mcpc 0.2.6
MCP Callback Protocol extension for enabling asynchronous tool callbacks and streaming updates from MCP clients
License: MIT
Requires Python: >=3.12
Keywords: async, callbacks, mcp, protocol, streaming

# MCPC - Model Context Protocol Callback

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MCPC is an extension to MCP (Model-Context-Protocol) that solves a critical limitation in LLM tool interactions: **enabling continued conversations while running tools background tasks**. It facilitates **asynchronous two-way communication** between LLMs and tools through the already existing MCP transport - no additional transport layer needed, while maintaining full backward compatibility.

<p align="center">
  <img src="assets/mcpc-demo_540.avif" alt="Conceptual demo of MCPC's continuous conversation flow between LLM and tools (UI not included)">
</p>

## What is MCPC?

MCPC is an **extension** to the MCP protocol, not a replacement. It builds upon the existing MCP infrastructure to add real-time two-way communication capabilities while maintaining full compatibility with standard MCP implementations.

MCPC solves a critical limitation in LLM tool interactions: **enabling continuous two-way communication while running background tasks**:

- Bidirectional communication between LLMs and tools using the same MCP transport
- Continuous conversation with LLMs during tool execution
- Real-time updates from background processes
- Asynchronous notifications when operations complete
- Support for indefinitely running tasks with streaming updates

## Compatibility Matrix

### Features

| Feature                           | Status             | Notes                                               |
| --------------------------------- | ------------------ | --------------------------------------------------- |
| STDIO Transport                   | ✅ Implemented     | Full support for standard input/output transport    |
| SSE Transport                     | ⚠️ Limited Support | Standard MCP operations only, MCPC features pending |
| MCPC Client → Standard MCP Server | ✅ Implemented     | Full backward compatibility                         |
| Standard MCP Client → MCPC Server | ✅ Implemented     | Automatic fallback to synchronous results           |

### Frameworks

| Framework                                                  | Status         | Notes                          |
| ---------------------------------------------------------- | -------------- | ------------------------------ |
| [FastMCP 😎](#basic-server-usage---fastmcp)                | ✅ Implemented | Recommended                    |
| [Standard MCP SDK Server](docs/standard-mcp-sdk-server.md) | ✅ Implemented | Works (Use FastMCP if you can) |

## Quick Installation

```bash
# With UV (recommended)
uv add mcpc

# With pip
pip install mcpc
```

## Basic Client Usage

```python
# Initialize the MCPC handler
mcpc_handler = MCPCHandler("my-provider")

# Define your event listener function
async def my_mcpc_listener(mcpc_message: MCPCMessage) -> None:
    print(f"Received MCPC message: {mcpc_message}")
    # Handle the message based on status
    if mcpc_message.type == "task" and mcpc_message.event == "complete":
        print(f"Task {mcpc_message.task_id} completed with result: {mcpc_message.result}")

# Add your event listener for MCPC Message
mcpc_handler.add_event_listener(my_mcpc_listener)

# Wrap the transport with MCPC event listeners
wrapped_transport = await mcpc_handler.wrap_streams(*transport)

# Create a ClientSession with the wrapped transport
session = await ClientSession(*wrapped_transport)

# Initialize MCPC features by checking for MCPC support
mcpc_supported = await mcpc_handler.init_mcpc(session)
if mcpc_supported:
    print(f"MCPC protocol v{mcpc_handler.protocol_version} supported")
```

## Basic Server Usage - FastMCP

```python
# Initialize MCPC helper
server = FastMCP("my-provider")
mcpc = MCPCHelper(server)

@server.tool()
async def process_data(url: str) -> dict:
    async def process_data_task():
        yield mcpc.create_task_event(
            event="update",
            tool_name="process_data",
            session_id=session_id,
            task_id=task_id,
            result=f"Processing {url}..."
        ))
        await asyncio.sleep(3) # Simulate processing time
        yield mcpc.create_task_event(
            event="complete",
            tool_name="process_data",
            session_id=session_id,
            task_id=task_id,
            result={
                YOUR_DATA_OBJECT
            }
        )

    # Start a background task - or run synchronous if no MCPC support
    collected_messages = await mcpc.start_task(task_id, process_data_task)

    # For standard MCP clients, return collected complete/failed messages
    if collected_messages:
        # Keep in mind that FastMCP differs from Standard MCP
        # by expecting direct result rather than TextContent
        # It has implications in that it returns
        # all messages as "one" item, rather than multiple
        return collected_messages

    # For MCPC clients, return immediate acknowledgment
    # See comment above on why this is not [TextContent]
    return mcpc.create_task_event(
        event="created",
        tool_name="process_data",
        session_id=session_id,
        task_id=task_id,
        result=f"Started processing data_id={data_id}. Updates will stream in real-time."
    )

if __name__ == "__main__":
    asyncio.run(server.run())
```

## Documentation

For detailed documentation, please see:

- [Non-FastMCP Server Guide](docs/standard-mcp-sdk-server.md) - basic usage of standard MCP SDK Server
- [API Reference](docs/api-reference.md) - Detailed API documentation
- [Protocol Details](docs/protocol-details.md) - Message structure and protocol information
- [Use Cases](docs/use-cases.md) - Example scenarios and use cases

## License

MIT