createPromptApiProvider

A LocalProvider backed by Chrome's built-in model (the Prompt API, LanguageModel).

A LocalProvider backed by Chrome’s built-in model (the Prompt API, LanguageModel). Reach for it when you build your own provider list; createLocalAdapter already tries it first. There is nothing to download per site once Chrome has the model.

Import

import { createPromptApiProvider } from "@webmcp-agents/local-agent";

Example

import {
  createLocalAdapter,
  createPromptApiProvider,
} from "@webmcp-agents/local-agent";

// Chrome's built-in model only, with no download fallback.
export const chromeOnly = createLocalAdapter({
  providers: [createPromptApiProvider()],
  label: "Chrome AI",
});

Options

None: createPromptApiProvider() takes no arguments.

Returns

A LocalProvider with id: "prompt-api", label: "Chrome built-in", toolsInline: true, and these members:

Member	Behaviour
`availability()`	No `LanguageModel`: `unavailable`. `"available"`: `available`. `"downloadable"` or `"downloading"`: `needs-download` with a note (Chrome downloads its model once, for every site). Anything else, or a throw: `unavailable`.
`prepare(onProgress, signal)`	Creates the base session (text in, text out), which starts Chrome’s download; `downloadprogress` events feed `onProgress`.
`generate(req)`	Clones the base session, prompts with a JSON `responseConstraint` (a message or one tool call), destroys the clone. Nothing streams.
`fits(req)`	Measures the prompt with `measureContextUsage` (or `measureInputUsage`) against `contextWindow` (or `inputQuota`), keeping 200 tokens for the reply. Without those, everything fits.
`dispose()`	Destroys the base session.
`broken`	Set when a session cannot be created; the adapter then moves to the next provider.

Notes

Tool calls come from JSON-constrained output, not the API’s native tools: those run execute inside prompt(), which would bypass the loop’s validation and the cursor’s pacing.
The system prompt rides on the first turn; tool results are sent as user turns (“Result of : …”).
Some devices report available but refuse to create a session. The provider then sets broken, remembers it in localStorage for this browser version (web-agents:prompt-api-broken), and throws “… Switching to the downloadable model.”
A preview API whose shape changes between Chrome versions: every call is guarded, and anything unexpected reads as unavailable. Desktop Chrome only.
Some devices give the model a window of about 1000 tokens; the loop shrinks the prompt until fits says yes.