Why Streamable HTTP Beats HTTP+SSE in MCP: Stability, Performance, and Simplicity

Background

Model Context Protocol (MCP) is a standard for communication between AI models and tools. The original transport used HTTP combined with Server‑Sent Events (SSE), which shows stability, performance and client‑side complexity issues in high‑concurrency scenarios.

Issues with HTTP + SSE

Long‑living connections – servers must keep persistent connections, consuming resources under load.

Message delivery limited to SSE – adds unnecessary overhead.

Infrastructure compatibility – firewalls and load balancers often terminate long SSE streams.

Streamable HTTP introduced in PR #206

The new transport replaces the dual‑channel design with a single, on‑demand HTTP endpoint that can return a normal response or stream data when needed. Key improvements:

Unified endpoint : removes the dedicated /sse path.

On‑demand streaming : server chooses between plain HTTP and streaming.

Session management : adds a session mechanism for stateful interactions.

Stability comparison

In a simulated test with 1 000 concurrent users, the SSE server required a separate TCP connection for each request, causing the number of open connections to explode, while Streamable HTTP reused a few dozen connections.

TCP connection count

Results show that HTTP + SSE continuously increases TCP connections, whereas Streamable HTTP keeps the count low by establishing connections only when needed.

Request success rate

When the number of concurrent users approaches the OS limit (≈1024 connections), the SSE server’s success rate drops sharply, while Streamable HTTP maintains a high success ratio.

Performance

Response‑time measurements (log scale) reveal that the SSE server’s latency grows from 0.0018 s to 1.5112 s as concurrency rises, whereas the Streamable HTTP server stays around 0.0075 s, benefiting from the high‑performance Higress gateway.

Client‑side complexity

Sample client code demonstrates that the SSE implementation must handle connection setup, reconnection logic and separate POST calls, whereas the Streamable HTTP client only sends a single POST request and processes the response.

Code examples

class SSEClient:
    def __init__(self, url: str, headers: dict = None):
        self.url = url
        self.headers = headers or {}
        self.event_source = None
        self.endpoint = None

    async def connect(self):
        # 1. Establish SSE connection
        async with aiohttp.ClientSession(headers=self.headers) as session:
            self.event_source = await session.get(self.url)
            # 2. Handle connection event
            print('SSE connection established')
            # 3. Process messages
            async for line in self.event_source.content:
                if line:
                    message = json.loads(line)
                    await self.handle_message(message)
                # 4. Error handling and reconnection
                if self.event_source.status != 200:
                    print(f'SSE error: {self.event_source.status}')
                    await self.reconnect()

    async def send(self, message: dict):
        # Requires an extra POST request
        async with aiohttp.ClientSession(headers=self.headers) as session:
            async with session.post(self.endpoint, json=message) as response:
                return await response.json()

    async def handle_message(self, message: dict):
        print(f'Received message: {message}')

    async def reconnect(self):
        print('Attempting to reconnect...')
        await self.connect()

class StreamableHTTPClient:
    def __init__(self, url: str, headers: dict = None):
        self.url = url
        self.headers = headers or {}

    async def send(self, message: dict):
        # 1. Send POST request
        async with aiohttp.ClientSession(headers=self.headers) as session:
            async with session.post(self.url, json=message,
                                    headers={'Content-Type': 'application/json'}) as response:
                # 2. Handle response
                if response.status == 200:
                    return await response.json()
                else:
                    raise Exception(f'HTTP error: {response.status}')

Conclusions

Streamable HTTP offers better stability, lower TCP connection usage, higher success rates under load, shorter and more predictable response times, and a simpler client implementation compared with the legacy HTTP + SSE transport.