---
title: config
sidebarTitle: config
---

# `fastmcp.apps.config`


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

### `app_config_to_meta_dict` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/config.py#L180" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
app_config_to_meta_dict(app: AppConfig | dict[str, Any]) -> dict[str, Any]
```


Convert an AppConfig or dict to the wire-format dict for ``meta["ui"]``.


## Classes

### `ResourceCSP` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/config.py#L20" 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/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/config.py#L56" 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.


### `AppConfig` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/config.py#L84" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>


Configuration for MCP App tools and resources.

Controls how a tool or resource participates in the MCP Apps extension.
On tools, ``resource_uri`` and ``visibility`` specify which UI resource
to render and where the tool appears.  On resources, those fields must
be left unset (the resource itself is the UI).

All fields use ``exclude_none`` serialization so only explicitly-set
values appear on the wire.  Aliases match the MCP Apps wire format
(camelCase).


### `PrefabAppConfig` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/config.py#L124" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>


App configuration for Prefab tools with sensible defaults.

Like ``app=True`` but customizable. Auto-wires the Prefab renderer
URI and merges the renderer's CSP with any additional domains you
specify.  The renderer resource is registered automatically.

Example::

    @mcp.tool(app=PrefabAppConfig())  # same as app=True

    @mcp.tool(app=PrefabAppConfig(
        csp=ResourceCSP(frame_domains=["https://example.com"]),
    ))


**Methods:**

#### `model_post_init` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/config.py#L140" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
model_post_init(self, __context: Any) -> None
```
