Two Approaches to AI Personalization
ChatGPT offers Custom Instructions and GPTs. Claude Code uses SKILL.md and CLAUDE.md files. Both allow personalizing AI behavior, but the approaches differ fundamentally. Understanding these differences will help you choose the right solution for your needs.
ChatGPT Custom Instructions: How They Work
The Concept
ChatGPT Custom Instructions is a personalization system built into the web interface. You define two text fields:
- "What would you like ChatGPT to know about you?": your personal context
- "How would you like ChatGPT to respond?": your response preferences
These instructions are automatically injected into every conversation.
Custom GPTs
GPTs go further: they combine custom instructions with actions (API calls), reference files, and a dedicated interface. A GPT is essentially an "application" built on ChatGPT.
Custom Instructions Limitations
- Size limited: approximately 1500 characters per field
- No versioning: impossible to track changes over time
- No collaboration: difficult to share and maintain as a team
- Web interface only: no integration with development tools
- Single active set: you cannot easily combine multiple instructions
Claude Code Skills: How They Work
The Concept
Claude Code skills are Markdown files (SKILL.md) stored in your filesystem. They are automatically loaded by Claude Code when referenced in your project's CLAUDE.md file.
Skills Architecture
my-project/
├── CLAUDE.md # Main file — references skills
├── .skills/
│ ├── code-review.md
│ ├── testing.md
│ └── documentation.md
└── src/
Each skill is an independent file, versioned with Git, shareable, and composable.
Skills Advantages
- Unlimited size: no artificial length constraints
- Native versioning: Git tracks every modification
- Collaboration: skills live in the repo, shared with the whole team
- Multi-editor: compatible with Cursor, Windsurf, VS Code, and Claude Code
- Composability: combine multiple skills based on context
Detailed Comparison
Accessibility
ChatGPT: accessible to everyone via the web interface. No technical skills required. Ideal for beginners.
Claude Code: requires minimum comfort with terminal and text files. More technical but more powerful.
Verdict: ChatGPT wins for general public accessibility.
Power and Flexibility
ChatGPT: limited in size and structure. GPTs add API actions but remain constrained by the interface.
Claude Code: total freedom of structure, size, and complexity. Support for nested, conditional, and contextual skills.
Verdict: Claude Code wins significantly for professional use cases.
Workflow Integration
ChatGPT: works in the browser. Copy-paste required to integrate results into other tools.
Claude Code: works directly in the terminal and code editor. Results are generated in your project files.
Verdict: Claude Code is clearly superior for developers and power users.
Sharing and Distribution
ChatGPT: GPTs are shareable via link. Custom Instructions require manual copy-paste.
Claude Code: skills are shared like any file — Git, email, marketplace. The open format (Markdown) guarantees portability.
Verdict: tie, but with an openness advantage for skills.
Security and Privacy
ChatGPT: your instructions are stored on OpenAI's servers. Conversations with Custom Instructions are subject to the same data policies.
Claude Code: skills are local files. They only leave your machine when sent with your prompt to the Claude API.
Verdict: skills offer better control over privacy.
Comparison Table
| Criteria | ChatGPT Custom Instructions | ChatGPT GPTs | Claude Code Skills | |---|---|---|---| | Max size | ~1500 chars | ~8000 chars | Unlimited | | Versioning | No | No | Native Git | | Collaboration | Difficult | Shareable link | Shared repo | | Multi-editor | No | No | Yes | | API actions | No | Yes | Via MCP | | Composability | No | Limited | Full | | Learning curve | Easy | Medium | Medium | | Cost | Free/Plus | Plus/Team | Claude API |
Which Tool to Choose?
Choose ChatGPT Custom Instructions if:
- You are just starting with AI
- Your needs are simple and personal
- You work primarily in the browser
- You do not need versioning or collaboration
Choose GPTs if:
- You need integrated API actions
- You want to share custom assistants
- Your workflow centers on ChatGPT
Choose Claude Code Skills if:
- You are a developer or power user
- You work in teams on complex projects
- You need composability and versioning
- You want portable skills across editors
- Data privacy is critical
The Future of AI Personalization
Both approaches are converging. ChatGPT is progressively adding file and project features. Claude Code is simplifying its interface. The important thing is choosing the tool that best integrates into your current workflow.
Explore our skill catalog on Skills Guides — they work with Claude Code, Cursor, Windsurf, and VS Code.