We introduce Lemur and Lemur-Chat, openly accessible language models optimizedfor both natural language and coding capabilities to serve as the backboneof versatile language agents. The evolution from language chat models tofunctional language agents demands that models not only master human interaction,reasoning, and planning but also ensure grounding in the relevant environments.This calls for a harmonious blend of language and coding capabilitiesin the models. Lemur and Lemur-Chat are proposed to address this necessity,demonstrating balanced proficiencies in both domains, unlike existingopen-source models that tend to specialize in either. Through meticulous pretrainingusing a code-intensive corpus and instruction fine-tuning on text and codedata, our models achieve state-of-the-art averaged performance across diversetext and coding benchmarks. Comprehensive experiments demonstrate Lemur’ssuperiority over existing open-source models and its proficiency across variousagent tasks involving human communication, tool usage, and interaction underfully- and partially- observable environments. The harmonization between naturaland programming languages enables Lemur-Chat to significantly narrow thegap with proprietary models on agent abilities, providing key insights into developingadvanced open-source agents adept at reasoning, planning, and operatingseamlessly across environments. Our model and code have been open-sourced athttps://github.com/OpenLemur/Lemur.