mcp_argument_unknown

No such argument. Answered with 400.

What it means

The call carried an argument the tool does not take. The schemas are closed on purpose (additionalProperties: false), so a misspelled parameter name is named rather than silently dropped.

What to do

Drop the argument the detail names, or correct its spelling against the tool's inputSchema, and call the tool again.

The body

{
  "type": "https://tigby.eu/problems/mcp_argument_unknown",
  "title": "No such argument",
  "status": 400,
  "detail": "…what was refused, naming this request's own values",
  "code": "mcp_argument_unknown",
  "doc_url": "https://docs.tigby.eu/errors/mcp_argument_unknown",
  "agent_hint": "…the next step, for software"
}

Branch on code. The detail names this request's own Handle, id or limit and is not part of the contract.