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fastmcp.server.apps

MCP Apps support — extension negotiation and typed UI metadata models. Provides constants and Pydantic models for the MCP Apps extension (io.modelcontextprotocol/ui), enabling tools and resources to carry UI metadata for clients that support interactive app rendering.

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Functions

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ui_to_meta_dict 

ui_to_meta_dict(ui: ToolUI | ResourceUI | dict[str, Any]) -> dict[str, Any]
Convert a UI model or dict to the wire-format dict for meta["ui"].

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resolve_ui_mime_type 

resolve_ui_mime_type(uri: str, explicit_mime_type: str | None) -> str | None
Return the appropriate MIME type for a resource URI. For ui:// scheme resources, defaults to UI_MIME_TYPE when no explicit MIME type is provided. This ensures UI resources are correctly identified regardless of how they’re registered (via FastMCP.resource, the standalone @resource decorator, or resource templates). Args:
  • uri: The resource URI string
  • explicit_mime_type: The MIME type explicitly provided by the user
Returns:
  • The resolved MIME type (explicit value, UI default, or None)

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Classes

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ResourceCSP

Content Security Policy for MCP App resources. Declares which external origins the app is allowed to connect to or load resources from. Hosts use these declarations to build the Content-Security-Policy header for the sandboxed iframe.

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ResourcePermissions

Iframe sandbox permissions for MCP App resources. Each field, when set (typically to {}), requests that the host grant the corresponding Permission Policy feature to the sandboxed iframe. Hosts MAY honour these; apps should use JS feature detection as a fallback.

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ToolUI

Typed _meta.ui for tools — links a tool to its UI resource. All fields use exclude_none serialization so only explicitly-set values appear on the wire. Aliases match the MCP Apps wire format (camelCase).

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ResourceUI

Typed _meta.ui for resources — rendering hints for UI-capable clients.