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This page is a quick lookup. For the authoritative API shape, read the W3C WebMCP spec and the WebMCP proposal.
The strict imperative WebMCP surface. Owned by the WebMCP specification, not by @mcp-b/* packages.

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Methods

MethodPurpose
registerTool(tool)Adds one tool to the registry. Requires name, description, and execute. Throws on duplicate names. Defaults inputSchema to { type: 'object', properties: {} } when omitted.
unregisterTool(name)Removes the named tool. No-op if the name is not registered.
The formal spec centers on registerTool() and unregisterTool(). See the W3C spec for the full ToolDescriptor shape, ContentBlock types, InputSchema constraints, and execution semantics.

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Example

From the @mcp-b/webmcp-polyfill README, using only the strict core API: 
import { initializeWebMCPPolyfill } from '@mcp-b/webmcp-polyfill';

initializeWebMCPPolyfill();

navigator.modelContext.registerTool({
  name: 'get-page-title',
  description: 'Get the current page title',
  inputSchema: { type: 'object', properties: {} },
  async execute() {
    return {
      content: [{ type: 'text', text: document.title }],
    };
  },
});
The testing and inspection companion to navigator.modelContext. Chromium exposes it in the native preview. The MCP-B polyfill can install a compatible shim.
This surface is not the stable consumer API. Chromium preview behavior may change while the standard discussion continues. See Spec Status and Limitations for current design status.

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Methods

MethodPurpose
listTools()Returns registered tool metadata as ModelContextTestingToolInfo[]. Does not return execute functions.
executeTool(name, argsJson, options?)Executes a tool. argsJson is a JSON string. Returns a serialized result string, or null when navigation interrupts the flow.
executeTool(name, source, options?)Executes a streamed tool. source contains a ReadableStream of chunks and a Promise of args.
registerToolsChangedCallback(callback)Registers a callback invoked when the tool list changes.
getCrossDocumentScriptToolResult()Returns cross-document declarative tool results as a serialized string.
Key behaviors:
  • listTools() returns metadata only, not execute functions.
  • inputSchema on returned tool info is a serialized JSON string, not a parsed object.
  • executeTool() takes a JSON string for ordinary (non-streamed) calls.

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Example

const tools = navigator.modelContextTesting?.listTools();

const result = await navigator.modelContextTesting?.executeTool(
  'search',
  JSON.stringify({ query: 'webmcp' })
);

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Native preview vs polyfill shim

BehaviorNative Chromium preview@mcp-b/webmcp-polyfill shim
Available with native navigator.modelContextYesOptional (installTestingShim)
Declarative tool visibilityYesNo
listTools().inputSchema formatJSON stringJSON string
executeTool(...)YesYes
getCrossDocumentScriptToolResult()Returns native resultsReturns "[]"
For shim configuration, see @mcp-b/webmcp-polyfill. For end-to-end testing patterns, see Test native and polyfill.

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Feature detection

Check for both APIs before use: 
if ('modelContext' in navigator) {
  // Standard WebMCP API available (native or polyfill)
  navigator.modelContext.registerTool({ /* ... */ });
}

if ('modelContextTesting' in navigator) {
  // Testing/inspection API available
  const tools = navigator.modelContextTesting.listTools();
}
For native preview availability, flags, and current browser support, see Browser support and flags.

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Where MCP-B begins

The browser standard owns the surface above. MCP-B adds a separate layer on top.
If you need…Page
Strict core runtime behavior in all browsers@mcp-b/webmcp-polyfill
Full runtime with prompts, resources, and transport@mcp-b/global
MCP-B extension methods (callTool, registerPrompt, registerResource, createMessage, elicitInput)@mcp-b/webmcp-ts-sdk reference
TypeScript contracts for the standard surface@mcp-b/webmcp-types
For why the core surface is kept separate from MCP-B additions, see Strict core vs MCP-B extensions.