> ## Documentation Index
> Fetch the complete documentation index at: https://fastmcp-docs-v3-beta2-feature-tracking.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# apps

# `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.

## Functions

### `ui_to_meta_dict` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/apps.py#L129" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python theme={null}
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"]`.

### `resolve_ui_mime_type` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/apps.py#L136" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python theme={null}
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)

## Classes

### `ResourceCSP` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/apps.py#L18" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

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.

### `ResourcePermissions` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/apps.py#L50" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

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.

### `ToolUI` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/apps.py#L77" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

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).

### `ResourceUI` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/apps.py#L110" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

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