Today I worked on planning Pezeta’s goal system. I experimented with using two AI agents to collaborate on breaking down the work: Claude Opus 4 as the coordinating agent with read/write access, and ChatGPT o3 through Repo Prompt for implementation planning.

The idea was to leverage each model’s strengths. Claude handles the file operations and manages the task structure, while ChatGPT analyzes the codebase and suggests implementation approaches. They literally chat with each other using the Repo Prompt MCP tool, with Claude asking questions and ChatGPT providing analysis. Sometimes I had to intervene to guide them back on track or stop them to wait for my approval on design decisions. It’s like having two developers pair programming, each bringing their own expertise to the planning session.

The approach surfaced some important design decisions. Instead of service objects, we’re going with Rails-idiomatic patterns: model methods, class methods on MonthlyAllocation, and callbacks for snapshots. Everything through ActiveRecord.

Planning Through Test Cases

The bulk of the work went into defining test specifications:

  • 7 test cases for SomedayGoal
  • 15 for MonthlyGoal (including top-up logic)
  • 17 for DeadlineGoal (with auto-advance scenarios)
  • 17 for ReserveGoal

Each test case addresses specific edge cases. What happens when a monthly goal already has surplus funds? How does a deadline goal behave when it’s past due? Getting these details right upfront matters.

All calculations will use “opening balance,” the balance at the start of the month, not the current balance. This prevents allocation amounts from changing as transactions flow through during the month, which matches how people think about budgets.

The session produced seven detailed task files ready for implementation. Each includes exact specifications, test cases, and implementation notes. The tasks cover Single Table Inheritance strategy, historical snapshots through callbacks, and model-driven allocation suggestions.

Tomorrow I’m letting Claude Code implement these tasks.