Democratizing Diplomacy: A Harness for Evaluating Any Large Language
  Model on Full-Press Diplomacy
Authors: name: Alexander Duffy; name: Samuel J Paech; name: Ishana Shastri; name: Elizabeth Karpinski; name: Baptiste Alloui-Cros; name: Tyler Marques; name: Matthew Lyle Olson
Published: 2025-08-10 21:07:08

Abstract: We present the first evaluation harness that enables any out-of-the-box,
local, Large Language Models (LLMs) to play full-press Diplomacy without
fine-tuning or specialized training. Previous work required frontier LLMs, or
fine-tuning, due to the high complexity and information density of Diplomacy's
game state. Combined with the high variance of matches, these factors made
Diplomacy prohibitive for study. In this work, we used data-driven iteration to
optimize a textual game state representation such that a 24B model can reliably
complete matches without any fine tuning. We develop tooling to facilitate
hypothesis testing and statistical analysis, and we present case studies on
persuasion, aggressive playstyles, and performance across a range of models. We
conduct a variety of experiments across many popular LLMs, finding the larger
models perform the best, but the smaller models still play adequately. We also
introduce Critical State Analysis: an experimental protocol for rapidly
iterating and analyzing key moments in a game at depth. Our harness
democratizes the evaluation of strategic reasoning in LLMs by eliminating the
need for fine-tuning, and it provides insights into how these capabilities
emerge naturally from widely used LLMs. Our code is available in the supplement
and will be open sourced.