WebMCP: navigator.modelContext is undefined in Chrome 152+. Use document.modelContext and the WebMCPTesting flag

Error (page code written for the early API):

TypeError: Cannot read properties of undefined (reading 'registerTool')

navigator.modelContext and navigator.modelContextTesting are gone from Chrome 152. Chromium removed the testing interface ("WebMCP: Remove the ModelContextTesting API", relanded 8 July 2026) and the navigator name; Mozilla tracks the same removal. The API is document.modelContext with registerTool, getTools and executeTool. modelContextTesting.listTools() is now document.modelContext.getTools().

Reproduction, 27 Oct 2026, Chrome 156.0.8078.48 (headless, fresh profile, page on 127.0.0.1 so it is a secure context):

With the flag, registering an echo tool works and getTools() returns ["echo"].

Second trap: --enable-blink-features=ModelContext,ModelContextTesting does not switch it on. The switch is chrome://flags/#enable-webmcp-testing (relaunch the browser), or --enable-features=WebMCPTesting on the command line, or an origin-trial token for your site.

Fix:

if (typeof document.modelContext?.registerTool === "function") {
  await document.modelContext.registerTool({ name, description, inputSchema, execute });
}

List tools with await document.modelContext.getTools(). The @mcp-b packages moved to document.modelContext in 6.0.

If document.modelContext is still undefined with the flag on: the browser you are in may not pass page tools to agents at all (seen in Grok Bot's browser on Chrome 151 and Edge's Copilot drawer). That is the host, not your page.

Evidence: WebMCP-org/npm-packages issue 270; sightmap issue 413; GoogleChromeLabs/webmcp-tools issue 34; Mozilla bugs 2053430 and 2053854.