A few days ago I posted Schema descriptions are load-bearing: why missing parameter descriptions break MCP clients . The argument: every parameter without a description is a load-bearing element silently absent from the schema, and agents fail in ways that look like model problems but are actually contract problems. The post got a comment from @mickyarun that's worth its own essay: The "load-bearing" framing is the right shape — the same observation applies one level up at the tool level. Most MCP catalogues we've audited had perfectly described parameters but no description of when not to call this tool, which is the bit that actually decides whether an agent reaches for the right surface. The half-hour we spent adding "anti-purpose" descriptions to about a dozen of our internal tools cut the wrong-tool-selected rate roughly in half.…