Fixed-Price Contracts and Agile Development Are Structurally Incompatible
The fixed-price contract is built on a specific assumption: that the scope of the work can be defined in advance with enough precision to price it accurately. The agile development model is built on a specific observation: that software requirements are not fully knowable in advance, and that attempting to define them completely before development begins produces worse outcomes than defining them incrementally in response to what is learned during development. These two positions are in direct conflict, and pretending otherwise is one of the more expensive mistakes in software delivery.
The failure mode of agile-in-name, fixed-price-in-practice is consistent. The contract specifies a scope that both parties know is incomplete. The team begins development and discovers, as always happens, that the requirements have gaps, assumptions that were wrong, and features that are more complex than they appeared. The client expects the original scope to be delivered for the original price. The vendor either absorbs the overrun, fights about what is in and out of scope, or delivers the original specification despite knowing it is not what the client actually needs. All three outcomes are bad. The disagreement is baked into the contract structure.
The alternative contract structures that actually support agile development are not complicated. A time-and-materials engagement with a defined team, a clear sprint cadence, and regular delivery of working software gives the client visibility into progress and the ability to redirect investment toward the highest-value work at any point. A fixed-budget engagement with a variable scope gives the client cost certainty while preserving the flexibility to change what gets built within that budget. Both require a client willing to engage as a collaborator rather than an auditor — to participate in sprint reviews, prioritize the backlog, and make trade-off decisions as the project progresses.
The client relationship that agile actually requires is one of continuous collaboration rather than periodic delivery. Vendors who sell this relationship honestly attract clients who understand it, and those engagements tend to produce good outcomes. Vendors who sell agile methodology while contracting on fixed scope tend to produce the worst of both models: the overhead of agile ceremonies with the inflexibility of waterfall delivery.