In this paper, we share the results of the piloting of national diagnostic assessments for strategic calculation with Grade 3 learners in South Africa. The diagnostic assessment pilot intervention was focused on promoting strategic use of calculation strategies and aimed to move learners on from the concrete one-to-one counting methods that persist in this grade and into higher grades. Working with a broader team of mathematics education specialists, including international leaders in the field and department of education representatives, we designed a series of assessments that focus on key calculation strategies such as bridging through ten. Each assessment, comprised a pre- and repeat post-assessment, was accompanied by interim lesson starters for teacher use in eight 10-min mental mathematics sessions. These were designed to develop learner fluency in each of the focal strategies and related skills. The pre- and post-assessments allow teachers to track improvement in student learning. The pilot focused on the "bridging through ten" strategy in seven classes across the Eastern Cape and Gauteng provinces. Positive results indicate that the format of these assessments paired with the lesson starters is potentially useful for broader national trialling and implementation. This work feeds into the need for diagnostic assessments that inform teaching focused on strategic efficiency rather than simply accepting correct answers produced through highly inefficient methods.