Harness Engineering 2.0 is here! After protocols for tools, agent communication, and agent interfaces, harnesses now have one too. Developers just built UHP (Unified Harness Protocol), a standardized interface for running agent tasks across any agent harness. HarnessRouter is its open-source server implementation. Here’s how it works: - The product backend sends a task with a `harness_id`. - HarnessRouter starts the selected harness in its own session. - The harness runs its native agent loop and streams back progress. - Files, artifacts, failures, and the final output return through the same interface. Without UHP, integrating every harness needs separate handling for task execution, sessions, streaming, files, recovery, and output formats. With UHP, Codex, Claude Code, Gemini CLI, Hermes, Pi, and other harnesses can plug into the same product backend while keeping their native tools, skills, models, and agent loops. This means developers can: - Switch harnesses without rebuilding the product integration. - Run the same task across different harnesses. - Add new harnesses without maintaining another execution stack. HarnessRouter currently supports 30 harness bases. Here’s the repo: (don’t forget to star it ⭐) As per official benchmarks, changing only the harness while keeping the model fixed reduced costs by 25% to 64% across several healthcare compliance tasks. And changing both the harness and model widened the difference to 99.8% between the highest and lowest-cost configurations. Developers can test their own workloads across different configurations using Harness Arena. Alibaba Cloud's Qoder has also adopted UHP, making it available as an agent backend through the HarnessRouter API. If you want to dive deeper, I already wrote an article that explains why model routing is different from harness routing and what supporting multiple agent runtimes actually involves. It also covers UHP, the local setup, a working API call, and how sessions and files work. Read it below.