System Instructions

What Is a System Instruction?

A system instruction defines the general behavior, role, rules, and output expectations of an AI application.

The user prompt supplies the immediate task. The system instruction supplies the behavior that should remain consistent while completing that task.

A Practical Example

Imagine a business email rewriting application.

The permanent application behavior might be:

You are a professional business communication assistant.

Rewrite messages so they are clear, friendly, and professional.

Preserve the original meaning.

Do not invent names, dates, promises, facts, or commitments.

Return only the rewritten message.

The individual user prompt might be:

hey lisa just checking if you saw my last email

The user does not need to repeat all the application rules every time. The server combines the permanent system instruction with the current user input.

Compare the Difference

The playground below sends the same user prompt in two different ways:

1. Once without a system instruction
2. Once with the system instruction

Run both versions and compare how much control the instruction provides.

How the Request Changes

The request without a system instruction contains:

{
    "model": "gemini-3.5-flash",
    "store": false,
    "input": "hey lisa just checking if you saw my last email"
}

The controlled version adds:

{
    "model": "gemini-3.5-flash",

    "system_instruction": "You are a professional business
    communication assistant...",

    "store": false,

    "input": "hey lisa just checking if you saw my last email"
}

The API endpoint is the same. The model is the same. The user prompt is the same. The added instruction changes the expected application behavior.

What Belongs in a System Instruction?

The Application Role

You are a business communication assistant.

The Primary Task

Rewrite messages so they are clear, professional, and friendly.

Important Rules

Preserve the user's original meaning.

Do not invent facts, names, dates, promises, or commitments.

The Required Output

Return only the rewritten message.

What Does Not Belong There?

Do not place information in the system instruction merely because it is long or inconvenient.

These normally belong in the user request:

• The message currently being rewritten
• The document currently being analyzed
• The topic selected by the user
• The customer's current form submission
• Temporary options selected for one request

System Instructions Are Not Security

A system instruction guides model behavior. It does not replace server-side security or application rules.

Do not rely on instructions to:

• Authenticate a user
• Protect an API key
• Enforce payment
• Prevent excessive API usage
• Validate uploaded files
• Decide which database records a user may access

Those responsibilities belong to your application code.

Avoid Conflicting Instructions

An instruction becomes unreliable when it contains rules that contradict one another.

For example:

Be extremely detailed.

Keep every answer under two sentences.

Explain every concept thoroughly.

The model must decide which requirement matters most.

A clearer version would be:

Explain the answer in no more than two concise paragraphs.

Include only the details needed to complete the task.

Do Not Overload the Instruction

More instructions do not automatically create better results.

Very long instructions can:

• Contain accidental contradictions
• Increase input-token usage
• Make important rules difficult to identify
• Become harder to test and maintain
• Attempt to handle unrelated tasks in one application

A focused application usually performs better with a focused instruction.

Test the Rules Individually

Do not test only the perfect input. Test each important rule.

Test Meaning Preservation

Tell John I agree to the price but I cannot begin until next week.

Test Missing Information

Tell them I will have it finished by

Test Dates and Numbers

The total is $1,275 and payment is due August 14.

Test Already-Good Input

Thank you for your time. I will review the proposal and respond
by Friday afternoon.

Watch for invented details, removed facts, unnecessary wording, and changes in meaning.

System Instructions Are Sent With Each Request

The instruction belongs to the interaction being created. Our PHP endpoint therefore includes it each time the controlled button is used. :contentReference[oaicite:1]{index=1}

In a finished application, users would not normally type or edit the instruction. It would be stored in a protected server file:

<?php

$systemInstruction = '
You are a professional business communication assistant.

Preserve the original meaning.
Do not invent information.
Return only the rewritten message.
';

The visible form would contain only the user input and approved options.

Practical Exercise

1. Run the default prompt without the instruction.
2. Run the same prompt with the instruction.
3. Compare the two results.
4. Change “professional and friendly” to “brief, professional, and direct.”
5. Run the controlled version again.
6. Add a rule requiring a subject line and observe the change.
7. Test a message containing a date, price, or specific promise.

Lesson Summary

A system instruction defines what an AI application does, how it behaves, which rules it follows, and what kind of output it returns.

The user prompt supplies the individual task. The server combines both pieces when preparing the Gemini request.

In the next lesson, we will stop asking Gemini for loosely formatted prose and require it to return structured JSON that an application can reliably process.