The only Kotlin Multiplatform library that bundles the Stockfish 18 chess engine as a native library for JVM, Android, iOS, and WebAssembly. Add one dependency and call the engine from common code: no manual binaries, no UCI subprocess wiring, no per-platform setup.

Stockfish is compiled from source and packaged for every target, so consumers ship a working engine with a single Gradle coordinate.

// Full — includes both large and small NNUE networks
implementation("fr.axl-lvy:stockfish-multiplatform:<version>")

// Lite — small NNUE network only, much smaller download size
implementation("fr.axl-lvy:stockfish-multiplatform-lite:<version>")

getStockfish() and search() are suspend functions, so call them from a coroutine:

suspend fun bestMove(): String {
  val engine = getStockfish()
  engine.setPosition(fen = "rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq - 0 1")
  val result = engine.search(depth = 20)
  return result.bestMove // e.g. "e7e5"
}

getStockfish() returns a shared singleton, so most apps never close it. If you do call close(), the next getStockfish() creates a fresh engine.

All targets are implemented and exercised in CI on every change.

Both modules expose the same API — the only difference is the bundled NNUE network:

The full variant re-evaluates positions with the large network when the small network's evaluation is uncertain, giving it a slight accuracy edge. The lite variant skips this and always uses the small network, resulting in a much smaller artifact.

Stockfish uses multi-threaded WebAssembly, which requires the browser to have SharedArrayBuffer enabled. Browsers only expose SharedArrayBuffer when the page is served with specific HTTP headers.

Kotlin/WASM with webpack — create webpack.config.d/headers.js in your module directory:

if (config.devServer) {
  config.devServer.headers = Object.assign(config.devServer.headers || {}, {
    "Cross-Origin-Opener-Policy": "same-origin",
    "Cross-Origin-Embedder-Policy": "credentialless"
  });
}

Production — configure your web server to add the same headers to all responses:

Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: credentialless

These headers are a browser requirement for any multi-threaded WebAssembly application, not specific to this library.

Runtime download: on WebAssembly the Stockfish engine is fetched at startup from the public CDN https://unpkg.com/stockfish@18.0.5/.... The first getStockfish() therefore requires network access, and your Content-Security-Policy must allow unpkg.com. This applies only to the WebAssembly target; JVM, Android, and iOS bundle the engine natively.

• Singleton: The native Stockfish bridge uses global static state — only one engine instance can exist per process. getStockfish() enforces this by returning the same instance on subsequent calls, unless the previous one was closed.
• Thread-safety: The high-level API (setPosition, search, setOption, postMessage) is mutex-serialized and safe to call from any coroutine. stop() and unsafePostMessage() are intentionally unguarded so they can be called while the mutex is held (e.g. to interrupt a search).
• stop() usage: Because search() holds the mutex until the engine emits bestmove, stop() must be called from a separate coroutine or thread.
• close() behavior: After close(), isClosed returns true and the next call to getStockfish() creates a fresh engine instance.
• Platform support: JVM (Linux/macOS/Windows), Android, iOS, and WebAssembly. All targets are implemented and exercised in CI.
• Closed engine: calling setPosition, setOption, postMessage, or search after close() throws IllegalStateException. If close() is called while a search() is in flight, that search() throws IllegalStateException instead of returning a result.