> ## Documentation Index > Fetch the complete documentation index at: https://docs.getbifrost.ai/llms.txt > Use this file to discover all available pages before exploring further. # MCP Apps > Serve the interactive interfaces of your MCP servers' tools to MCP clients through Bifrost's gateway, while keeping app-only tools away from models. ## Overview An **MCP App** is a tool that comes with its own interface. Next to the tool, the server offers a `ui://` resource that holds the app's HTML, and an MCP client that supports MCP Apps shows it in the conversation, for example an interactive chart or form instead of a block of text. Some of an app's tools exist only for its interface, such as a button that refreshes the data the app shows. These **app-only tools** are meant to be called by the app, never by a model. A server marks a tool's interface with its `ui://` resource in the tool's metadata, and marks a tool app-only by making it visible to the app but not the model. Bifrost also recognises interfaces declared in the OpenAI Apps SDK's form. Bifrost carries MCP Apps through its [MCP gateway](https://docs.getbifrost.ai/mcp/gateway-auth.md): * **Apps reach clients that can show them.** A server's own endpoint serves its app tools and `ui://` resources as the server sent them. * **App-only tools never reach a model.** They are listed only on a server's own endpoint, to clients that declared MCP Apps support, and never offered to models in chat completions, the Responses API or code mode. * **Servers don't collide.** On the shared `/mcp` endpoint and Virtual MCPs, each server's `ui://` resources are kept apart, so two servers that ship apps with the same URI don't overwrite each other. * **Your access rules apply.** A virtual key, Virtual MCP or request filter that can't reach a server can't load its apps either. There is nothing to turn on. Bifrost serves MCP Apps whenever a connected server offers them. *** ## How apps reach clients What a caller gets depends on where it connects: | Where the caller connects | Tools with an interface | App-only tools | `ui://` resources | | - | - | - | - | | A server's own endpoint, `/mcp/` | Listed, with their interface | Listed and callable for clients that declared MCP Apps support | As the server sent them, for example `ui://budget/planner.html` | | The shared `/mcp` endpoint | Listed, with their interface | Never listed | Kept apart per server, for example `ui://budget/budget/planner.html` | | A [Virtual MCP](https://docs.getbifrost.ai/mcp/virtual-mcps.md) endpoint | Listed, with their interface | Never listed | Kept apart per server | | Models in chat completions, the Responses API or [code mode](https://docs.getbifrost.ai/mcp/code-mode.md) | Offered as plain tools, without the interface | Never offered | Not offered | Connect an MCP client to a server's own endpoint, `/mcp/`, to use that server's apps in full. The endpoint uses the server's endpoint slug (see [Direct MCP client endpoints](https://docs.getbifrost.ai/mcp/virtual-mcps.md#direct-mcp-client-endpoints)), or `//mcp/` under a `server.base_path`. It accepts the same credentials as `/mcp`, and the credential must reach that server. On the shared `/mcp` endpoint, a client can still show an app's interface for a tool it calls. An app that calls its own app-only tools needs the server's own endpoint. *** ## Which clients show the interface Every caller gets a tool's interface along with the tool. A client that supports MCP Apps and connects on MCP version `2026-07-28`, which carries the client's capabilities on every request, gets everything: the interface and the app-only tools. A client on an earlier MCP version can still show an app from a server on an earlier version. From a server on `2026-07-28`, the app may not load for it, since Bifrost can pass its MCP Apps support to that server only on `2026-07-28` requests. Either way, app-only tools are served only to `2026-07-28` clients, so an app that calls them won't work in full on an earlier version. A client that doesn't support MCP Apps shows the tool's result as text instead, since MCP Apps require every tool to return a text result as well. The server can run any MCP version Bifrost supports. Bifrost declares MCP Apps support to each server when it connects, on `2025-06-18`, `2025-11-25` and `2026-07-28` alike, so the server knows it can send its apps. See [Supported Protocol Versions](https://docs.getbifrost.ai/mcp/overview.md#supported-protocol-versions). *** ## Tool names apps use An app calls tools by the names its own server gave them, for example `refresh`, while Bifrost lists them under the server's name, as `budget-refresh`. Bifrost accepts both: * **On a server's own endpoint,** the server's own name resolves to that server's tool. * **On the shared `/mcp` endpoint and Virtual MCPs,** the server's own name resolves when exactly one server the caller can reach has a tool by that name that models may call. When several do, the call is refused and lists the full names. App-only tools never resolve there. A tool's full name always takes precedence over a server's own name. *** ## Who can reach an app An app is reachable through the same rules as the server's tools: the servers a [virtual key](https://docs.getbifrost.ai/features/governance/mcp-tools.md) is assigned to, the servers a [Virtual MCP](https://docs.getbifrost.ai/mcp/virtual-mcps.md) draws from, and any [request filters](https://docs.getbifrost.ai/mcp/filtering.md). A caller that can't reach a server sees none of its tools, apps or `ui://` resources. A server's resource allow-list also applies to its `ui://` resources. Each server exposes all of its resources by default. If you limit what a server exposes, include the `ui://` resources of its apps, or clients can't load them. > [tab] Web UI > > 1. Open **MCP Servers** and select the server. > 2. Open the **Prompts & Resources** tab. > 3. Make sure the app's `ui://` resources are among the exposed resources. > 4. Save the server. > > ![Prompts and resources tab on an MCP server](https://mintcdn.com/bifrost/2WkmI1Hm1IrA_f0n/media/ui-mcp-prompts-resources.png?fit=max\&auto=format\&n=2WkmI1Hm1IrA_f0n\&q=85\&s=687e4b1b75bdc78334d9dec1bb949ebb) > [tab] config.json > > Include the app's `ui://` resources in `resources_to_expose` on the server's `client_configs[]` entry: > > ```json theme={null} > { > "mcp": { > "client_configs": [ > { > "name": "budget", > "connection_type": "http", > "connection_string": "https://budget.example.com/mcp", > "tools_to_execute": ["*"], > "resources_to_expose": ["ui://budget/planner.html", "file:///policy.md"] > } > ] > } > } > ``` > > | Field | Type | Required | Description | > | - | - | - | - | > | `resources_to_expose` | string\[] | No | Resource URIs and URI templates served through the gateway, including `ui://` resources. `["*"]` exposes everything and `[]` exposes nothing. Omitted, a new server exposes everything and an existing server keeps its stored list. | See [Resources and Prompts](https://docs.getbifrost.ai/mcp/resources-and-prompts.md#choosing-what-a-server-exposes) for how the allow-lists work. *** ## Code mode A server in [code mode](https://docs.getbifrost.ai/mcp/code-mode.md) is reached through the code-mode tools on the gateway, including its own endpoint. Its tools aren't listed one by one, so clients see none of its app tools there. Keep a server whose apps you want to serve out of code mode. *** ## Logs Every call from an app is recorded in **MCP Logs** like any other tool call, including calls to app-only tools that Bifrost refuses. A call by a server's own tool name that Bifrost can't resolve on the shared `/mcp` endpoint or a Virtual MCP is refused before it runs, and isn't logged. Open the entry and expand **More details** to see what Bifrost knew about the call, under **MCP Apps**: | Marker | Meaning | | - | - | | **Caller hosts MCP Apps** | The client declared MCP Apps support. | | **App-only tool** | The tool is meant for the app's interface only. | | **Opens `ui://…`** | The tool comes with an interface, at this resource. | | **Called as ``** | The client called the tool by its server's own name. | ![MCP log entry showing MCP Apps markers](https://mintcdn.com/bifrost/FbINXxF0HH6IyjIV/media/ui-mcp-logs-apps-markers.png?fit=max\&auto=format\&n=FbINXxF0HH6IyjIV\&q=85\&s=50bfe1a9bcfea11e78461220cb60f182) *** ## Troubleshooting ### The app doesn't show in the client * **The client connects to a Virtual MCP or the shared `/mcp` endpoint.** Connect it to the server's own endpoint, `/mcp/`, to use the app in full. * **The client doesn't support MCP Apps.** It shows the tool's text result instead. * **The app calls app-only tools, and the client isn't on MCP `2026-07-28`.** App-only tools are hidden from it, so the app can't work in full. Check that the client supports the newest MCP version. * **The server's resource allow-list leaves out the app's `ui://` resource.** Add it to the exposed resources. * **The server is in code mode.** Its app tools aren't listed on the gateway. ### An app-only tool isn't found App-only tools are listed and callable only on the server's own endpoint, for clients that declared MCP Apps support. Models, the shared `/mcp` endpoint and Virtual MCPs never reach them. ### A call by the tool's short name is refused On the shared `/mcp` endpoint or a Virtual MCP, there are two causes: * **More than one server you can reach has a tool by that name.** Call it by its full name, such as `budget-refresh`, or connect to the server's own endpoint. * **The only match is an app-only tool.** App-only tools work only on the server's own endpoint, whatever name you call them by. Connect the client to `/mcp/`. *** ## Next Steps > [card] [Resources and Prompts](https://docs.getbifrost.ai/mcp/resources-and-prompts.md) > > Serve your servers' prompts and resources > [card] [Virtual MCPs](https://docs.getbifrost.ai/mcp/virtual-mcps.md) > > Bundle servers behind one endpoint > [card] [Gateway Authentication](https://docs.getbifrost.ai/mcp/gateway-auth.md) > > Connect MCP clients to Bifrost > [card] [Code Mode](https://docs.getbifrost.ai/mcp/code-mode.md) > > Let models call tools through code This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.