---
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#L181" 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"]``.


### `is_model_visible` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/config.py#L188" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
is_model_visible(component: FastMCPComponent) -> bool
```


Whether a component may be shown to, or invoked by, the model.

Visibility is a declaration, and the MCP Apps spec puts the filtering on
the host — so ``tools/list`` carries app-only tools and the host keeps
them from the model. That division only works where a host stands between
the server and the model.

It does not hold for surfaces a server drives itself. A search result or
a code-mode catalog reaches the model as ordinary tool output, and a
call-tool proxy invokes on a name the model supplies; nothing downstream
can filter either. Those surfaces have to apply the declaration here.

A component with no ``visibility`` is visible: the field marks the
exception, and the spec's default is both audiences.


## Classes

### `ResourceCSP` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/apps/config.py#L21" 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#L57" 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#L85" 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#L125" 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#L141" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

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