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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/⅔)
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#

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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)#

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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"
        }
    }
)