Assessing additionality is a critical element of project-level greenhouse gas (GHG) accounting to ensure that reported mitigation outcomes genuinely result from an intervention and would not have occurred in its absence. However, additionality in agrifood systems remains difficult to evaluate due to complex biophysical, socioeconomic and policy drivers, limited data, and uncertainty in baseline projections. Existing approaches applied by climate funds, standards and protocols offer valuable guidance but are not fully tailored to the specific characteristics of agrifood interventions supported through development finance.This report reviews the concept of additionality and examines how it is addressed across major climate finance frameworks, with a view to identifying practices relevant for project-level GHG accounting in agriculture. It proposes a practical approach to integrating ex ante additionality assessment into the Environmental eXternalities ACcounting Tool (EX-ACT) tool, including guidance for constructing credible baselines, applying realistic adoption rates of climate-smart practices and managing uncertainty. The document also outlines key limitations and considerations regarding scope, methodological boundaries and practical implementation challenges when applying additionality assessment within EX-ACT. The report does not prescribe a definitive methodology but highlights areas where further development, refinement and institutional alignment would be needed to strengthen additionality assessment for agrifood mitigation projects within EX-ACT tool application.