navigator.modelContext is deprecated. Move to document.modelContext

If you built anything against WebMCP before late July 2026, you wrote navigator.modelContext. That is no longer where the API lives.

What changed and why

On 21 July 2026 the specification moved the interface from navigator to document. The reasoning is sound: tools belong to a document, not to a browser. A tab showing your checkout and a tab showing your blog expose different capabilities, and navigator is shared across the whole browsing context in a way that made that awkward to express.

Chrome deprecated navigator.modelContext in version 150. The origin trial continues to serve it — which is the dangerous part, because your code keeps working right up until it does not.

The migration

// Before
await navigator.modelContext.registerTool({ /* … */ });

// After
await document.modelContext.registerTool({ /* … */ });

// During the overlap, resolve once and log when you land on the old path
function modelContext() {
  if ("modelContext" in document) return document.modelContext;
  if ("modelContext" in navigator) {
    console.warn("[webmcp] falling back to the deprecated navigator.modelContext");
    return (navigator as any).modelContext;
  }
  return null;
}

const mc = modelContext();
if (mc) await mc.registerTool({ /* … */ });

Keep the warning. It is the only thing that will tell you which of your deployments is still on the old path once the trial stops serving it.

Everything to grep for

Expect more of this

WebMCP is a Draft Community Group Report and is not on the standards track. A rename of this size in the same year the feature shipped experimentally is not an anomaly, it is what pre-standard means.

Two habits keep the cost bounded:

• Wrap the API once. A single registerTool wrapper in your codebase turns the next relocation into a one-file change instead of a search-and-replace across a monorepo.
• Define tools as data. Keep names, descriptions and schemas in a plain object, separate from the registration call. When the call signature moves, the definitions do not.

// lib/webmcp.ts — the only file that knows where the API lives
export type ToolDef = {
  name: string;
  description: string;
  inputSchema: object;
  annotations?: { readOnlyHint?: boolean; untrustedContentHint?: boolean };
  execute: (args: any, ctx: { signal: AbortSignal }) => Promise<unknown>;
};

export async function register(tool: ToolDef, signal?: AbortSignal) {
  const mc = "modelContext" in document ? document.modelContext : null;
  if (!mc) return false;
  await mc.registerTool(tool, signal ? { signal } : undefined);
  return true;
}