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Reject notifications carrying an unknown or expired session

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## Motivation and Context

In `StreamableHTTPTransport#handle_post`, the response and regular-request branches reject
a message whose session no longer exists (the response branch via `session_exists?`,
the regular path via `validate_and_touch_session`), but the notification branch dispatched unconditionally.
A notification carrying a present-but-unknown (or expired-but-not-yet-reaped) `Mcp-Session-Id`
therefore passed the `missing_session_id` gate and the ownership gate (which returns true
when the session does not exist) and reached `dispatch_notification` with no session,
running against the shared `Server` instead of a live `ServerSession`. With the default no-op notification
handlers this had no visible effect, but a custom notification handler would then run without
a live session behind it.

`handle_post` now checks `session_active?` in the notification branch, mirroring the other two branches.
`session_active?` rejects an unknown or expired session (evicting the expired one) without extending its lifetime,
so a stale session ID yields `404 Session not found` and a live session is dispatched exactly as before.

## How Has This Been Tested?

New tests in `test/mcp/server/transports/streamable_http_transport_test.rb` assert that
a notification with an unknown session ID is rejected with `404 Session not found`
and that a notification carrying a live session ID is still accepted with `202`.
The new rejection test fails against the previous code (it returned `202`) and passes now.

## Breaking Changes

None for a conforming client: a client that has completed `initialize` sends its notifications with
the live session ID and is unaffected. Only a notification bearing an unknown or expired session ID,
previously accepted with `202`, now receives `404 Session not found`.
This commit is contained in:
Koichi ITO 2026-07-10 01:01:15 +09:00
commit 6236ea1b34
2 changed files with 39 additions and 0 deletions
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5
lib/mcp/server/transports/streamable_http_transport.rb
View file

@ -517,6 +517,11 @@ module MCP
end
if notification?(body)
# Reject a notification carrying an unknown or expired session ID instead of
# dispatching it against the shared `Server`. Mirrors the response and regular-request
# branches; without it a custom notification handler could run without a live session.
return session_not_found_response if !@stateless && !session_active?(session_id)
dispatch_notification(body_string, session_id)
handle_accepted
elsif response?(body)

34
test/mcp/server/transports/streamable_http_transport_test.rb
View file

@ -525,6 +525,40 @@ module MCP
assert_equal "Missing session ID", body["error"]["message"]
end
test "rejects notification with an unknown session ID in stateful mode" do
request = create_rack_request(
"POST",
"/",
{ "CONTENT_TYPE" => "application/json", "HTTP_MCP_SESSION_ID" => "does-not-exist" },
{ jsonrpc: "2.0", method: "notifications/initialized" }.to_json,
)
response = @transport.handle_request(request)
assert_equal 404, response[0]
body = JSON.parse(response[2][0])
assert_equal "Session not found", body["error"]["message"]
end
test "accepts a notification carrying a live session ID in stateful mode" do
init_request = create_rack_request(
"POST",
"/",
{ "CONTENT_TYPE" => "application/json" },
{ jsonrpc: "2.0", method: "initialize", id: "init", params: initialize_params }.to_json,
)
session_id = @transport.handle_request(init_request)[1]["Mcp-Session-Id"]
notification = create_rack_request(
"POST",
"/",
{ "CONTENT_TYPE" => "application/json", "HTTP_MCP_SESSION_ID" => session_id },
{ jsonrpc: "2.0", method: "notifications/initialized" }.to_json,
)
response = @transport.handle_request(notification)
assert_equal 202, response[0]
end
test "rejects response without session ID in stateful mode" do
request = create_rack_request(
"POST",