zeromcp 1.3.0
Zero-dependency MCP server implementation
Requires Python: >=3.11

# zeromcp

**Minimal MCP server implementation in pure Python.**

A lightweight, handcrafted implementation of the [Model Context Protocol](https://modelcontextprotocol.io/) focused on what most users actually need: exposing tools with clean Python type annotations.

## Features

- ✨ **Zero dependencies** - Pure Python, standard library only
- 🎯 **Type-safe** - Native Python type annotations for everything
- 🚀 **Fast** - Minimal overhead, maximum performance
- 🛠️ **Handcrafted** - Written by a human<sup>[1](#ai-usage)</sup>, verified against the spec
- 🌐 **HTTP/SSE transport** - Streamable responses
- 📡 **Stdio transport** - For legacy clients
- 📦 **Tiny** - Less than 1,000 lines of code

## Installation

```bash
pip install zeromcp
```

Or with uv:

```bash
uv add zeromcp
```

## Quick Start

```python
from typing import Annotated
from zeromcp import McpServer

mcp = McpServer("my-server")

@mcp.tool
def greet(
    name: Annotated[str, "Name to greet"],
    age: Annotated[int | None, "Age of person"] = None
) -> str:
    """Generate a greeting message"""
    if age:
        return f"Hello, {name}! You are {age} years old."
    return f"Hello, {name}!"

if __name__ == "__main__":
    mcp.serve("127.0.0.1", 8000)
```

Then manually test your MCP server with the [inspector](https://github.com/modelcontextprotocol/inspector):

```bash
npx -y @modelcontextprotocol/inspector
```

Once things are working you can configure the `mcp.json`:

```json
{
  "mcpServers": {
    "my-server": {
      "type": "http",
      "url": "http://127.0.0.1/mcp"
    }
  }
}
```

## Stdio Transport

For MCP clients that only support stdio transport:

```python
from zeromcp import McpServer

mcp = McpServer("my-server")

@mcp.tool
def greet(name: str) -> str:
    """Generate a greeting"""
    return f"Hello, {name}!"

if __name__ == "__main__":
    mcp.stdio()
```

Then configure in `mcp.json` (different for every client):

```json
{
  "mcpServers": {
    "my-server": {
      "command": "python",
      "args": ["path/to/server.py"]
    }
  }
}
```

## Type Annotations

zeromcp uses native Python `Annotated` types for schema generation:

```python
from typing import Annotated, Optional, TypedDict, NotRequired

class GreetingResponse(TypedDict):
    message: Annotated[str, "Greeting message"]
    name: Annotated[str, "Name that was greeted"]
    age: Annotated[NotRequired[int], "Age if provided"]

@mcp.tool
def greet(
    name: Annotated[str, "Name to greet"],
    age: Annotated[Optional[int], "Age of person"] = None
) -> GreetingResponse:
    """Generate a greeting message"""
    if age is not None:
        return {
            "message": f"Hello, {name}! You are {age} years old.",
            "name": name,
            "age": age
        }
    return {
        "message": f"Hello, {name}!",
        "name": name
    }
```

## Union Types

Tools can accept multiple input types:

```python
from typing import Annotated, TypedDict

class StructInfo(TypedDict):
    name: Annotated[str, "Structure name"]
    size: Annotated[int, "Structure size in bytes"]
    fields: Annotated[list[str], "List of field names"]

@mcp.tool
def struct_get(
    names: Annotated[list[str], "Array of structure names"]
         | Annotated[str, "Single structure name"]
) -> list[StructInfo]:
    """Retrieve structure information by names"""
    return [
        {
            "name": name,
            "size": 128,
            "fields": ["field1", "field2", "field3"]
        }
        for name in (names if isinstance(names, list) else [names])
    ]
```

## Error Handling

```python
from zeromcp import McpToolError

@mcp.tool
def divide(
    numerator: Annotated[float, "Numerator"],
    denominator: Annotated[float, "Denominator"]
) -> float:
    """Divide two numbers"""
    if denominator == 0:
        raise McpToolError("Division by zero")
    return numerator / denominator
```

## Resources

Expose read-only data via URI patterns. Resources are serialized as JSON.

```python
from typing import Annotated

@mcp.resource("file://data.txt")
def read_file() -> dict:
    """Get information about data.txt"""
    return {"name": "data.txt", "size": 1024}

@mcp.resource("file://{filename}")
def read_any_file(
    filename: Annotated[str, "Name of file to read"]
) -> dict:
    """Get information about any file"""
    return {"name": filename, "size": 2048}
```

## Prompts

Expose reusable prompt templates with typed arguments.

```python
from typing import Annotated

@mcp.prompt
def code_review(
    code: Annotated[str, "Code to review"],
    language: Annotated[str, "Programming language"] = "python"
) -> str:
    """Review code for bugs and improvements"""
    return f"Please review this {language} code:\n\n```{language}\n{code}\n```"
```

## CORS

By default, zeromcp allows CORS requests from localhost origins (`localhost`, `127.0.0.1`, `::1`) on **any port**. This allows tools like the MCP Inspector or local AI tools to communicate with your MCP server.

```python
from zeromcp import McpServer

mcp = McpServer("my-server")

# Default: allow localhost on any port
mcp.cors_allowed_origins = mcp.cors_localhost

# Allow all origins (use with caution)
mcp.cors_allowed_origins = "*"

# Allow specific origins
mcp.cors_allowed_origins = [
    "http://localhost:3000",
    "https://myapp.example.com",
]

# Disable CORS (blocks all browser cross-origin requests)
mcp.cors_allowed_origins = None

# Custom logic
mcp.cors_allowed_origins = lambda origin: origin.endswith(".example.com")
```

Note: CORS only affects browser-based requests. Non-browser clients like `curl` or MCP desktop apps are unaffected by this setting.

## Supported clients

The following clients have been tested:

- [Claude Code](https://code.claude.com/docs/en/mcp#installing-mcp-servers)
- [Claude Desktop](https://modelcontextprotocol.io/docs/develop/connect-local-servers#installing-the-filesystem-server) (_stdio only_)
- [Visual Studio Code](https://code.visualstudio.com/docs/copilot/customization/mcp-servers)
- [Roo Code](https://docs.roocode.com/features/mcp/using-mcp-in-roo) / [Cline](https://docs.cline.bot/mcp/configuring-mcp-servers) / [Kilo Code](https://kilocode.ai/docs/features/mcp/using-mcp-in-kilo-code)
- [LM Studio](https://lmstudio.ai/docs/app/mcp)
- [Jan](https://www.jan.ai/docs/desktop/mcp#configure-and-use-mcps-within-jan)
- [Gemini CLI](https://geminicli.com/docs/tools/mcp-server/#how-to-set-up-your-mcp-server)
- [Cursor](https://cursor.com/docs/context/mcp)
- [Windsurf](https://docs.windsurf.com/windsurf/cascade/mcp)
- [Zed](https://zed.dev/docs/ai/mcp) (_stdio only_)
- [Warp](https://docs.warp.dev/knowledge-and-collaboration/mcp#adding-an-mcp-server)

_Note_: generally the `/mcp` endpoint is preferred, but not all clients support it correctly.

<a name="ai-usage"></a>
<sup>1</sup>README and some of the tests written by Claude