Researcher discovers unofficial access to GPT-5.5 through OpenAI's Codex API backdoor

Simon Willison found that OpenAI's Codex API—a code-writing tool the company built years ago—still grants access to GPT-5.5, a more advanced model than the public GPT-4o release, despite no official announcement or changelog documenting this capability.

The Codex endpoint appears to be a legacy interface that was never fully deprecated; users calling it receive GPT-5.5's responses without needing to pay for a separate model tier or join a waitlist, bypassing OpenAI's usual product gatekeeping.

For developers and businesses relying on OpenAI's APIs, this means some teams may already have access to a newer, more capable model than they realize—but also raises questions about whether OpenAI intended this exposure or will close it, potentially breaking existing integrations that depend on the Codex endpoint.