SidConArena: An Environment Evaluating Agents in Open-Ended, Positive-Sum Bargaining Game

Evaluating LLM agents requires dynamic environments that go beyond static reasoning and zero-sum games. Real-world economic interaction is often open-ended and mixed-motive: agents must negotiate, create positive-sum surplus, compete for scarce assets, and plan under delayed returns. We introduce SidConArena, a new benchmark framework for evaluating LLM agents in open-ended, positive-sum bargaining. SidConArena formalizes a multi-player economy as a finite-horizon partially observable stochastic game with three coupled phases: natural-language negotiation with binding trades, deterministic converter-based production, and sealed-bid auctions for long-term assets. The framework combines structured observations, a phase-specific reference agent, a validated function-call interface, and asynchronous execution, enabling free-form interaction while preserving rule-grounded evaluation. Homogeneous and heterogeneous rollouts show variation in economic outcomes under the shared reference implementation. Selected trajectories illustrate valuation inconsistencies, passive bargaining, and limited long-horizon planning.