WebMCP Demo Store — one-file server, two modes, one benchmark

Same store, same tools, two delivery modes:

• WebMCP mode — the page exposes its catalog through MCP tool calls (MCP-over-HTTP /mcp, plus Chrome-native document.modelContext), so an agent reads only what the task needs.

• Plain mode — the same store as an ordinary website; the agent parses full HTML pages, as every agent must today.

Benchmark (150-product catalog, identical tasks): 79% less action bytes, 42% fewer input tokens, 8/8 tasks correct in both modes. DOM mode wins on small catalogs then collapses; WebMCP wins with age. Crossover report: report.html → WebMCP_Comparison_Report.pdf.

Run it

python3 server.py                    # plain mode      → http://localhost:9501
MCP=1 PORT=9502 python3 server.py    # WebMCP mode     → http://localhost:9502 + /mcp

stdlib-only (http.server, json, uuid, threading) — no pip install, no dependencies.

Related MCP server: Commerce MCP Server

Live demos

• plain: https://demonomcp.converso.work

• WebMCP: https://demowithmcp.converso.work (.well-known/mcp discovery, /mcp endpoint)

• launch package (deck, whitepaper, reports): https://demowithmcp.converso.work/launch.html

Chrome-native WebMCP test (13 Oct 2026)

Chrome 154 + --enable-features=WebMCP exposes document.modelContext on HTTPS origins (about:blank excluded).

• Registered the 5-tool store contract via the IMPERATIVE API (registerTool): search_products, add_to_cart, get_cart, checkout (server-backed; all fetches in-page).

• Standard client flow verified: getTools() discovery → executeTool() calls → orders 1017/1018 placed, T2/T4/T8 picks correct (HP Pavilion 14, Razer DeathAdder, OnePlus 12R).

• 154 build quirks found & worked around:

  1. executeTool(tool, args) requires args as a JSON string (object form throws "Failed to parse input arguments"); return values are JSON strings too — JSON.parse both sides.

  2. Tools can't be re-registered under the same name (InvalidStateError) — reload the page to reset.

  3. Flag/feature name: --enable-features=WebMCP works headless; docs' chrome://flags/#enable-webmcp-testing is the manual path. Note: API is document.modelContext (moved from navigator since July 2026 build).

• Takeaway: the demo's tool contract is compatible with Chrome's native WebMCP, not just our JSON-RPC shim. When Chrome ships the client agent (Gemini-in-Chrome), the store is already agent-ready.

Repo layout

License

MIT.