NASA revised its Commercial Crew Program agreement with Boeing to reduce the number of planned Starliner missions from six to four. The agency announced the change on Monday, citing a contract update that shifts the final two flights into an optional category.

The original contract, valued at $4.5 billion, called for six operational missions of Boeing’s CST-100 Starliner capsule after certification. NASA said the new structure keeps four missions in place while allowing flexibility for two additional flights if needed. The adjustment follows years of schedule slips, technical problems and financial strain for the spacecraft program.

Boeing’s work on Starliner has accumulated more than $2 billion in charges since 2016. The capsule has undergone repeated redesigns and testing setbacks. Its most recent crewed trip, flown last year, encountered propulsion system issues that left its first astronaut team on the International Space Station for about nine months before their return on a separate vehicle.

NASA said the next Starliner flight will be uncrewed. It will serve as the third orbital test mission for the spacecraft, continuing efforts to validate systems that have struggled to meet operational requirements. The agency described the upcoming flight as part of an ongoing development phase rather than the start of routine service.

The mission, designated Starliner-1, will transport cargo to the International Space Station. NASA said Boeing and the agency are working toward a launch no earlier than April 2026. The schedule remains dependent on progress with propulsion fixes and other updates prompted by the previous mission’s anomalies.

The revised contract underscores the contrast between Boeing’s performance and that of SpaceX, the other Commercial Crew provider. SpaceX has carried NASA astronauts on regular rotations since 2020, while Boeing continues to pursue full certification.

NASA said its decision to scale back Starliner’s required missions reflects program realities while leaving room for added flights if the capsule completes testing and meets safety expectations. The agency did not announce further changes to timelines or funding.