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5373f01d6f* chore: bump ext-apps to 1.1.2 Changes since 1.1.1: - feat: add ui/download-file method for host-mediated file downloads (#475) - pdf-server: robust path validation + folder/file root support (#497) - ci: auto-fix prettier formatting in pre-commit hook and CI (#498) * fix: sync local SDK build into node_modules for examples npm workspaces hoists example dependencies to the root node_modules, but installs the published registry copy of @modelcontextprotocol/ext-apps instead of linking to the local source. This causes type-check failures when examples use features not yet published to npm. Add scripts/link-self.mjs which copies the freshly-built dist/ and package.json into the hoisted node_modules copy after each SDK build, ensuring examples always type-check against the latest local types. See: https://github.com/npm/feedback/discussions/774 * Revert "fix: sync local SDK build into node_modules for examples" This reverts commit e3387e8434f3def030f5a223a70632092c929119. |
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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.