At the time of this study, the American Diabetes Association generally recommended an HbA1c target of <7.0% (it was <7.5% at the time this study took place, for individuals younger than 18 years. The especific target varies depending on the individual). The DCA 2000 HbA1c Analyzer (Siemens-Bayer) was used for point of care HbA1c analysis, it has an analytical measurement range for HbA1c of 2.5% to 14.0%. HbA1c values are collected via fingerstick and blood assay at routine diabetes care visits and values were extracted from the medical record at each clinic visit during the study period.

Diabetes-specific family impact was measured using the Diabetes Family Impact Scale (DFIS), a 15-item measure of the impact of diabetes on family members' activities and relationships. The scale range from the minimum to maximum possible score is 0-100. A higher score represents a better outcome.

Family impact was measured using The Pediatric Quality of Life Family Impact Module (Peds QL-FI), a 36-item measure of the impact of parenting a child with a chronic medical condition on family functioning and parent QOL. The scale range from the minimum to the maximum score is 0-100. A higher score represents a better outcome.

Family communication was measured by the parent-report Helping for Health Inventory (HHI), a 15-item questionnaire assessing perceptions of parental help around the teen's diabetes management. The scale range from the minimum to maximum possible score is 15-75. A lower score represents a better outcome.

Family communication was measured using the adolescent-report Helping for Health Inventory (HHI), a 15-item questionnaire assessing perceptions of parental help around the teen's diabetes management. The scale range from the minimum to maximum possible score is 15-75. A lower score represents a better outcome.

Family conflict was measured using the Diabetes Family Conflict Scale - Revised (DFCS), a measure of diabetes-specific conflict in families with adolescents with type 1 diabetes. The scale range from the minimum to maximum possible score is 19-57. A lower score represents a better outcome.

Family conflict was measured using the Diabetes Family Conflict Scale - Revised (DFCS), a measure of diabetes-specific conflict in families with adolescents with type 1 diabetes. The scale range from the minimum to maximum possible score is 19-57. A lower score represents a better outcome.

Diabetes burden was measured using the Problem Areas in Diabetes (PAID-T) parent-report, a measure of how bothersome day-to-day problems are for parents of adolescents with type 1 diabetes. The scale range from the minimum to maximum possible score is 26-156. A lower score represents better a outcome.

Diabetes burden was measured using the Problem Areas in Diabetes-Teen (PAID-T) self-report, a measure of how bothersome day-to-day problems are for adolescents with type 1 diabetes. The scale range from the minimum to maximum possible score is 26-156. A lower score represents better a outcome.

Parent-report of adolescent self-management was measured using the 24-item Diabetes Self-Management Profile Self-Report (DSMP). Parents completed the version appropriate to their child's current diabetes regimen (conventional insulin regimen, 24 items; flexible insulin regimen, 24 items). The scale range from the minimum to maximum possible score is 0-86. A higher score represents a better outcome.

Self-management was measured using the Self-Care Inventory-Revised (SCI-R), a 15-item measure of the frequency that adolescents engage in diabetes management behaviors on a 5-point scale from Never to Always. The scale range from the minimum to maximum possible score is 5-75. A higher score represents a better outcome.

The Diabetes Strengths and Resilience measure is a self-report assessment of positive behaviors related to diabetes resilience for youth with type 1 diabetes, such as perceived competence to manage the demanding diabetes regimen, to adapt to the unpredictability of diabetes, and to seek help and support with diabetes challenges. The scale range from the minimum to maximum possible score is 0-48. A higher value on this scale represents a better outcome.

Adolescent quality of life was measured using the Monitoring Individual Needs in Diabetes Youth Questionnaire (MIND-Youth/MY-Q), a 33-item measure of diabetes-specific health-related QOL. The scale range from the minimum to maximum possible score is 0-100. A higher score represents a better outcome.

Adolescents answered 3 items adapted from the Parent-Youth Relationship Index of the National Longitudinal Study of Youth-1997, a measure of parent-adolescent relationship quality. Item scale ranges from minimum to maximum score of 1-5 for each item, with a higher score indicating a better outcome.

Intervention process from baseline to follow-up was analyzed using a general linear mixed model, using the item "How often does he/she praise you for doing well?" Time since first baseline clinic visit was computed and analyzed using a mixed model with time, arm, and the time-arm interaction term. The outcome measure data are reported as slopes by arm.

*NOTE: Intervention process measures were administered biweekly, and we are reporting the calculated mean change per day with 95% confidence intervals.

Usual diabetes care and study-related data collection, without use of app during the study period. They will be offered an opportunity to try the app and share their feedback with the study team after completing follow-up data collection.

Total of all reporting groups

Objective measurement of adherence to diabetes regimen measured by blood glucose monitoring frequency (a well-accepted surrogate of overall adherence), obtained via blood glucose meter downloads. The average daily frequency was calculated over the 14 days prior to the assessment at the baseline visit.

n represents number with complete data.

At the time of this study, the American Diabetes Association generally recommended an HbA1c target of <7.5% for individuals younger than 18 years (specific target varies depending on the individual). The DCA 2000 HbA1c Analyzer (Siemens-Bayer) was used for point of care HbA1c analysis, it has an analytical measurement range for HbA1c of 2.5% to 14.0%. HbA1c values are collected via fingerstick and blood assay at routine diabetes care visits and values were extracted from the medical record at each clinic visit during the study period.

n is the number with an HbA1c value available at this timepoint.

Diabetes-specific family impact was measured using the Diabetes Family Impact Scale (DFIS), a 15-item measure of the impact of diabetes on family members' activities and relationships. The scale range from the minimum to maximum possible score is 0-100. A higher score represents a better outcome.

n represents number with complete data to calculate a score.

Family impact was measured using the Diabetes Family Impact Scale (DFIS), a 15-item measure of the impact of diabetes on family members' activities and relationships. The scale ranges from 0-100. A higher score represents a better outcome.

Family communication was measured by the parent-report Helping for Health Inventory (HHI), a 15-item measure of perceptions of parental help around the teen's diabetes management. The scale range from the minimum to maximum possible score is 15-75. A lower score represents a better outcome.

n represents number with complete data to calculate a score.

Family communication was measured with the Helping for Health Inventory (HHI), a 15-item measure of perceptions of parental help around the teen's diabetes management. The scale range from the minimum to maximum possible score is 15-75. A lower score represents a better outcome.

n represents number with complete data to calculate a score.

Family conflict was measured using the Diabetes Family Conflict Scale - Revised (DFCS), a measure of diabetes-specific conflict in families with adolescents with type 1 diabetes. The scale range from the minimum to maximum possible score is 19-57. A lower score represents a better outcome.

Family conflict was measured using the Diabetes Family Conflict Scale-Revised (DFCS), a measure of diabetes-specific conflict in families with adolescents with type 1 diabetes. The scale range from the minimum to maximum possible score is 19-57. A lower score represents a better outcome.

Diabetes burden was measured using the Problem Areas in Diabetes (PAID-T) parent-report, a measure of how bothersome day-to-day problems are for parents of adolescents with type 1 diabetes. The scale range from the minimum to maximum possible score is 26-156. A lower score represent better a outcome.

Diabetes burden was measured using the Problem Areas in Diabetes-Teen (PAID-T) self-report, a measure of how bothersome day-to-day problems are for adolescents with type 1 diabetes. The scale range from the minimum to maximum possible score is 26-156. A lower score represents better a outcome.

Parent report of adolescent's self-management was measured using the 24-item Diabetes Self-Management Profile Self-Report (DSMP). The version appropriate to their child's current diabetes regimen (conventional insulin regimen, 24 items; flexible insulin regimen, 24 items) was administered. The scale range from the minimum to maximum possible score is 0-86. A higher score represents a better outcome.

Parent-report of adolescent self-management was measured using the 24-item Diabetes Self-Management Profile Self-Report (DSMP). Parents completed the version appropriate to their child's current diabetes regimen (conventional insulin regimen, 24 items; flexible insulin regimen, 24 items). The scale range from the minimum to maximum possible score is 0-86. A higher score represents a better outcome.

The Diabetes Strengths and Resilience (DSTAR) measure is a self-report assessment of positive behaviors related to diabetes resilience for youth with type 1 diabetes, such as perceived competence to manage the demanding diabetes regimen, to adapt to the unpredictability of diabetes, and to seek help and support with diabetes challenges. The scale range from the minimum to maximum possible score is 0-48. A higher score represents a better outcome.

n represents number with complete data to calculate a score.

Adolescent quality of life was measured using the Monitoring Individual Needs in Diabetes Youth Questionnaire (MIND-Youth/MY-Q), a 33-item measure of diabetes-specific health-related QOL. The scale range from the minimum to maximum possible score is 0-100. A higher score represents a better outcome.

n represents number with complete data to calculate a score.

Feasibility of the app measured by percentage of participants that engaged with or used the app at least twice a week during the intervention period.

Participants in the intervention arm completed the Usefulness, Satisfaction, and Ease of Use Questionnaire (USE), a measure of the users' perceived usefulness of, satisfaction with, and ease of use of a particular technology. The item scale ranges from the minimum to maximum possible score is 1-7, with a higher score representing a better outcome. Acceptability measured as percentage of participants who selected a score of at least 4 (out of 7) on the item "I am satisfied with it," indicating it was at least somewhat acceptable.

The number of participants that felt the intervention was well-received was collected for Adolescents and Parents. To determine if the intervention was well-received, verbal responses from qualitative interviews with were coded for types of participant feedback by the study team. We coded these data qualitatively and classified them as Positive, Negative, or Neutral. Positive responses indicate the intervention was well-received.

n represents number we were able to reach and obtain follow-up interview data

Objective measurement of adherence to diabetes regimen measured by blood glucose monitoring frequency (a well-accepted surrogate of overall adherence), obtained via blood glucose meter downloads. The average daily frequency was calculated over the 14 days prior to the assessment at the baseline visit.

n represents number with complete data to calculate a score.

At the time of this study, the American Diabetes Association generally recommended an HbA1c target of <7.0% (it was <7.5% at the time this study took place, for individuals younger than 18 years. The especific target varies depending on the individual). The DCA 2000 HbA1c Analyzer (Siemens-Bayer) was used for point of care HbA1c analysis, it has an analytical measurement range for HbA1c of 2.5% to 14.0%. HbA1c values are collected via fingerstick and blood assay at routine diabetes care visits and values were extracted from the medical record at each clinic visit during the study period.

n is the number with an HbA1c value available at this timepoint.

Diabetes-specific family impact was measured using the Diabetes Family Impact Scale (DFIS), a 15-item measure of the impact of diabetes on family members' activities and relationships. The scale range from the minimum to maximum possible score is 0-100. A higher score represents a better outcome.

n represents number with complete data to calculate a score.

Family impact was measured using The Pediatric Quality of Life Family Impact Module (Peds QL-FI), a 36-item measure of the impact of parenting a child with a chronic medical condition on family functioning and parent QOL. The scale range from the minimum to the maximum score is 0-100. A higher score represents a better outcome.

n represents number with complete data to calculate a score.

Family communication was measured by the parent-report Helping for Health Inventory (HHI), a 15-item questionnaire assessing perceptions of parental help around the teen's diabetes management. The scale range from the minimum to maximum possible score is 15-75. A lower score represents a better outcome.

n represents number with complete data to calculate a score.

Family communication was measured using the adolescent-report Helping for Health Inventory (HHI), a 15-item questionnaire assessing perceptions of parental help around the teen's diabetes management. The scale range from the minimum to maximum possible score is 15-75. A lower score represents a better outcome.

n represents number with complete data to calculate a score.

Family conflict was measured using the Diabetes Family Conflict Scale - Revised (DFCS), a measure of diabetes-specific conflict in families with adolescents with type 1 diabetes. The scale range from the minimum to maximum possible score is 19-57. A lower score represents a better outcome.

represents number with complete data to calculate a score.

Family conflict was measured using the Diabetes Family Conflict Scale - Revised (DFCS), a measure of diabetes-specific conflict in families with adolescents with type 1 diabetes. The scale range from the minimum to maximum possible score is 19-57. A lower score represents a better outcome.

n represents number with complete data to calculate a score.

Diabetes burden was measured using the Problem Areas in Diabetes (PAID-T) parent-report, a measure of how bothersome day-to-day problems are for parents of adolescents with type 1 diabetes. The scale range from the minimum to maximum possible score is 26-156. A lower score represents better a outcome.

n represents number with complete data to calculate a score.

Diabetes burden was measured using the Problem Areas in Diabetes-Teen (PAID-T) self-report, a measure of how bothersome day-to-day problems are for adolescents with type 1 diabetes. The scale range from the minimum to maximum possible score is 26-156. A lower score represents better a outcome.

n represents number with complete data to calculate a score.

Parent-report of adolescent self-management was measured using the 24-item Diabetes Self-Management Profile Self-Report (DSMP). Parents completed the version appropriate to their child's current diabetes regimen (conventional insulin regimen, 24 items; flexible insulin regimen, 24 items). The scale range from the minimum to maximum possible score is 0-86. A higher score represents a better outcome.

n represents number with complete data to calculate a score.

Self-management was measured using the Self-Care Inventory-Revised (SCI-R), a 15-item measure of the frequency that adolescents engage in diabetes management behaviors on a 5-point scale from Never to Always. The scale range from the minimum to maximum possible score is 5-75. A higher score represents a better outcome.

n represents number with complete data to calculate a score.

The Diabetes Strengths and Resilience measure is a self-report assessment of positive behaviors related to diabetes resilience for youth with type 1 diabetes, such as perceived competence to manage the demanding diabetes regimen, to adapt to the unpredictability of diabetes, and to seek help and support with diabetes challenges. The scale range from the minimum to maximum possible score is 0-48. A higher value on this scale represents a better outcome.

n represents number with complete data to calculate a score.

Adolescent quality of life was measured using the Monitoring Individual Needs in Diabetes Youth Questionnaire (MIND-Youth/MY-Q), a 33-item measure of diabetes-specific health-related QOL. The scale range from the minimum to maximum possible score is 0-100. A higher score represents a better outcome.

n represents number with complete data to calculate a score.

Adolescents answered 3 items adapted from the Parent-Youth Relationship Index of the National Longitudinal Study of Youth-1997, a measure of parent-adolescent relationship quality. Item scale ranges from minimum to maximum score of 1-5 for each item, with a higher score indicating a better outcome.

Intervention process from baseline to follow-up was analyzed using a general linear mixed model, using the item "How often does he/she praise you for doing well?" Time since first baseline clinic visit was computed and analyzed using a mixed model with time, arm, and the time-arm interaction term. The outcome measure data are reported as slopes by arm.

*NOTE: Intervention process measures were administered biweekly, and we are reporting the calculated mean change per day with 95% confidence intervals.

Usual diabetes care and study-related data collection, without use of app during the study period. They will be offered an opportunity to try the app and share their feedback with the study team after completing follow-up data collection.