Modbus MCP: lets an AI agent read Modbus devices, and it refuses to write unless you switch writes on
Writes stay off unless the server is started with them enabled, and a write is sent once and never retried, so a lost reply comes back as "may have been applied, check first." That is how a tool that can touch a live register should behave.
For controls and instrumentation techs: let an agent pull register values and device IDs for troubleshooting without giving it a way to change anything.