Purpose This paper aims to examine the processes of sustainability reporting assurance (SRA) and the influence they have on shaping perception from disclosures. Given the evidence of inconsistencies and ambiguities in assurance processes, this paper examines how legitimacy is attained and maintained at different stages of SRA. Design/methodology/approach Evidence collected from 23 semi-structured interviews with assurance providers (APs), consultants, professionals and non-governmental organisations (NGOs) (non-APs) was used to conduct a thematic analysis from the perspectives of interviewees. Findings APs and non-APs are united in recognising the value of SRA, although, perspectives on transparency between the two groups differ. Experience and industry knowledge are essential to SRA delivery with non-APs preferring accounting APs. Nevertheless, non-APs are concerned about the role of companies in deciding assurance scope, as it can affect scrutiny. APs favour data accuracy (as opposed to data relevance) assurance due to team dynamics and internal review influences, with the latter also restricting assurance innovation. APs are interested in accessing better evidence and stakeholder engagement evaluations. Providing advisory services was not rejected by all APs. The perspectives of APs and non-APs demonstrate how progress in SRA has gained pragmatic legitimacy with noticeable gaps that serve to undermine attainment of moral legitimacy. Research limitations/implications SRA is a developing practice that will adopt changes as it continues to mature; some of these changes could impact findings in this research. General perspectives on SRA were sought from interviewees, this affected the ability for an in-depth focus on any of the range of interesting SRA issues that arose over the course of the research. Interviews were conducted with relevant parties in the SRA space that operate in the UK. Perspectives from parties outside the UK were not solicited. Practical implications Companies make an important decision to commission SRA. Findings in this research have highlighted specific non-APs issues of concern that can be useful in structuring operations and reporting regimes to facilitate assurance procedures. The findings will also be helpful to APs as they can direct more emphasis on stakeholder concerns towards demonstrating greater stakeholder accountability. Regulatory and standard setters can enact appropriate policies that can potentially drive the practice forward for assessment of cognitive legitimacy. Social implications The findings provide relevant account of stakeholder voices on the quality of corporate disclosures that has a direct effect on the wellbeing of communities and sustainability of societies. Collective stakeholder input on expectations can shape sustainability discourse. Originality/value This research demonstrates the applicability of financial audit quality indicators in SRA processes, extends the debate around the effectiveness of new audit fields and highlights the challenges of maintaining legitimacy with different audiences.