LABSCO SUMMARY

It refuses to replace the tool list, and that is the point.

Most proxies flatten an upstream server into one generic call tool and lose the schemas; here the invariant is stated and tested — upstream names, descriptions and schemas survive, and no call_upstream_tool is registered. Read the version line before relying on the advanced policies: ambiguous_if and confirm_if are parsed for forward compatibility but are not implemented in v0.1.0, so what works today is required-field elicitation and sensitive-field blocking.

What it is

A transparent MCP proxy. It forwards an upstream server's tools untouched, and adds one behaviour on the call path: when a required argument is missing, it elicits that value from the user through the client, merges it into the arguments, and only then calls upstream.

What you get

Upstream tools kept exactly as they are — same names, descriptions and input schemas, visible in native tools/list, with no synthetic wrapper tool and no upstream_ prefix
Missing required fields requested from the user through the client's MCP elicitation capability, with your own prompt copy per tool
A definite outcome when elicitation is unavailable: if the client cannot elicit, or the user declines or cancels, it returns a structured result instead of calling upstream with holes
Required fields from two sources: the upstream JSON Schema required list, plus per-tool extras declared in YAML for call-time validation, in schema order with the configured ones appended
A credential guard that runs first — a missing required field that looks like a secret blocks form-mode elicitation and returns a tool_call_blocked result, while complete explicit input is still forwarded
Either kind of upstream: an HTTP upstream.url, or a upstream.command with args and string environment variables

Requirements

Python via uvx mcp-elicitation-proxy --config config.yaml; the config path can also come from MCP_ELICITATION_PROXY_CONFIG. The YAML needs exactly one of upstream.url or upstream.command — they are mutually exclusive — and carries the elicitation, policies and per-tool tools blocks. The client entry is the same uvx command; the README suggests the shorter local alias elicitation-proxy. A repeatable smoke test against @modelcontextprotocol/server-everything is documented with MCP Inspector. This is version 0.1.0.

Setup effort

One command — uvx mcp-elicitation-proxy --config config.yaml

See installation options

USE CASES

01

Stopping a tool call that is missing an argument

Let the proxy ask the user for the missing field instead of forwarding an incomplete call upstream.

02

Requiring a field the upstream schema does not

Declare it per tool in the YAML so call-time validation catches it even though upstream marks it optional.

03

Keeping secrets out of a prompt box

Let the sensitive-field policy block elicitation for credential-shaped arguments and return a structured refusal.

04

Adding elicitation to a server you cannot modify

Point the proxy at the existing HTTP or command-launched server and leave its tool surface untouched.

BEFORE YOU INSTALL

What to know first

Requires

uvx on PATH

INSTALLATION

Connect your client

Server setup

Configuration file

JSON

{
  "mcpServers": {
    "elicitation-proxy": {
      "command": "uvx",
      "args": [
        "mcp-elicitation-proxy",
        "--config",
        "<YOUR_PATH_TO_CONFIG>"
      ]
    }
  }
}

<YOUR_PATH_TO_CONFIG> is the full path of your config.yaml — the file that names the upstream MCP server the proxy sits in front of (an HTTP url or a command, as in the project's examples). You can set MCP_ELICITATION_PROXY_CONFIG instead.

uvx

TERMINAL

uvx mcp-elicitation-proxy --config config.yaml

MCP Elicitation Proxy