Whether those seeking asylum can be believed is a central concern in both public discourse and institutional processes. As a result, credibility assessments have become an important component of the latter. This article contributes to existing scholarship on credibility assessments by critically examining the discourse and related 'language ideologies' underlying them. The examination includes published tribunal decisions on appeals of institutional rejections of asylum-seeker applications, and the tribunal's official credibility assessment guidelines. It considers how constructions of language and diversity affect the way credibility is assessed in visa decision-making. In the application process, sole authorship of the texts produced is discursively assigned to the asylum-seekers. This discourse is problematic as it constructs credibility as attaching to them alone. However, this contradicts the sociolinguistic realities: the texts produced in this setting are institutionally controlled and result from the interaction of multiple participants. The examination also demonstrates how the essentialisation of culture and linguistic diversity can create implausibility. Institutional discourse thus creates serious challenges for applicants, who must communicate 'credibly' to gain protection, even though this communication and its evaluation are far from wholly within their control.