09 · The system
The harness
The model is the engine. The harness is the car: sandbox, permissions, memory, retries, compaction, traces, evals. This is the actual product.
In English
People say “the model.” The thing that ships is the harness: permissions, sandboxes, what to load into context, retries, traces, tests. The model is a stateless function the harness calls.
- Try this
- Click each layer. Read the “without / with” pair. That gap is the product.
- Keep this
- The model is the engine. The harness is the car.
Lab · explode the product
Stateless engine
Model
A completion API: tokens in, tokens out. No files, no clock, no memory of the last request. Treating this as the product is the usual failure.
- Without a harness
- You paste a prompt into a playground and hope.
- With a harness
- The model is a function the harness calls, with a budget, a timeout, and a trace id.
In 2025 the industry settled on a word for the runtime around the model: the harness. It is the loop, the tool surface, the sandbox, memory and skills, compaction, permissions, traces, and evals. The model is necessary. The harness is the product.
Two teams with the same model and different harnesses do not ship the same agent. Click a layer.