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## Motivation and Context Companion to the stateless lifecycle work (SEP-2575, modelcontextprotocol/modelcontextprotocol#2575) for the 2026-07-28 MCP spec release. The modern lifecycle removed the HTTP GET listening stream; `subscriptions/listen` replaces it as a long-lived POST that opts in to server change notifications. The Python SDK ships the server side of this as its SEP-2575 event-bus work (python-sdk PR /modelcontextprotocol/python-sdk#3035); the TypeScript SDK landed it in PR modelcontextprotocol/typescript-sdk#2321. Wire behavior, per the draft schema: - The request carries a REQUIRED `notifications` filter (`SubscriptionFilter`): `toolsListChanged`, `promptsListChanged`, `resourcesListChanged`, and `resourceSubscriptions` (URI list, replacing the legacy `resources/subscribe` RPC). Every type is opt-in; the server MUST NOT send types the client did not request. - The first stream message is `notifications/subscriptions/acknowledged`, reporting the subset of requested types the server agreed to honor. Honoring reads the capability FLAGS that promise delivery (`listChanged`, `subscribe`), the same derivation `server/discover` uses for its era-aware capability stripping; the mere presence of a primitive's capability is not enough. - Every notification delivered on the stream carries the correlating `io.modelcontextprotocol/subscriptionId` (the listen request id) in `_meta`. - A graceful teardown (transport `close`) sends a `SubscriptionsListenResult` response before closing the stream, stamped with the REQUIRED 2026-07-28 `resultType` at its construction site (it never passes through the dispatch path); an abrupt disconnect sends nothing. Implementation: - `StreamableHTTPTransport` intercepts `subscriptions/listen` on the modern path (after header and envelope validation) and serves it as a long-lived SSE stream, using the same register-and-return body proc pattern as the legacy GET stream. Subscriptions live in an in-process registry keyed by the listen request id; fan-out hooks into `send_notification` ahead of the legacy delivery, so a resource updated by one session's tool call also reaches modern subscribers. The matching snapshot is taken under the transport mutex, but stream writes happen outside it, matching the legacy delivery paths: a slow or stalled subscriber must not block the transport. Duplicate subscription ids close the new stream instead of double-registering. - Concurrent listen streams are capped (`max_listen_subscriptions:`, default 1000, `nil` to opt out); a listen request past the cap is rejected with HTTP 503, like the `max_sessions` guard against session floods. Each stream holds an open connection for its lifetime, so without a bound an unauthenticated client can retain unbounded connections. - Each stream is kept alive by an SSE comment frame written every `listen_keepalive_interval:` seconds (default 15, matching the TypeScript SDK; `nil` to opt out when an upstream proxy pings the stream). A silently dropped connection would otherwise hold its capped slot until the next fan-out write failed, so on a quiet server the cap would ratchet down permanently; the periodic write detects the dead peer and frees the slot. A comment frame cannot corrupt an interleaved notification's JSON, and the write happens outside the transport mutex, reusing the legacy GET stream's keepalive mechanism. - Every SSE response now carries `x-accel-buffering: no`, which the spec asks of SSE streams and both reference SDKs send: a buffering reverse proxy would hold events back instead of delivering them as they are written, and on a listen stream it would also swallow the keepalive frames a dropped peer is detected by. The header rides the shared `SSE_HEADERS`, so the legacy streams gain it too. - stdio does not serve the stream, matching the Python SDK's stream-pair behavior: no server handler is registered, so the method answers `-32601`. - `Server#discover` becomes era-aware about notification delivery: `listChanged`/`subscribe` capability flags promise delivery over `subscriptions/listen` streams in the modern lifecycle, so they are stripped when the transport does not serve that RPC (the new `Transport#serves_subscriptions_listen?` seam, true for `StreamableHTTPTransport`). - The conformance fixture defines the diagnostic triggers the `server-stateless` scenario calls (`test_trigger_tool_change` / `test_trigger_prompt_change`): each broadcasts its list-changed notification to the listen streams and returns, mirroring the suite's TypeScript reference fixture, which mutates nothing either. Out of scope, noted for follow-ups: an external event bus for multi-worker deployments, and the client-side listen driver (the Python reference is python-sdk PR modelcontextprotocol/python-sdk#3047). Refs #389. ## How Has This Been Tested? New tests in `test/mcp/server/transports/streamable_http_transport_test.rb` cover: the SSE response with the acknowledgement as the first event (including the honored-subset reduction for unsupported types and for capability entries lacking the delivery flag), the `notifications` filter and envelope requirements (400 responses), opt-in-only delivery with the correlating `subscriptionId`, URI-scoped `resources/updated` delivery, per-subscription ids across concurrent streams, the graceful close result carrying `resultType: "complete"`, duplicate-id rejection, the 503 past the concurrent stream cap, the keepalive (writing the comment frame outside the mutex, freeing a dead peer's slot, no thread when the interval is `nil`, and the positive-or-nil interval validation), and `x-accel-buffering: no` on both the listen stream and the legacy GET stream. `test/mcp/server/transports/stdio_transport_test.rb` asserts `-32601` over stdio, and `test/mcp/server_test.rb` covers the era-aware capability stripping in `server/discover` for both transport kinds. `bundle exec rake` (tests, RuboCop, and conformance baseline) passes. Against the conformance fixture server at `--spec-version 2026-07-28`, the `server-stateless` subscription checks all report SUCCESS: the acknowledgement, `subscriptionId` tagging, and filter-containment MUSTs, plus both list-changed SHOULD checks driven by the new trigger tools. The `--requirements 2025-11-25` server leg passes 78/78, unchanged. ## Breaking Changes None. The method was previously unhandled (`-32601` everywhere); the only observable change to existing responses is that `server/discover` no longer advertises `listChanged`/`subscribe` flags on transports that cannot deliver those notifications in the modern lifecycle, which `server/discover` has not shipped in a gem release with anyway. |
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| .. | ||
| client.rb | ||
| client_runner.rb | ||
| expected_failures.yml | ||
| README.md | ||
| server.rb | ||
| server_runner.rb | ||
MCP Conformance Tests
Validates the Ruby SDK's conformance to the MCP specification using @modelcontextprotocol/conformance.
Prerequisites
- Node.js (for
npx) bundle installcompleted
Running the Tests
Run all scenarios
bundle exec rake conformance
Runs both server and client conformance tests in sequence. Server conformance starts the
conformance server and tests it. Client conformance spawns test servers for each scenario
and invokes conformance/client.rb against them. Scenarios listed in expected_failures.yml
are allowed to fail without affecting the exit code.
Environment variables
| Variable | Description | Default |
|---|---|---|
PORT |
Server port (server conformance only) | 9292 |
SCENARIO |
Run a single scenario by name | (all) |
SPEC_VERSION |
Filter scenarios by spec version | (all) |
VERBOSE |
Show raw JSON output when set | (off) |
# Run a single server scenario
bundle exec rake conformance SCENARIO=ping
# Use a different port with verbose output
bundle exec rake conformance PORT=3000 VERBOSE=1
# Start the server on a specific port
bundle exec rake conformance_server PORT=3000
Start the server and test separately
# Terminal 1: start the server
bundle exec rake conformance_server
# Terminal 2: run all scenarios
npx @modelcontextprotocol/conformance@alpha server --url http://localhost:9292/mcp
# Terminal 2: run a single scenario
npx @modelcontextprotocol/conformance@alpha server --url http://localhost:9292/mcp --scenario ping
Keeps the server alive between test runs, which avoids the startup overhead when iterating on a single scenario. Stop the server with Ctrl+C when done.
List available scenarios
bundle exec rake conformance_list
Prints all scenario names that can be passed to SCENARIO.
SDK Tier Report
The MCP SDK Tier system requires SDK maintainers to self-assess and report results to the SDK Working Group via modelcontextprotocol/modelcontextprotocol issues.
To generate a full tier assessment report, use the /mcp-sdk-tier-audit slash command from
the modelcontextprotocol/conformance
repository with the conformance server running:
# Terminal 1 (this repository): start the conformance server
bundle exec rake conformance_server
# Terminal 2 (conformance repository): run the tier audit skill as a slash command in Claude Code
/mcp-sdk-tier-audit /path/to/modelcontextprotocol/ruby-sdk http://localhost:9292/mcp
The skill evaluates conformance pass rate, issue label taxonomy, triage metrics, documentation coverage, and policy compliance, then produces a markdown report suitable for tier advancement submissions.
File Structure
conformance/
server.rb # Conformance server (Rack + Puma, default port 9292)
server_runner.rb # Starts the server, runs npx conformance server, exits with result code
client.rb # Conformance client (invoked by npx conformance client)
client_runner.rb # Runs npx conformance client, exits with result code
expected_failures.yml # Baseline of known-failing scenarios (server and client)
README.md # This file
Known Limitations
Known-failing scenarios are registered in conformance/expected_failures.yml. They are allowed to
fail without affecting the exit code and are tracked to catch regressions.
These are shown in the output of bundle exec rake conformance.