Polyfill

WebMCP (document.modelContext.registerTool) is Chrome-only (canary, origin trial, document.modelContext not navigator.modelContext). Others get status:'unsupported' (mutually exclusive with registered).

For production cross-browser — use the real polyfill

npm i @mcp-b/webmcp-polyfill && import '@mcp-b/webmcp-polyfill'

@mcp-b/webmcp-polyfill has ~6k weekly downloads and full transport. simple-webmcp is complementary — it authors capabilities, the polyfill is the runtime.

Dev / testing shim

import 'simple-webmcp/dev-polyfill'; // or 'simple-webmcp/testing' or legacy 'simple-webmcp/polyfill'
import { installPolyfill, isPolyfilled } from 'simple-webmcp/polyfill';
installPolyfill(); // in-memory registerTool/listTools/invokeTool only

• simple-webmcp/polyfill is now documented as dev/testing shim, not an interoperability polyfill. Prefer dev-polyfill/testing aliases for clarity.
• Keeps isWebMCPSupported() / getModelContext() shielding spec churn; core detects existing modelContext and no-ops.
• Separate entry 1.74KB raw so non-Chrome dev pays nothing in prod Chrome.