Performance tasks enable the development of not just the knowledge but also the skills of students. Unfortunately, teachers find it challenging to prepare their performance tasks. To help teachers, an authentic framework for the preparation of performance tasks should be utilized. GRASPS, as an assessment model, can help teachers develop authentic assessment. In this study, the researcher aimed (1) to determine the profile of the teacher-respondents in terms of sex, age, civil status, years in service, and highest educational attainment; (2) to recognize the challenges in the preparation of performance tasks in mathematics in terms of goal, role, audience, situation, product, and standards and criteria for success (GRASPS); (3) to identify if there is a significant difference in the challenges in the preparation of performance tasks in mathematics in terms of GRASPS when respondents are grouped according to their profile; and lastly, (4) to propose a GRASPS-based guide performance tasks in mathematics. Through descriptive research design, the study was conducted and evaluated. The public secondary mathematics teachers answered a survey questionnaire. The level of challenge in the preparation of performance tasks in mathematics were identified. In terms of goal, the challenges in the preparation of performance tasks are challenging. On the other hand, the role, audience, situation, product and standards, and criteria for success are found to be very challenging. Also, based on the results of this study, the researcher found that there is no significant difference in the challenges in the preparation of performance tasks in mathematics in terms of GRASPS when respondents are grouped according to their profile. Thus, based on the most challenging indicators, the proposed GRASPS-based guide performance tasks in mathematics were crafted and designed to help teachers develop authentic assessment and to aid 21st-century learners develop metacognition.