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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@ -1844,6 +1844,109 @@ module MCP
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refute_predicate(client, :connected?)
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end
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def test_connect_modern_probes_discover_and_adopts_the_modern_lifecycle
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discover_stub = stub_discover
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result = client.connect(mode: :modern)
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assert_requested(discover_stub)
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assert_predicate(client, :connected?)
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assert_predicate(client, :modern?)
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assert_equal(["2026-07-28"], result["supportedVersions"])
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assert_equal(result, client.server_info)
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assert_equal("2026-07-28", client.protocol_version)
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end
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def test_modern_requests_carry_the_envelope_and_matching_header
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stub_discover
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client.connect(
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mode: :modern,
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client_info: { name: "my-app", version: "9.9" },
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capabilities: { elicitation: {} },
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)
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request_stub = stub_request(:post, url).with do |req|
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body = JSON.parse(req.body)
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meta = body.dig("params", "_meta")
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body["method"] == "tools/list" &&
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req.headers["Mcp-Protocol-Version"] == "2026-07-28" &&
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meta == {
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"io.modelcontextprotocol/protocolVersion" => "2026-07-28",
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"io.modelcontextprotocol/clientInfo" => { "name" => "my-app", "version" => "9.9" },
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"io.modelcontextprotocol/clientCapabilities" => { "elicitation" => {} },
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}
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end.to_return(
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status: 200,
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headers: { "Content-Type" => "application/json" },
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body: { result: { tools: [] } }.to_json,
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)
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client.send_request(request: { jsonrpc: "2.0", id: "1", method: "tools/list" })
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assert_requested(request_stub)
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end
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def test_connect_modern_fails_without_a_mutual_modern_version
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stub_request(:post, url).with do |req|
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JSON.parse(req.body)["method"] == "server/discover"
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end.to_return(
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status: 200,
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headers: { "Content-Type" => "application/json" },
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body: { result: { supportedVersions: ["2025-11-25"] } }.to_json,
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)
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error = assert_raises(RequestHandlerError) { client.connect(mode: :modern) }
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assert_includes(error.message, "no mutually supported modern protocol version")
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refute_predicate(client, :connected?)
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refute_predicate(client, :modern?)
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end
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def test_connect_auto_falls_back_to_the_legacy_handshake_on_discovery_errors
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stub_request(:post, url).with do |req|
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JSON.parse(req.body)["method"] == "server/discover"
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end.to_return(
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status: 200,
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headers: { "Content-Type" => "application/json" },
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body: { error: { code: -32601, message: "Method not found" } }.to_json,
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)
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init_stub = stub_initialize
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notification_stub = stub_notification
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result = client.connect(mode: :auto)
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assert_requested(init_stub)
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assert_requested(notification_stub)
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refute_predicate(client, :modern?)
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assert_predicate(client, :connected?)
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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_discovery_lacks_a_mutual_modern_version
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# Rollout tolerance: a server may answer discovery while only serving legacy versions;
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# auto mode falls back to the legacy handshake.
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stub_request(:post, url).with do |req|
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JSON.parse(req.body)["method"] == "server/discover"
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end.to_return(
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status: 200,
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headers: { "Content-Type" => "application/json" },
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body: { result: { supportedVersions: ["2025-11-25"] } }.to_json,
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)
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init_stub = stub_initialize
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stub_notification
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client.connect(mode: :auto)
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assert_requested(init_stub)
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refute_predicate(client, :modern?)
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assert_predicate(client, :connected?)
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end
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def test_connect_rejects_unknown_modes_and_legacy_versions_for_modern_mode
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assert_raises(ArgumentError) { client.connect(mode: :bogus) }
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assert_raises(ArgumentError) { client.connect(mode: :modern, protocol_version: "2025-11-25") }
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end
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def test_reconnect_after_close
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stub_initialize
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stub_notification
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@ -1949,6 +2052,24 @@ module MCP
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.to_return(status: 202, body: "")
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end
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def stub_discover
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stub_request(:post, url).with do |req|
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JSON.parse(req.body)["method"] == "server/discover"
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end.to_return(
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status: 200,
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headers: { "Content-Type" => "application/json" },
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body: {
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result: {
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supportedVersions: ["2026-07-28"],
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capabilities: { tools: {} },
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_meta: { "io.modelcontextprotocol/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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}.to_json,
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)
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end
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def stub_request(method, url)
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WebMock.stub_request(method, url)
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end
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