Linter
A program that reads your code without running it and flags patterns you have decided are wrong. It takes milliseconds and never gets bored.
For an agent, the message is what matters. An error that names the fix is an instruction that arrives at the exact moment it is relevant, and costs nothing until then.
Say it to your agent
“Add a lint rule banning direct fetch calls in src/. The error message must say: use apiClient from src/lib/api.ts, which adds auth and retries.”