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@mcp-b/webmcp-polyfill installs the strict WebMCP core on navigator.modelContext when the browser does not ship it yet. It is the runtime choice for teams that want the browser-facing standard surface without MCP-B-only extensions.

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When to use this package

  • You want registerTool and unregisterTool in browsers today.
  • You are building a site or reusable library that wants to stay close to the W3C surface.
  • You want to pair runtime behavior with @mcp-b/webmcp-types for type inference.

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When not to use this package

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Where it sits in the package graph

This package sits directly above @mcp-b/webmcp-types. The full runtime in @mcp-b/global wraps the polyfill when it initializes.

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First step

Initialize the polyfill before your app registers tools. The reference page documents the initialization options, testing shim behavior, and the strict-core lifecycle.

Reference

Initialization options, testing shim details, and runtime behavior.

First tool tutorial

Build your first strict-core tool.

Native vs polyfill vs global

Compare the three runtime strategies.

Test native and polyfill

Validate your code against both native and polyfilled paths.