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Refuse server-to-client requests in the modern lifecycle per SEP-2575

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## Motivation and Context

The modern lifecycle removes server-initiated JSON-RPC requests: a stateless server has no connection to send them on,
and SEP-2322 multi round-trip `input_required` results replace what sampling and elicitation covered before.
`StdioTransport#send_request` already says so, refusing outright when its session is modern.
`StreamableHTTPTransport#send_request` says nothing, so a modern handler that reaches for elicitation
or sampling falls through to the session lookup and is told `Session not found: <uuid>` against a session id it never chose.
Failing is right, the explanation is not: `handle_modern` mints a session per request without ever registering it in `@sessions`,
so that lookup cannot succeed for a modern request no matter what the handler does.

`send_request` now refuses a modern-era session up front with the message `StdioTransport` uses, which turns
an incidental registration detail into a stated rule and leaves the rest of the method a legacy-only path by construction.
Callers see the same `RuntimeError` class as before, only with a message naming the actual constraint.

## How Has This Been Tested?

A new test in `test/mcp/server/transports/streamable_http_transport_test.rb` sends `elicitation/create` from
a session constructed with `era: :modern` and asserts the SEP-2575 message. It sits with the existing guard tests for stateless mode,
a missing `session_id`, and an unknown session, which still pass unchanged.
`bundle exec rake` is green, and both conformance legs pass their baseline check.

## Breaking Changes

None. The call already failed; only the message changes.
This commit is contained in:
Koichi ITO 2026-08-11 03:21:33 +09:00
commit bea636c007
1 changed files with 22 additions and 0 deletions
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12
test/mcp/server/transports/streamable_http_transport_test.rb
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@ -2939,6 +2939,18 @@ module MCP
assert_equal("Stateless mode does not support server-to-client requests.", error.message)
end
test "send_request refuses to run at all in the modern lifecycle" do
# SEP-2575 removes server-initiated requests, so a modern handler reaching for elicitation or sampling
# is told why rather than being sent down a path that fails on an unrelated session lookup.
modern_session = ServerSession.new(server: @server, transport: @transport, session_id: "modern-1", era: :modern)
error = assert_raises(RuntimeError) do
@transport.send_request("elicitation/create", { message: "hi" }, session_id: "modern-1", server_session: modern_session)
end
assert_equal("Server-initiated requests are not available in the modern lifecycle (SEP-2575).", error.message)
end
test "send_request raises error when session_id is not provided" do
error = assert_raises(RuntimeError) do
@transport.send_request("sampling/createMessage", { "messages" => [] })