The polyfill
The document.modelContext (WebMCP) polyfill in @webmcp-agents/core, including declarative <form toolname> tools; @webmcp-agents/polyfill is deprecated and re-exports it.
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> and createWebAgents 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:
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 |
Side-effect entry that installs the polyfill on import, for a page without the provider. |
installWebMCPPolyfill |
Install the polyfill unless the browser has WebMCP; returns the active ModelContext. |
PolyfillModelContext |
The installed document.modelContext: registerTool, unregisterTool, getTools, executeTool with { signal }, the non-standard cancelTool, events, declarative forms and the confirmation pause. |
loadWebMCPPolyfill |
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 }).executeToolaccepts 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 asdata-tool-form-active/data-tool-submit-active. SubmitEvent.agentInvokedandrespondWith(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 Chromefill → submit → toolactivated.@webmcp-agents/corehandles both. - No
:tool-form-active/:tool-submit-activepseudo-classes: use thedata-tool-…attributes (core’scursor.cssstyles both, in separate rules). - Unregistering a tool doesn’t cancel its running calls; in Chrome it does.