This chapter explores the influence of third-party assurance on the sentiment and subjectivity of corporate sustainability reports. As sustainability reporting grows in importance for corporate transparency and stakeholder engagement, the role of assurance—whether limited or reasonable and provided by audit or non-audit firms—has become increasingly critical in shaping report narratives. Utilizing advanced sentiment analysis methodologies, including BERT-based models, we analyze the tonal qualities and factual content of sustainability reports. Findings indicate that reports without assurance exhibit more positive sentiment, while those with limited or reasonable assurance reflect varying degrees of narrative objectivity and sentiment neutrality. Reports assured by audit firms tend to convey more neutral and comprehensive narratives, emphasizing factuality over subjective tones, compared to those assured by non-audit entities. This analysis contributes to the understanding of how assurance practices impact the perceived credibility and faithful representation of narrative sustainability disclosures. By combining natural language processing insights with empirical data, our study underscores the transformative role of assurance in enhancing nonfinancial disclosures quality and fostering accountability. The results provide policy and practical implications for the discussion on mandating third-party assurance of nonfinancial disclosures.