Currently, Google Gemini 2.5 Pro and OpenAI GPT-4.1 API are widely recognized for having the largest input capacities (“context windows”) among mainstream chat AIs, supporting up to 1 million tokens for a single conversation or session. This capacity is unparalleled—allowing users to provide extremely long documents, extensive chat histories, or massive codebases for the AI to process in one session.
By comparison:
- The public-facing ChatGPT (web/Plus) and GPT-4o models support up to 128,000 tokens in their latest premium tiers.
- Gemini 2.5 Pro has a 1 million token limit per session (roughly 750,000 words) and is often cited as “nearly unbeatable” for very long-form dialogue or input-heavy tasks.
- API/professional users can access even greater context windows with GPT-4.1 API (up to 1 million tokens) or GPT-5 (up to 400,000 tokens).
For the longest continuous input and reference per session, Gemini 2.5 Pro and OpenAI’s GPT-4.1 API options are currently the leading choices, with capabilities far beyond other commercial models in late 2025.The chat AI with the largest input allowance currently is Google Gemini 2.5 Pro, which supports a context window of up to 1 million tokens per chat session—far exceeding most competitors. OpenAI’s GPT-4.1 API also offers a 1 million token context window for professional/enterprise users, but GPT-4o in ChatGPT Plus is limited to 128,000 tokens. For long ongoing conversations or processing massive text inputs, Gemini 2.5 Pro and GPT-4.1 API have the highest known limits among mainstream chat AIs as of September 2025.