This study reports the selection, adaptation, and validation of an assessment instrument for written narrative skills in Spanish‑heritage learners in grades 4 and 5 (ages 8-11). The instrument consists of two picture‑story sequences, each accompanied by a narrative text designed to be intelligible across Spanish varieties (both Latin‑American and Peninsular). Content validity was established by a panel of experts in linguistics and education, and the texts were pilot‑tested with heritage‑language students representing different varieties to confirm their comprehensibility. Reliability was examined by administering the instrument to two independent samples of monolingual Argentine students. Retelling scores showed no significant differences either between the two student groups or between the two picture sequences, indicating that performance was not enhanced simply by familiarity with the task procedure. These findings support the instrument’s validity and reliability as a measure of written‑narrative development (story grammar), also in heritage‑language contexts.