Open up the source code of any agent framework, harness, etc. Hermes, Copilot, Pi, Opencode or whatever.
You will find tools, tools everywhere. 

Examples: 

https://github.com/NousResearch/hermes-agent/tree/main/tools
https://github.com/anomalyco/opencode/tree/dev/packages/opencode/src/tool

These are bash hooks, file viewers, file editors,...