checkpoints

Two thousand evals against every checkpoint is ten million prompts per run and the difference between a four-hour and a two-day turnaround. The pipeline as a batch inference job with a content-addressed cache, the sharding that keeps GPUs busy, and the reproducibility rules that let a regression be believed.

Every 30 minutes a thousand GPUs write a terabyte in a burst that must finish in a minute, then nothing until the next. The burst arithmetic that sizes the write tier, the two-stage path to durable storage, retention that keeps the right checkpoints, resharding so a 512-GPU checkpoint resumes on 256, and lineage.

An agent thinks for a second, runs a tool for a minute, and repeats a hundred times over an hour. The session state machine and where each piece lives, the sandbox per session with its cold-start budget, the KV cache that should survive the tool call, and checkpoints that resume a step rather than an hour.

A three-month run on 8,000 GPUs is a large bet against everything that can happen to one building. The recovery point and the recovery time as numbers, the checkpoint replication that sets the first, the second cluster and its warm state that set the second, and the drills that make the numbers true.