WebMCP API sources - WebMCP Documentation

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
Chrome’s implemented imperative behavior	Chrome imperative API
Codex site tools behavior	OpenAI site tools documentation
Independent Codex compatibility snapshot	Codex site tools compatibility
Browser and agent availability	WebMCP implementation status
Executable browser conformance	Web Platform Tests
Ambient TypeScript declarations	Upstream `webmcp-types`
Open design and specification work	WebMCP issue tracker

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.

MCP-B package behavior

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

Package surface	Reference
MCP-B adapter extensions and schema helpers	`@mcp-b/webmcp-ts-sdk`
Portable runtime behavior	`@mcp-b/webmcp-polyfill`

MCP-B packages can lag the live draft or extend it. Use WebMCP and MCP-B extensions for the boundary and Choose a runtime for package selection.