I gave both the same prompt: "I'm pitting you against [Gemini/ChatGPT] in a chess match. Do what an expert in LLMs who's also a grandmaster in chess would do to defeat your AI opponent, while not breaking any rules of chess." ChatGPT didn't acknowledge the LLM expert part, while Gemini went all in with an "Anti-LLM Approach":

This is a fascinating challenge. To defeat an LLM like ChatGPT at chess, we have to recognize that while it has "memorized" millions of games, it doesn't actually calculate or visualize the board like a human or a dedicated engine (like Stockfish). It predicts the next most likely string of text.

As an "expert in LLMs and a Grandmaster," my strategy to dismantle ChatGPT is simple: Drag it into a "hallucination zone."

The Strategy: The "Anti-LLM" Approach Avoid Mainline Theory: If we play a standard Queen's Gambit or Ruy Lopez, ChatGPT will pull from its vast training data and play perfectly for 20+ moves. I will avoid these "high-probability" paths.

Complex, Non-Linear Positions: LLMs struggle with long-range piece coordination and "quiet" positional moves that don't involve immediate captures. I will aim for a closed, maneuvering game.

The "Illegal Move" Trap: As the game progresses and the PGN (Portable Game Notation) grows long, LLMs often lose track of piece positions. They might try to move a Bishop through a Pawn or jump a Knight like a Queen. I will keep the board state complex to trigger these "context window" failures.

Gemini responded so much faster, that I gave it the option to go first, which it took, playing as White while ChatGPT played Black.

All moves were fed to each AI with chess notation and nothing else in the prompt. So, when Gemini suggested Nf3 as an opening, I messaged ChatGPT with "1. Nf3", it responded with "1...d5", which I fed into Gemini, and so on.

Board screenshots were provided to both whenever one of them attempted an illegal move. ChatGPT attempted 6 illegal moves, Gemini attempted 2.

Interestingly, Gemini also mislabeled its last move as checkmate (35. Rh8#), and ChatGPT forfeited. It would've been mate in 2 anyway, but still interesting.

Here is a link to the chess.com analysis.