As stakeholders increasingly rely on Corporate Social Responsibility (CSR) information to assess firm value and long-term performance, firms face growing pressure to disclose their environmental, social, and governance (ESG) activities. However, in the absence of mandatory disclosure requirements, companies often selectively report favorable information, resulting in inconsistencies in content, format, and comparability across firms. This voluntary and strategic reporting behavior undermines stakeholders' ability to accurately evaluate corporate performance. To address these gaps, standardized frameworks such as the Global Reporting Initiative (GRI) have emerged, offering structured guidelines to improve transparency, comparability, and accountability in sustainability reporting. Against this backdrop, this study examines the relationship between the quality of CSR reports, adoption of GRI guidelines, and the efficiency of a firm’s investments by examining a set of companies listed on China A-shares. This study addresses four key questions: (1) Do GRI adoption and the quality of CSR reports impact a firm’s investment efficiency? (2) How does a firm’s efficiency of investment and firm-specific characteristics affect a firm’s GRI adoption and the quality of their CSR reports? (3) Does GRI adoption strengthen the relationship between the quality of CSR reports and investment efficiency? and (4) How might a company’s financing constraints and excess cash reserves moderate the impact of GRI disclosures on investment behavior? Using a panel dataset of 30,080 firm-year observations from 2008–2022, the study applies a Three-Stage Least Squares (3SLS) approach to address endogeneity and explore both direct and reciprocal relationships. Empirical results show that high-quality CSR reporting and GRI adoption significantly improves investment efficiency, especially for reducing under-investment, by enhancing transparency and guiding capital allocation. However, in cases of over-investment, these practices may reflect symbolic compliance, where firms strategically adopt GRI and enhance disclosures without substantively impacting sustainability issues. The effect of GRI is strengthened when firms hold excess cash, encouraging more disciplined investment; but is weakened under financial constraints. Moreover, a bidirectional relationship is observed, with investment inefficiency also influencing the quality of CSR reports and GRI adoption, suggesting dynamic resource allocation strategies. Robustness checks, including weak instrument tests, error term correlations, and comparisons with 2SLS models, support the validity of these findings. Results further indicate that GRI and the quality of CSR reports have a stronger effect on investment efficiency among non-state-owned enterprises (non-SOEs), while GRI primarily enhances CSR report quality through policy-driven compliance among SOEs. This study contributes to the literature on sustainable corporate governance and offers practical implications for firms and policymakers seeking to improve ESG disclosure, stakeholder trust, and investment efficiency through standardized reporting.