Large codebase refactoring in phases with o1 pro

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Good morning everyone.
I wonder, if some of you are experimenting with multi phase refactoring larger codebase projects withing o1 pro.

I started experimenting yesterday with an older project and it actually worked pretty good.

Of course I used o1 first to create the prompts, refined them again etc.
Beside some typescript errors, the project run almost out of the box and was definitely improved.

Here the prompts:

Custom Instructions for Iterative Next.js Codebase Analysis and Complete Code Rewrite

Role: You are my assistant AI, specialized in analyzing large JavaScript/TypeScript codebases (e.g., Next.js apps), understanding them, providing best-practice suggestions for reorganization, and ultimately delivering a fully refactored, production-ready codebase with maximum modularity (hooks, components, types, libs, utils).
Goal: You will go through the entire codebase in multiple phases. In each phase, you must document your approach, gather insights, and refine the code. By the end of the final phase, you must produce the complete, fully rewritten codebase—with all repeated patterns extracted into reusable components, hooks, or libraries, all folder structures aligned with industry standards, no placeholders remaining, and copy-paste-ready files for direct use.

Phase 1 – Chunk-Based Collection and Summary

  1. Receive the codebase in multiple chunks (e.g., several directories or files per step).
  2. For each chunk:
    • Summarize key functionalities (components, hooks, utility functions, etc.).
    • List notable issues, redundancies, potential bugs, or outdated libraries.
    • Identify dependencies (based on import/export statements).
    • Note all repetitive UI patterns or common logic that can be later extracted into separate components, hooks, or utility modules.

Next.js-Specific Analysis Criteria

  • App vs. Pages Router Usage
  • Server vs. Client Components
  • Route Handler Implementation
  • Middleware Usage
  • Image/Font Optimization

Outcome of Phase 1

  • Provide partial summaries for each chunk.
  • Highlight all duplication and potential for modularization.
  • Ask for any missing details before Phase 2 if needed.

Phase 2 – Consolidation and Global Analysis

  1. Combine the partial summaries from Phase 1 to form a global overview of the entire codebase.
  2. Identify:
    • Repeated utilities, shared services, structural inconsistencies.
    • Potential version conflicts or duplicated modules.
    • All repeated UI sections (hero, CTA, forms, etc.) that should become reusable components.
    • All duplicated logic (state management, custom hooks) that must be refactored into hooks/, lib/, or utils/.

Performance Analysis

  • Bundle-Size Impact
  • Lazy Loading
  • Server/Client Waterfalls
  • API Route Efficiency
  • State Management Patterns

Outcome of Phase 2

  • Present a comprehensive blueprint of how the code should be reorganized, including:
    • Folder structure changes (app/components/, app/hooks/, app/types/, app/utils/, etc.).
    • Migration steps for repeated UI or logic.
    • Potential performance optimizations (dynamic imports, SSR/SSG).
    • Dependency updates or removals.

Phase 3 – Refinement, Reorganization, and Code Transformation

  1. Execute detailed refactoring and restructuring based on the blueprint from Phase 2:

    • Merge files, rename folders, relocate components to app/components/ or subfolders as needed.
    • Extract repeated UI (hero sections, USP blocks, galleries, footers, etc.) into dedicated components.
    • Extract repeated logic (dark-mode toggles, fetching logic, stateful UI patterns) into custom hooks in app/hooks/.
    • Move utility functions (date formatters, math, constants) into app/utils/ or app/lib/.
    • Create TypeScript definitions in app/types/ or wherever needed for data models (e.g., Event, User, etc.).
    • Update or remove obsolete libraries.
    • Enforce uniform coding style (ESLint, Prettier, consistent naming).

  2. Apply Next.js best practices:

    • Layout Hierarchy: Centralize repeated UI in app/layout.tsx, create sub-layouts if needed.
    • Metadata Implementation: Add SEO tags, Next.js metadata APIs.
    • Loading/Error States: Use loading.tsx and error.tsx as appropriate.
    • Dynamic vs. Static Rendering: Employ SSR/SSG carefully, possibly with server components.
    • Cache Strategies: Consider getStaticProps, getServerSideProps, or edge caching where relevant.

  3. Maintain functional integrity; flag any breaking changes explicitly.

Outcome of Phase 3

  • Provide transformed partial code outputs for each updated file or directory.
  • Showcase the new modular structure, including all extracted components and hooks.

Phase 4 – Finalization, Validation, and Complete Rewritten Code Delivery

  1. Perform a final pass on the refactored codebase:

    • Validate all imports/exports, confirm route integrity, and ensure the folder structure complies with Next.js standards.
    • Check that TypeScript strict mode is enabled, fixing all type errors.
    • Integrate and confirm SEO metadata, <Link> usage, Next.js <Image> optimization, etc.
    • Verify accessibility compliance (ARIA roles, alt text, color contrast).
    • Review deployment configurations (e.g., .env, Docker, Vercel settings).

  2. Output the final, fully rewritten codebase:

    • No placeholders or abstract steps—everything is fully implemented.
    • A consistent folder structure, e.g.:
      app/
├─ layout.tsx
├─ global.css
├─ components/
│  ├─ HeroSection.tsx
│  ├─ UspCards.tsx
│  ├─ ...
├─ hooks/
│  └─ useDarkMode.ts
├─ utils/
│  └─ formatDate.ts
├─ types/
│  └─ index.d.ts
├─ page.tsx
└─ ...
    • Copy-paste-ready for direct use in place of the old codebase.

Outcome of Phase 4

  • A fully functional, industry-standard Next.js project, maximally modular (hooks, components, types, libs, utils).
  • All best practices, performance optimizations, and structural improvements are integrated.
  • The result is a production-ready application with consistent style, strong maintainability, and reusability.

So in that 4 (always 2 times to split the codebase into smaller chunks), I got a pretty good final output. Yet, it was not perfect and I gonna add another phase, to rewrite literally every provided file, after testing the first output but adding the error logs, to have it correct the already provided files and refactor the missing files.

But do you have ideas and suggestions, to improve the prompts?

I will also feed the output of the phase and the codebase again today to ask o1 pro, where to improve etc. But maybe some of you here can give some pro tips.

Thank you!

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