---
sidebar_position: 9
sidebar_label: "Build a Plugin"
title: "Build a Hermes Plugin"
description: "Step-by-step guide to building a complete Hermes plugin with tools, hooks, data files, and skills"
---

# Build a Hermes Plugin

This guide walks through building a complete Hermes plugin from scratch. By the end you'll have a working plugin with multiple tools, lifecycle hooks, shipped data files, and a bundled skill — everything the plugin system supports.

## What you're building

A **calculator** plugin with two tools:
- `calculate` — evaluate math expressions (`2**16`, `sqrt(144)`, `pi * 5**2`)
- `unit_convert` — convert between units (`100 F → 37.78 C`, `5 km → 3.11 mi`)

Plus a hook that logs every tool call, and a bundled skill file.

## Step 1: Create the plugin directory

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

## Step 2: Write the manifest

Create `plugin.yaml`:

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

This tells Hermes: "I'm a plugin called calculator, I provide tools and hooks." The `provides_tools` and `provides_hooks` fields are lists of what the plugin registers.

Optional fields you could add:
```yaml
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
```

## Step 3: Write the tool schemas

Create `schemas.py` — this is what the LLM reads to decide when to call your tools:

```python
"""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"],
    },
}
```

**Why schemas matter:** The `description` field is how the LLM decides when to use your tool. Be specific about what it does and when to use it. The `parameters` define what arguments the LLM passes.

## Step 4: Write the tool handlers

Create `tools.py` — this is the code that actually executes when the LLM calls your tools:

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

import json
import math

# 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}"})
```

**Key rules for handlers:**
1. **Signature:** `def my_handler(args: dict, **kwargs) -> str`
2. **Return:** Always a JSON string. Success and errors alike.
3. **Never raise:** Catch all exceptions, return error JSON instead.
4. **Accept `**kwargs`:** Hermes may pass additional context in the future.

## Step 5: Write the registration

Create `__init__.py` — this wires schemas to handlers:

```python
"""Calculator plugin — registration."""

import logging

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)
```

**What `register()` does:**
- Called exactly once at startup
- `ctx.register_tool()` puts your tool in the registry — the model sees it immediately
- `ctx.register_hook()` subscribes to lifecycle events
- `ctx.register_cli_command()` registers a CLI subcommand (e.g. `hermes my-plugin <subcommand>`)
- `ctx.register_command()` registers an in-session slash command (e.g. `/myplugin <args>` inside CLI / gateway chat) — see [Register slash commands](#register-slash-commands) below
- `ctx.dispatch_tool(name, arguments)` — call any other tool (built-in or from another plugin) with the parent agent's context (approvals, credentials, task_id) wired up automatically. Useful from slash-command handlers that need to invoke `terminal`, `read_file`, or any other tool as if the model had called it directly.
- If this function crashes, the plugin is disabled but Hermes continues fine

**`dispatch_tool` example — a slash command that runs a tool:**

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

def register(ctx):
    ctx.register_command("scan", handle_scan, help="Find files matching a glob")
```

The dispatched tool goes through the normal approval, redaction, and budget pipelines — it's a real tool invocation, not a shortcut around them.

## Step 6: Test it

Start Hermes:

```bash
hermes
```

You should see `calculator: calculate, unit_convert` in the banner's tool list.

Try these prompts:
```
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?
```

Check plugin status:
```
/plugins
```

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

## Your plugin's final structure

```
~/.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)
```

Four files, clear separation:
- **Manifest** declares what the plugin is
- **Schemas** describe tools for the LLM
- **Handlers** implement the actual logic
- **Registration** connects everything

## What else can plugins do?

### Ship data files

Put any files in your plugin directory and read them at import time:

```python
# 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)
```

### Bundle skills

Plugins can ship skill files that the agent loads via `skill_view("plugin:skill")`. Register them in your `__init__.py`:

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

```python
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)
```

The agent can now load your skills with their namespaced name:

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

**Key properties:**
- Plugin skills are **read-only** — they don't enter `~/.hermes/skills/` and can't be edited via `skill_manage`.
- Plugin skills are **not** listed in the system prompt's `<available_skills>` index — they're opt-in explicit loads.
- Bare skill names are unaffected — the namespace prevents collisions with built-in skills.
- When the agent loads a plugin skill, a bundle context banner is prepended listing sibling skills from the same plugin.

:::tip Legacy pattern
The old `shutil.copy2` pattern (copying a skill into `~/.hermes/skills/`) still works but creates name collision risk with built-in skills. Prefer `ctx.register_skill()` for new plugins.
:::

### Gate on environment variables

If your plugin needs an API key:

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

If `WEATHER_API_KEY` isn't set, the plugin is disabled with a clear message. No crash, no error in the agent — just "Plugin weather disabled (missing: WEATHER_API_KEY)".

When users run `hermes plugins install`, they're **prompted interactively** for any missing `requires_env` variables. Values are saved to `.env` automatically.

For a better install experience, use the rich format with descriptions and signup URLs:

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

| Field | Required | Description |
|-------|----------|-------------|
| `name` | Yes | Environment variable name |
| `description` | No | Shown to user during install prompt |
| `url` | No | Where to get the credential |
| `secret` | No | If `true`, input is hidden (like a password field) |

Both formats can be mixed in the same list. Already-set variables are skipped silently.

### Conditional tool availability

For tools that depend on optional libraries:

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

### Register multiple hooks

```python
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)
```

### Hook reference

Each hook is documented in full on the **[Event Hooks reference](/docs/user-guide/features/hooks#plugin-hooks)** — callback signatures, parameter tables, exactly when each fires, and examples. Here's the summary:

| Hook | Fires when | Callback signature | Returns |
|------|-----------|-------------------|---------|
| [`pre_tool_call`](/docs/user-guide/features/hooks#pre_tool_call) | Before any tool executes | `tool_name: str, args: dict, task_id: str` | ignored |
| [`post_tool_call`](/docs/user-guide/features/hooks#post_tool_call) | After any tool returns | `tool_name: str, args: dict, result: str, task_id: str, duration_ms: int` | ignored |
| [`pre_llm_call`](/docs/user-guide/features/hooks#pre_llm_call) | Once per turn, before the tool-calling loop | `session_id: str, user_message: str, conversation_history: list, is_first_turn: bool, model: str, platform: str` | [context injection](#pre_llm_call-context-injection) |
| [`post_llm_call`](/docs/user-guide/features/hooks#post_llm_call) | Once per turn, after the tool-calling loop (successful turns only) | `session_id: str, user_message: str, assistant_response: str, conversation_history: list, model: str, platform: str` | ignored |
| [`on_session_start`](/docs/user-guide/features/hooks#on_session_start) | New session created (first turn only) | `session_id: str, model: str, platform: str` | ignored |
| [`on_session_end`](/docs/user-guide/features/hooks#on_session_end) | End of every `run_conversation` call + CLI exit | `session_id: str, completed: bool, interrupted: bool, model: str, platform: str` | ignored |
| [`on_session_finalize`](/docs/user-guide/features/hooks#on_session_finalize) | CLI/gateway tears down an active session | `session_id: str \| None, platform: str` | ignored |
| [`on_session_reset`](/docs/user-guide/features/hooks#on_session_reset) | Gateway swaps in a new session key (`/new`, `/reset`) | `session_id: str, platform: str` | ignored |

Most hooks are fire-and-forget observers — their return values are ignored. The exception is `pre_llm_call`, which can inject context into the conversation.

All callbacks should accept `**kwargs` for forward compatibility. If a hook callback crashes, it's logged and skipped. Other hooks and the agent continue normally.

### `pre_llm_call` context injection

This is the only hook whose return value matters. When a `pre_llm_call` callback returns a dict with a `"context"` key (or a plain string), Hermes injects that text into the **current turn's user message**. This is the mechanism for memory plugins, RAG integrations, guardrails, and any plugin that needs to provide the model with additional context.

#### Return format

```python
# 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
```

Any non-None, non-empty return with a `"context"` key (or a plain non-empty string) is collected and appended to the user message for the current turn.

#### How injection works

Injected context is appended to the **user message**, not the system prompt. This is a deliberate design choice:

- **Prompt cache preservation** — the system prompt stays identical across turns. Anthropic and OpenRouter cache the system prompt prefix, so keeping it stable saves 75%+ on input tokens in multi-turn conversations. If plugins modified the system prompt, every turn would be a cache miss.
- **Ephemeral** — the injection happens at API call time only. The original user message in the conversation history is never mutated, and nothing is persisted to the session database.
- **The system prompt is Hermes's territory** — it contains model-specific guidance, tool enforcement rules, personality instructions, and cached skill content. Plugins contribute context alongside the user's input, not by altering the agent's core instructions.

#### Example: Memory recall plugin

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

import httpx

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)
```

#### Example: Guardrails plugin

```python
"""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)
```

#### Example: Observer-only hook (no injection)

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

import logging
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)
```

#### Multiple plugins returning context

When multiple plugins return context from `pre_llm_call`, their outputs are joined with double newlines and appended to the user message together. The order follows plugin discovery order (alphabetical by plugin directory name).

### Register CLI commands

Plugins can add their own `hermes <plugin>` subcommand tree:

```python
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,
    )
```

After registration, users can run `hermes my-plugin status`, `hermes my-plugin config`, etc.

**Memory provider plugins** use a convention-based approach instead: add a `register_cli(subparser)` function to your plugin's `cli.py` file. The memory plugin discovery system finds it automatically — no `ctx.register_cli_command()` call needed. See the [Memory Provider Plugin guide](/docs/developer-guide/memory-provider-plugin#adding-cli-commands) for details.

**Active-provider gating:** Memory plugin CLI commands only appear when their provider is the active `memory.provider` in config. If a user hasn't set up your provider, your CLI commands won't clutter the help output.

### Register slash commands

Plugins can register in-session slash commands — commands users type during a conversation (like `/lcm status` or `/ping`). These work in both CLI and gateway (Telegram, Discord, etc.).

```python
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",
    )
```

After registration, users can type `/mystatus` in any session. The command appears in autocomplete, `/help` output, and the Telegram bot menu.

**Signature:** `ctx.register_command(name: str, handler: Callable, description: str = "")`

| Parameter | Type | Description |
|-----------|------|-------------|
| `name` | `str` | Command name without the leading slash (e.g. `"lcm"`, `"mystatus"`) |
| `handler` | `Callable[[str], str \| None]` | Called with the raw argument string. May also be `async`. |
| `description` | `str` | Shown in `/help`, autocomplete, and Telegram bot menu |

**Key differences from `register_cli_command()`:**

| | `register_command()` | `register_cli_command()` |
|---|---|---|
| Invoked as | `/name` in a session | `hermes name` in a terminal |
| Where it works | CLI sessions, Telegram, Discord, etc. | Terminal only |
| Handler receives | Raw args string | argparse `Namespace` |
| Use case | Diagnostics, status, quick actions | Complex subcommand trees, setup wizards |

**Conflict protection:** If a plugin tries to register a name that conflicts with a built-in command (`help`, `model`, `new`, etc.), the registration is silently rejected with a log warning. Built-in commands always take precedence.

**Async handlers:** The gateway dispatch automatically detects and awaits async handlers, so you can use either sync or async functions:

```python
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")
```

:::tip
This guide covers **general plugins** (tools, hooks, slash commands, CLI commands). For specialized plugin types, see:
- [Memory Provider Plugins](/docs/developer-guide/memory-provider-plugin) — cross-session knowledge backends
- [Context Engine Plugins](/docs/developer-guide/context-engine-plugin) — alternative context management strategies
:::

### Distribute via pip

For sharing plugins publicly, add an entry point to your Python package:

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

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

### Distribute for NixOS

NixOS users can install your plugin declaratively if you provide a `pyproject.toml` with entry points:

**Entry-point plugins** (recommended for distribution):
```nix
# 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 ];
  })
];
```

**Directory plugins** (no `pyproject.toml` needed):
```nix
services.hermes-agent.extraPlugins = [
  (pkgs.fetchFromGitHub {
    owner = "you";
    repo = "hermes-my-plugin";
    rev = "v1.0.0";
    hash = "sha256-...";
  })
];
```

See the [Nix Setup guide](/docs/getting-started/nix-setup#plugins) for complete documentation including overlay usage and collision checking.

## Common mistakes

**Handler doesn't return JSON string:**
```python
# 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})
```

**Missing `**kwargs` in handler signature:**
```python
# Wrong — will break if Hermes passes extra context
def handler(args):
    ...

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

**Handler raises exceptions:**
```python
# 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)})
```

**Schema description too vague:**
```python
# 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."
```