Bug: Undefined 'summaryParts' error when using GPT-5.4 model

Bug Report

Description

When using GPT-5.4 as the model in Codex CLI, the agent crashes with an undefined 'summaryParts' error during response processing. This does not occur with GPT-5.3-codex.

Steps to Reproduce

1. Configure Codex CLI to use gpt-5.4 as the model
2. Run a prompt that triggers a multi-step response (e.g., codex "refactor this function to use async/await")
3. Observe the error during response summary generation

Expected Behavior

The agent should process the GPT-5.4 response and display the summary without errors.

Actual Behavior

The CLI crashes with an error similar to:

TypeError: Cannot read properties of undefined (reading 'summaryParts')

This appears to happen when the response handler tries to access summaryParts on the response object, which is structured differently in GPT-5.4 compared to GPT-5.3-codex.

Environment

• Codex CLI version: latest (installed via npm)
• Model: gpt-5.4
• OS: macOS (Darwin 25.2.0)
• Node.js: v22.x

Additional Context

GPT-5.4 appears to return a different response structure than what the summary generation code expects. The summaryParts field may have been renamed or restructured in the GPT-5.4 API response format. This is likely a compatibility issue where the Codex CLI response parser needs to handle the updated response schema from newer models.

Related issues with GPT-5.4 compatibility: #13773, #13864