AI & LLMs
What is a diff?
A diff is a record of exactly what changed between two versions of a file: the lines added, the lines removed, and nothing else.
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Technical definition
A diff is the computed difference between two versions of a text file, expressed as a set of line-level additions and deletions with surrounding context. Diffs underpin version control review, merges, and patch application.
In AI coding, diffs serve a second purpose. Instead of regenerating an entire file to change part of it, a model emits a diff describing the specific edit. The platform applies it. Output length drops from the size of the file to the size of the change.
In plain terms
A diff is a change list. Rather than handing you a new copy of a document and letting you find what moved, it tells you: this line went away, this line arrived.
Why it matters
For anyone paying for AI by usage, diffs are a cost mechanism.
Every token a model writes is billed. Changing one line in a 500-line file by rewriting the file spends 500 lines of output. Changing it with a diff spends a few. On a project you edit dozens of times, the difference compounds.
There is a quality effect too. A regenerated file is a chance for the model to alter something you did not ask about. A targeted edit leaves the rest of the file untouched.
And diffs make review possible. Reading a change list takes seconds. Comparing two versions of a whole file does not.
