ToolSource

Where tools come from. Use the native one, or write your own.

interface ToolSource {
  discover(signal?: AbortSignal): Promise<ToolSnapshot>;
  execute(call: ToolCall, revision: number, signal?: AbortSignal): Promise<ToolResult>;
  subscribe(listener: () => void): () => void;
  dispose(): void;
}

The native source

import { createWebMCPSource } from 'actionwire';

const source = createWebMCPSource();

It reads document.modelContext, and it:

• Discovers with getTools() and keeps the native handle for each tool.
• Skips any tool whose window is not the current window. Frames are out of scope for this release.
• Parses the input schema once and reuses it. It does not write a second schema.
• Listens for toolchange and re-discovers.
• Validates arguments against the tool schema before it calls the handler.
• Reads readOnlyHint and consequentialHint into the tool definition.

Construction does not throw on a browser with no WebMCP. The first discover() throws UNSUPPORTED_WEBMCP instead.

NOTE

The native source sends arguments in the format each discovered tool declares: a JSON string for the older Chromium build, an object for the newer one. Both work with no configuration.

ToolDefinition

Field	Type	Meaning
id	string	Unique key used by the bridge to run the tool.
name	string	Name the model sees.
description	string	What the model reads to choose the tool.
inputSchema	Record<string, Json>	JSON Schema object. Must be type: "object".
readOnly	boolean | undefined	From readOnlyHint. Missing means unsafe.
consequential	boolean | undefined	From consequentialHint. true always prompts.

Ids and names must be unique and non-empty. Supported schema dialects are JSON Schema 2020-12, the default, and draft-07.

Revisions

ToolSnapshot carries a revision. It rises when the tool list changes in a way that matters, including a re-registration that keeps the same name and schema.

The bridge passes the revision it planned against into execute. If the source has moved on, the call fails with STALE_TOOLS and the handler does not run. This is what makes an approval expire. See Confirmations.

If you write a source, raise revision and call your subscribers whenever your tool list changes. Skipping that lets an old approval run against a new tool.

Writing a source

Follow the worked example under Without native WebMCP. The short contract:

• discover returns the current snapshot. Throw AgentError on failure.
• execute returns a ToolResult with ok: false and a code on failure. Do not throw for an ordinary tool error.
• subscribe returns its own unsubscribe function.
• dispose releases everything and makes later calls fail.