Finalize server/discover and Add Client Modern Lifecycle Support per SEP-2575
## Motivation and Context Final step of the stateless lifecycle (SEP-2575, modelcontextprotocol/modelcontextprotocol#2575) for the 2026-07-28 MCP spec release, now that the server core and both server transports can serve modern traffic. Server side, `Server#discover` reaches its spec-final shape: - `supportedVersions` advertises modern versions only, matching the TypeScript and Python SDKs: legacy versions are negotiated via `initialize`, not selected from discovery. This also closes an interop hazard of the interim response: auto-negotiating TS and Python clients treat any `DiscoverResult` as proof of a modern server and do not fall back to `initialize`, so advertising only legacy versions made them fail outright. - The `ttlMs`/`cacheScope` cache hints are REQUIRED on `DiscoverResult` (unlike the opt-in SEP-2549 hints on list and read results), so the spec defaults (`0`, `"private"`) fill in when the server was not configured with `ttl_ms`/`cache_scope`, and the configured SEP-2549 values are reused otherwise. - The server identity rides in the result `_meta` as the optional `io.modelcontextprotocol/serverInfo` stamp instead of a top-level `serverInfo` field, following spec PR modelcontextprotocol/modelcontextprotocol#3002 (the TypeScript SDK stamps `_meta` the same way and the Python `DiscoverResult` carries no top-level `serverInfo`, so the top-level placement was invisible to both). `Client#discover` and the `server_implementation` reader read the `_meta` stamp with a top-level fallback for servers built against the frozen SEP text, and the key is exposed as `RequestEnvelope::SERVER_INFO_META_KEY`. Client side, both bundled transports learn the modern lifecycle, and `MCP::Client` negotiates it automatically by default: - `MCP::Client#discover` sends `server/discover` and returns a `DiscoverResult` struct (`supported_versions`, `capabilities`, `server_info`, `instructions`, `ttl_ms`, `cache_scope`). It works before `connect`; the stdio transport's connect guard exempts `server/discover` and spawns the subprocess on demand. - `connect` gains `mode:` and defaults to automatic negotiation, joining the Python, Go, and C# SDKs (the TypeScript SDK keeps a legacy default): transports whose `connect` declares `mode:` (the bundled HTTP and stdio transports) receive `:auto`, which probes `server/discover` and falls back to the legacy handshake on any discovery failure, including a successful discovery without a mutually supported modern version (rollout tolerance for servers that answer discovery while only serving legacy versions). Custom transports whose `connect` does not declare `mode:` keep receiving the historical legacy call shape, detected by keyword introspection; a bare `**kwargs` deliberately does not count, so wrappers and test doubles that absorb arbitrary keywords stay on the legacy shape. An explicit `:modern`/`:auto` against such a transport raises `ArgumentError` instead of silently downgrading. - The mode vocabulary is `:legacy`/`:auto` (shared with the Python and TypeScript SDKs) plus an explicit `:modern`, instead of folding a version-string pin into `mode` as Python and TypeScript do: this client has carried the scalar `protocol_version:` keyword since the handshake landed, so a second version-bearing input would make the API ambiguous about which one wins. `mode:` stays the strategy and `protocol_version:` stays the version, the same orthogonal split as the Go and C# scalar version pins; Python's `mode="2026-07-28"` is expressed here as `mode: :modern` with `protocol_version: "2026-07-28"`. - An explicit legacy-generation `protocol_version` (e.g. `"2025-11-25"`) pins the legacy handshake without a probe, so an explicitly requested version is never overridden by the default negotiation adopting the modern lifecycle. `mode: :legacy` forces the classic handshake unconditionally; prefer it for spawn-per-invocation CLI tools and when using server-initiated requests, which exist only on the legacy lifecycle. - Because the raw `connect` return value and `server_info` mirror the wire result, their shape depends on the negotiated era (`InitializeResult` vs `DiscoverResult`). New era-independent readers absorb that difference: `MCP::Client#protocol_version` (negotiated or adopted version; backed by a new stdio transport reader and the existing HTTP one), `#server_capabilities`, `#instructions`, and `#server_implementation`, which reads the spec-final `_meta` `io.modelcontextprotocol/serverInfo` stamp with a top-level fallback and returns `nil` when a modern server does not identify itself. `server_info` remains the permanent raw window to everything the readers do not cover. - The stdio `server/discover` probe is bounded by a new `DEFAULT_DISCOVER_PROBE_TIMEOUT` (5 seconds, matching the C# SDK) when no `read_timeout` was configured: a compliant legacy server answers the probe with `-32601` immediately, but a non-compliant one that silently drops unknown methods must read as legacy evidence for the auto fallback instead of blocking `connect` forever. Regular requests keep the unbounded default. - In modern mode, the new shared `MCP::Client::ModernEnvelope` stamps the SEP-2575 `_meta` triple onto every request (never onto notifications, whose `_meta` carries no envelope), reusing the reserved key names from `MCP::RequestEnvelope`. The HTTP transport sends the matching `MCP-Protocol-Version` header and keeps the existing `Mcp-Method`/`Mcp-Name` mirror headers; no `Mcp-Session-Id` is ever attached because `initialize` is never sent. - The conformance client pins `mode: :legacy`: the referee validates the legacy `initialize` handshake and its scenario servers do not expect a preceding probe. Resolves #389. ## How Has This Been Tested? New tests in `test/mcp/server_test.rb` pin the required cache hints (spec defaults and configured SEP-2549 values); the existing discover assertions across `server_test.rb` and both transport test files were updated to the modern-only `supportedVersions`. New tests in `test/mcp/client_test.rb` cover the `DiscoverResult` struct mapping, `ValidationError`/`ServerError` on malformed and error responses, and the mode resolution contract: the `:auto` default on transports declaring `mode:`, the unchanged legacy call shape for transports without it, the legacy pin for an explicit stable `protocol_version`, `ArgumentError` for explicit non-legacy modes on transports without `mode:`, unknown-mode rejection, and the era-independent readers against legacy results, modern results, the `_meta` `serverInfo` stamp, and absence. New tests in `test/mcp/client/http_test.rb` cover modern adoption via discovery, envelope stamping with the matching header on subsequent requests, the no-mutual-version failure, both auto-mode fallbacks, and argument validation. New tests in `test/mcp/client/stdio_test.rb` cover pre-connect `server/discover`, modern adoption with envelope stamping and no `initialize` frame, the auto-mode fallback, the probe timeout default applying only while probing, and the fallback when a server never answers the probe. ## Breaking Changes `server/discover` responses change shape (modern-only `supportedVersions` plus the required `ttlMs`/`cacheScope`); this method has not shipped in a gem release yet, so no released behavior changes. On the bundled HTTP and stdio transports, `MCP::Client#connect` without `mode:` now probes `server/discover` before the legacy handshake: against legacy servers this adds one round trip and an unknown-method entry in their logs with an unchanged final result, and once a server serves the modern lifecycle the connection adopts it, changing the raw result shape from `InitializeResult` to `DiscoverResult`. Pass `mode: :legacy` or an explicit legacy-generation `protocol_version` to keep the previous behavior unconditionally, or use the era-independent readers, which are stable across both lifecycles. Custom transports without `mode:` and direct `transport.connect` calls are unaffected.
This commit is contained in:
Koichi ITO
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1 changed files with 1203 additions and 151 deletions
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@ -1414,6 +1414,198 @@ module MCP
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[transport, server_thread, [stdin_read, stdin_write, stdout_read, stdout_write]]
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end
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def test_send_request_allows_server_discover_before_connect
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stdin_read, stdin_write = IO.pipe
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stdout_read, stdout_write = IO.pipe
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stderr_read, _ = IO.pipe
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Open3.stubs(:popen3).returns([stdin_write, stdout_read, stderr_read, mock_wait_thread])
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transport = Stdio.new(command: "ruby", args: ["server.rb"])
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server_thread = Thread.new do
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discover_request = JSON.parse(stdin_read.gets)
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response = {
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jsonrpc: "2.0",
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id: discover_request["id"],
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result: { supportedVersions: ["2026-07-28"], ttlMs: 0, cacheScope: "private" },
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}
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stdout_write.puts(JSON.generate(response))
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stdout_write.flush
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end
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# No prior `connect`: `server/discover` (SEP-2575) is sessionless discovery.
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response = transport.send_request(request: { jsonrpc: "2.0", id: "d1", method: "server/discover" })
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assert_equal(["2026-07-28"], response.dig("result", "supportedVersions"))
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refute_predicate(transport, :connected?)
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ensure
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server_thread&.join
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[stdin_read, stdin_write, stdout_read, stdout_write, stderr_read].each do |io|
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io.close unless io.closed?
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end
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end
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def test_connect_modern_probes_discover_and_stamps_requests
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stdin_read, stdin_write = IO.pipe
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stdout_read, stdout_write = IO.pipe
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stderr_read, _ = IO.pipe
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Open3.stubs(:popen3).returns([stdin_write, stdout_read, stderr_read, mock_wait_thread])
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transport = Stdio.new(command: "ruby", args: ["server.rb"])
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received = []
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server_thread = Thread.new do
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discover_request = JSON.parse(stdin_read.gets)
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received << discover_request
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stdout_write.puts(JSON.generate({
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jsonrpc: "2.0",
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id: discover_request["id"],
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result: {
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supportedVersions: ["2026-07-28"],
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capabilities: {},
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serverInfo: { name: "test-server", version: "1.0" },
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ttlMs: 0,
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cacheScope: "private",
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},
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}))
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stdout_write.flush
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tools_request = JSON.parse(stdin_read.gets)
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received << tools_request
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stdout_write.puts(JSON.generate({ jsonrpc: "2.0", id: tools_request["id"], result: { tools: [] } }))
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stdout_write.flush
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end
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result = transport.connect(mode: :modern, client_info: { name: "my-app", version: "9.9" })
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response = transport.send_request(request: { jsonrpc: "2.0", id: "t1", method: "tools/list" })
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assert_equal(["2026-07-28"], result["supportedVersions"])
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assert_predicate(transport, :modern?)
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assert_predicate(transport, :connected?)
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assert_equal("2026-07-28", transport.protocol_version)
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assert_equal("t1", response["id"])
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# No `initialize` was sent; both frames carry the SEP-2575 envelope.
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assert_equal(["server/discover", "tools/list"], received.map { |frame| frame["method"] })
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triple = received[1].dig("params", "_meta")
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assert_equal("2026-07-28", triple["io.modelcontextprotocol/protocolVersion"])
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assert_equal({ "name" => "my-app", "version" => "9.9" }, triple["io.modelcontextprotocol/clientInfo"])
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assert_equal({}, triple["io.modelcontextprotocol/clientCapabilities"])
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ensure
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server_thread&.join
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[stdin_read, stdin_write, stdout_read, stdout_write, stderr_read].each do |io|
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io.close unless io.closed?
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end
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end
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def test_connect_auto_falls_back_to_the_legacy_handshake
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stdin_read, stdin_write = IO.pipe
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stdout_read, stdout_write = IO.pipe
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stderr_read, _ = IO.pipe
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Open3.stubs(:popen3).returns([stdin_write, stdout_read, stderr_read, mock_wait_thread])
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transport = Stdio.new(command: "ruby", args: ["server.rb"])
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server_thread = Thread.new do
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discover_request = JSON.parse(stdin_read.gets)
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stdout_write.puts(JSON.generate({
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jsonrpc: "2.0",
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id: discover_request["id"],
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error: { code: -32601, message: "Method not found" },
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}))
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stdout_write.flush
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init_request = JSON.parse(stdin_read.gets)
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stdout_write.puts(JSON.generate({
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jsonrpc: "2.0",
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id: init_request["id"],
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result: {
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protocolVersion: "2025-11-25",
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capabilities: {},
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serverInfo: { name: "test-server", version: "1.0" },
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},
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}))
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stdout_write.flush
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# `notifications/initialized`
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stdin_read.gets
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end
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result = transport.connect(mode: :auto)
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assert_equal("2025-11-25", result["protocolVersion"])
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assert_equal("2025-11-25", transport.protocol_version)
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refute_predicate(transport, :modern?)
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assert_predicate(transport, :connected?)
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ensure
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server_thread&.join
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[stdin_read, stdin_write, stdout_read, stdout_write, stderr_read].each do |io|
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io.close unless io.closed?
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end
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end
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def test_connect_auto_bounds_the_discover_probe_when_no_read_timeout_is_configured
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transport = Stdio.new(command: "ruby", args: ["server.rb"])
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transport.stubs(:start)
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observed_timeout = nil
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transport.define_singleton_method(:probe_discover) do
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observed_timeout = @read_timeout
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raise RequestHandlerError.new("probe timed out", {}, error_type: :internal_error)
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end
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transport.define_singleton_method(:connect_legacy) do |**|
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{ "protocolVersion" => "2025-11-25" }
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end
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result = transport.connect(mode: :auto)
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assert_equal(Stdio::DEFAULT_DISCOVER_PROBE_TIMEOUT, observed_timeout)
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assert_nil(transport.instance_variable_get(:@read_timeout))
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assert_equal("2025-11-25", result["protocolVersion"])
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end
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def test_connect_auto_falls_back_when_the_server_never_answers_the_probe
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stdin_read, stdin_write = IO.pipe
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stdout_read, stdout_write = IO.pipe
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stderr_read, _ = IO.pipe
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Open3.stubs(:popen3).returns([stdin_write, stdout_read, stderr_read, mock_wait_thread])
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# A configured `read_timeout` also bounds the probe; the server below reads
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# the `server/discover` request and never responds, which must count as
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# legacy evidence instead of blocking `connect` forever.
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transport = Stdio.new(command: "ruby", args: ["server.rb"], read_timeout: 0.05)
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server_thread = Thread.new do
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# Silently swallow the probe.
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stdin_read.gets
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init_request = JSON.parse(stdin_read.gets)
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stdout_write.puts(JSON.generate({
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jsonrpc: "2.0",
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id: init_request["id"],
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result: {
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protocolVersion: "2025-11-25",
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capabilities: {},
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serverInfo: { name: "test-server", version: "1.0" },
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},
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}))
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stdout_write.flush
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# `notifications/initialized`
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stdin_read.gets
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end
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result = transport.connect(mode: :auto)
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assert_equal("2025-11-25", result["protocolVersion"])
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refute_predicate(transport, :modern?)
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ensure
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server_thread&.join
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[stdin_read, stdin_write, stdout_read, stdout_write, stderr_read].each do |io|
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io.close unless io.closed?
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end
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end
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def mock_wait_thread
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thread = mock("wait_thread")
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thread.stubs(:alive?).returns(true)
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