On 30 June 2026 Anthropic launched Claude Sonnet 5 at USD 2.00 per million input tokens and USD 10.00 per million output tokens, and said plainly that those were introductory rates. They ran through 31 August 2026. From 1 September the model was to sit at USD 3.00 and USD 15.00 — a rise of 50 percent on every token, input and output alike.
We put that on the watchlist because it was the rare kind of pricing event you can settle without argument. Two numbers, one date, one page. The question read: does Claude Sonnet 5 rise by 50 percent on 31 August 2026? The settlement condition was the figure printed on Anthropic’s own pricing page.
It does not. On 10 August 2026 Anthropic made the introductory price permanent. The pricing documentation now carries a note stating that the USD 2.00 and USD 10.00 rates are the standard price, and that the scheduled increase to USD 3.00 and USD 15.00 on 1 September 2026 will not occur. The model table shows USD 2.00 input and USD 10.00 output, with batch rates of USD 1.00 and USD 5.00 and cache hits at USD 0.20. The Sonnet 5 launch post carries the same amendment as a dated changelog entry.
The watchlist resolves NO. Not “not yet” — the scheduled event has been cancelled at source, three weeks before its trigger date, by the only party able to cancel it.
Two points of precision are owed here. First, our question said 31 August, and the step-up was actually scheduled for 1 September; 31 August was the last day of the old price, not the first day of the new one. The framing was loose. Second, and more importantly, the direction of our implied expectation was wrong. Publishing a dated question about a scheduled increase carries a bias towards believing the schedule. We believed it. Anthropic’s stated schedule was clear, the sunset date was in the launch material, and nothing in the intervening six weeks suggested a reversal. It reversed anyway.
What it means is narrower than the headlines around it suggest. Sonnet 5 now sits permanently at a third below Sonnet 4.6, which remains at USD 3.00 and USD 15.00. Against Opus 5 at USD 5.00 and USD 25.00, the gap for high-volume agent work is now structural rather than promotional, which changes how a team should reason about model selection: you can plan a twelve-month budget on the Sonnet 5 line without a September cliff written into it. Anyone who had already migrated workloads off Sonnet 5 in anticipation of the rise has done work they did not need to do, and that is a real cost, even if a small one.
One caveat worth stating. “Permanent” is a vendor’s word, not a contract. Anthropic has removed a specific dated increase; it has not undertaken never to reprice Sonnet 5. The tokenizer note in the same pricing document is a reminder that effective cost per unit of work can move without the headline rate moving at all — Claude 4.7 and later models use a tokenizer that produces roughly 30 percent more tokens for the same text. Sonnet 5 is in that cohort. A stable price per million tokens is not the same as a stable price per page.
Subscription plan pricing is unchanged by this. The reversal is an API-side event.
We will keep a successor question on the watchlist: does the Sonnet 5 base rate change at all before 30 June 2027? Same settlement condition, same page.