V4A 1.0.1

V4A.Net

A .NET library for applying V4A diffs (the patch format used by OpenAI’s apply-patch tool) to text files.

V4A.Net implements the same patch application semantics used by OpenAI’s apply-patch workflow: patched text is generated with context hunks and applied via fuzzy context matching rather than strict line numbers. This library lets .NET applications interpret and apply those patches reliably. ([OpenAI Platform][1])

Purpose

Modern AI tools like OpenAI’s apply-patch API return structured patch operations with diffs in a context-based format. Consumers of that API must parse and apply these patches locally. V4A.Net provides a faithful .NET implementation of this diff format’s application logic, matching the expectations of the apply-patch specification. (OpenAI Platform)

Installation

Install via NuGet:

dotnet add package V4A

Usage

Apply a patch to existing content

using V4A;

string original = "line1\nline2\nline3\n";
string diff = string.Join("\n", new[] {
    "@@ line1",
    "-line2",
    "+updated",
    " line3"
});

// Apply contextual update
string result = DiffApplier.ApplyDiff(original, diff);

// result:
// line1
// updated
// line3

Create new content from a diff

Diffs in create mode consist only of + prefixed lines:

string diff = string.Join("\n", new[] {
    "+hello",
    "+world",
    "+"
});

string result = DiffApplier.ApplyDiff(
    input: "",
    diff: diff,
    mode: DiffApplier.ApplyDiffMode.Create);

// result:
// hello
// world

How it works

V4A.Net supports two main diff formats:

• Update mode: Applies a set of context hunks with @@ anchors, fuzzy matching, deletions (-) and additions (+).
• Create mode: Interprets all lines as new content (only + prefixes allowed).

When contextual diffs can’t be matched exactly, the library attempts fuzzy matches and throws detailed exceptions on failure.

Attribution

This library is a C# port of the Python apply_diff implementation from the openai-agents-python repository. The original Python version was authored by Kazuhiro Sera and released under the MIT license. Source implementations from the Python SDK serve as reference for semantics and behavior. ([OpenAI Platform][1])

License

MIT