This article examines how preschool teachers balance fidelity and adaptation when implementing a research-based mathematics education innovation for toddlers (1–3 years old). The empirical material comprises 43 video-documented teaching activities planned and enacted independently by 34 preschool teachers. The analysis focuses on the pedagogical and mathematical fidelity to the core components of the innovation in these teaching activities. Findings show that pedagogical fidelity to the core component of mathematizing, in the sense of embedding mathematics meaningfully in activities, consistently required productive adaptations. Sometimes these productive adaptations were combined with mathematical fidelity, sometimes not. The study contributes to understanding how fidelity and adaptation can coexist productively — or non-productively — in implementation of an early mathematics education innovation. The impact sheet to this article can be accessed at https://doi.org/10.1163/26670127-bja10036 under Supplementary Materials. © Hanna Palmér et al., 2026. Published with license by Koninklijke Brill BV. This is an open access article distributed under the terms of the CC BY 4.0 license.