MCP streamable HTTP vs stdio transport
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Contents
Short answer. stdio runs the server as a subprocess of the client and is the default for local tools; streamable HTTP exposes the server at a URL for remote or shared use, with optional Server-Sent Events for streaming. The older HTTP+SSE transport (two endpoints) is deprecated in favor of streamable HTTP.
Comparison
| stdio | Streamable HTTP | |
|---|---|---|
| Where the server runs | Same machine, child process | Anywhere |
| Sessions | Implicit (process lifetime) | Optional Mcp-Session-Id header |
| Auth | Inherits the user's environment | OAuth 2.1 / bearer tokens |
| Multiple clients | One per process | Many |
| Streaming | Line-delimited JSON-RPC | SSE responses when needed |
Details
- A streamable HTTP server accepts
POST /mcpwith JSON-RPC and may answer withapplication/jsonortext/event-stream. Stateless servers can ignore sessions entirely. - Clients must send
MCP-Protocol-Versionafter initialization; servers should tolerate its absence. - CORS headers are required if browser-based clients connect.
Sources
- MCP specification, Transports (checked 2026-09-10).