Multi-harness RL guide: LFM2.5 jumps 42% to 54% with 31% fewer tool calls

SergioPaniego · x · 2026-10-01

Adithya S K released an open guide to multi-harness RL, built on the observation that the same model behaves differently across agent harnesses. The method trains any model with RL on any task set inside real-world harnesses like Claude Code, Codex, and OpenCode, without changing harness or training code. Trained across four harnesses, LFM2.5-2.6B improved from 42% to 54% while making 31% fewer tool calls.

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