[Schema][Server] Add MCP Apps extension (io.modelcontextprotocol/ui) support (#281)

* feat: add MCP Apps extension (io.modelcontextprotocol/ui) support

Adds support for the MCP Apps protocol extension, which lets servers
expose interactive HTML UIs as resources rendered by spec-conforming
clients (e.g. Goose) in sandboxed iframes.

- Generic extension plumbing: ServerExtensionInterface and
  Builder::enableExtension(), with extensions advertised under
  capabilities.extensions during the initialize handshake.
  ClientCapabilities/ServerCapabilities gain an extensions field.
- Schema DTOs under Mcp\Schema\Extension\Apps: the McpApps marker,
  UiToolMeta, ToolVisibility enum, UiResourceContentMeta,
  UiResourceCsp, and UiResourcePermissions (with empty-object
  presence markers per spec).
- A worked weather example under examples/server/mcp-apps/ with a
  minimal spec-conforming view (ui/initialize handshake, postMessage
  JSONRPCMessage objects, and ResizeObserver-driven size reporting).
- docs/extensions.md guide with pointers from index.md and
  examples.md to the ext-apps repository for the TypeScript SDK and
  richer view-side patterns.
- Unit and inspector snapshot test coverage.

Spec: https://github.com/modelcontextprotocol/ext-apps

* Type UiToolMeta visibility as ToolVisibility enum

Also fix UiResourceContentMeta::jsonSerialize() return phpdoc.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Address review nitpicks for MCP Apps extension

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Apply PR review feedback for MCP Apps extension

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Add regression tests and CHANGELOG entry for capabilities extensions

- Pin the is_array() guards on extensions in ServerCapabilities and
  ClientCapabilities::fromArray() with explicit non-array payload tests.
- Pin the isset()-rejects-null contract in UiResourcePermissions::fromArray().
- Surface the new extensions field on both capability classes in CHANGELOG.
- Inline the ?: idiom in Builder::build() to match the surrounding style.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
Christopher Hertel 2026-06-01 09:35:54 +01:00 • committed by GitHub
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@ -10,6 +10,8 @@ All notable changes to `mcp/sdk` will be documented in this file.
* [BC Break] Renamed `Mcp\Schema\Resource` to `Mcp\Schema\ResourceDefinition`. No alias.
* [BC Break] Renamed `Mcp\Capability\Registry\Loader\ArrayLoader` to `Mcp\Capability\Registry\Loader\ReflectedElementLoader`.
* [BC Break] Bump default protocol version to `2025-11-25`
* Add support for MCP Apps extension in schema and server
* Add `extensions` to `ServerCapabilities` and `ClientCapabilities` and `Builder::enableExtension()`
* Allow overriding the default name pattern for Discovery
* Add configurable session garbage collection (`gcProbability`/`gcDivisor`)
* Add `ChainLoader` to compose multiple `LoaderInterface` implementations via explicit ordering.