Rails Foundation Open-Sources lemans, the Ruby Agent Harness Behind Its Agents on Rails Benchmarks

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Announced August 24 on the official Rails blog, lemans is the harness that produced every Agents on Rails number published so far, now open source. The team started on Harbor and found it merely good-enough, so with Rails Foundation backing they wrote a Ruby-native replacement designed as a library rather than only a CLI, with a component-based architecture allowing custom backends, storage providers and concurrent executors to be plugged in. The same post reports four new model runs including Sonnet 5, Terra, an open-weight Qwen runnable locally, and one unnamed model. This matters as the first serious agent harness native to a non-Python web framework ecosystem.