<![CDATA[Purpose:This study investigates how Specialized Content Knowledge (SCK) operates as a dynamic, domain-specific construct to address critical gaps in understanding how cognitive content subdomains influence real-time teacher noticing, error diagnosis, and professional identity shifts among secondary mathematics educators. Method:An explanatory sequential mixed-methods design was conducted with secondary mathematics teachers ( ; nested qualitative subsample ). Quantitative data were gathered using an adapted Mathematical Knowledge for Teaching (MKT) diagnostic instrument and analyzed via Item Response Theory (IRT) and Multilevel Regression, while qualitative phases utilized classroom video recordings and 60-minute video-stimulated recall interviews analyzed via two-stage coding in NVivo ( ). Findings:Results demonstrate that higher SCK directly dictates procedural fluency across Polya’s problem-solving stages, specifically enabling solution verification ("looking back"). Qualitative video analysis revealed that advanced SCK empowers real-time teacher noticing and adaptive, on-the-spot error diagnosis during algebraic instruction, serving as a primary catalyst for transforming teacher identity from passive curriculum deliverers into autonomous knowledge workers. Practical implications:The findings offer actionable strategies for teacher education and professional development programs to transition from generic pedagogy toward video-based error analysis, domain-specific noticing protocols, and reflective field observations. Originality/value:This research provides a novel, practice-based empirical framework that directly links cognitive content subdomains (SCK) to live classroom noticing, interactive decision-making, and affective professional identity transformation.]]>
