Adapting the Interface, Not the Model: Runtime Harness Adaptation for Deterministic LLM Agents

LLM agents are shaped not only by their language models, but also by the runtime harness that mediates observation, tool use, action execution, feedback interpretation, and trajectory control. While existing agent adaptation methods mainly update model parameters, many failures in deterministic, rule-governed domains stem from mismatches at the model--environment interface. We propose Life-Harness, a lifecycle-aware runtime harness that improves frozen LLM agents without changing model weights or evaluation environments. Life-Harness evolves from training trajectories by converting recurring interaction failures into reusable interventions across environment contracts, procedural skills, action realization, and trajectory regulation, and remains fixed for evaluation on unseen tasks. On seven deterministic environments from $\tau$-bench, $\tau^2$-bench, and AgentBench, Life-Harness improves 116 out of 126 model--environment settings across 18 model backbones, with an average absolute improvement of 14.6 percentage points (pp), corresponding to an 88.5\% relative improvement. Harnesses evolved only from Qwen3-4B-Instruct trajectories transfer to 17 other models, showing that \method{} captures reusable environment-side structure rather than model-specific behavior. These results position runtime interface adaptation as a complementary alternative to model-centric agent training. Code is available at \href{https://anonymous.4open.science/r/Life-Harness-Anonymous-3FA2/}{Anonymous GitHub}.