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.
103 lines
4.1 KiB
Ruby
103 lines
4.1 KiB
Ruby
# frozen_string_literal: true
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module MCP
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module Methods
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INITIALIZE = "initialize"
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PING = "ping"
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LOGGING_SET_LEVEL = "logging/setLevel"
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# Sessionless capability discovery (MCP 2026-07-28 draft, SEP-2575).
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SERVER_DISCOVER = "server/discover"
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PROMPTS_GET = "prompts/get"
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PROMPTS_LIST = "prompts/list"
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COMPLETION_COMPLETE = "completion/complete"
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RESOURCES_LIST = "resources/list"
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RESOURCES_READ = "resources/read"
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RESOURCES_TEMPLATES_LIST = "resources/templates/list"
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RESOURCES_SUBSCRIBE = "resources/subscribe"
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RESOURCES_UNSUBSCRIBE = "resources/unsubscribe"
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TOOLS_CALL = "tools/call"
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TOOLS_LIST = "tools/list"
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ROOTS_LIST = "roots/list"
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SAMPLING_CREATE_MESSAGE = "sampling/createMessage"
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ELICITATION_CREATE = "elicitation/create"
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# Notification methods
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NOTIFICATIONS_INITIALIZED = "notifications/initialized"
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NOTIFICATIONS_TOOLS_LIST_CHANGED = "notifications/tools/list_changed"
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NOTIFICATIONS_PROMPTS_LIST_CHANGED = "notifications/prompts/list_changed"
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NOTIFICATIONS_RESOURCES_LIST_CHANGED = "notifications/resources/list_changed"
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NOTIFICATIONS_RESOURCES_UPDATED = "notifications/resources/updated"
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NOTIFICATIONS_ROOTS_LIST_CHANGED = "notifications/roots/list_changed"
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NOTIFICATIONS_MESSAGE = "notifications/message"
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NOTIFICATIONS_PROGRESS = "notifications/progress"
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NOTIFICATIONS_CANCELLED = "notifications/cancelled"
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NOTIFICATIONS_ELICITATION_COMPLETE = "notifications/elicitation/complete"
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class MissingRequiredCapabilityError < StandardError
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attr_reader :method
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attr_reader :capability
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def initialize(method, capability)
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super("Server does not support #{capability} (required for #{method})")
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@method = method
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@capability = capability
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end
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end
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class << self
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def notification?(method)
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method.is_a?(String) && method.start_with?("notifications/")
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end
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def ensure_capability!(method, capabilities)
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case method
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when PROMPTS_GET, PROMPTS_LIST
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require_capability!(method, capabilities, :prompts)
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when NOTIFICATIONS_PROMPTS_LIST_CHANGED
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require_capability!(method, capabilities, :prompts)
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require_capability!(method, capabilities, :prompts, :listChanged)
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when RESOURCES_LIST, RESOURCES_TEMPLATES_LIST, RESOURCES_READ
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require_capability!(method, capabilities, :resources)
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when NOTIFICATIONS_RESOURCES_LIST_CHANGED
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require_capability!(method, capabilities, :resources)
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require_capability!(method, capabilities, :resources, :listChanged)
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when RESOURCES_SUBSCRIBE, RESOURCES_UNSUBSCRIBE, NOTIFICATIONS_RESOURCES_UPDATED
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require_capability!(method, capabilities, :resources)
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require_capability!(method, capabilities, :resources, :subscribe)
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when TOOLS_CALL, TOOLS_LIST
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require_capability!(method, capabilities, :tools)
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when NOTIFICATIONS_TOOLS_LIST_CHANGED
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require_capability!(method, capabilities, :tools)
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require_capability!(method, capabilities, :tools, :listChanged)
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when LOGGING_SET_LEVEL, NOTIFICATIONS_MESSAGE
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require_capability!(method, capabilities, :logging)
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when COMPLETION_COMPLETE
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require_capability!(method, capabilities, :completions)
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when ROOTS_LIST
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require_capability!(method, capabilities, :roots)
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when SAMPLING_CREATE_MESSAGE
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require_capability!(method, capabilities, :sampling)
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when ELICITATION_CREATE
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require_capability!(method, capabilities, :elicitation)
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when INITIALIZE, PING, SERVER_DISCOVER, NOTIFICATIONS_INITIALIZED, NOTIFICATIONS_ROOTS_LIST_CHANGED,
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NOTIFICATIONS_PROGRESS, NOTIFICATIONS_CANCELLED, NOTIFICATIONS_ELICITATION_COMPLETE
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# No specific capability required.
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end
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end
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private
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def require_capability!(method, capabilities, *keys)
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name = keys.join(".") # :resources, :subscribe -> "resources.subscribe"
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has_capability = capabilities.dig(*keys)
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return if has_capability
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raise MissingRequiredCapabilityError.new(method, name)
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end
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end
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end
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end
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