# MCP Apps
An **MCP App** is a tool with a face: alongside its data, the tool points at an HTML
document the host renders as an interactive surface.
Two parts, always two parts:
1. **A tool** that does the work and returns data, like any other tool.
2. **A `ui://` resource** containing the HTML the host shows for it.
The tool carries a `_meta.ui.resourceUri` reference to the resource. The host fetches
it with `resources/read`, renders it in a **sandboxed iframe**, and pushes the tool's
result into that iframe via `postMessage`. Your server never sends or receives any
`ui/*` messages: that traffic is between the host and the iframe. You serve a tool
and an HTML document; the host does the theater.
The SDK ships this as the built-in `Apps` extension (`io.modelcontextprotocol/ui`).
If [Extensions](https://py.sdk.modelcontextprotocol.io/advanced/extensions/index.md) are new to you, skim that page first. One minute,
then come back.
## A clock with a face
```python title="server.py" hl_lines="19 22 30 32"
# docs_src/apps/tutorial001.py
from mcp import Client
from mcp.client import advertise
from mcp.server.apps import APP_MIME_TYPE, EXTENSION_ID, Apps, client_supports_apps
from mcp.server.mcpserver import MCPServer
from mcp.server.mcpserver.context import Context
CLOCK_HTML = """\
Clock
...
"""
apps = Apps()
@apps.tool(resource_uri="ui://clock/app.html", description="The current time.")
def get_time(ctx: Context) -> str:
now = "2026-06-26T12:00:00Z"
if not client_supports_apps(ctx):
return f"The time is {now}."
return now
apps.add_html_resource("ui://clock/app.html", CLOCK_HTML, title="Clock")
mcp = MCPServer("clock", extensions=[apps])
async def main() -> None:
async with Client(mcp, extensions=[advertise(EXTENSION_ID, {"mimeTypes": [APP_MIME_TYPE]})]) as client:
result = await client.call_tool("get_time", {})
print(result.content)
# [TextContent(text='2026-06-26T12:00:00Z')]
```
Four moves:
* `Apps()`: one instance holds your UI-bound tools and their resources.
* `@apps.tool(resource_uri="ui://clock/app.html")`: a regular tool, plus the
`_meta.ui.resourceUri` stamp. Everything `@mcp.tool()` accepts (name, title,
description, ...) passes through.
* `apps.add_html_resource("ui://clock/app.html", CLOCK_HTML)`: the matching
resource, served as `text/html;profile=mcp-app`. That exact MIME type is what
tells a host "this is an app, render it".
* `MCPServer("clock", extensions=[apps])`: opt in. The server now advertises
`io.modelcontextprotocol/ui` under `capabilities.extensions`.
The HTML itself listens for the host's `postMessage` and shows the result. For real
apps, use the official [`@modelcontextprotocol/ext-apps`](https://github.com/modelcontextprotocol/ext-apps)
browser SDK inside your HTML. It gives you `ontoolresult`, `callServerTool`,
`getHostContext`, and `onhostcontextchanged` instead of raw message events.
## Graceful degradation
Not every client renders apps. The spec is blunt about what that means for you:
> Tools **MUST** return a meaningful `content` array even when UI is available.
The model reads `content`; the iframe is for humans. A UI-capable host still feeds
the text result to the model, and a text-only client gets *only* that. So the
canonical pattern is one tool, two answers. Look at `get_time` again:
```python title="server.py" hl_lines="23-27"
# docs_src/apps/tutorial001.py
from mcp import Client
from mcp.client import advertise
from mcp.server.apps import APP_MIME_TYPE, EXTENSION_ID, Apps, client_supports_apps
from mcp.server.mcpserver import MCPServer
from mcp.server.mcpserver.context import Context
CLOCK_HTML = """\
Clock
...
"""
apps = Apps()
@apps.tool(resource_uri="ui://clock/app.html", description="The current time.")
def get_time(ctx: Context) -> str:
now = "2026-06-26T12:00:00Z"
if not client_supports_apps(ctx):
return f"The time is {now}."
return now
apps.add_html_resource("ui://clock/app.html", CLOCK_HTML, title="Clock")
mcp = MCPServer("clock", extensions=[apps])
async def main() -> None:
async with Client(mcp, extensions=[advertise(EXTENSION_ID, {"mimeTypes": [APP_MIME_TYPE]})]) as client:
result = await client.call_tool("get_time", {})
print(result.content)
# [TextContent(text='2026-06-26T12:00:00Z')]
```
`client_supports_apps(ctx)` is `True` only when the client declared the
`io.modelcontextprotocol/ui` extension **and** listed `text/html;profile=mcp-app`
in its `mimeTypes` settings. The field is required, so a client that omits it
does not count. That is exactly what `main()` in the same file declares: the
client half of the negotiation, and the rich answer comes back.
!!! warning
Never return a placeholder like `"[Rendered UI]"` as the only content. If the
fallback text is useless, the tool is useless to every text-only client and to
the model itself. Write the sentence.
## Locking the iframe down
The resource side carries the security metadata: what the iframe may load, which
browser permissions it wants, how it would like to be framed:
```python title="server.py" hl_lines="9 19-22"
# docs_src/apps/tutorial002.py
from mcp.server.apps import Apps, ResourceCsp, ResourcePermissions
from mcp.server.mcpserver import MCPServer
DASHBOARD_HTML = "Dashboard"
apps = Apps()
@apps.tool(resource_uri="ui://dashboard/app.html", visibility=["app"])
def refresh_dashboard() -> str:
"""Refresh the dashboard data."""
return "refreshed"
apps.add_html_resource(
"ui://dashboard/app.html",
DASHBOARD_HTML,
title="Dashboard",
csp=ResourceCsp(connect_domains=["https://api.example.com"]),
permissions=ResourcePermissions(clipboard_write={}),
domain="dashboard.example.com",
prefers_border=True,
)
mcp = MCPServer("dashboard", extensions=[apps])
```
`csp` and `permissions` are **requests to the host**, not server behaviour. The host
builds the iframe's Content-Security-Policy and Permissions-Policy from them, and it
may refuse. Feature-detect in your JS rather than assuming a grant.
`ResourceCsp`, field by field (Python name, wire key, what the host does with it):
| Python | Wire (`_meta.ui.csp`) | Controls |
|---|---|---|
| `connect_domains` | `connectDomains` | `connect-src`: where `fetch`/XHR may go |
| `resource_domains` | `resourceDomains` | `img-src`, `style-src`, ...: static assets |
| `frame_domains` | `frameDomains` | `frame-src`: nested iframes |
| `base_uri_domains` | `baseUriDomains` | `base-uri`: what `` may point at |
`ResourcePermissions`: each field requests a browser permission for the iframe.
| Python | Wire (`_meta.ui.permissions`) |
|---|---|
| `camera` | `camera` |
| `microphone` | `microphone` |
| `geolocation` | `geolocation` |
| `clipboard_write` | `clipboardWrite` |
!!! note
CSP and permissions live on the **resource**, never on the tool. The spec's tool
metadata has no slot for them, and hosts ignore them there. The SDK makes the
mistake unrepresentable: `@apps.tool()` simply has no `csp` parameter.
### Visibility
`visibility=["app"]` on a tool says "this exists for the iframe, not the model":
* `"model"`: the model may call it.
* `"app"`: the iframe may call it (via `callServerTool`).
* Omitted: both, which is the default.
Filtering is the **host's** job. Your server lists app-only tools in `tools/list`
like any other; the host hides them from the model. Don't filter server-side.
## The rules the SDK enforces
All of these fail at startup, not in production:
* A `resource_uri` or resource URI that isn't `ui://...` is a `ValueError` at
decoration/registration time.
* A tool bound to a URI with **no matching registered resource** is a `ValueError`
when `MCPServer(extensions=[apps])` consumes the extension. A tool advertising
HTML that 404s on `resources/read` is a misconfiguration, so it refuses to
construct.
* `meta={"ui": ...}` on `@apps.tool()` is a `ValueError`. The decorator owns
`_meta["ui"]`; say it with `resource_uri=` and `visibility=`. Other `meta=` keys
merge fine alongside.
Neither the TypeScript ext-apps SDK nor FastMCP catches any of these today; we'd
rather you find out before a host does.
## Beyond inline HTML
`add_html_resource` covers the common case: a string of HTML. For anything else,
HTML on disk or generated content, build the resource yourself and hand it over:
```python title="server.py" hl_lines="12 18"
# docs_src/apps/tutorial003.py
from pathlib import Path
from mcp.server.apps import Apps
from mcp.server.mcpserver import MCPServer
from mcp.server.mcpserver.resources import FileResource
REPORT_HTML = Path(__file__).parent / "report.html"
apps = Apps()
@apps.tool(resource_uri="ui://report/app.html")
def refresh_report() -> str:
"""Refresh the report data."""
return "report refreshed"
apps.add_resource(FileResource(uri="ui://report/app.html", name="report", path=REPORT_HTML))
mcp = MCPServer("report", extensions=[apps])
```
`add_resource` fills in the `text/html;profile=mcp-app` MIME type when the resource
doesn't set one explicitly, and rejects an explicit mismatch: a `ui://` resource
under any other MIME type is one no host will render.
!!! tip
Targeting a pre-GA host that still reads the deprecated flat
`_meta["ui/resourceUri"]` key? Merge it yourself:
`@apps.tool(resource_uri="ui://x", meta={"ui/resourceUri": "ui://x"})`.
The nested `ui` object is the spec shape; the flat key is on its way out.
## See it run
The `apps` story in `examples/stories/` is this page as a runnable pair: a server
with a UI-bound clock tool and a client that negotiates Apps, reads the tool's
`_meta.ui.resourceUri`, fetches the HTML, and calls the tool.
```bash
uv run python -m stories.apps.client
```