The Six Systems of a Harness
Click or tab through any component in the diagram. Requests flow down from the task; observations flow back up through the loop.
Harness Quality Simulator
Same model, different harness. Set the maturity of each system, then run a simulated 12-step task and watch where the run breaks down. Scores use a simple compounding model: each step must survive every subsystem.
Adjust the sliders and press Run simulated task to see how far the agent gets.
Design Primitives
Recurring primitives from harness engineering practice — small ideas that compound across long tasks.
Fresh context per step
Rebuild the prompt from durable state instead of letting a raw transcript grow. Stale context is the quietest failure mode in long-horizon agents.
Legible tools
Few, well-named tools with strict schemas and rich error messages beat many overlapping ones. The error message is part of the interface.
Externalized plans
Plans, todo lists, and subgoal files live outside the model so progress survives context resets and can be inspected by humans.
Verification over trust
Never accept the agent's claim of success. Run tests, diff outputs, and check invariants after every meaningful action.
Budgets everywhere
Cap steps, tokens, retries, and spend. A harness that cannot stop an agent cannot be trusted to run one.
Recoverable failure
Design for retry: idempotent actions, checkpoints, and summaries so a failed step costs one step, not the whole run.
Harness Design Checklist
Score your own agent scaffold. Check what you have, then copy the checklist to share or file as an issue.