Claude Sonnet 4.5

Claude Sonnet 4.5 (API name claude-sonnet-4-5-20250929) was released on 29 September 2025 and remains available, listed by Anthropic among its legacy models. It is the oldest Claude still on sale, and the last Sonnet built on the pre-4.6 tokenizer and the pre-adaptive reasoning design. The technical dividing line runs straight through this model. Sonnet 4.5 has a 200,000-token context window and a 64,000-token output ceiling, where its successor jumped to a million and 128,000. Its extended thinking is entirely manual: the developer switches deliberation on and sets a token budget. From Sonnet 4.6 onward the model decides that for itself, and from Opus 4.7 onward the classic sampling controls — temperature, top_p, top_k — stop being accepted at all. Sonnet 4.5 still takes them. Pricing is unchanged from its successor at $3 per million input tokens and $15 per million output, which means the newer model is strictly the better buy on capacity; what keeps 4.5 in service is code written against its exact behaviour. Anthropic's training data cutoff for it is July 2025, with knowledge most reliable through January 2025. It was also the first model for which Anthropic offered Bedrock global and regional endpoints — the routing split that every later Claude inherited. Anthropic guarantees availability until at least 29 September 2026; that is a floor, not an announced shutdown.

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Anthropic's "not sooner than" dates are a floor, not a shutdown — and Sonnet 4.5's floor is 46 days away

8/14/2026 · Anthropic — Model deprecations

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