# System Prompt Patterns Cheat Sheet
_The role, constraint and format blocks that make a system prompt hold_
A system prompt is the only instruction the model cannot be talked out of mid-conversation. These are the blocks that belong in one and the order they work best in.
> Difficulty: intermediate  
> Version: 1.0  
> Updated: 2026-07-22  
> Categories: Prompt Engineering  
> Tags: Claude, Openai, System Prompt

Source: https://invitationbuddy.com/cheat-sheet/system-prompt-patterns-cheat-sheet

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## The five blocks
Written in this order because each one narrows the space left by the one above it.
| Block | What it does | Failure it prevents |
| --- | --- | --- |
| Role | Who the model is and who it is talking to  Generic, unplaceable tone |
| Capability | What it may do, and with which tools  Inventing actions it cannot take |
| Constraint | Hard limits — length, scope, forbidden topics  Scope creep on long threads |
| Format | The exact output shape expected  Prose where you needed JSON |
| Refusal | What to do when the request falls outside scope  Confident answers to the unanswerable |

## Phrasings that measurably work
| Instead of | Write | Why |
| --- | --- | --- |
| "Be concise" | "Answer in at most 3 sentences"  A number is checkable; an adjective is not |
| "Do not hallucinate" | "If the context does not contain the answer, say so"  Names the observable behaviour, not the internal state |
| "Use good formatting" | "Return a markdown table with columns X, Y"  Removes every degree of freedom you did not want |
| "Think step by step" | "Work through the steps in  tags first"  Gives the reasoning somewhere to live |
| "Be helpful" | Delete the line  Costs tokens and changes nothing |

## Order matters
Put the output format LAST. Models weight the end of the system prompt heavily when deciding how to shape a reply, and a format spec buried above three paragraphs of role description is the first thing dropped on a long conversation.

## FAQs
**Should the system prompt or the user message hold the instructions?**
Anything that must survive the whole conversation goes in the system prompt — role, constraints, output format. Anything about this one request goes in the user message. Instructions placed in a user turn get diluted as the thread grows.

**How long should a system prompt be?**
As long as it needs to be and no longer. Every token is re-sent on every turn, so it is the most expensive text in your application. Delete any line you cannot name a failure for.

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_Generated from https://invitationbuddy.com/cheat-sheet/system-prompt-patterns-cheat-sheet_