What is alternatives for "fastmcp dev apps" to launch MCP apps ready Web UI server, to check interoperability.

[itaru2622]

I started to test MCP apps with FastMCP for mcp-app server,
and want to test with other UI Server besides FastMCP dev UI to keep interoperability.

For example, as far as legacy MCP (tool, resource, prompt), modelcontextprotocol/inspector and others can be used.
but how about mcp apps?

the above inspector seems not supporting mcp app yet.
modelcontextprotocol/ext-apps/app-bridge.ts seems to be used partially by fastmcp dev UI but I couldn't find easy way to use it as standanlone UI server, like modelcontextprotocol/inspector.

Is there any OSS for UI server which supports mcp app and tool.

[LarryHu0217]

There are now two OSS options for this.

1. MCP Inspector v2

The current Inspector supports MCP Apps directly. Start it with:

npx @modelcontextprotocol/inspector

Connect it to the Streamable HTTP endpoint of the FastMCP server, then open the Apps tab, select the app tool, provide its arguments, and choose Open App. The Inspector fetches the ui:// resource and renders it through the MCP Apps sandbox/AppBridge flow.

For automation, the Inspector also documents a CLI-first workflow using --app-info, followed by a deep link into the web renderer:
https://github.com/modelcontextprotocol/inspector/blob/672f9f41c548487a468b9e7007d2f9de14da5a69/docs/mcp-app-review.md

2. The official ext-apps basic host

The MCP Apps repository includes a reference host that can connect to arbitrary HTTP MCP servers:

git clone https://github.com/modelcontextprotocol/ext-apps.git
cd ext-apps
npm install
SERVERS='["http://localhost:8000/mcp"]' npm start

Then open http://localhost:8080. Its host implementation includes tool selection, tool calls, AppBridge setup, and a double-iframe sandbox:
https://github.com/modelcontextprotocol/ext-apps/tree/10195ad91851502134930e9b80ec2c04e277a720/examples/basic-host

For current interoperability testing, I would use Inspector first because it exposes the protocol and App metadata. Use basic-host when you want a small reference host implementation to compare against FastMCP's dev UI. The basic-host is an example/test host, not a production host distribution.