local civil society organizations have been driving engagement with municipal open data, as a form of data-driven activism. • The notion of "bi-directional open data" is developed here to characterize emerging possibilities for data openness between governments and the public. • We recommend that cities adopt a new philosophy of data openness that embraces civic participation with open data, leading towards city-citizen data relations. Municipal open data projects are motivated by a desire to democratize data access and knowledge production, strengthen transparency, and advance cities socially and economically. However, their effects and implications are insufficiently analyzed. This paper examines civic engagement in open data in Cape Town, South Africa, the continent's first municipal-level open data initiative. Findings reveal how local civil society organizations have been driving engagement with municipal open data as part of their recent turn towards technology and data-driven forms of public engagement and activism. This analysis highlights the important role of the "smart civil society organization"-occupying a position between the smart city and smart citizen-that is developing significant capacity to produce and share data about the city's informal settlements with stakeholders in government, the private sector, and wider society. Minimal engagement with or recognition of civil society efforts illustrates the limits to the city's philosophy of data openness, which is largely restricted to releasing selected government datasets to the public. The notion of "bi-directional open data" is developed here to characterize emerging possibilities for data openness between governments and the public. This may be particularly relevant for cities like Cape Town with a highly active, capable, and data-literate civil society.