Are Harnesses Worth the Cost?
Martin Fowler, Tuesday, October 13th, 2026
Experiment finds a behavioral harness made a cheap coding model as reliable as a frontier model at 33% lower cost.
Naresh Jain had coding agents build the same API service with and without a behavioral harness of architectural contracts and deterministic test feedback.
The frontier model passed all 178 acceptance tests in all 10 runs either way, and the harness only raised its token use about 10.5 times and its cost about 7 times.
The cost-optimized model succeeded in only 3 of 10 unharnessed runs but 10 of 10 with the harness.
Even with higher token use, harnessed runs of the smaller model cost $0.273 versus $0.406 for the frontier model, suggesting harnesses are a practical way to get dependable results from smaller models.