---
slug: reference/polyfill
description: "The document.modelContext (WebMCP) polyfill in @webmcp-agents/core, including declarative <form toolname> tools; @webmcp-agents/polyfill is deprecated and re-exports it."
---

# The polyfill

A `document.modelContext` (WebMCP) polyfill for browsers without native support, including declarative `<form toolname>` tools. It ships in `@webmcp-agents/core` as `@webmcp-agents/core/polyfill`, and [`<WebAgentsProvider>`](/docs/reference/react/web-agents-provider) and [`createWebAgents`](/docs/reference/core/create-web-agents) load it themselves where the browser has no WebMCP (their `polyfill` option, default `"auto"`), as a chunk of its own that browsers with WebMCP never download. It steps aside when the browser has the real API.

Most apps never import it. A page that registers tools without the provider imports it directly:

```ts
import "@webmcp-agents/core/polyfill/auto"; // or:
import { installWebMCPPolyfill } from "@webmcp-agents/core/polyfill";
const modelContext = installWebMCPPolyfill();
```

## `@webmcp-agents/polyfill` is deprecated

The package re-exports these entries (`@webmcp-agents/polyfill` is `@webmcp-agents/core/polyfill`, `@webmcp-agents/polyfill/auto` is `@webmcp-agents/core/polyfill/auto`), so code written against it keeps working. To move on: uninstall it, and delete the `import("@webmcp-agents/polyfill/auto")` and any "ready" state that waited for it; the provider does both.

## APIs

| API | Description |
|---|---|
| [`@webmcp-agents/core/polyfill/auto`](/docs/reference/polyfill/auto) | Side-effect entry that installs the polyfill on import, for a page without the provider. |
| [`installWebMCPPolyfill`](/docs/reference/polyfill/install-webmcp-polyfill) | Install the polyfill unless the browser has WebMCP; returns the active `ModelContext`. |
| [`PolyfillModelContext`](/docs/reference/polyfill/polyfill-model-context) | The installed `document.modelContext`: `registerTool`, `unregisterTool`, `getTools`, `executeTool` with `{ signal }`, the non-standard `cancelTool`, events, declarative forms and the confirmation pause. |
| [`loadWebMCPPolyfill`](/docs/reference/core/load-webmcp-polyfill) | From `@webmcp-agents/core`: load it only where the browser has no WebMCP, as the provider does. |

## What it covers

- `registerTool(tool, { signal })`, `unregisterTool(name)`, `getTools()`, `executeTool(nameOrTool, args, { signal })`. `executeTool` accepts both its own `(name, object)` and Chrome's `(tool, JSON string)` shapes.
- Events on `document.modelContext`: `toolchange`, `toolactivated`, `toolcancel` (the last two with `.toolName`).
- Declarative `<form toolname …>` tools: input schema from the controls, `toolautosubmit`, and the confirmation pause, mirrored as `data-tool-form-active` / `data-tool-submit-active`.
- `SubmitEvent.agentInvoked` and `respondWith(promise)`.
- Non-standard `cancelTool(name)`, so dev tooling can cancel a call it didn't start.

## Differences from Chrome

- Event order: the polyfill fires `toolactivated → fill → submit`, native Chrome `fill → submit → toolactivated`. `@webmcp-agents/core` handles both.
- No `:tool-form-active` / `:tool-submit-active` pseudo-classes: use the `data-tool-…` attributes (core's `cursor.css` styles both, in separate rules).
- Unregistering a tool doesn't cancel its running calls; in Chrome it does.

## See also

- [Browsers without WebMCP](/docs/more/polyfill) and [Installation](</docs/installation>)
- [`@webmcp-agents/core`](/docs/reference/core) ([`loadWebMCPPolyfill`](/docs/reference/core/load-webmcp-polyfill), [WebMCP types](/docs/reference/core/webmcp-types), [`executeTool`](/docs/reference/core/execute-tool))
- [`@webmcp-agents/react`](/docs/reference/react) ([JSX attributes](/docs/reference/react/jsx-attributes), [`useToolAnswer`](/docs/reference/react/use-tool-answer))