McpServer.layerHttp: optional legacy GET SSE for idle list-change notifications #10436

Feature request

Please consider an opt-in standalone GET SSE stream for sessionful MCP protocol revisions on McpServer.layerHttp, so an idle connected client can receive notifications/tools/list_changed (and the other list-change notifications).

This is a request for a currently documented limitation, not a claim that returning 405 violates the protocol: the GET event stream is optional. Effect 4.1.2 explicitly excludes GET SSE in the layerHttp documentation.

Reproduction and observed behavior

Environment: effect@4.1.2, @effect/platform-node@4.1.2, Node 26.12.1, and the independent @modelcontextprotocol/sdk@1.32.2 client.

1. Run a long-lived McpServer.layerHttp endpoint with protocols: [McpProtocol.v2025_11_25] and a registered toolkit.
2. Connect the SDK Client with StreamableHTTPClientTransport, and register a ToolListChangedNotificationSchema handler.
3. Initialization, listing and tool calls succeed; the client retains its negotiated session id.
4. The SDK opens an idle GET stream on the same endpoint. The endpoint returns 405.
5. Update the registered tool description while the client is idle. A subsequent listTools sees the new description, but the notification handler receives no tools/list_changed (a separate 5-second wait expires).
6. The same dynamic catalog and SDK client over layerStdio / StdioClientTransport receive the notification without reconnecting.

The downstream proof exercises real HTTP calls and real stdio in a child process, including a live implementation transition. We keep the notification assertion on stdio and document the HTTP limitation rather than implement an alternate transport in the application.

Desired behavior

With an explicit option enabling legacy GET SSE, retain the stream under the MCP session owner and deliver server-initiated notifications independently of an active POST. Scope closure, session termination and client disconnect should close it. This need not include replay, event resumption or session expiry.

An application can change a tool's description or side-effect hints after replacing its implementation. Existing clients need to learn about that change before their next call; polling listTools on every call is not equivalent to the notification mechanism.

This differs from #8060, which covered notifications emitted during an active request, and #8919, which covered optional subscriptions for the 2026 protocol. Stdio and the newer subscription transport remain usable alternatives.