In an experiment involving qualified greenhouse gas (GHG) assurers, we examine how GHG internal control quality (weak vs. strong) affects assurers conservatism in planning decisions under two different emissions disclosure requirements (voluntary vs. mandatory). We find that weak internal control quality increases the scope of evidence-gathering and site-visit emission coverage. In site-visit decisions, the effect of internal control quality on assurers conservatism is less pronounced under a mandatory disclosure requirement, where heightened accountability pressures may exist. We show that while assurers perceive firms with strong internal control quality as more reliable and less likely to be materially misstated, these perceptions do not reduce the scope of site-visit emission coverage, demonstrating costly assurance conservatism under a mandatory disclosure requirement. This effect, however, is not observed in evidence-gathering decisions. Our results provide insights into the effects of client-related factors on assurers judgment conservatism, thus offering implications for policymakers and assurance firms.