Diff-Based Tool Output

When a tool edits a file, return only the changed lines or a unified diff instead of the full file content. The model does not need to re-read the entire file to understand the modification.

How It Works

The tool computes the diff between the original and modified file states and returns that diff in a standard format (unified diff, git diff, or structured patch). The model receives a compact representation that highlights exactly what changed.

Benefits

• drastically reduces token volume for file edit operations
• makes changes explicit and reviewable
• avoids model confusion from re-reading large unchanged sections

Example

Instead of returning 500 lines of file content after an edit:

{
  "file": "file.py",
  "diff": "@@ -10,3 +10,4 @@\n def foo():\n+    pass\n"
}

See Also

• Grep-Result Tool Output — another technique for returning minimal tool output.
• Structured Tool Output — diffs can be wrapped in structured schemas.
• Stateful Tracking in Agent Loops — tracking file state enables diff computation.

Tools

• Aider — coding assistant that uses diffs and concise edit formats to minimize context size.
• GitHub Copilot edits — returns edit diffs rather than full file rewrites.