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Add a pluggable server extension API with MCP Apps (#3003)

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Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>
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Marcelo Trylesinski 2026-06-29 11:58:05 +02:00 • committed by GitHub
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examples/stories/apps/README.md
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# apps
MCP Apps: a tool result carries a `_meta.ui` reference to a `ui://` resource
that the host renders as an interactive surface. The story will register a
`@ui` resource and return it from a tool.
MCP Apps: a tool carries a `_meta.ui.resourceUri` reference to a `ui://`
resource that the host renders as an interactive surface. The server opts in via
the `Apps` extension (`io.modelcontextprotocol/ui`); the client negotiates it by
advertising the `text/html;profile=mcp-app` MIME type.
**Status: not yet implemented** ([#2896](https://github.com/modelcontextprotocol/python-sdk/issues/2896)).
The `extensions` capability map is not yet surfaced on `MCPServer`, so a server
cannot advertise Apps support and a client cannot negotiate it.
## Run it
```bash
# stdio (default — the client spawns the server as a subprocess)
uv run python -m stories.apps.client
# HTTP — the client self-hosts the server on a free port, runs, then tears it down
uv run python -m stories.apps.client --http
```
## What to look at
- `server.py` `MCPServer("apps-example", extensions=[apps])` — the extension
advertises `io.modelcontextprotocol/ui` under `ServerCapabilities.extensions`
and contributes the UI-bound tool and its `ui://` resource. `MCPServer` itself
never learns about "ui"; it applies a closed set of contributions.
- `server.py` `@apps.tool(resource_uri=...)` — stamps `_meta.ui.resourceUri` on
the tool; `add_html_resource` registers the matching `ui://` resource at
`text/html;profile=mcp-app`.
- `server.py` `client_supports_apps(ctx)` — SEP-2133 graceful degradation: a
client that did not negotiate Apps gets a text-only result.
- `client.py` `Client(target, extensions={...})` — the client advertises Apps
support so the server returns the UI-enabled result, then reads the tool's
`_meta.ui.resourceUri` and fetches that resource.
## Spec
[MCP Apps — extensions](https://modelcontextprotocol.io/specification/draft/extensions/apps)
· [SEP-2133 — extensions capability](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/2133)
## See also
`custom_methods/` (registering a non-spec method without an extension).