GPT 5.1 Prompting Guide

GPT-5.1 introduces several meaningful differences from the GPT-4 model family. For a comprehensive prompting reference, we recommend reviewing the official guide:

OpenAI Cookbook – GPT-5.1 Prompting Guide

Below is a Synthflow-specific summary. When migrating an agent to GPT-5.1, we strongly recommend reading the full guide in detail.

Verbosity & Persistence

GPT-5.1 can be more verbose than previous models.
If you notice overly long responses, add explicit constraints to your system instructions—for example:

“Be concise. Respond in under two sentences because you are on a phone call.”

You may also reinforce persistence expectations using a dedicated instruction block, such as:

1
<persistence>
2
You are an agent. Continue working until the user’s query is completely resolved before ending your turn.
3
Only yield when you are confident the problem is fully solved.
4
Never stop due to uncertainty—research or deduce the most reasonable approach and proceed.
5
Do not ask the user to confirm assumptions; choose the most reasonable assumption, continue, and document it afterward.
6
</persistence>

Efficiency

1
<efficiency>
2
Efficiency is critical. You have a time limit.
3
Plan carefully, make tool calls deliberately, and verify outputs to avoid waste.
4
</efficiency>

Being explicit about efficiency helps GPT-5.1 reduce unnecessary steps, retries, and tool usage.

Continue Until Fully Resolved

To reduce latency and avoid back-and-forth turn-taking, you may include a directive such as:

“Continue working until the task is fully resolved.”

GPT-5.1 typically adheres closely to this instruction.

Prompt Format Guidance

Structuring your prompts clearly is especially important with GPT-5.1. We recommend using sectioned prompts:

• Role
• Instructions
• Tools and Constraints (e.g., parallel reads, sequential writes)
• Output Format and Length Requirements
• Edge Cases (e.g., what to do if data is missing)

Overly Strict Wording

GPT-5.1 follows instructions with extremely high precision. Overly rigid or contradictory language can lead to unexpected outcomes.

In addition to Markdown formatting, many teams have found that structured XML-like blocks—for example:

1
<instruction_spec> ... </instruction_spec>

This helps improve instruction clarity and make sections easier to reference and maintain.

Repetition Cleanup

Before finalizing a prompt, paste it into GPT-5.1 and ask it to check for:

• redundancy
• conflicting instructions
• opportunities to simplify wording

Because GPT-5.1 adheres so strictly to instructions, small contradictions in your prompt can have outsized effects.

Prompt Optimizer

OpenAI provides a built-in prompt optimizer: OpenAI Prompt Optimizer

If you’re encountering issues with your prompt, you can use the following template to request optimization:

When asked to optimize prompts, explain which specific phrases should be added or removed to elicit the desired behavior or prevent undesired behavior.

Prompt: [PROMPT] Desired behavior: [DESIRED BEHAVIOR] Undesired behavior: [UNDESIRED BEHAVIOR]

While keeping as much of the existing prompt as possible, what minimal edits would help the model reliably produce the desired behavior?