A pelican for GPT-5.5 via the semi-official Codex backdoor API

· ai coding · Source ↗

TLDR

  • GPT-5.5 launched April 23, 2026 in Codex and ChatGPT but not the API yet; Simon Willison built a plugin to access it via the Codex CLI subscription endpoint.

Key Takeaways

  • OpenAI officially supports third-party tools using the /backend-api/codex/responses endpoint, confirmed by Romain Huet and Peter Steinberger; Claude Code, Pi, and Opencode already use it.
  • Willison built llm-openai-via-codex, a plugin for the LLM CLI tool that routes prompts through an existing Codex subscription, installed with llm install llm-openai-via-codex.
  • GPT-5.5 API pricing is $5/1M input and $30/1M output tokens, exactly double GPT-5.4’s $2.50/$15; GPT-5.5 Pro goes to $30/$180.
  • Reasoning effort matters: xhigh used 9,322 reasoning tokens vs. 39 at default, took nearly 4 minutes, and produced qualitatively better SVG output.
  • Ethan Mollick’s review found the jagged frontier still holds: GPT-5.5 excels unpredictably, with weaknesses that remain hard to anticipate.

Why It Matters

  • Any developer with a paid OpenAI plan can now route CLI prompts through GPT-5.5 before the official API ships, using the open-source Codex server endpoint.
  • At 2x the price of GPT-5.4, GPT-5.5 positions similarly to Opus vs. Sonnet: a premium tier, not a replacement for the cheaper model.
  • OpenAI’s public endorsement of subscription-level API access by third-party tools is a direct contrast to Anthropic’s decision to block OpenClaw from the same pattern.

Simon Willison, Simon Willison’s Weblog · 2026-04-23 · Read the original

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