MCP Transports

MCP Transports Overview

Feature	stdio	sse (Server-Sent Events)	streamable-http
Bidirectional	❌ No	❌ No	✅ Yes
Protocol	OS-level stdin/stdout over TCP	HTTP (one-way stream over HTTP/1.1)	HTTP (usually chunked transfer or HTTP/2/3)
Data Format	Text (line-delimited), JSON	JSON/Text (EventStream)	Arbitrary bytes, JSON, binary, I/O streams
Latency	🟢 Low (no network)	🟡 Medium (HTTP overhead, server push model)	🟢 Low-to-medium (depends on server impl)
Streaming Support	⚠️ Simulated (by polling stdout)	✅ Yes (server pushes tokens as events)	✅ Yes (true duplex stream)
Transport Overhead	🟢 Minimal	🟡 Medium (HTTP headers, reconnection)	🟡 Higher (duplex control, buffers)
Concurrency	❌ Usually single process/thread	✅ Supported with multiple HTTP connections	✅ Natively concurrent
Error Handling	❌ Basic (process exit codes)	🟡 Limited (need to parse events)	✅ Custom error/status codes possible
Tooling Complexity	🟢 Simple subprocess	🟡 Moderate (SSE client lib required)	🔴 High (custom protocol or server code needed)
Server Implementation	CLI app or local executable	Web server with SSE support	Custom backend (e.g., FastAPI with async streams)
Ideal Use Case	Local models or CLI tools	Hosted LLMs with streaming (e.g., OpenAI)	Agent/toolchain with rich, stateful interaction
Transport Type	IPC/OS process-based	Unidirectional HTTP stream	Bidirectional over HTTP
Compression Support	❌ None	⚠️ Limited (gzip encoding possible)	✅ Full control over compression
Backpressure Handling	❌ Minimal (buffer overflow risk)	⚠️ Poor (SSE lacks flow control)	✅ Good (can implement windowing/chunking)

Server Code Snippet

from mcp.server.fastmcp import FastMCP

mcp = FastMCP("Server1")

# start the server with default stdio transport
mcp.run(transport="stdio")

# start the server with streamable HTTP transport
mcp.run(transport="streamable-http")

# start the server with Server-Sent Events (SSE) transport
mcp.run(transport="sse") # http://127.0.0.1:8000/sse

Client Code Snippet (langchain adapter)

from langchain_mcp_adapters.client import MultiServerMCPClient

client = MultiServerMCPClient(
    {
        "server1": {
            "command": "python",
            "args": ["server1.py"],
            "transport": "stdio",
        },
        "server2": {
            "url": "http://localhost:8000/mcp",
            "transprot": "streamable_http"
        },
        "server3":{
            "url": "http://localhost:8000/sse",
            "transport": "sse"
        }
    }
)