Claude API 300K Output Tokens: Complete Guide to Long-Form Generation (2026)
The Claude API now supports up to 300,000 output tokens per request — roughly 460 pages of text in a single API call — but only through the Message Batches API with a specific beta header. The synchronous API remains capped at 64K tokens. This guide explains exactly how to enable 300K output, which models support it, when to use it, and what it costs. What Are Claude API 300K Output Tokens? Claude API 300K output tokens refers to Anthropic’s maximum per-request generation limit, available on Claude Sonnet 4.6, Opus 4.6, and Opus 4.7 via the asynchronous Message Batches API. At approximately 650 words per 1,000 tokens, 300,000 tokens translates to roughly 195,000 words — the equivalent of a 460-page technical document or a full software codebase migration in a single API call. This capability is unlocked by passing the output-300k-2026-03-24 beta header with your batch request; without it, even Sonnet 4.6 caps at 64K tokens on synchronous calls. The 300K limit represents a 4.7× increase over the previous 64K ceiling and is the highest output token limit of any major LLM API in 2026 — GPT-4o Long Output tops out at 64K, and Gemini 1.5 Pro at 8K. For enterprises running document generation, codebase analysis, or legal drafting pipelines, this change fundamentally alters the economics of LLM-based automation. ...