mcp-transport-prototype

A prototype implementation to understand building custom MCP server that is accessible over stdio and SSE transport. The goal here is to understand SSE and stdio transports.

SSE Transport?

Server-Sent Events (SSE) is a simple, efficient way to push real-time updates from the server to the client over HTTP.

• Unlike WebSockets, SSE is one-way: the server can stream data to the client, but not vice versa.
• It works over standard HTTP and is well-suited for scenarios like live updates, notifications, or streaming responses.

In this prototype, SSE is used to stream JSON-RPC responses from the MCP server to connected clients.

Unlike stdio, SSE runs the MCP server as a separate HTTP service. This makes it well-suited for use cases like web dashboards or browser-based apps that need to receive streamed updates from the server over the network.

sse_transport.py

It sets up an MCP server over HTTP using Server-Sent Events (SSE).

• Clients send request via a POST API
• Clients can listen to response using an SSE stream

See in action

# Run FastAPI server in terminal 1
python main.py --transport sse

# Open an SSE connection in terminal 2
curl http://localhost:8000/stream/test

# Send client POST reqeuest in terminal 2
curl -X POST http://localhost:8000/send/test \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "params": {
      "name": "Amish"
    }
  }'

Output

event: mcp_response
data: {"jsonrpc": "2.0", "id": 1, "result": {"echo": {"name": "Amish"}}}

What is stdio?

Stdio (short for standard input/output) is a traditional way for programs to communicate by reading from and writing to the terminal or connected processes.

• stdin (standard input): receives input data — usually from the keyboard, a file, or another program.
• stdout (standard output): sends output data — usually printed on the terminal or piped to another program.

In this prototype, the MCP server reads JSON-RPC requests line-by-line from stdin, processes them, and returns responses via stdout.

Compared to SSE, stdio-based MCP servers feel more like in-process tools. They're ideal when you're launching the MCP server as a subprocess, such as from a command-line tool, an LLM agent, or a background script that needs to send and receive structured data efficiently without using a network.

stdio_transport.py

It sets up an MCP server that communicates using standard input/output (stdio)

• Reads requests from stdin
• Writes the response back to stdout

See in action

# Run in-process MCP server
uv run main.py --transport stdio
# Send client request over STDIN
{"jsonrpc": "2.0", "id": 1, "params": {"name": "Amish"}}

Output

{"jsonrpc": "2.0", "id": 1, "result": {"echo": {"name": "Amish"}}}  # STDOUT