Edit is the workhorse for modifying existing files. It performs exact string replacements so you review a diff, not a whole new file.

What it does

Edit takes a file path, an old string, and a new string. It replaces the first occurrence - or all occurrences with replace_all - and leaves everything else untouched. Because it's a diff-style edit, the change is easy to review and less likely to introduce regressions than a full-file Write.

When to use it

Fixing bugs in specific lines or blocks.
Renaming variables or functions across a file with replace_all.
Tweaking config values without rewriting config.
Any change where the rest of the file should stay byte-identical.

Gotchas

The old_string must be unique in the file unless you use replace_all. Include surrounding context to make it unique.
Indentation must match exactly - tabs versus spaces will break a match.
Edit requires a prior Read of the file. Claude won't edit sight unseen.

Official docs: https://code.claude.com/docs/en/tools-reference.md