# Custom instruction

**Source:** https://promtable.com/glossary/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.

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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

- 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.

## Related terms

- [system-prompt](https://promtable.com/glossary/system-prompt)
- [persona-prompting](https://promtable.com/glossary/persona-prompting)
- [gpt-store](https://promtable.com/glossary/gpt-store)

*Last updated: 2026-06-01*
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Original page: https://promtable.com/glossary/custom-instruction
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