| Filename | Latest commit message | Latest commit date |
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5bbf277882* chore: bump ext-apps to 1.1.0 Features: - registerAppResource now returns the registered resource (#370) - Accept UIResourceMeta in both resources/list and resources/read (#410) - callServerTool throws helpful error when called with a string (#449) - Add double-connect guard to prevent protocol message handling errors (#450) Bug fixes: - Only ignore messages that lack jsonrpc 2.0 in message-transport (#448) - Align basic-host dark mode styles (#438) Spec: - Change Host <> Sandbox communication protocol to SHOULD (#435) - Clarify UIResourceMeta in both list and read responses (#410) * fix(debug-server): add missing npm package config (main, bin, types, exports) Align debug-server package.json with all other server examples: - main: dist/server.js (was server.ts) - types: dist/server.d.ts - bin: mcp-server-debug -> dist/index.js - exports with types - files: only dist (remove source server.ts) |
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| src | ||
| .gitignore | ||
| index.html | ||
| package.json | ||
| README.md | ||
| sandbox.html | ||
| serve.ts | ||
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| vite.config.ts | ||
Example: Basic Host
A reference implementation showing how to build an MCP host application that connects to MCP servers and renders tool UIs in a secure sandbox.
This basic host can also be used to test MCP Apps during local development.
Key Files
index.html/src/index.tsx- React UI host with tool selection, parameter input, and iframe managementsandbox.html/src/sandbox.ts- Outer iframe proxy with security validation and bidirectional message relaysrc/implementation.ts- Core logic: server connection, tool calling, and AppBridge setup
Getting Started
npm install
npm run start
# Open http://localhost:8080
By default, the host application will try to connect to an MCP server at http://localhost:3001/mcp. You can configure this behavior by setting the SERVERS environment variable with a JSON array of server URLs:
SERVERS='["http://localhost:1234/mcp", "http://localhost:5678/mcp"]' npm run start
Architecture
This example uses a double-iframe sandbox pattern for secure UI isolation:
Host (port 8080)
└── Outer iframe (port 8081) - sandbox proxy
└── Inner iframe (srcdoc) - untrusted tool UI
Why two iframes?
- The outer iframe runs on a separate origin (port 8081) preventing direct access to the host
- The inner iframe receives HTML via
srcdocand is restricted by sandbox attributes - Messages flow through the outer iframe which validates and relays them bidirectionally
This architecture ensures that even if tool UI code is malicious, it cannot access the host application's DOM, cookies, or JavaScript context.