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[Doc] Clarify that StreamableHTTPTransport requires a single-process server

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## Motivation and Context

`StreamableHTTPTransport` stores session and SSE stream state in memory using
instance variables (`@sessions`, `@mutex`). This design requires all HTTP requests
to be handled within the same process to share state.

Process-based servers like Unicorn fork separate worker processes that do not share memory,
which breaks session management and SSE connections.
This requirement is not obvious from the current documentation.

## How Has This Been Tested?

Documentation-only change.

## Breaking Changes

None.
This commit is contained in:
Koichi ITO 2026-04-03 20:42:59 +09:00
commit 9ebb42a536
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README.md
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@ -303,6 +303,13 @@ transport = MCP::Server::Transports::StreamableHTTPTransport.new(server, session
### Usage
> [!IMPORTANT]
> `MCP::Server::Transports::StreamableHTTPTransport` stores session and SSE stream state in memory,
> so it must run in a single process. Use a single-process server (e.g., Puma with `workers 0`).
> Multi-process configurations (Unicorn, or Puma with `workers > 0`) fork separate processes that
> do not share memory, which breaks session management and SSE connections.
> Stateless mode (`stateless: true`) does not use sessions and works with any server configuration.
#### Rails Controller
When added to a Rails controller on a route that handles POST requests, your server will be compliant with non-streaming