The SpyCatcher-SpyTag reaction is a fast, specific method to covalently link two proteins. We developed SpyLock - a 'lockable' SpyCatcher that can be reversibly inhibited to control ligation with a SpyTag. SpyLock’s reactivity is restored upon reaction with a reducing agent. SpyLock can be fused with SpyCatcher to create BiLockCatcher, allowing for the sequential incorporation of two different Fabs containing SpyTag, resulting in bispecific prototypes for functional screening. This technology integrates seamlessly with Bio Rad’s Pioneer™ Antibody Discovery Platform and is applicable to all recombinant antibodies.