One hack is to end the prompt with: following solid architecture principles.
EG:
Step 1: Define problem in PROBLEM.md
Step 2: Ask agent to gather scope from codebase and update PROBLEM.md
Step 3: Ask agent to create a plan following design and architecture best practices (solid, etc) and update PROBLEM.md
Step 4: Ask agent to implement PROBLEM.md
EG: Step 1: Define problem in PROBLEM.md Step 2: Ask agent to gather scope from codebase and update PROBLEM.md Step 3: Ask agent to create a plan following design and architecture best practices (solid, etc) and update PROBLEM.md Step 4: Ask agent to implement PROBLEM.md