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Public story · 2026-07-01 · high
Sampling-parameter calls now fail outright, and safety refusals return as normal-looking 200 responses.
Why now: Latent Space is already calling June 30 the start of a release wave, with Fable 5 and loosened creative controls expected next.
Anthropic shipped Claude Sonnet 5 on June 30, calling it a drop-in replacement for Sonnet 4.6, per Simon Willison's read of the platform docs.
The new tokenizer emits roughly 30% more tokens for the same input text. Token budgets, context limits, and cost projections built against Sonnet 4.6 are already off by nearly a third.
Manual extended thinking now returns HTTP 400. So does setting temperature, top_p, or top_k to anything but the default, since adaptive thinking runs by default now. These calls don't degrade quietly, they fail outright.
Sonnet 5 is the first model at this tier with real-time cybersecurity safeguards. A refusal now comes back as HTTP 200 with a stop_reason of refusal. Error handling that only checks status codes will read that refusal as a normal completion and pass along an empty response.
Yes, the pricing is good: $2/$10 intro rates through August 31, a 1M-token context window, and 128k max output. There's no Priority Tier, which matters if you were counting on it.
None of that makes this a drop-in. Recount your token budgets and add a check for a refusal stop_reason before you flip the switch.
Each link below shares sources, entities, or timing with this story.
Claude Code uses Sonnet / Shared entities / Same source domain / What happened next / Tension
Linked by a graph relationship (Claude Code uses Sonnet); both cover Anthropic, HTTP, Simon Willison, Sonnet; reported by the same outlet (simonwillison.net).
Anthropic released Fable / Shared entities / Same source domain / Earlier coverage / Tension
Linked by a graph relationship (Anthropic released Fable); both cover Anthropic, Fable, Simon Willison, Willison; reported by the same outlet (simonwillison.net).
Claude Code uses Sonnet / Shared entities / Same source domain / What happened next
Linked by a graph relationship (Claude Code uses Sonnet); both cover Anthropic, August, Latent Space, Simon Willison; reported by the same outlet (simonwillison.net).
Opus built by Anthropic / Shared entities / Same source domain / What happened next
Linked by a graph relationship (Opus built by Anthropic); both cover Anthropic, Simon Willison, Sonnet, Willison; reported by the same outlet (simonwillison.net).
Claude Code uses Sonnet / Shared entities / Same source domain / What happened next
Linked by a graph relationship (Claude Code uses Sonnet); both cover Anthropic, Fable, Simon Willison, Willison; reported by the same outlet (simonwillison.net).
Claude Code uses Sonnet / Shared entities / Shared topic / What happened next
Linked by a graph relationship (Claude Code uses Sonnet); both cover Anthropic, Build, Fable, Sonnet; overlapping topics (anthropic, cost).
Linked by a graph relationship (Claude Code uses Sonnet); both cover Anthropic, August, Claude Sonnet, Sonnet; overlapping topics (context, sonnet).
Claude Code uses Sonnet / Shared entities / Same source domain / Shared topic / Earlier coverage
Linked by a graph relationship (Claude Code uses Sonnet); both cover Anthropic, Fable, Simon Willison; reported by the same outlet (simonwillison.net).