Stateless MCP: One Request per Tool Call¶

A tool call is a single self-describing HTTP request, so any MCP server instance can answer it without shared session state.

The 2026-07-28 Model Context Protocol specification retired the initialize/initialized exchange and the Mcp-Session-Id header. Each request now carries its protocol version, client identity, and client capabilities in a _meta object, and names its operation in the Mcp-Method and Mcp-Name HTTP headers. The maintainers state the consequence directly: "any request can now land on any server instance behind a plain round-robin load balancer without needing shared storage."

The payoff is conditional. It lands when you run more than one instance, when your tool surface is small enough that context cost is not the binding constraint, and when you do not depend on the features this revision deprecated. Outside those conditions the revision is a migration bill.

What changed on the wire¶

Three headers carry what the handshake used to establish. SEP-2243 standardizes two of them; it treats the third as already in circulation, since "clients already include headers like Mcp-Protocol-Version".

Mcp-Method mirrors the JSON-RPC method, for example tools/call. SEP-2243 requires it on all requests and notifications.
Mcp-Name mirrors params.name or params.uri — the tool, resource, or prompt — and SEP-2243 requires it on tools/call, resources/read, and prompts/get.
MCP-Protocol-Version names the spec revision, for example 2026-07-28. It predates this revision; with no handshake to settle it, "each request carries an Mcp-Protocol-Version header" (AgentCore Gateway).

Because the method and target sit in headers, a gateway, rate limiter, or web application firewall routes and meters on them "instead of parsing JSON bodies" (MCP blog). The headers duplicate fields in the body, and SEP-2243 closes the gap that would open if the two disagreed: servers that process the request body "MUST reject requests where the values specified in the headers do not match the values in the request body". Optional Mcp-Param-* headers, bound to tool parameters via x-mcp-header, "follow the same validation rules as standard headers".

Two more changes follow from dropping the session (MCP blog). Server-initiated elicitation, sampling, and roots requests no longer need a held-open stream: under Multi Round-Trip Requests the server returns resultType: "input_required" and the client retries the original call with inputResponses attached. List responses from tools/list, prompts/list, resources/list, and resources/read now carry ttlMs and cacheScope, so clients cache tool catalogs across reconnects.

Why it works¶

Removing protocol sessions deletes an affinity requirement rather than deleting work. Under the older spec the server minted an Mcp-Session-Id during initialize, so every later request from that client had to reach the instance holding that state, enforced with sticky load balancing, a shared session store, or gateway body inspection. Making each request self-describing means the request carries everything a handler needs, which is the ordinary HTTP scaling property applied to a protocol that had opted out of it (MCP blog).

State that must persist is relocated, not removed. The maintainers' pattern is to "mint an explicit handle from a tool and have the model pass it back as an argument", which they argue "works better than session state hidden in the transport - the model can see the handle and thread it between tools" (MCP blog).

When this backfires¶

One instance, low traffic: sticky routing costs nothing when there is nothing to be sticky about, so the migration buys no scaling and adds work.
Context-bound agents: statelessness does not reduce tool-schema tokens. Anthropic measured a five-server setup at roughly 55,000 tokens across 58 tool definitions, and 134,000 tokens internally before optimization (Anthropic). Deferred tool loading addresses that cost separately.
Genuinely stateful tools: handles now ride in tool arguments the model must carry forward. A dropped or mangled handle turns a transport guarantee into a prompt-reliability problem.
Servers built on deprecated features: Roots, Sampling, Logging, and the legacy HTTP+SSE transport are deprecated with a minimum twelve-month window, and Tasks moved into an extension (MCP blog). Those servers migrate twice.
Gateways that route on headers without enforcing them: the routing benefit assumes the server rejects header and body disagreement. Without that check, Mcp-Name becomes an authorization surface that need not match what the server executes.

Migrating a running server¶

JSON-RPC bodies, tool and resource semantics, and the endpoint shape all stay. Four assumptions have to go:

That an initialize call preceded this request. Read protocol version and client info from _meta on every request instead.
That a session ID identifies the caller. Mint an explicit handle from a tool where continuity is needed.
That capabilities were negotiated once. Clients that want them up front call server/discover.
That a long-lived stream is available for server-initiated prompts. Replace it with Multi Round-Trip Requests.

Emit and validate the two routing headers on every request. The maintainers concede "there will be some migration cost, especially for developers that did depend on session identifiers" (MCP blog).

Example¶

The release candidate published the two shapes side by side, reproduced by Simon Willison.

Before — two requests, the first only to mint a session:

POST /mcp HTTP/1.1
Content-Type: application/json

{"jsonrpc":"2.0","id":1,"method":"initialize",
 "params":{"protocolVersion":"2025-11-25","capabilities":{},
 "clientInfo":{"name":"my-app","version":"1.0"}}}

POST /mcp HTTP/1.1
Mcp-Session-Id: 1868a90c-3a3f-4f5b
Content-Type: application/json

{"jsonrpc":"2.0","id":2,"method":"tools/call",
 "params":{"name":"search","arguments":{"q":"otters"}}}

After — one request that describes itself:

POST /mcp HTTP/1.1
MCP-Protocol-Version: 2026-07-28
Mcp-Method: tools/call
Mcp-Name: search
Content-Type: application/json

{"jsonrpc":"2.0","id":1,"method":"tools/call",
 "params":{"name":"search","arguments":{"q":"otters"},
 "_meta":{"io.modelcontextprotocol/clientInfo":{"name":"my-app","version":"1.0"}}}}

Key Takeaways¶

The 2026-07-28 revision removed the initialize handshake and Mcp-Session-Id, so a tool call is one self-describing request.
The two headers SEP-2243 adds, Mcp-Method and Mcp-Name, let gateways route and meter without reading the body, and servers reject header and body disagreement.
Operationally you drop the session store, the sticky routing, and a chunk of client and server code.
Statelessness does not reduce tool-definition token cost, which is the separate reason teams reach for skills or a shell.
Roots, Sampling, Logging, and HTTP+SSE are deprecated on a twelve-month clock, so migration is scheduled work rather than optional work.