v0.2.0

Realigns the library with the current WebMCP
spec. This is a clean break — the public surface (<WebMCPProvider>, useMcpTool,
useWebMCPStatus) is unchanged in shape, but the underlying WebMCP wiring has breaking
changes.

Breaking changes

• API moved to document.modelContext. The registration API is now installed and detected
  on document.modelContext instead of navigator.modelContext. Native detection is
  document-only — navigator.modelContext is no longer read or written. (The testing/consumer
  API stays on navigator.modelContextTesting.)

• registerTool returns a Promise. The polyfill's registerTool(tool, options?) now
  returns a Promise<undefined> that rejects on:

  • an empty/missing name, a name longer than 128 chars, or a name not matching
    ^[A-Za-z0-9_.-]+$;
  • a duplicate name already registered;
  • an empty/missing description;
  • an execute that is not a function;
  • a non-serializable inputSchema;
  • an exposedTo entry that is not a parseable, potentially-trustworthy origin.

  Already-aborted AbortSignal passed to registerTool resolves as a no-op (registration is
  skipped), matching native Chrome 151. (Note: WebMCP spec PR #202 specifies rejection; native had
  not shipped that as of Chrome 151. Revisit if native changes.)

  useMcpTool routes these rejections into state.error and onError, except AbortError
  (lifecycle teardown), which is ignored.

• Unregistration is AbortSignal-only. There is no unregisterTool. Tools are removed by
  aborting the AbortSignal passed via registerTool's options; useMcpTool does this on
  unmount.

• Single-argument handlers. Tool execute(input) and user handler(args) now take a single
  argument. ModelContextClient and requestUserInteraction are removed entirely.

• ToolAnnotations narrowed to { readOnlyHint?, untrustedContentHint? }. The classic MCP
  hints (destructiveHint, idempotentHint, openWorldHint) and annotations.title are
  removed.

• Top-level title? added to the tool config and descriptor (replacing the dropped
  annotations.title).

• exposedTo?: string[] added to the tool config and RegisterToolOptions for cross-frame
  origin visibility. Changing exposedTo re-registers the tool.

• toolchange event. document.modelContext is an EventTarget that fires a bare
  toolchange event (no detail) on tool register/unregister. addEventListener("toolchange", ...) and an ontoolchange handler are both supported.

Retained library extensions

• outputSchema (Zod output / JSON-Schema outputSchema) is kept as a documented library
  extension, not part of the spec descriptor.
• structuredContent on CallToolResult is retained.
• Handlers always return a CallToolResult with a content array (including error results
  with isError: true) — a deliberate convention layered over the spec's looser return type so
  results bridge cleanly to desktop MCP clients.

Notes

• The already-aborted-signal behavior was confirmed against native Chrome 151: native skips
  registration and resolves with undefined (a no-op) rather than rejecting, so the polyfill
  matches. (WebMCP spec PR #202 specifies rejection; native had not shipped that as of Chrome 151.
  Revisit if native changes.)