Custom instruction
Custom instructions are user-level or persona-level system prompts that personalize an LLM's behavior across sessions — ChatGPT Custom Instructions, Claude Style / Projects, Gemini Gems system prompts, Poe Bot definitions are 2026 implementations.
Custom instructions move common preferences out of every chat: 'I'm a Turkish-speaking iOS engineer, answer in Turkish for chat but use English for code'; 'be concise, no preamble'. Two flavors: account-level (apply to every chat) and persona-level (apply only to a specific custom GPT / Gem / Project). Production patterns for builders: combine custom instruction + knowledge attachment + tool list to ship purpose-built personas without backend code. Trade-offs: custom instructions are not bulletproof — adversarial users can extract them ([[prompt-leakage]]), and instructions don't survive into long conversations as well as the base [[system-prompt]] would in API use. Best for personal use + medium-trust persona deployments.
When to use custom instruction
- Personal account-level preferences.
- Lightweight persona customization.
Common mistakes
- Storing secrets in custom instructions — adversarial prompts can extract them.
- Treating custom instructions as security boundary — they're a UX layer, not access control.
FAQ
What is custom instruction?
Custom instructions are user-level or persona-level system prompts that personalize an LLM's behavior across sessions — ChatGPT Custom Instructions, Claude Style / Projects, Gemini Gems system prompts, Poe Bot definitions are 2026 implementations.
When should I use custom instruction?
Personal account-level preferences. Lightweight persona customization.
What are the most common mistakes with custom instruction?
Storing secrets in custom instructions — adversarial prompts can extract them. Treating custom instructions as security boundary — they're a UX layer, not access control.
Related terms
- System prompt — A system prompt is the high-priority instruction block that defines a model's role, constraints, and default behaviors for an entire conversation.
- Persona prompting — Persona prompting is the practice of assigning the model a specific identity, expertise, and audience in the system prompt to steer voice, tone, and answer depth.
- GPT Store — The GPT Store is OpenAI's public marketplace of custom GPTs created by users — discoverable, browseable, and (for top builders) monetizable. By 2026 it hosts 3M+ public GPTs across categories.
Last updated: 2026-06-01. Raw markdown: https://promtable.com/glossary/custom-instruction.md.