> ## Documentation Index
> Fetch the complete documentation index at: https://docs.mcp-b.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# WebMCP API sources

> Quick lookup for the document-first WebMCP surface and its compatibility APIs.

`document.modelContext` belongs to the evolving WebMCP Community Group
proposal. Use the source that owns the behavior you need.

## Platform and implementation sources

| Need                                             | Source                                                                                                          |
| ------------------------------------------------ | --------------------------------------------------------------------------------------------------------------- |
| Proposed interface, dictionaries, and algorithms | [WebMCP Community Group draft](https://webmachinelearning.github.io/webmcp/)                                    |
| Chrome's implemented imperative behavior         | [Chrome imperative API](https://developer.chrome.com/docs/ai/webmcp/imperative-api)                             |
| Codex site tools behavior                        | [OpenAI site tools documentation](https://learn.chatgpt.com/docs/webmcp)                                        |
| Independent Codex compatibility snapshot         | [Codex site tools compatibility](/reference/webmcp/codex-site-tools)                                            |
| Browser and agent availability                   | [WebMCP implementation status](https://github.com/webmachinelearning/webmcp/blob/main/implementation-status.md) |
| Executable browser conformance                   | [Web Platform Tests](https://wpt.fyi/results/webmcp)                                                            |
| Ambient TypeScript declarations                  | [Upstream `webmcp-types`](https://github.com/webmachinelearning/webmcp-types)                                   |
| Open design and specification work               | [WebMCP issue tracker](https://github.com/webmachinelearning/webmcp/issues)                                     |

<Warning>
  The proposal and browser implementations can differ while changes land. Use the Community Group
  draft for the proposed API, and use implementation docs and tests for browser compatibility.
</Warning>

## MCP-B package behavior

MCP-B package references describe the code shipped by this project. They do
not redefine the proposal.

| Package surface                        | Reference                                                       |
| -------------------------------------- | --------------------------------------------------------------- |
| Type contracts and compatibility types | [`@mcp-b/webmcp-types`](/packages/webmcp-types/reference)       |
| Portable runtime behavior              | [`@mcp-b/webmcp-polyfill`](/packages/webmcp-polyfill/reference) |
| Extended server behavior               | [`@mcp-b/webmcp-ts-sdk`](/packages/webmcp-ts-sdk/reference)     |

MCP-B preserves some compatibility surfaces that differ from the live draft.
The upstream `webmcp-types` package provides ambient proposal declarations;
the scoped `@mcp-b/webmcp-types` package adds MCP-B schema inference and
compatibility types. They are separate packages.

Use [WebMCP and MCP-B extensions](/explanation/strict-core-vs-mcp-b-extensions)
for the boundary and [Choose a runtime](/how-to/choose-runtime) for package
selection.