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@mcp-b/global is the MCP-B runtime entry point layered on top of WebMCP. It orchestrates the polyfill, creates a BrowserMcpServer, sets up transport, and replaces navigator.modelContext with the server instance. After initialization, navigator.modelContext still supports the strict WebMCP surface, but it also exposes MCP-B extension methods such as prompts, resources, sampling, elicitation, and transport-facing helpers. 
npm install @mcp-b/global

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Package selection

PackageUse When
@mcp-b/webmcp-typesYou need compile-time types only
@mcp-b/webmcp-polyfillYou want the WebMCP standard path without extensions
@mcp-b/globalYou want the MCP-B runtime with transport, prompts, resources, and extension APIs
For the reasoning behind this split, see Strict Core vs. MCP-B Extensions.

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Installation

npm install @mcp-b/global

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Minimal example

FormatSizeDependencies
IIFE~285KB minifiedAll bundled
ESM~16KBExternal dependencies required

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Auto-initialization

Importing @mcp-b/global in a browser environment auto-initializes navigator.modelContext. This behavior is controlled by window.__webModelContextOptions. 
<script>
  window.__webModelContextOptions = {
    autoInitialize: true,
    transport: {
      tabServer: { allowedOrigins: ['https://myapp.com'] },
    },
  };
</script>
<script src="https://unpkg.com/@mcp-b/global@latest/dist/index.iife.js"></script>
To prevent auto-initialization: 
<script>
  window.__webModelContextOptions = { autoInitialize: false };
</script>
<script src="https://unpkg.com/@mcp-b/global@latest/dist/index.iife.js"></script>
<script>
  MCPB.initializeWebModelContext();
</script>

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Functions

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initializeWebModelContext(options?)

Initializes the global adapter. Replaces navigator.modelContext with a BrowserMcpServer instance. 
import { initializeWebModelContext } from '@mcp-b/global';

initializeWebModelContext({
  transport: {
    tabServer: { allowedOrigins: ['https://example.com'] },
  },
});
Behavior:
  • No-op in non-browser environments (SSR safe).
  • Idempotent: calling multiple times after first initialization is a no-op.
  • Skipped if navigator.modelContext is already a BrowserMcpServer (cross-bundle guard).
Initialization sequence:
  1. Calls initializeWebMCPPolyfill() to install navigator.modelContext and navigator.modelContextTesting (skipped if native API exists).
  2. Saves a reference to the current navigator.modelContext as native.
  3. Creates a BrowserMcpServer with { native } so core operations mirror to the underlying context.
  4. Syncs existing tools from the native/polyfill context into the server.
  5. Replaces navigator.modelContext with the BrowserMcpServer instance.
  6. Creates and connects the transport.

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cleanupWebModelContext()

Tears down the adapter, closes transport, and restores navigator.modelContext to its pre-initialization state. 
import { cleanupWebModelContext } from '@mcp-b/global';

cleanupWebModelContext();
After cleanup, initializeWebModelContext() can be called again to re-initialize.

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Configuration

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WebModelContextInitOptions

OptionTypeDefaultDescription
transportTransportConfigurationAuto-detectTransport layer configuration
autoInitializebooleantrueWhether to auto-initialize on import
nativeModelContextBehavior'preserve' | 'patch''preserve'How to handle an existing native modelContext. Both modes wrap with BrowserMcpServer.
installTestingShimboolean | 'always' | 'if-missing''if-missing'Controls navigator.modelContextTesting installation

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TransportConfiguration

OptionTypeDefaultDescription
tabServerPartial<TabServerTransportOptions> | falseAutoTab transport options, or false to disable
iframeServerPartial<IframeChildTransportOptions> | falseAutoIframe transport options, or false to disable
Transport is auto-selected based on context:
ContextTransport Used
Inside an iframeIframeChildTransport
Main windowTabServerTransport
// Restrict to specific origins
initializeWebModelContext({
  transport: {
    tabServer: { allowedOrigins: ['https://myapp.com'] },
  },
});

// Disable tab transport (iframe only)
initializeWebModelContext({
  transport: { tabServer: false },
});

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Methods on navigator.modelContext

After initialization, navigator.modelContext is a BrowserMcpServer instance. It exposes: Strict WebMCP methods (mirrored to native/polyfill):
MethodDescription
registerTool(tool)Add a single tool
unregisterTool(name)Remove a tool by name
Extension methods (MCP-B only):
MethodDescription
listTools()List registered tools
callTool(params)Execute a tool by name
registerResource(descriptor)Register an MCP resource
registerPrompt(descriptor)Register an MCP prompt
createMessage(params)Request LLM sampling
elicitInput(params)Request user input
For the MCP-B-only methods on navigator.modelContext, see @mcp-b/webmcp-ts-sdk reference. For the standard-owned browser surface, see WebMCP standard API.

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Tool routing

@mcp-b/global mirrors the strict tool lifecycle down to the underlying native or polyfill context so browser-facing tooling can still see registered tools. During initialization it can also backfill existing tools from navigator.modelContextTesting when that shim is available.

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Type exports

import type {
  CallToolResult,
  InputSchema,
  ModelContext,
  ModelContextCore,
  ModelContextOptions,
  NativeModelContextBehavior,
  ToolAnnotations,
  ToolDescriptor,
  ToolListItem,
  ToolResponse,
  TransportConfiguration,
  WebModelContextInitOptions,
} from '@mcp-b/global';