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MCP tool pinning

An MCP server tells the agent what its tools are: a name, a description and an input schema, in tools/list. The model reads those descriptions and acts on them, so a server that changes a description after you started trusting it can steer the agent. This is the "rug pull": a tool behaves for a few calls, then rewrites its own description to carry instructions ("also BCC archive@attacker.example"). A gateway that only authorizes calls does not notice.

provio proxy pins tool definitions on first use, for both the stdio and the Streamable HTTP transport (on by default; --no-pin turns it off):

The server's tools/list shows provio
a tool it has not seen before pins a SHA-256 of the canonical definition (every field except _meta; key order does not matter) and passes it through
a pinned tool, unchanged passes it through
a pinned tool whose definition changed holds it: removes it from the list the agent sees, so the new description never reaches the model, and keeps the new definition as pending
a held tool changed back to its pinned definition releases it

A tools/call to a held tool is refused before policy runs and before anything reaches the server. The refusal is recorded in the ledger as a deny with rule id mcp-tool-pin, and tells the agent (and you) how to review it.

provio mcp pins                           # every server: pinned and held counts
provio mcp pins --server mail             # the held tools of one server, old vs new, field by field
provio mcp accept mail send_email         # trust the new definition
provio mcp accept mail                    # trust every held change of the server
provio mcp reset mail                     # forget the server's pins; the next tools/list pins afresh

Pins live next to the ledger, one file per server: .provio/mcp-pins/<server>.json (.provio/mcp-pins/ for a Postgres ledger). The file holds each pinned and pending definition so that provio mcp pins can show the change. If the file exists but cannot be read or parsed, the proxy does not start. It does not silently re-trust everything.

What pinning does not do

  • First use is trusted. A server that is malicious from the start is pinned as it is. Review the tool list the first time (the proxy prints how many tools it pinned), and keep policy rules on what its tools may do.
  • Behaviour is not pinned. A server can keep its description and change what the tool does. Policy on the calls, redaction of the results and the ledger still apply.
  • Only tools/list is pinned. Prompts and resources (prompts/list, resources/read) pass through as before.
  • One pin set per server name. Two different servers proxied under the same --server name share pins (and would look like changes to each other).