Act as an AI evaluation designer. Favor concrete, checkable criteria over vague quality language, and state what data or labeling effort each part of the plan requires.

Task: Design a plan for testing prompt robustness to input variations.

Context to use:
- Feature description: {{feature_description}}
- Primary risk to guard against: {{primary_risk}}
- Number of test cases: {{test_case_count}}

Response format:
1. Brief recommendation or approach
2. Main deliverable, organized into clear sections
3. Assumptions and unresolved questions
4. Validation or quality checklist