The only harness moat
The only moat unique to building a harness is owning the model. Every other harness builder is reselling someone else's model, and reselling doesn't produce a moat from the harness itself.
A harness is the loop that turns a model into an agent. It manages tool calls, context, permissions, and the interface the user works through. That loop is real engineering, but it does not by itself create a durable advantage.
What owning the model buys
A model owner designs the model and the harness as one product. The model is trained on the harness's own tool-call format and context-handling behavior, so the two fit without translation.
The harness also reaches internal interfaces the public API never exposes. Its marginal cost is compute, not the retail API price every other harness pays.
A competitor on the public API can copy the harness surface: the tools, the loop, the interface. It cannot copy the co-design, the private interfaces, or the cost structure.
Everyone else is a reseller
A harness that doesn't own a model depends on an API it doesn't control. This holds whether it builds on one provider or five.
Spreading across providers gets sold as neutrality, as if model-agnostic were a moat. It lowers the risk of one provider cutting you off, but it adds dependencies instead of removing them.
It also gives away leverage. Every provider you resell can undercut you with its own first-party harness, built on the advantages above.
What resellers actually compete on
Resellers win on ordinary software moats, none specific to AI. A known product with existing users gets tried first, while a new one has to earn every user.
The longer someone uses a harness, the more of their settings, history, and connected tools live inside it. Moving to another harness means setting all of that up again.
A harness built for one field can beat a general-purpose one inside that field. It encodes the field's data formats, its regulations, and the systems its work runs through, which takes domain experts to get right.
A general-purpose provider won't do this. Any single field is too small a market to pull effort away from the model and the harness that serve everyone.
These are real businesses. They are the same moats any software company has without a model involved.
The platform tax
The reseller position is worse than the usual app-on-a-platform story. The provider sets the price, decides which features exist, and controls the roadmap you build against.
On most platforms the owner stays out of your category. Here the platform owner ships a first-party harness that competes with you directly, using interfaces it doesn't hand you.
What follows
If you don't own a model, you are a software company sitting on a model API. Compete on distribution, switching costs, and domain depth, and price in that your supplier will enter your market with advantages you can't match.