The Care Act 2014 gave English local authorities a duty to ‘shape’ social care markets and encouraged them to work co-productively with stakeholders. Grid-group cultural theory is used here to explain how local authorities have undertaken market shaping, based on a four-part typology of rules and relationships. The four types are: procurement (strong rules, weak relationships); managed market (strong rules, strong relationships); open market (weak rules, weak relationships); and partnership (weak rules, strong relationships). Qualitative data from English local authorities show that they are using different types of market shaping in different parts of the care market (e.g. residential vs home care), and shifting types over time. Challenges to the sustainability of the care system (rising demand, funding cuts, workforce shortages) are pulling local authorities towards the two ‘strong rules’ approaches which run against the co-productive thrust of the Care Act. Some local authorities are experimenting with hybrids of the two ‘weak rules’ approaches but the rival cultural biases of different types mean that hybrid approaches risk antagonising providers and further unsettling an unstable market.