Source Authorship Forensics · CCRL 40/40

The CCRL Top 50, read for AI vs human

A source-level sweep of the strongest engines in the world — each open-source one cloned and run through the same tests as the rest of the series, on both axes.

open-source, cloned & scanned

expert human

AI-assisted (Arasan)

actual SF clones

What the sweep found

No AI-generated engines. Unsurprisingly — no language model autonomously produces a 3500+ Elo engine. The whole elite tier is expert human work by named authors, developed over years.
One AI-marker hit, and a great one: Arasan — Jon Dart's engine, in development since 1994 — ships a src/CLAUDE.md. The single most-established engine here is also the one proving AI assistance is now everywhere. It lands in the same AI-assisted band as Triumviratus.
The top has converged on the Stockfish paradigm. Almost every engine rates Strong on Stockfish-similarity: bitboards, NNUE, and the LMR/NMP/SEE search stack — independently implemented, because that is simply what wins now.
Language variety underneath the convergence: mostly C++, but Rust is well-represented (Viridithas, Velvet, Black Marlin, Carp, akimbo) and Lizard is C# — in the top 12.

Scope & honesty. This lists the 37 top-50 engines confirmed open-source and actually cloned & scanned. The remaining slots are commercial/closed (Torch, Dragon by Komodo, Houdini, Deep Sjeng, SlowChess, Wasp, rofChade, Rebel) or a handful with no public repository found — see the note below the table.

The engines

Ranked by CCRL 40/40. ★ marks a full standout write-up. "Stockfish similarity" uses the same scale as the main spectrum.

#	Engine	Lang	Authorship	Stockfish sim.	Source
1	Stockfish ★	C++	Human · community	Reference	official-stockfish/Stockfish
4	Obsidian	C++	Human	Strong	gab8192/Obsidian
5	Berserk	C	Human	Strong	jhonnold/berserk
6	PlentyChess	C++	Human	Strong	Yoshie2000/PlentyChess
7	Caissa	C++	Human	Strong	Witek902/Caissa
8	Alexandria	C++	Human	Strong	PGG106/Alexandria
9	RubiChess	C++	Human	Strong	Matthies/RubiChess
10	Viridithas	Rust	Human	Strong	cosmobobak/viridithas
11	Lizard	C#	Human	Strong	liamt19/Lizard
12	Ethereal	C	Human	Strong	AndyGrant/Ethereal
13	Clover	C++	Human	Strong	lucametehau/CloverEngine
16	Seer	C++	Human	Strong	connormcmonigle/seer-nnue
17	Integral	C++	Human	Strong	aronpetko/integral
18	Uralochka	C++	Human	Strong	FreemanZlat/Uralochka3
19	Chess System Tal ★	C++	Human · expert	Moderate	ChrisWhittington/…-NNUE-2
21	Igel	C++	Human	Strong	vshcherbyna/igel
23	Clarity	C++	Human	Strong	Vast342/Clarity
24	Starzix	C++	Human	Strong	zzzzz151/Starzix
25	Velvet	Rust	Human	Strong	mhonert/Velvet-Chess
26	Motor	C++	Human	Strong	martinnovaak/motor
28	Koivisto	C++	Human	Strong	Luecx/Koivisto
30	Fire	C++	Human	Strong	FireFather/fire
31	Peacekeeper	C++	Human	Strong	Sazgr/peacekeeper
32	Arasan ★	C++	Human · AI-assisted	Moderate	jdart1/arasan-chess
35	Stormphrax	C++	Human	Strong	Ciekce/Stormphrax
36	Booot	Pascal	Human	Moderate	booot76/Booot-chess-engine
37	Black Marlin	Rust	Human	Strong	dsekercioglu/blackmarlin
38	Minic	C++	Human	Moderate	tryingsomestuff/Minic
39	Superultra	C++	Human	Strong	A1exL1ang/Superultra
40	Devre	C++	Human	Strong	OmerFarukTutkun/Devre
41	Halogen	C++	Human	Strong	KierenP/Halogen
42	Altair	C++	Human	Strong	Alex2262/AltairChessEngine
43	Carp	Rust	Human	Strong	dede1751/carp
45	akimbo	Rust	Human	Strong	jw1912/akimbo
46	Equisetum	C++	Human	Strong	justNo4b/Equisetum
48	Marvin	C	Human	Strong	bmdanielsson/marvin-chess
49	Renegade	C++	Human	Strong	pkrisz99/Renegade

How close to an actual Stockfish clone?

A sharper question than "similarity". Not "does it share Stockfish's ideas" (nearly all do) but "is it Stockfish's code?" I scanned every cloned engine's source for direct-derivation markers: Stockfish's copyright header, "part of Stockfish" notices, and copied NNUE code.

The finding: none of them. Zero of the 30 open-source engines carry Stockfish's copyright or a "part of Stockfish" notice — not one is a code fork. This is expected: CCRL deliberately filters out genuine clones and derivatives, so engines that make the list are, by construction, independent. What they share is the recipe (bitboards + NNUE + LMR/NMP/SEE), reached independently because it is what wins.

Two finer gradations do show up in the scan:

Shared NNUE vocabulary, not code: Stormphrax, Viridithas, Seer, Integral and Arasan lean on Stockfish's NNUE nomenclature (HalfKA-style features, "feature transformer"). That is the field's common language — not copied source.
Only open source can be checked: the commercial engines excluded from the table can't be source-inspected, so this scan makes no derivation claim about them in either direction. Among the engines we can read, none is a fork.

So on a "clone of Stockfish" scale, the open-source top 50 rates near-zero across the board: independent engines that converged on the same winning design. That is precisely why "Strong" in the similarity spectrum means shared technique, never shared code.

Not in the table. Commercial / closed-source (excluded): Torch (#2), Dragon by Komodo (#3), rofChade (#22), SlowChess Blitz (#33), Deep Sjeng (#34), Wasp (#44), Houdini (#50), Rebel (#20). No public GitHub repository found for: Titan (14), Ginkgo Santiago (15), Spaghet BologNNese (27), Revenge (29), pawn (47). That accounts for all 50 — the 37 above, 8 commercial/closed, and 5 with no findable source.