Street outreach is a violence prevention model in which organizations hire residents with strong relationships and local expertise to mediate violent conflicts in their communities. We present the results of a two-year co-design engagement between a street outreach organization and academic researchers in which we collaboratively designed, built, and deployed a mobile application (the Street Peace app) to support street outreach workers (SOWs). Three different street outreach organizations in Chicago adopted the app for two months. Results suggest that the app supported SOWs' transformative justice practices to build a counter-structure to traditional policing, which is historically oppressive to Black communities. The SOWs used the app to mediate potentially violent conflicts without police involvement, build community through in-person events, and extend their communities of care through positive stories and narratives that countered harmful stereotypes about Black criminality. By affording SOWs more agency over their communication with each other, the app enabled SOWs to connect their strengths and scale their existing practices that combat structural oppression and prefigure liberatory futures.