GPT-5.6, the successor to GPT-5.5, has been accessed by external developers within the Codex environment, according to Beating Monitor. The new model features a context window of 1.5 million tokens, a 43% increase over GPT-5.5's 1.05 million tokens. Initial traces of GPT-5.6 appeared on April 28, with developers noting successful invocations via ChatGPT Pro's OAuth authentication. Real-world tests confirmed GPT-5.6's capability to handle inputs exceeding 900,000 tokens, with the model self-identifying as running on openai/gpt-5.6. The model supports an enhanced reasoning level and offers a fast mode with improved speed. Development is reportedly in full swing, with an expected release in early June. Polymarket currently estimates an 85% probability of GPT-5.6 being released before June 30.