> ## 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.

# Spec status and limitations

> What the WebMCP proposal offers today and where implementations have room to grow.

WebMCP is an active community-group proposal, not a completed W3C
standard. It defines the direction of the browser API, but several
details are still a work in progress.

Browser APIs, declarative attributes, testing surfaces, iframe policy,
concurrency, cancellation, and output-schema behavior may continue to change.

Implementations should track the current [W3C draft](https://webmachinelearning.github.io/webmcp/),
keep domain operations separate from the browser registration adapter, use
feature detection, and test both native and polyfill paths. MCP-B extensions
must remain clearly distinguished from the standard surface.

New code uses `document.modelContext`. `navigator.modelContext` remains a
deprecated compatibility alias, while `navigator.modelContextTesting` is a
testing and inspection surface. Adapter boundaries reduce migration cost.

See [Try the native Chrome preview](/tutorials/first-native-preview) and
[Test native and polyfill](/how-to/test-native-and-polyfill).

## Upstream sources to track

Use first-party sources for compatibility decisions:

| Source                                                                                       | Owns                                                                                                                               |
| -------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------- |
| [W3C WebMCP draft](https://webmachinelearning.github.io/webmcp/)                             | Standard API shape, `document.modelContext`, `registerTool()`, permissions policy, security model                                  |
| [Chrome WebMCP imperative API](https://developer.chrome.com/docs/ai/webmcp/imperative-api)   | Current Chrome preview behavior for `getTools()`, `executeTool()`, `AbortSignal` cleanup, and `navigator.modelContext` deprecation |
| [Chrome WebMCP declarative API](https://developer.chrome.com/docs/ai/webmcp/declarative-api) | Form annotation behavior and declarative schema synthesis                                                                          |
| [Chrome WebMCP tool security](https://developer.chrome.com/docs/ai/webmcp/security)          | Browser-facing tool security guidance                                                                                              |
| [MCP SEP-2106](https://modelcontextprotocol.io/seps/2106-json-schema-2020-12)                | MCP schema changes for JSON Schema 2020-12, output schemas, and `structuredContent`                                                |

This release line follows those sources by making `document.modelContext` the
primary surface, preferring `getTools()` plus `executeTool()`, using
`AbortSignal` for cleanup, and keeping legacy MCP-B helpers as compatibility
APIs.

`outputSchema` is not part of the current W3C/Chrome WebMCP tool dictionary.
MCP-B keeps it as helper metadata for TypeScript inference, structured MCP
responses, and package/runtime adapters. Do not rely on native browser WebMCP
to validate tool output against it.