GPT-5.1 Prompting Guide

GPT-5.1 introduces several meaningful differences from the GPT-4 model family. Read OpenAI’s GPT-5.1 prompting guide for the full reference. This page summarizes what changes when you run an agent on GPT-5.1 in Synthflow.

You can change the LLM at any time from General configuration in the agent editor.

Verbosity and 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 can also reinforce persistence expectations using a dedicated instruction block:

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

Efficiency

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

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<instruction_spec> ... </instruction_spec>

This helps improve instruction clarity and makes 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. 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?