gptdiff-js

A browser-first JavaScript port of gptdiff, scoped to the two core APIs — generateDiff and smartapply — wired to NanoGPT for LLM completions (including "Sign in with NanoGPT" OAuth PKCE).

Everything runs in the browser (and in Node 18+): no filesystem, no build step, zero runtime dependencies. The diff engine is a faithful port of the Python implementation; it operates on an in-memory { path: content } file map instead of a directory on disk.

See it in action: live browser demos → (source in gptdiff-js-examples).

The gptdiff family — gptdiff (CLI + Python API) · gptdiff-js (you are here) · gptdiff-js-examples (live browser demos)

import { generateDiff, smartapply, buildEnvironment } from 'gptdiff-js';

const files = { 'greet.py': 'def greet():\n    print("hello")\n' };

// 1. Ask the model for a unified diff
const diff = await generateDiff(buildEnvironment(files), 'Say goodbye instead of hello');

// 2. Apply it with AI-powered conflict resolution
const updated = await smartapply(diff, files);
console.log(updated['greet.py']);

Install / run

It's plain ESM — point an import at src/index.js, or open index.html from a static server:

npx serve .        # then visit the printed URL and try the demo
npm test           # run the unit suite (Node's built-in test runner)
npm run test:live  # hit NanoGPT for real (requires env vars, see below)

Configuration

Configuration comes from environment variables (Node) or setEnv(...) overrides (browser). OAuth sign-in sets the API key override for you.

Variable	Purpose	Default
GPTDIFF_LLM_API_KEY	NanoGPT API key (sk-nano-…)	— (required for live calls)
GPTDIFF_LLM_BASE_URL	OpenAI-compatible base URL	https://nano-gpt.com/api/v1/
GPTDIFF_MODEL	Model id	xiaomi/mimo-v2.5-pro-ultraspeed

import { setEnv } from 'gptdiff-js';
setEnv('GPTDIFF_LLM_API_KEY', 'sk-nano-…'); // browser, no process.env

Sign in with NanoGPT (OAuth 2.0 + PKCE)

src/oauth.js implements the NanoGPT OAuth PKCE flow using Web Crypto. In a browser:

import { registerClient, beginSignIn, completeSignIn } from 'gptdiff-js/oauth';

// On page load — finishes the redirect and stores the access token as
// the GPTDIFF_LLM_API_KEY override automatically:
await completeSignIn();

// To start sign-in (clientId from a one-time dynamic registration):
const { client_id } = await registerClient({
  clientName: 'My App',
  redirectUri: location.origin + location.pathname,
});
await beginSignIn({ clientId: client_id }); // redirects to NanoGPT

Low-level helpers are also exported: generatePkce, buildAuthorizeUrl, exchangeCodeForToken, generateCodeChallenge.

API

generateDiff(environment, goal, opts?) → Promise<string>

Builds the prompt, calls the LLM, and returns the unified diff extracted from the ```diff block(s) of the response.

• opts.model, opts.temperature, opts.maxTokens, opts.apiKey, opts.baseUrl, opts.prepend, opts.images
• opts.callLlm — inject a custom/mock completion client (used heavily in tests).

smartapply(diffText, files, opts?) → Promise<{ path: content }>

Applies a diff to an in-memory file map with per-file, LLM-assisted conflict resolution (runs files concurrently). Handles creation, modification, and deletion; <think>…</think> and reasoning preambles are stripped automatically. Returns a new map; deleted files are omitted.

• opts.model, opts.apiKey, opts.baseUrl, opts.maxTokens
• opts.callLlmForApply — inject a custom/mock single-file applier.

applyDiff(files, diffText) → { changed, files }

Deterministic, no-LLM patch application (the strict "basic" applier). changed is true iff something was created, modified, or deleted; files is the updated map (equal to the input on failure — no partial writes).

Other exports

parseDiffPerFile, buildEnvironment, colorCodeDiff, swallowReasoning, stripBadOutput, extractDiffBlocks, callLlm, resolveApiKey, resolveBaseUrl, getEnv, setEnv, and the oauth namespace.

Differences from the Python package

• No filesystem. applyDiff/smartapply take and return a { path: content } map rather than reading/writing a project directory.
• Scope. Only generateDiff + smartapply (and their dependencies) are ported — not the gptdiff/gptpatch/plangptdiff CLIs.
• Dependency injection replaces Python's monkeypatch: pass callLlm / callLlmForApply to test without a network.
• fetch + Web Crypto replace openai/requests/tiktoken.

License

MIT