PowerUp Coaching — Living Knowledge Base

Insight: Spec-First Vibe Coding — Multi-Model Architecture Review Before Writing a Line of Code

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“Vibe coding without a spec is just vibing. The spec is what makes the AI useful rather than just fast.” — Kasimir Hedstrom

Session context: 2026-01-08_Mastermind — Kasimir Hedstrom shared his evolved AI-assisted development workflow, which challenges the popular narrative that vibe coding means letting AI write whatever it wants.

Core Idea

“Vibe coding” — using AI to write code rapidly based on natural language descriptions — has become a popular frame for AI-assisted development. The popular version suggests you can simply describe what you want and let the AI build it. The problem with this approach is that the quality of AI-generated code is bounded by the quality of the architecture decisions embedded in the prompt — and most conversational prompts embed no architecture decisions at all.

The spec-first approach adds a critical upstream step: before writing a single line of code, write a product requirements document (PRD) that specifies what the tool should do, what it should explicitly not do, and what constraints apply (language, deployment context, data privacy requirements). Then — before implementation begins — run this PRD through multiple AI models simultaneously and ask each to critique the architecture. Claude, ChatGPT, and Gemini each have different structural blind spots: Claude tends to flag edge cases, ChatGPT surfaces usability concerns, Gemini pushes back on technical constraints. Getting all three critiques before committing to an architecture catches problems that any single model would miss.

The resulting workflow is more structured than the popular vibe coding narrative, but the time investment is small relative to the cost of architectural problems discovered mid-build. A 20–30 minute multi-model architecture review replaces days of refactoring. The PRD becomes the equivalent of a prompt brief for a writing project: the higher the quality of the specification, the higher the quality of the critique, the higher the quality of the eventual build.

This pattern is the development-specific application of the multi-model synthesis principle: use parallel models to surface different types of risk and constraint, then resolve them before committing to a direction.

Practical Application

The Spec-First Workflow:

  1. Write a one-page PRD before touching any code: what it does, what it explicitly doesn’t do, deployment context, language/framework constraints, data privacy requirements, performance expectations.
  2. Submit the PRD simultaneously to Claude, ChatGPT, and Gemini with the prompt: “Review this architecture spec. What are the most significant architectural risks, constraints I haven’t considered, or decisions I’ll regret once I’m mid-build?”
  3. Compare the three critiques. Note what each model flagged that the others didn’t.
  4. Resolve the top 3–5 architectural concerns in the PRD before starting implementation.
  5. Only then: begin vibe coding with the revised PRD as the system prompt context.

Evolution Across Sessions

This builds on Insight - Use a PRD-First Workflow to Build Apps Without Getting Lost (September 2025), which established the PRD-first development principle. The new development is the multi-model architecture review — adding a parallel critique pass across three models before implementation begins. This is a more specific and more powerful version of the PRD-first insight, combining the architectural specification discipline with the multi-model deliberation pattern that emerged from the group’s January sessions.

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