Custom Instructions (ChatGPT) and System Prompts (most AI interfaces) let you write a persistent block of text that loads at the start of every conversation. More control than automatic memory — but limited in length and scope.
A well-structured system prompt captures: your role and title, domain knowledge and expertise level, communication preferences, output format requirements, and recurring constraints your work involves.
Example system prompt structure
Role: I am a [job title] at [company type]. I work on [main tasks].
Domain: [key domain knowledge, terminology, tools I use]
Communication preferences: [format, tone, length preferences]
Important context: [recurring constraints or facts to remember]
Output defaults: [how I want responses formatted by default]
The core limitation: ChatGPT Custom Instructions cap at 1,500 characters (~250 words). That's not enough to capture your full professional context. And you have to manually paste or re-enter this whenever you switch models — there's no sync across ChatGPT, Claude, and Gemini.
Explicit control
Free
Works immediately
1,500 char limit (ChatGPT)
No cross-model sync
No version history
Manual re-paste per tool
Best for: Power users on a single AI tool who want control over context without a third-party tool. Read our full guide to Claude custom instructions or our comparison of Custom Instructions vs SmarterContext for a deeper breakdown.