running the real library in your browser — nothing here is faked
webmcp-verified
Agent tools that answer only from your data, cost fewer tokens to discover, and keep a receipt. Everything below runs the actual npm package client-side — no server, no mock.
1 · grounded, not invented
Ask the shop's quoting tool something
It answers from a declared rate card. Ask for something that isn't on it and it returns a diagnostic — it will not make up a price.
2 · cheaper to discover
The token cost of choosing a tool
An agent loads the tool list before it picks one. The primary numbers below are served — schema in the list (what this library registers on an MCP/WebMCP host, since MCP requires it) — where only the long-form help text is deferred, pulled in full only for the tool the agent uses. Lighter, underneath: schema-deferred — only if your host lets you omit inputSchema — an upper bound most hosts can't actually offer.
Measured on 14 real tools from 5 official MCP servers — not a made-up surface. Descriptions are verbatim from each server's source:
1 tool14 tools14 tools
3 · a receipt for every answer
Prove what it said
Every answer emits a tamper-evident receipt — a fingerprint of exactly what was returned, from which source. Ask a quote above, then check or tamper with it here.
Ask a quote in section 1 first, then a receipt shows up here.
honest scope
The token counts use a ~4-char/token gauge, so absolute numbers are approximate — the % and the break-even are the robust figures.
Lean discovery loses under 4 tools on this surface (the describe round-trip costs more than it saves), and loses again once the agent describes a second tool. A few-of-many-tools win, not magic.
This is a cost lever, not an accuracy one. If an agent picks the wrong tool, a cheaper menu won't fix it — two tools with blurry boundaries stay blurry once the schema loads. The fix for that is fewer, genuinely distinct tools: turn variations on the same question into a parameter instead of a new tool. (Credit to the practitioners on r/mcp who made this point sharply.)
Grounding stops the tool inventing a value off your source. It can't make your source data correct — that's still on you.
The fingerprint is tamper-evidence, not a cryptographic signature.