@maxhealth.tech/prefab / mcp / APPS_EXTENSION

Variable: APPS_EXTENSION

const APPS_EXTENSION: "io.modelcontextprotocol/ui" = 'io.modelcontextprotocol/ui';

Defined in: mcp/resource.ts:149

Capability key for the MCP Apps extension (versioned independently of core).

The identifier is …/ui, and the trap here is that …/apps looks right and is wrong. The normative source is the MCP Apps spec, which states "This extension is identified as: io.modelcontextprotocol/ui" in both the current 2026-01-26 revision and the draft, and the reference implementation agrees: ext-apps/src/server/index.ts exports EXTENSION_ID = "io.modelcontextprotocol/ui".

io.modelcontextprotocol/apps appears only as illustrative "e.g." text in the Rust and C# SDKs' generic ServerCapabilities.extensions doc comments (and their test fixtures), showing the SHAPE of the extensions map rather than naming this extension. Copying it from there declares the capability under a key no host looks up, which fails silently: rendering still works, because hosts fall back to detecting apps via _meta.ui plus the MIME type, so only the SEP-2133 declaration is lost.

Duplicated as a literal rather than imported, because prefab ships zero dependencies.