor . --ci

name: calculator
version: 1.0.0
description: Math calculator — evaluate expressions and convert units
provides_tools:
  - calculate
  - unit_convert
provides_hooks:
  - post_tool_call

author: Your Name
requires_env:          # gate loading on env vars; prompted during install
  - SOME_API_KEY       # simple format — plugin disabled if missing
  - name: OTHER_KEY    # rich format — shows description/url during install
    description: "Key for the Other service"
    url: "https://other.com/keys"
    secret: true
capabilities:          # privileged host surfaces you request (consent flow)
  - tools.override     # replace built-in tools (needs user consent)
  - llm.model_override # choose the model for host-owned LLM calls

def register(ctx):
    if ctx.has_capability("tools.override"):
        ctx.register_tool(..., override=True)
    else:
        ctx.register_tool(...)   # register under a non-conflicting name

[project.entry-points."hermes_agent.plugins"]
calculator = "my_pkg:register"

[project.entry-points."hermes_agent.plugin_capabilities"]
"calculator.tools.override" = "my_pkg:register"

# plugin.yaml — manifest v2 example
name: my-plugin
version: 1.2.0
manifest_version: 2
api_version: 1
license: MIT
homepage: https://github.com/owner/my-plugin
tags: [gateway, demo]
requires_plugins:
  - id: other-plugin
    version_range: ">=1.0,<2"
python_dependencies:
  - "somepkg>=1.0,<2"     # surfaced, never auto-installed
config_schema:
  api_url: {type: str, default: "", description: "Service endpoint"}

"""Tool schemas — what the LLM sees."""

CALCULATE = {
    "name": "calculate",
    "description": (
        "Evaluate a mathematical expression and return the result. "
        "Supports arithmetic (+, -, *, /, **), functions (sqrt, sin, cos, "
        "log, abs, round, floor, ceil), and constants (pi, e). "
        "Use this for any math the user asks about."
    ),
    "parameters": {
        "type": "object",
        "properties": {
            "expression": {
                "type": "string",
                "description": "Math expression to evaluate (e.g., '2**10', 'sqrt(144)')",
            },
        },
        "required": ["expression"],
    },
}

UNIT_CONVERT = {
    "name": "unit_convert",
    "description": (
        "Convert a value between units. Supports length (m, km, mi, ft, in), "
        "weight (kg, lb, oz, g), temperature (C, F, K), data (B, KB, MB, GB, TB), "
        "and time (s, min, hr, day)."
    ),
    "parameters": {
        "type": "object",
        "properties": {
            "value": {
                "type": "number",
                "description": "The numeric value to convert",
            },
            "from_unit": {
                "type": "string",
                "description": "Source unit (e.g., 'km', 'lb', 'F', 'GB')",
            },
            "to_unit": {
                "type": "string",
                "description": "Target unit (e.g., 'mi', 'kg', 'C', 'MB')",
            },
        },
        "required": ["value", "from_unit", "to_unit"],
    },
}

"""Tool handlers — the code that runs when the LLM calls each tool."""

# Safe globals for expression evaluation — no file/network access
_SAFE_MATH = {
    "abs": abs, "round": round, "min": min, "max": max,
    "pow": pow, "sqrt": math.sqrt, "sin": math.sin, "cos": math.cos,
    "tan": math.tan, "log": math.log, "log2": math.log2, "log10": math.log10,
    "floor": math.floor, "ceil": math.ceil,
    "pi": math.pi, "e": math.e,
    "factorial": math.factorial,
}

def calculate(args: dict, **kwargs) -> str:
    """Evaluate a math expression safely.

    Rules for handlers:
    1. Receive args (dict) — the parameters the LLM passed
    2. Do the work
    3. Return a JSON string — ALWAYS, even on error
    4. Accept **kwargs for forward compatibility
    """
    expression = args.get("expression", "").strip()
    if not expression:
        return json.dumps({"error": "No expression provided"})

    try:
        result = eval(expression, {"__builtins__": {}}, _SAFE_MATH)
        return json.dumps({"expression": expression, "result": result})
    except ZeroDivisionError:
        return json.dumps({"expression": expression, "error": "Division by zero"})
    except Exception as e:
        return json.dumps({"expression": expression, "error": f"Invalid: {e}"})

# Conversion tables — values are in base units
_LENGTH = {"m": 1, "km": 1000, "mi": 1609.34, "ft": 0.3048, "in": 0.0254, "cm": 0.01}
_WEIGHT = {"kg": 1, "g": 0.001, "lb": 0.453592, "oz": 0.0283495}
_DATA = {"B": 1, "KB": 1024, "MB": 1024**2, "GB": 1024**3, "TB": 1024**4}
_TIME = {"s": 1, "ms": 0.001, "min": 60, "hr": 3600, "day": 86400}

def _convert_temp(value, from_u, to_u):
    # Normalize to Celsius
    c = {"F": (value - 32) * 5/9, "K": value - 273.15}.get(from_u, value)
    # Convert to target
    return {"F": c * 9/5 + 32, "K": c + 273.15}.get(to_u, c)

def unit_convert(args: dict, **kwargs) -> str:
    """Convert between units."""
    value = args.get("value")
    from_unit = args.get("from_unit", "").strip()
    to_unit = args.get("to_unit", "").strip()

    if value is None or not from_unit or not to_unit:
        return json.dumps({"error": "Need value, from_unit, and to_unit"})

    try:
        # Temperature
        if from_unit.upper() in {"C","F","K"} and to_unit.upper() in {"C","F","K"}:
            result = _convert_temp(float(value), from_unit.upper(), to_unit.upper())
            return json.dumps({"input": f"{value} {from_unit}", "result": round(result, 4),
                             "output": f"{round(result, 4)} {to_unit}"})

        # Ratio-based conversions
        for table in (_LENGTH, _WEIGHT, _DATA, _TIME):
            lc = {k.lower(): v for k, v in table.items()}
            if from_unit.lower() in lc and to_unit.lower() in lc:
                result = float(value) * lc[from_unit.lower()] / lc[to_unit.lower()]
                return json.dumps({"input": f"{value} {from_unit}",
                                 "result": round(result, 6),
                                 "output": f"{round(result, 6)} {to_unit}"})

        return json.dumps({"error": f"Cannot convert {from_unit} → {to_unit}"})
    except Exception as e:
        return json.dumps({"error": f"Conversion failed: {e}"})

"""Calculator plugin — registration."""

from . import schemas, tools

logger = logging.getLogger(__name__)

# Track tool usage via hooks
_call_log = []

def _on_post_tool_call(tool_name, args, result, task_id, **kwargs):
    """Hook: runs after every tool call (not just ours)."""
    _call_log.append({"tool": tool_name, "session": task_id})
    if len(_call_log) > 100:
        _call_log.pop(0)
    logger.debug("Tool called: %s (session %s)", tool_name, task_id)

def register(ctx):
    """Wire schemas to handlers and register hooks."""
    ctx.register_tool(name="calculate",    toolset="calculator",
                      schema=schemas.CALCULATE,    handler=tools.calculate)
    ctx.register_tool(name="unit_convert", toolset="calculator",
                      schema=schemas.UNIT_CONVERT, handler=tools.unit_convert)

    # This hook fires for ALL tool calls, not just ours
    ctx.register_hook("post_tool_call", _on_post_tool_call)

def handle_scan(ctx, raw_args: str):
    """Implement /scan by invoking the terminal tool through the registry."""
    result = ctx.dispatch_tool("terminal", {"command": f"find . -name '{raw_args}'"})
    return result  # returned to the caller's chat UI

def register(ctx):
    # Handlers receive a single raw_args string; close over ctx via a lambda.
    ctx.register_command(
        "scan",
        lambda raw: handle_scan(ctx, raw),
        description="Find files matching a glob",
    )

def register(ctx):
    endpoint = ctx.get_config("endpoint", default="https://example.invalid")
    retries = ctx.get_config("retry.attempts", default=3)

    ctx.set_config("endpoint", endpoint)
    ctx.set_config("retry.attempts", retries)

def register(ctx):
    cursor = ctx.state.get("cursor", default={"page": 0})
    ctx.state.set("cursor", {"page": cursor["page"] + 1})

hermes

What's 2 to the power of 16?
Convert 100 fahrenheit to celsius
What's the square root of 2 times pi?
How many gigabytes is 1.5 terabytes?

/plugins

Plugins (1):
  ✓ calculator v1.0.0 (2 tools, 1 hooks)

HERMES_PLUGINS_DEBUG=1 hermes plugins list

hermes logs --level WARNING | grep -i plugin

~/.hermes/plugins/calculator/
├── plugin.yaml      # "I'm calculator, I provide tools and hooks"
├── __init__.py      # Wiring: schemas → handlers, register hooks
├── schemas.py       # What the LLM reads (descriptions + parameter specs)
└── tools.py         # What runs (calculate, unit_convert functions)

# In tools.py or __init__.py
from pathlib import Path

_PLUGIN_DIR = Path(__file__).parent
_DATA_FILE = _PLUGIN_DIR / "data" / "languages.yaml"

with open(_DATA_FILE) as f:
    _DATA = yaml.safe_load(f)

from plugins.plugin_storage import plugin_data_dir, plugin_db

# <hermes home>/plugin-data/<name>/ — created on first use
state_file = plugin_data_dir("my-plugin") / "state.json"

# Or a SQLite database at <data dir>/data.db (WAL mode, thread-friendly)
conn = plugin_db("my-plugin")
conn.execute("CREATE TABLE IF NOT EXISTS runs (id TEXT PRIMARY KEY)")

~/.hermes/plugins/my-plugin/
├── __init__.py
├── plugin.yaml
└── skills/
    ├── my-workflow/
    │   └── SKILL.md
    └── my-checklist/
        └── SKILL.md

from pathlib import Path

def register(ctx):
    skills_dir = Path(__file__).parent / "skills"
    for child in sorted(skills_dir.iterdir()):
        skill_md = child / "SKILL.md"
        if child.is_dir() and skill_md.exists():
            ctx.register_skill(child.name, skill_md)

skill_view("my-plugin:my-workflow")   # → plugin's version
skill_view("my-workflow")              # → built-in version (unchanged)

# plugin.yaml — simple format (backwards-compatible)
requires_env:
  - WEATHER_API_KEY

# plugin.yaml — rich format
requires_env:
  - name: WEATHER_API_KEY
    description: "API key for OpenWeather"
    url: "https://openweathermap.org/api"
    secret: true

# tools.py
from tools.lazy_deps import ensure, FeatureUnavailable

def my_tool_handler(args, **kwargs):
    try:
        ensure("my-plugin.my-backend")   # key must be in LAZY_DEPS
    except FeatureUnavailable as exc:
        return {"error": str(exc)}

    import my_backend_sdk   # safe now
    ...

_client = None

def get_client():
    global _client
    if _client is not None:
        return _client
    _client = ExpensiveClient(...)   # ← TOCTOU race
    return _client

from plugins.plugin_utils import lazy_singleton, SingletonSlot

# Zero-arg accessor → decorate it:
@lazy_singleton
def get_client():
    return ExpensiveClient(load_config())   # runs exactly once

client = get_client()    # safe across threads
get_client.reset()       # drop the instance (tests / teardown)

# Accessor that takes a build argument → use a slot:
_slot: SingletonSlot = SingletonSlot()

def get_client(config=None):
    return _slot.get(lambda: ExpensiveClient(resolve(config)))

def reset_client():
    _slot.reset()

ctx.register_tool(
    name="my_tool",
    schema={...},
    handler=my_handler,
    check_fn=lambda: _has_optional_lib(),  # False = tool hidden from model
)

def register(ctx):
    ctx.register_tool(
        name="browser_navigate",             # same name as the built-in
        toolset="plugin_my_browser",         # your own toolset namespace
        schema={...},
        handler=my_custom_navigate,
        override=True,                       # explicit opt-in
    )

plugins:
  entries:
    my-plugin:                    # the plugin's registry key from `hermes plugins list`
      allow_tool_override: true

def register(ctx):
    ctx.register_hook("pre_tool_call", before_any_tool)
    ctx.register_hook("post_tool_call", after_any_tool)
    ctx.register_hook("pre_llm_call", inject_memory)
    ctx.register_hook("on_session_start", on_new_session)
    ctx.register_hook("on_session_end", on_session_end)

# Dict with context key
return {"context": "Recalled memories:\n- User prefers dark mode\n- Last project: hermes-agent"}

# Plain string (equivalent to the dict form above)
return "Recalled memories:\n- User prefers dark mode"

# Return None or don't return → no injection (observer-only)
return None

hooks:
  output_spill:
    enabled: true          # default: true
    max_chars: 10000       # default; set higher to opt out of spilling
    preview_head: 500      # chars shown at the top of the preview
    preview_tail: 500      # chars shown at the bottom of the preview
    # directory: null      # default: $HERMES_HOME/hook_outputs

"""Memory plugin — recalls relevant context from a vector store."""

MEMORY_API = "https://your-memory-api.example.com"

def recall_context(session_id, user_message, is_first_turn, **kwargs):
    """Called before each LLM turn. Returns recalled memories."""
    try:
        resp = httpx.post(f"{MEMORY_API}/recall", json={
            "session_id": session_id,
            "query": user_message,
        }, timeout=3)
        memories = resp.json().get("results", [])
        if not memories:
            return None  # nothing to inject

        text = "Recalled context from previous sessions:\n"
        text += "\n".join(f"- {m['text']}" for m in memories)
        return {"context": text}
    except Exception:
        return None  # fail silently, don't break the agent

def register(ctx):
    ctx.register_hook("pre_llm_call", recall_context)

"""Guardrails plugin — enforces content policies."""

POLICY = """You MUST follow these content policies for this session:
- Never generate code that accesses the filesystem outside the working directory
- Always warn before executing destructive operations
- Refuse requests involving personal data extraction"""

def inject_guardrails(**kwargs):
    """Injects policy text into every turn."""
    return {"context": POLICY}

def register(ctx):
    ctx.register_hook("pre_llm_call", inject_guardrails)

"""Analytics plugin — tracks turn metadata without injecting context."""

logger = logging.getLogger(__name__)

def log_turn(session_id, user_message, model, is_first_turn, **kwargs):
    """Fires before each LLM call. Returns None — no context injected."""
    logger.info("Turn: session=%s model=%s first=%s msg_len=%d",
                session_id, model, is_first_turn, len(user_message or ""))
    # No return → no injection

def register(ctx):
    ctx.register_hook("pre_llm_call", log_turn)

def cap_find_output(tool_name, args, **kwargs):
    """Rewrite terminal find commands to cap their output."""
    command = args.get("command", "")
    if tool_name == "terminal" and command.startswith("find "):
        return {
            "args": {**args, "command": command + " | head -100"},
            "source": "my-plugin",
            "reason": "cap find output",
        }
    return None  # leave the call unchanged

def register(ctx):
    ctx.register_middleware("tool_request", cap_find_output)

def _my_command(args):
    """Handler for hermes my-plugin <subcommand>."""
    sub = getattr(args, "my_command", None)
    if sub == "status":
        print("All good!")
    elif sub == "config":
        print("Current config: ...")
    else:
        print("Usage: hermes my-plugin <status|config>")

def _setup_argparse(subparser):
    """Build the argparse tree for hermes my-plugin."""
    subs = subparser.add_subparsers(dest="my_command")
    subs.add_parser("status", help="Show plugin status")
    subs.add_parser("config", help="Show plugin config")
    subparser.set_defaults(func=_my_command)

def register(ctx):
    ctx.register_tool(...)
    ctx.register_cli_command(
        name="my-plugin",
        help="Manage my plugin",
        setup_fn=_setup_argparse,
        handler_fn=_my_command,
    )

def _handle_status(raw_args: str) -> str:
    """Handler for /mystatus — called with everything after the command name."""
    if raw_args.strip() == "help":
        return "Usage: /mystatus [help|check]"
    return "Plugin status: all systems nominal"

def register(ctx):
    ctx.register_command(
        "mystatus",
        handler=_handle_status,
        description="Show plugin status",
    )

async def _handle_check(raw_args: str) -> str:
    result = await some_async_operation()
    return f"Check result: {result}"

def register(ctx):
    ctx.register_command("check", handler=_handle_check, description="Run async check")

def register(ctx):
    def _handle_deliver(raw_args: str):
        result = ctx.dispatch_tool(
            "delegate_task",
            {
                "goal": raw_args,
                "toolsets": ["terminal", "file", "web"],
            },
        )
        return result

    ctx.register_command(
        "deliver",
        handler=_handle_deliver,
        description="Delegate a goal to a subagent",
    )

def register(ctx):
    def on_blocked(*, task_id, reason=None, **kw):
        # Runs in the worker process; ctx._cli_ref is None here.
        ctx.dispatch_tool("kanban_comment", {
            "task_id": task_id,
            "comment": f"[{ctx.profile_name}] auto-noted block: {reason}",
        })
    ctx.register_hook("kanban_task_blocked", on_blocked)

def register(ctx):
    async def _on_approve(ack, body, action):
        # ack within 3 seconds — slack_bolt requirement.
        await ack()
        # body["channel"]["id"], body["user"]["id"], body["message"]["ts"]
        # action["action_id"], action["value"]
        sweep_id = (action.get("value") or "").split("|", 1)[-1]
        # ...do the deterministic work, then post a follow-up.

    ctx.register_slack_action_handler("inbox_sweep_approve", _on_approve)

def register(ctx):
    def _wire(native, adapter):
        # native: the platform's client/app object (see table below)
        # adapter: the platform adapter instance (treat as read-only)
        # Import platform SDKs HERE so register() works without them.
        ...

    ctx.register_platform_handler("discord", _wire)

def register(ctx):
    def _wire(application, adapter):
        from telegram.ext import CallbackQueryHandler

        async def _on_button(update, context):
            query = update.callback_query
            await query.answer()
            # ...handle "myplugin:*" callbacks

        application.add_handler(
            CallbackQueryHandler(_on_button, pattern=r"^myplugin:")
        )

    ctx.register_platform_handler("telegram", _wire)

def register(ctx):
    def _wire(bot, adapter):
        async def on_raw_reaction_add(payload):
            ...  # e.g. reaction-based voting / moderation

        bot.add_listener(on_raw_reaction_add, "on_raw_reaction_add")

    ctx.register_platform_handler("discord", _wire)

# plugins/model-providers/acme/__init__.py
from providers import register_provider
from providers.base import ProviderProfile

register_provider(ProviderProfile(
    name="acme",
    aliases=("acme-inference",),
    display_name="Acme Inference",
    env_vars=("ACME_API_KEY", "ACME_BASE_URL"),
    base_url="https://api.acme.example.com/v1",
    auth_type="api_key",
    default_aux_model="acme-small-fast",
    fallback_models=("acme-large-v3", "acme-medium-v3"),
))

# plugins/model-providers/acme/plugin.yaml
name: acme-provider
kind: model-provider
version: 1.0.0
description: Acme Inference — OpenAI-compatible direct API

# plugins/platforms/myplatform/adapter.py
from gateway.platforms.base import BasePlatformAdapter

class MyPlatformAdapter(BasePlatformAdapter):
    async def connect(self): ...
    async def send(self, chat_id, text): ...
    async def disconnect(self): ...

def check_requirements():
    import os
    return bool(os.environ.get("MYPLATFORM_TOKEN"))

def _env_enablement():
    import os
    tok = os.getenv("MYPLATFORM_TOKEN", "").strip()
    if not tok:
        return None
    return {"token": tok}

def register(ctx):
    ctx.register_platform(
        name="myplatform",
        label="MyPlatform",
        adapter_factory=lambda cfg: MyPlatformAdapter(cfg),
        check_fn=check_requirements,
        required_env=["MYPLATFORM_TOKEN"],
        # Auto-populate PlatformConfig.extra from env so env-only setups
        # show up in `hermes gateway status` without SDK instantiation.
        env_enablement_fn=_env_enablement,
        # Opt in to cron delivery: `deliver=myplatform` routes to this var.
        cron_deliver_env_var="MYPLATFORM_HOME_CHANNEL",
        emoji="💬",
        platform_hint="You are chatting via MyPlatform. Keep responses concise.",
    )

# plugins/platforms/myplatform/plugin.yaml
name: myplatform-platform
label: MyPlatform
kind: platform
version: 1.0.0
description: MyPlatform gateway adapter
requires_env:
  - name: MYPLATFORM_TOKEN
    description: "Bot token from the MyPlatform console"
    password: true
optional_env:
  - name: MYPLATFORM_HOME_CHANNEL
    description: "Default channel for cron delivery"
    password: false

# plugins/memory/my-memory/__init__.py
from agent.memory_provider import MemoryProvider

class MyMemoryProvider(MemoryProvider):
    @property
    def name(self) -> str:
        return "my-memory"

    def is_available(self) -> bool:
        import os
        return bool(os.environ.get("MY_MEMORY_API_KEY"))

    def initialize(self, session_id: str, **kwargs) -> None:
        self._session_id = session_id

    def sync_turn(self, user_content, assistant_content, *,
                  session_id="", messages=None) -> None:
        ...

    def prefetch(self, query, *, session_id="") -> str:
        ...

    def get_tool_schemas(self) -> list[dict]:
        return []   # required @abstractmethod — see full guide

def register(ctx):
    ctx.register_memory_provider(MyMemoryProvider())

# plugins/context_engine/my-engine/__init__.py
from agent.context_engine import ContextEngine

class MyContextEngine(ContextEngine):
    @property
    def name(self) -> str:
        return "my-engine"

    def update_from_response(self, usage) -> None: ...
    def should_compress(self, prompt_tokens: int = None) -> bool: ...
    def compress(self, messages, current_tokens=None, focus_topic=None,
                 force=False, memory_context="") -> list: ...

def register(ctx):
    ctx.register_context_engine(MyContextEngine())

# plugins/image_gen/my-imggen/__init__.py
from agent.image_gen_provider import ImageGenProvider

class MyImageGenProvider(ImageGenProvider):
    @property
    def name(self) -> str:
        return "my-imggen"

    def is_available(self) -> bool: ...
    def generate(self, prompt: str, aspect_ratio="landscape", **kwargs) -> dict:
        # returns success_response(...) / error_response(...)
        ...

def register(ctx):
    ctx.register_image_gen_provider(MyImageGenProvider())

# plugins/image_gen/my-imggen/plugin.yaml
name: my-imggen
kind: backend
version: 1.0.0
description: Custom image generation backend

mcp_servers:
  filesystem:
    command: "npx"
    args: ["-y", "@modelcontextprotocol/server-filesystem", "/home/user/projects"]
    timeout: 120

  linear:
    url: "https://mcp.linear.app/sse"
    auth:
      type: "oauth"

# ~/.hermes/hooks/long-task-alert/HOOK.yaml
name: long-task-alert
description: Send a push notification when a long task finishes
events:
  - agent:end

# ~/.hermes/hooks/long-task-alert/handler.py
async def handle(event_type: str, context: dict) -> None:
    if context.get("duration_seconds", 0) > 120:
        # send notification …
        pass

hooks:
  - event: post_tool_call
    command: "notify-send 'Tool ran: {tool_name}'"
    when:
      tools: [terminal, patch, write_file]

hermes skills tap add myorg/skills-repo
hermes skills search my-workflow --source myorg/skills-repo
hermes skills install myorg/skills-repo/my-workflow

tts:
  provider: voxcpm
  providers:
    voxcpm:
      type: command
      command: "voxcpm --ref ~/voice.wav --text-file {input_path} --out {output_path}"
      output_format: mp3
      voice_compatible: true

# pyproject.toml
[project.entry-points."hermes_agent.plugins"]
my-plugin = "my_plugin_package"

pip install hermes-plugin-calculator
# Plugin auto-discovered on next hermes startup

# User's configuration.nix
services.hermes-agent.extraPythonPackages = [
  (pkgs.python312Packages.buildPythonPackage {
    pname = "my-plugin";
    version = "1.0.0";
    src = pkgs.fetchFromGitHub {
      owner = "you";
      repo = "hermes-my-plugin";
      rev = "v1.0.0";
      hash = "sha256-...";  # nix-prefetch-url --unpack
    };
    format = "pyproject";
    build-system = [ pkgs.python312Packages.setuptools ];
  })
];

services.hermes-agent.extraPlugins = [
  (pkgs.fetchFromGitHub {
    owner = "you";
    repo = "hermes-my-plugin";
    rev = "v1.0.0";
    hash = "sha256-...";
  })
];

# Wrong — returns a dict
def handler(args, **kwargs):
    return {"result": 42}

# Right — returns a JSON string
def handler(args, **kwargs):
    return json.dumps({"result": 42})

# Wrong — will break if Hermes passes extra context
def handler(args):
    ...

# Right
def handler(args, **kwargs):
    ...

# Wrong — exception propagates, tool call fails
def handler(args, **kwargs):
    result = 1 / int(args["value"])  # ZeroDivisionError!
    return json.dumps({"result": result})

# Right — catch and return error JSON
def handler(args, **kwargs):
    try:
        result = 1 / int(args.get("value", 0))
        return json.dumps({"result": result})
    except Exception as e:
        return json.dumps({"error": str(e)})

# Bad — model doesn't know when to use it
"description": "Does stuff"

# Good — model knows exactly when and how
"description": "Evaluate a mathematical expression. Use for arithmetic, trig, logarithms. Supports: +, -, *, /, **, sqrt, sin, cos, log, pi, e."