Introducing the curse of coordination. Agents perform 50% worse in teams than working alone. People building human-AI collaboration today don't realize why current LLMs fail to be good teammates. We built CooperBench to study this. For humans, we recognize that teamwork isn't just the sum of individual capability. Communication and coordination often outweigh raw skill. But for AI? We're only hill-climbing benchmarks that evaluate solo technical abilities. CooperBench A benchmark to evaluate agent cooperation in realistic software teamwork tasks. The setup is intuitive: two agents, two tasks, two VMs, one chat channel (agents can send over arbitrary text, even the entire patch they wrote). We evaluate whether the merged solution from both agents passes the requirements of both tasks. The curse of coordination The most striking result: agents perform 50% worse in teams (black line) than working alone (blue line). Why is this happening? Is it because they can't use the communication tool? No. They spent 20% of their time sending messages. The problem? Those messages were repetitive, vague, ignored questions, or straight-up hallucinated. But bad communication is only part of the story. We found two deeper failures: Commitment: Agents don't do what they promised. Expectations: Agents don't expect others to keep promises either. Without these, cooperation collapses. However, there is a silver lining We also find emergent coordination behaviors, e.g. role division, resource division, and negotiation, which gives us hope that we can use reinforcement learning to improve coordination. What's next? It is true that highly-engineered multi-agent orchestration could largely sidestep the coordination problem. However, we care more about the AI's capability: if we truly want AI to be our teammates, we need them to be natively capable of effective communicating and coordinating. Two agents on software tasks is just the beginning. The real goal: agents that can cooperate with us well enough to actually empower us. CooperBench is our first step. If you're working on this too, let's talk.