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What’s New

GPT-5.3 Codex, GPT-5.4, GPT-5.4 Pro, GPT-5.5, and GPT-5.5 Pro all use 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.
phase is not available in the Chat Completions API. The Chat Completions format cannot represent multiple output items with distinct phases in a single response. Use the Responses API for full phase support.

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The phase Field

phase appears on assistant output messages and has three possible values:
ValueMeaning
nullNo phase specified (default)
"commentary"Intermediate assistant message
"final_answer"The final closeout message
phase is only valid on assistant messages. Do not add phase to user or system messages.

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Why It Matters

For models like gpt-5.3-codex, gpt-5.4, gpt-5.4-pro, gpt-5.5, and gpt-5.5-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.

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Usage

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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. 
{
  "model": "openai/gpt-5.4",
  "input": [
    {
      "type": "message",
      "role": "user",
      "content": [
        {
          "type": "input_text",
          "text": "Refactor the auth module"
        }
      ]
    }
  ]
}
The response will include phase on assistant output messages: 
{
  "output": [
    {
      "type": "message",
      "role": "assistant",
      "content": [
        {
          "type": "output_text",
          "text": "I'll start by analyzing..."
        }
      ],
      "phase": "commentary"
    },
    {
      "type": "message",
      "role": "assistant",
      "content": [
        {
          "type": "output_text",
          "text": "Here's the refactored code..."
        }
      ],
      "phase": "final_answer"
    }
  ]
}
For follow-up requests, include the assistant output items with their phase intact: 
{
  "model": "openai/gpt-5.4",
  "input": [
    {
      "type": "message",
      "role": "user",
      "content": [
        {
          "type": "input_text",
          "text": "Refactor the auth module"
        }
      ]
    },
    {
      "type": "message",
      "role": "assistant",
      "content": [
        {
          "type": "output_text",
          "text": "I'll start by analyzing..."
        }
      ],
      "phase": "commentary"
    },
    {
      "type": "message",
      "role": "assistant",
      "content": [
        {
          "type": "output_text",
          "text": "Here's the refactored code..."
        }
      ],
      "phase": "final_answer"
    },
    {
      "type": "message",
      "role": "user",
      "content": [
        {
          "type": "input_text",
          "text": "Now add unit tests"
        }
      ]
    }
  ]
}

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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.

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Implementation Pattern

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

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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 silently ignore 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.

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Supported Models

Modelphase Support
openai/gpt-5.5Supported
openai/gpt-5.5-proSupported
openai/gpt-5.4Supported
openai/gpt-5.4-proSupported
openai/gpt-5.3-codexSupported
Other OpenAI modelsSilently ignored (safe to pass)
Non-OpenAI modelsNot applicable
Passing phase to OpenAI models that don’t support it (like gpt-4o) is safe — OpenAI silently ignores the field. You do not need to filter phase based on the model.

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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, GPT-5.4 Pro, GPT-5.5, and GPT-5.5 Pro

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Resources