GPT-5.4 Migration Guide

What’s New

GPT-5.4, GPT-5.4 Pro, and GPT-5.3 Codex introduce the phase field on assistant messages. This field is critical for multi-turn agentic workflows — it tells the model whether an assistant message is intermediate commentary or the final answer.

OpenRouter supports phase in the Responses API.

The phase Field

phase appears on assistant output messages and has three possible values:

Why It Matters

For models like gpt-5.3-codex, gpt-5.4, and gpt-5.4-pro, correctly preserving phase on assistant messages is required for optimal performance. If phase metadata is dropped when reconstructing conversation history, significant performance degradation can occur — including early stopping on longer-running tasks.

Usage

Responses API

When using the Responses API, assistant output items include phase. You must persist these items verbatim and pass them back in subsequent requests.

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{
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  "model": "openai/gpt-5.4",
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  "input": [
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    {
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      "type": "message",
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      "role": "user",
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      "content": [
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        {
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          "type": "input_text",
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          "text": "Refactor the auth module"
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        }
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      ]
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    }
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  ]
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}

The response will include phase on assistant output messages:

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{
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  "output": [
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    {
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      "type": "message",
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      "role": "assistant",
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      "content": [
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        {
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          "type": "output_text",
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          "text": "I'll start by analyzing..."
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        }
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      ],
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      "phase": "commentary"
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    },
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    {
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      "type": "message",
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      "role": "assistant",
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      "content": [
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        {
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          "type": "output_text",
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          "text": "Here's the refactored code..."
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        }
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      ],
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      "phase": "final_answer"
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    }
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  ]
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}

For follow-up requests, include the assistant output items with their phase intact:

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{
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  "model": "openai/gpt-5.4",
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  "input": [
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    {
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      "type": "message",
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      "role": "user",
7
      "content": [
8
        {
9
          "type": "input_text",
10
          "text": "Refactor the auth module"
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        }
12
      ]
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    },
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    {
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      "type": "message",
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      "role": "assistant",
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      "content": [
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        {
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          "type": "output_text",
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          "text": "I'll start by analyzing..."
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        }
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      ],
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      "phase": "commentary"
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    },
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    {
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      "type": "message",
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      "role": "assistant",
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      "content": [
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        {
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          "type": "output_text",
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          "text": "Here's the refactored code..."
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        }
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      ],
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      "phase": "final_answer"
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    },
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    {
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      "type": "message",
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      "role": "user",
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      "content": [
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        {
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          "type": "input_text",
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          "text": "Now add unit tests"
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        }
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      ]
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    }
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  ]
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}

Chat Completions API

The Chat Completions API does not support phase in responses. A single chat completion response can only contain one message per choice, so there is no way to represent the separate commentary and final answer output items that models like GPT-5.4 produce.

If you need phase support for multi-turn agentic workflows, use the Responses API instead.

Implementation Pattern

When building an integration with the Responses API, persist your output items verbatim, including phase on assistant messages:

Key Rules

1. Preserve phase on assistant messages — When you receive a response with phase, store it and send it back on subsequent requests.
2. Do not add phase to user messages — phase is only valid on assistant messages. The Responses API will reject requests with phase on user messages.
3. Do not drop phase — Omitting phase from assistant messages in multi-turn conversations will degrade model performance.
4. Use the Responses API — phase requires the Responses API. The Chat Completions API cannot represent multi-phase output.

Supported Models

Breaking Changes

None. The phase field is additive:

• Existing requests without phase continue to work on all models
• Models that don’t support phase silently ignore it
• No changes are required unless you want to take advantage of improved multi-turn performance with GPT-5.3 Codex, GPT-5.4, and GPT-5.4 Pro

Resources

• Prompt Guidance for GPT-5.4 — OpenAI’s official guide covering prompt patterns and migration tips for GPT-5.4, including completeness checks, verification loops, tool persistence, and structured outputs.
• OpenAI Responses API Reference
• Codex CLI Integration Guide
• OpenRouter API Documentation
• OpenRouter Codex Models