The writing test of the Third Comparative and Explanatory Study implemented by the Latin American Laboratory for Assessment of the Quality of Education constitutes one of the few international experiences of large-scale assessment of writing. In order to strengthen these experiences, the theoretical framework adopted in this test, the task applied, the rubric and the coding process are reviewed, combining qualitative and quantitative techniques. The results show problems related to validity and reliability, and allow to formulate recommendations for future international assessments of writing. These include an articulation between the theoretical framework, the objectives and the scope of the assessment, a process of thorough review of the topics and syntax of the prompts, and the use of strategies to obtain evidences of validity and reliability at the different stages of the assessment process.