[Schema] SEP-1865 MCP Apps: Content types, ui:// scheme, tool metadata fields #350

Implements the schema portion of SEP-1865 (MCP Apps) for the MCP Spec 2026-07-28 release.

Tracked by umbrella #335. Depends on SEP-2133 extensions framework.

Spec summary

MCP Apps lets servers ship interactive HTML UIs to hosts. Servers predeclare UI resources under the ui:// URI scheme with MIME type text/html;profile=mcp-app, and tools reference these templates via metadata so hosts can prefetch and cache them. UIs run in sandboxed iframes and communicate bidirectionally with the host using MCP JSON-RPC over postMessage.

Spec lives at modelcontextprotocol/ext-apps.

PHP SDK changes

• Recognize text/html;profile=mcp-app MIME type.
• Recognize ui:// URI scheme (additions to URI handling under src/Schema/).
• Add _meta-style tool metadata field tying a tool to a UI resource (e.g. mcp-apps-namespaced output template ref).
• Surface the schema in tool result formatters so tools/call responses can include the UI linkage.

Related

• Umbrella [2026-07-28] Extensions framework + Tasks + MCP Apps #335
• Server-side capability + builder in separate issue