Add Server-Side MCP Apps Helpers per SEP-1865
## Motivation and Context SEP-1865 "MCP Apps - Interactive User Interfaces for MCP" is Final on the Extensions Track: (https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1865) an optional extension, negotiated through the SEP-2133 `capabilities.extensions` mechanism, that lets a server ship HTML user interfaces which the host renders for tool results. The normative spec lives in the ext-apps repository (`specification/2026-01-26/apps.mdx`). The Python SDK implements it in-SDK (python-sdk#3003, the `Apps` extension); the TypeScript ecosystem ships it as the separate `@modelcontextprotocol/ext-apps` package on top of the core SDK's SEP-2133 support. Server-side support is a thin convention layer over primitives this SDK already has (SEP-2133 extension capabilities, `_meta` on tools and resources, free-form resource URIs and MIME types). New `MCP::Apps` module: - Wire vocabulary constants shared with the reference implementations: `EXTENSION_ID` (`"io.modelcontextprotocol/ui"` - note `/ui`, not `/apps`), `RESOURCE_MIME_TYPE` (`"text/html;profile=mcp-app"`), `URI_SCHEME` (`"ui://"`), and the legacy flat meta key `RESOURCE_URI_META_KEY` (`"ui/resourceUri"`). - `Apps.capability(mime_types:)` builds the `capabilities.extensions` fragment for `support_extensions` on the server or `connect` on the client. - `Apps.ui_resource(uri:, name:, ...)` builds an `MCP::Resource` for a UI template, enforcing the spec's MUSTs (a `ui://` URI; the parameterized HTML MIME type by default). - `Apps.tool_meta(resource_uri:, visibility:, meta:, legacy:)` builds the `_meta.ui.resourceUri` linkage (optionally also emitting the legacy flat alias, matching the reference server helper that keeps both shapes in sync), merged non-destructively into caller-supplied meta. - `Apps.client_supports?(client_capabilities, mime_type:)` gates the text-only fallback the extension requires for clients that did not declare the capability (symbol/string tolerant). Everything else the extension defines (the sandboxed iframe, the `ui/*` postMessage bridge methods, consent for UI-initiated calls) is HOST responsibility and out of scope for a server SDK; a server only ever receives ordinary `resources/read` and `tools/call` requests. The README section states this boundary. Resolves #376. ## How Has This Been Tested? New `test/mcp/apps_test.rb` pins the wire vocabulary and covers the capability fragment, `ui_resource` scheme validation and MIME default, `tool_meta` non-destructive merging, visibility, the legacy alias, and `client_supports?` across symbol/string keys, mimeTypes narrowing, and missing declarations. New integration tests in `test/mcp/server_test.rb` drive a full server: the `initialize` result advertises the extension, tools/list serializes `_meta.ui.resourceUri`, `resources/read` serves the HTML template, and `client_supports?` distinguishes hosts with and without the declared extension. `bundle exec rake` (tests, RuboCop, and conformance baseline) passes. Also verified end-to-end over stdio: a client connecting with the extension declared negotiates the capability, discovers the tool linkage via `_meta.ui.resourceUri`, and reads the `ui://` template as `text/html;profile=mcp-app` content. ## Breaking Changes None. Purely additive helper module; no existing code paths change.
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Koichi ITO
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@ -8,6 +8,7 @@ require_relative "mcp/version"
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module MCP
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autoload :Annotations, "mcp/annotations"
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autoload :Apps, "mcp/apps"
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autoload :Cancellation, "mcp/cancellation"
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autoload :CancelledError, "mcp/cancelled_error"
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autoload :Client, "mcp/client"
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109
lib/mcp/apps.rb
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109
lib/mcp/apps.rb
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@ -0,0 +1,109 @@
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# frozen_string_literal: true
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module MCP
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# Server-side helpers for the MCP Apps extension (SEP-1865, Extensions Track, Final):
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# interactive user interfaces delivered as `ui://` HTML template resources that hosts
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# render for tool results. The extension is negotiated per SEP-2133 through
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# `capabilities.extensions` on both sides; a server declares {EXTENSION_ID}, registers
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# UI templates as ordinary resources, and links tools to templates via `_meta`.
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#
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# Everything else defined by the extension (the sandboxed iframe, the `ui/*`
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# postMessage bridge methods, consent for UI-initiated calls) is HOST responsibility:
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# a server only ever receives ordinary `resources/read` and `tools/call` requests.
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#
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# Because the extension is optional, a UI-enabled tool MUST still return a meaningful
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# text-only result for clients that did not declare the capability;
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# use {Apps.client_supports?} to branch.
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#
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# @example Declaring an Apps-enabled server
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# capabilities = MCP::Server::Capabilities.new
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# capabilities.support_tools
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# capabilities.support_resources
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# capabilities.support_extensions(MCP::Apps.capability)
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# server = MCP::Server.new(
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# name: "weather_server",
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# capabilities: capabilities,
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# resources: [MCP::Apps.ui_resource(uri: "ui://weather-server/dashboard", name: "weather_dashboard")],
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# )
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# server.define_tool(
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# name: "get_weather",
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# meta: MCP::Apps.tool_meta(resource_uri: "ui://weather-server/dashboard"),
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# ) { |server_context:|
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# ...
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# }
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#
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# https://github.com/modelcontextprotocol/ext-apps/blob/main/specification/2026-01-26/apps.mdx
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module Apps
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# Reverse-DNS extension identifier (note: `/ui`, not `/apps`), shared wire vocabulary with
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# the reference `@modelcontextprotocol/ext-apps` package and the Python SDK's `Apps` extension.
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EXTENSION_ID = "io.modelcontextprotocol/ui"
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# UI template resources MUST use this parameterized MIME type.
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RESOURCE_MIME_TYPE = "text/html;profile=mcp-app"
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# UI template resource URIs MUST use this scheme.
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URI_SCHEME = "ui://"
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# Legacy flat `_meta` key linking a tool to its template; the canonical shape is
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# the nested `_meta.ui.resourceUri`. {Apps.tool_meta} can emit both.
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RESOURCE_URI_META_KEY = "ui/resourceUri"
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extend self
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# The `capabilities.extensions` fragment advertising Apps support. Pass to
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# `MCP::Server::Capabilities.new(extensions: ...)` or `support_extensions(...)`,
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# or merge into a client's `connect(capabilities: { extensions: ... })`.
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def capability(mime_types: [RESOURCE_MIME_TYPE])
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{ EXTENSION_ID => { mimeTypes: mime_types } }
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end
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# Builds an `MCP::Resource` for a UI template, enforcing the spec's MUSTs:
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# a `ui://` URI and the `text/html;profile=mcp-app` MIME type by default.
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def ui_resource(uri:, name:, mime_type: RESOURCE_MIME_TYPE, **rest)
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unless uri.is_a?(String) && uri.start_with?(URI_SCHEME)
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raise ArgumentError, "MCP Apps template URIs must start with #{URI_SCHEME.inspect} (got #{uri.inspect})"
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end
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Resource.new(uri: uri, name: name, mime_type: mime_type, **rest)
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end
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# Builds the tool `_meta` linking a tool to its UI template, merged non-destructively into
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# caller-supplied `meta`. `visibility` restricts who sees the tool (an array of `"model"` / `"app"`).
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# `legacy: true` also writes the flat `"ui/resourceUri"` alias, matching the reference server helper
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# that keeps both key shapes in sync for older hosts.
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def tool_meta(resource_uri:, visibility: nil, meta: nil, legacy: false)
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unless resource_uri.is_a?(String) && resource_uri.start_with?(URI_SCHEME)
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raise ArgumentError, "resource_uri must start with #{URI_SCHEME.inspect} (got #{resource_uri.inspect})"
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end
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ui_entry = { resourceUri: resource_uri }
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ui_entry[:visibility] = visibility if visibility
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merged = (meta || {}).merge(ui: ui_entry)
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merged[RESOURCE_URI_META_KEY.to_sym] = resource_uri if legacy
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merged
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end
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# Whether the client declared Apps support for `mime_type` in its `capabilities.extensions`
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# (symbol or string keys). UI-enabled tools use this to fall back to a text-only result for
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# clients without the extension.
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def client_supports?(client_capabilities, mime_type: RESOURCE_MIME_TYPE)
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extensions = read_key(client_capabilities, :extensions)
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declaration = read_key(extensions, EXTENSION_ID)
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return false unless declaration.is_a?(Hash)
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mime_types = read_key(declaration, :mimeTypes)
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# A declaration without mimeTypes advertises the extension without narrowing.
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return true if mime_types.nil?
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mime_types.is_a?(Array) && mime_types.include?(mime_type)
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end
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private
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def read_key(hash, key)
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return unless hash.is_a?(Hash)
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value = hash[key.to_sym]
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value.nil? ? hash[key.to_s] : value
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end
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end
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end
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