[ochafik]

Summary

Middle-ground counterpart to #798, along the lines of Option 3 in #797 and of #796: instead of deprecating App.registerTool in favor of WebMCP, keep both tool surfaces first-class and have the SDK bridge them in both directions.

• Apps → WebMCP: tools registered with App.registerTool() are mirrored into document.modelContext when the document provides one, so a WebMCP-enabled host (one that consumes document.modelContext, e.g. natively) sees them without speaking the Apps tools/list channel. The wire path is untouched; mirroring is additive.
• WebMCP → Apps: tools the page registers on document.modelContext are served over the existing tools/list / tools/call iframe channel — the SDK declares the tools capability on the View's behalf when the page carries WebMCP tools at connect time, and relays toolchange as notifications/tools/list_changed. installWebMcpPolyfill() is the explore: add a thin bridge between WebMCP and MCP Apps Tools #796 "drop-in" for WebMCP-authored views that must also run where neither the browser nor the host provides document.modelContext.

No new wire messages, no host-side API changes, no deprecations: hosts keep bridge.listTools() / callTool(), views keep registerTool(), and a WebMCP-only host like the ones #797 anticipates reads document.modelContext as it would on any page. Dedup rules: registered tools shadow same-named WebMCP entries (which also keeps a tool's own mirror from being echoed back), only the frame's own tools are served, and the spec text says a host able to discover a view's tools both ways consumes exactly one surface per view. Both behaviors are per-App opt-outs (mirrorToolsToWebMcp / bridgeWebMcpTools).

Spec: the informative "Relation to WebMCP" section becomes a normative "WebMCP Interoperability" section with the WebMCP ↔ MCP mapping table (same shapes as #798: results as {content: [], structuredContent}, non-object values wrapped as {result: v}, thrown errors as isError: true).

Like #798, this is meant to make the option concrete for discussion, not to merge as-is — but unlike #798 it is non-breaking, so it would not need a major release.

Test plan

bun test src passes (253, including 8 new interop tests driving a real Server ↔ App pair over InMemoryTransport with the SDK polyfill as the model context); tsc --noEmit and typedoc --treatValidationWarningsAsErrors are clean. Not verified: native WebMCP (Chrome), real third-party hosts, the browser e2e suite.

Known limits / open questions

• Mirror re-registration on tool.update() aborts-then-registers; an implementation whose signal-triggered unregistration is asynchronous could reject the re-registration as a duplicate (works with the SDK polyfill; same pattern as Prototype WebMCP for View-exposed tools, to illustrate #797 #798).
• A model context that appears only after connect() is picked up for mirroring at the next reconnect or tool mutation, but the tools capability cannot be declared retroactively — WebMCP tools registered late on a page that had none at connect time are not served (capability is fixed at ui/initialize).
• toolchange fired by the App's own mirror registrations currently produces an extra notifications/tools/list_changed beside the one registerTool()/enable()/… already send.
• Native registration inside the sandbox needs the tools Permissions Policy delegated (feat: add support for 'tools' permission #691); where it is not, mirroring fails gracefully and tools/list remains the fallback.

cc @liady @idosal

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commit: 6044135

[ochafik]

superseded by #829