The user's actual question - Variable data - Anything that changes per request

Claude: system prompts are very influential, prefer XML tags inside - GPT: system prompts work but model can ignore at high creativity - Gemini: similar to Claude For Claude specifically, this format works well: You are an assistant for [Company]. <rules - Only answer questions about our product - Refuse anything off-topic politely - Cite source URLs when possible </rules <style - Professional but friendly - Use markdown headers </style

There's no hard limit but past 1000 tokens, attention dilutes. Aim for 200-800 tokens.

Wishy-washy: "Be helpful" — too vague - Contradictory rules: "Be concise" + "Provide examples" - No format spec: model picks one randomly - No refusal pattern: model can be tricked off-topic

Iterate. Look at 10 real outputs, identify failure mode, update prompt.

Read [what is prompt engineering](https://vibecodingturkey.com/en/rehberler/what-is-prompt-engineering) for full techniques.

What Is a System Prompt 2026 — Guide

Q: Format

In API: json { "model": "claude-sonnet-4-6", "system": "You are a helpful coding assistant.", "messages": [{"role": "user", "content": "How do I sort an array?"}] }

Q: What Goes in a System Prompt

Role: who is the AI? ("You are a senior code reviewer.") - Constraints: what can't it do? ("Don't write more than 3 paragraphs.") - Style: how should it write? ("Use clear, technical English.") - Tools: what tools are available? - Examples: few-shot examples - Knowledge: durable facts ("This codebase uses Next.js 14.")

Q: Length

There's no hard limit but past 1000 tokens, attention dilutes. Aim for 200-800 tokens.

Q: Common Mistakes

Wishy-washy: "Be helpful" — too vague - Contradictory rules: "Be concise" + "Provide examples" - No format spec: model picks one randomly - No refusal pattern: model can be tricked off-topic

Q: Tuning

Iterate. Look at 10 real outputs, identify failure mode, update prompt.

Q: Next Step

Read [what is prompt engineering](https://vibecodingturkey.com/en/rehberler/what-is-prompt-engineering) for full techniques.

What Is a System Prompt 2026

A system prompt is a special category of input that sets the AI's persona, constraints, and durable context. Distinct from the user's question. Format In API: json { "model": "claude-sonnet-4-6", "system": "You are a helpful coding assistant.", "messages": {"role": "user", "content": "How do I sort an array?"} } What Goes in a System Prompt - Role: who is the AI? "You are a senior code reviewer."…

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