c145876bb9
## Motivation and Context SEP-2575 (modelcontextprotocol/modelcontextprotocol#2575, merged for the 2026-07-28 spec release) makes MCP stateless, removing the `initialize`/`notifications/initialized` handshake in favor of per-request `_meta`-carried negotiation. The full lifecycle rewrite is unmerged in both the TypeScript SDK (the closed #2063/#2064/#2251 prototype stack, on hold since 2026-06-08) and the Python SDK, but two pieces of its wire contract stayed stable across every prototype iteration and are purely additive: - The `server/discover` method, which servers MUST implement: sessionless capability discovery returning `supportedVersions`, `capabilities`, `serverInfo`, and `instructions`. The new `Server#discover` handler reuses the same data `init` already builds, but is state-free and idempotent: it stores no client info, never marks a session initialized, and responds regardless of declared capabilities or initialization state (added to the no-capability group in `Methods.ensure_capability!`). `serverInfo` is returned unfiltered because discovery precedes version negotiation; the draft's `ttlMs`/`cacheScope` fields on `DiscoverResult` are left to the SEP-2549 work. On `StreamableHTTPTransport`, a `server/discover` POST is exempt from the `Mcp-Session-Id` requirement and the `MCP-Protocol-Version` header check, like `initialize`, so clients can probe a server with a bare POST in both stateful and stateless modes. - The error code vocabulary: the new `MCP::ErrorCodes` module exports `MISSING_REQUIRED_CLIENT_CAPABILITY = -32003` and `UNSUPPORTED_PROTOCOL_VERSION = -32004`. Constants only; nothing raises them yet (that belongs to the 2026-07-28 lifecycle work), and the existing inline `-32042` of `URLElicitationRequiredError` is intentionally left untouched. The removal of `initialize`, `ping`, and `logging/setLevel` that SEP-2575 also specifies is deliberately NOT implemented: the SDK targets protocol versions up to 2025-11-25, where those methods are required. Part of #389. ## How Has This Been Tested? - `test/mcp/server_test.rb`: `server/discover` returns the supported versions, the server's capabilities, server info, and instructions; it responds before `initialize` and with empty capabilities; it does not mark the session initialized (a subsequent `initialize` still succeeds); `instructions` is omitted when the server has none; and `define_custom_method(method_name: "server/discover")` now raises `MethodAlreadyDefinedError`. - `test/mcp/methods_test.rb`: `ensure_capability!` does not raise for `SERVER_DISCOVER` with empty capabilities. - `test/mcp/server/transports/streamable_http_transport_test.rb`: a `server/discover` POST without a session ID returns 200 with the discover result in both stateful and stateless modes, without issuing an `Mcp-Session-Id`. - New `test/mcp/error_codes_test.rb` pins the -32003/-32004 values. `bundle exec rake` (tests, RuboCop, and conformance baseline) passes. ## Breaking Changes None at the wire level for existing clients. One narrow API consequence: servers that previously registered their own `"server/discover"` handler via `define_custom_method` now get `MethodAlreadyDefinedError` because the method is built in.
13 lines
412 B
Ruby
13 lines
412 B
Ruby
# frozen_string_literal: true
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require "test_helper"
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module MCP
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class ErrorCodesTest < ActiveSupport::TestCase
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test "exposes the SEP-2575 stateless lifecycle error codes" do
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# The exact values are wire vocabulary shared with other SDKs.
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assert_equal(-32021, ErrorCodes::MISSING_REQUIRED_CLIENT_CAPABILITY)
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assert_equal(-32022, ErrorCodes::UNSUPPORTED_PROTOCOL_VERSION)
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end
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end
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end
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