Claude Sonnet 5 is the next generation of Anthropic's Sonnet family, released on June 30, 2026. The official announcement calls it "the most agentic Sonnet model yet," and it became the default model for Free and Pro plans at launch. This guide covers what changed, how pricing works, and what existing users and developers should watch for when upgrading — based on the official announcement and developer docs.
🟢 Model references match the current lineup · model notice · Fable subscription
Fable 5 and 5.1 subscription (updated September 7, 2026): Claude Fable 5.1, released September 1, 2026, is the current Fable model and Fable 5 is now legacy. Plan terms are the same for both — Max and Team Premium plans include Fable at up to 50% of the weekly usage limit; Pro and Team Standard use usage credits (
Update (September 27, 2026): Claude Opus 5.5 was released on September 22, 2026 and is now the current Opus model (API claude-opus-5-5, $4 per million input tokens and $20 per million output tokens, down from Opus 5's $5/$25). Anthropic says it performs at the level of Fable 5.1 on most work and costs 40% less to run than Opus 5 on typical workloads. Opus 5 remains available but is marked legacy in the official model docs. Opus 5 prices and comparisons below reflect the pre-launch state and will be updated. Details: Claude Opus 5.5 complete guide.
Key specs at a glance
| Item | Claude Sonnet 5 |
|---|---|
| Release date | June 30, 2026 |
| API model ID | claude-sonnet-5 |
| Context window (how much conversation and documents the model holds at once) | 1M tokens — both the default and the maximum; no smaller variant |
| Max output | 128k tokens |
| Pricing (per million tokens) | $2 input / $10 output — now the standard price. Announced at launch as introductory pricing through August 31, 2026, but the scheduled September 1, 2026 increase to $3/$15 was cancelled (official pricing docs, September 2, 2026) |
| Thinking | Adaptive thinking (the model decides on its own when to think deeply) on by default |
| Availability | All plans — default for Free and Pro; Claude Code and the API |
What actually improved
According to the official announcement, Sonnet 5 can make plans, use tools like browsers and terminals, and run autonomously at a level that, just a few months ago, required larger and more expensive models. Compared with its predecessor Sonnet 4.6, it improves substantially on reasoning, tool use, coding, and knowledge work, and comes close to the higher-tier Opus 5 at a lower price. Anthropic also reports that safety assessments found an overall lower rate of undesirable behaviors than Sonnet 4.6.
Put simply: the "sensible mid-size sedan" of the lineup just inherited much of the top trim's driving performance. If you are deciding between models, see our model selection guide and the model comparison.
Pricing — and the tokenizer change that is easy to miss
API pricing is $2 input / $10 output per million tokens. It was announced at launch as an introductory rate through August 31, 2026, with a move to $3 / $15 (Sonnet 4.6's standard rate) from September 1 — but Anthropic's official pricing docs now state that $2 / $10 is the standard price and the scheduled increase will not occur. The announcement also notes up to 90% savings with prompt caching (a discount for resending the same prompt prefix) and 50% with batch processing (grouping non-urgent requests).
One thing developers must know: per the official docs, Sonnet 5 uses a new tokenizer (the rule that splits text into tokens), and the same text produces approximately 30% more tokens. Even with unchanged per-token pricing, the cost of an equivalent request can differ; a max_tokens budget tuned for Sonnet 4.6 may truncate output; and the 1M window holds less text than before. The official guidance is to recount tokens against Sonnet 5 rather than reuse figures measured on earlier models.
Developer migration — three things that now return a 400
The docs describe Sonnet 5 as a drop-in upgrade for Sonnet 4.6 with exactly three behavior changes:
- Adaptive thinking is on by default. No configuration needed; the model thinks deeply when it decides it should.
- Manual extended thinking is removed. Requests with
thinking: {type: "enabled", budget_tokens: N}return a 400 error. - Sampling parameters are locked. Setting
temperature,top_p, ortop_kto non-default values returns a 400 error.
Also note that Priority Tier (a dedicated-capacity option) is not available on Sonnet 5. Aside from these, code that already runs on Sonnet 4.6 needs no other changes, per the docs. For choosing between API models, see the API model comparison guide.
How each plan gets it
- Free & Pro: Sonnet 5 is the default model — nothing to configure.
- claude.ai chat (paid plans): per the official help center, Opus 5 and Sonnet 5 support a 1M-token context window in chat on all paid plans; Opus 4.8, 4.7, 4.6 and Sonnet 4.6 support 500K in chat.
- Claude Code: requires v2.1.197 or later; the
sonnetalias resolves to Sonnet 5, and it always runs with the 1M window on the API. For costs, see our Claude Code pricing guide.
Cybersecurity safeguards — a first for Sonnet
Sonnet 5 is the first Sonnet-tier model with real-time cybersecurity safeguards enabled by default. Requests involving prohibited or high-risk cybersecurity topics may be refused — and on the API, a refusal comes back as a successful HTTP 200 response with stop_reason: "refusal", not an error. If you run automated pipelines, handle that response shape explicitly.
FAQ
How long does the introductory price last?
It was announced as running through August 31, 2026, with a rise to $3 input / $15 output from September 1. That increase did not happen: the official pricing docs (checked September 2, 2026) state that $2 / $10 is now the standard price and the scheduled increase will not occur.
Will my Sonnet 4.6 code work as is?
Mostly, yes. Remove manual extended thinking and non-default sampling parameters (both now return 400 errors), and revisit max_tokens and cost estimates given the roughly 30% higher token counts.
Is there an extra charge for the 1M context window?
No. Per the official docs, 1M is the default for Sonnet 5 — no beta header and standard pricing.
This article was originally written on July 4, 2026 based on Anthropic's official announcement and developer documentation (platform.claude.com, support.claude.com), and was updated on September 2, 2026 to reflect that Anthropic cancelled the planned September 1 price increase and confirmed $2 / $10 as Sonnet 5's standard price. Pricing, availability, and lineup details may change further — please re-check the official docs before making important decisions.
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- Claude Opus 4.7: The Complete Guide to New Features and Specs
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- Claude Fable 5 & Mythos 5: The Complete Guide
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