Businesses say they want a growth partner. They run a procurement process designed for agencies. They get agencies. They are then disappointed when the relationship behaves like an agency relationship.
The brief is the first place this shows up. Agency briefs are deliverable-shaped: produce X, by Y, for Z. Partner briefs are outcome-shaped: move this metric, here is the context, propose the path.
The contract is the second. Agency contracts price the deliverable. Partner contracts price the outcome — or at least the shared risk of pursuing it.
The success metric is the third. Agencies are measured on delivery. Partners are measured on the business moving.
If you want a partner, change the brief, the contract, and the metric. If you keep the agency-shaped process, you will keep getting an agency-shaped relationship — no matter what the relationship is called on the master services agreement.