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The Killfish chess engine, developed by the Mexican programmer Antonio Calzada, is a high-performance Stockfish derivative designed primarily for fast time controls and practical engine competition. Within the contemporary ecosystem of NNUE-based engines, Killfish has emerged as a specialised tool for bullet and blitz play, combining the raw tactical power of Stockfish with targeted refinements and frequent, carefully curated builds by its author.(X (formerly Twitter))

As a derivative, Killfish inherits the fundamental architecture of Stockfish—bitboard representation, alpha–beta search with sophisticated pruning, and an NNUE (Efficiently Updatable Neural Network) evaluation framework. On top of this foundation, Calzada is known for a hands-on, experimental approach: successive “PB” (private build) versions of Killfish are tested intensively in private and public rating environments, and only the most stable, performant iterations are released more widely. This iterative workflow is visible both in forum discussions and in the regular announcements of new PB builds.(outskirts.altervista.org)

From a competitive standpoint, Killfish has achieved notable success in independent testing. In large round-robin events featuring many of the strongest contemporary engines, versions such as Killfish PB 090225 have taken clear first place against elite opposition, an indication that the engine is not merely theoretically strong, but also robust across thousands of practical games.(Facebook) Other testing environments have likewise adopted Killfish as a benchmark Stockfish derivative, using it as a reference point when evaluating new engines or versions, which underscores its perceived reliability and strength among engine testers.(chessengines.blogspot.com)

The engine’s performance is particularly visible in bullet rating lists of Stockfish derivatives, where Killfish consistently occupies the very top positions. In such lists, compiled from many thousands of high-quality games under strict testing conditions, Killfish PB builds have achieved a clear Elo lead over a field composed almost exclusively of modern NNUE engines. The underlying statistics—large game samples, balanced colour assignment and rigorous time controls—indicate that Killfish’s superiority in these settings is statistically meaningful rather than anecdotal. This makes the engine especially attractive for users who focus on bullet and blitz competition, or who rely on rapid analysis workflows.

Beyond raw strength, Killfish is closely integrated into the broader ecosystem curated by Calzada. Through the ChessEngines AC Telegram channel, the author disseminates binaries, experimental versions, test results and a wide range of auxiliary tools for serious engine users. The channel has grown into a specialised hub for computer-chess enthusiasts, and Killfish is one of its central offerings, often released together with companion engines such as SpecTral and Dark SisTer.(Telegram) This mode of distribution fosters an active feedback loop between developer and community, which in turn accelerates the identification of regressions, corner cases and promising new ideas.

For practical users—engine testers, correspondence players and advanced analysts—Killfish offers a combination of attributes that are particularly valuable:

• Tactical sharpness and stability at very fast time controls, making it a natural choice for bullet gauntlets and stress-testing new hardware or opening books.
• Strong compatibility with Stockfish-style workflows, allowing users to integrate Killfish into existing GUIs, test frameworks and opening suites without needing to change infrastructure.
• Rapid development cadence, with frequent PB builds that incorporate the latest ideas from the Stockfish codebase as well as Calzada’s own search and evaluation experiments.(outskirts.altervista.org)

In the broader landscape of modern chess engines, Killfish can thus be viewed as a focused, high-end derivative that bridges the gap between mainstream Stockfish releases and experimental research engines. It retains the theoretical depth and endgame accuracy characteristic of top NNUE programs, while emphasising practical improvements for speed chess and large-scale automated testing. For organisations maintaining rating lists and for individual users seeking a powerful companion to Stockfish itself, Killfish by Antonio Calzada stands out as a mature, rigorously tested and competitively validated choice.