Agent Harness vs Loop Engineering
Short answer
An agent harness is the system that validates, authorizes, executes, records, and returns observations for the actions a model proposes. Loop engineering decides how that harness iterates: the goal, validation, stop rule, budget, and feedback.
Why it matters
The model proposes; the harness acts. A strong harness without a loop runs one safe step at a time; a loop without a harness iterates fast but unsafely. Production agents need both — the harness for safety per action, the loop for direction across many actions.
Practical checklist
- Decide what the harness validates and authorizes before any action runs
- Record every action and its observation so failures can be audited
- Wrap the harness with the loop's stop rule and budget cap
- Require human approval for irreversible actions
- Keep the model proposing actions, not executing them directly
Example
A code agent: the harness runs only allow-listed commands, records each result, and refuses writes outside the repo. The loop wraps that with goal = make CI green, stop after 5 failures, and escalate to a human before merge.
Common failure modes
Letting the model execute directly instead of proposing to a harness
A harness with no loop — safe but directionless
A loop with no harness — fast but unsafe
No record of actions, so failures cannot be audited