Anthropic quietly adds Claude Opus 5.5 to its Foundation Models SDK
A single commit to Anthropic's ClaudeForFoundationModels repository introduces a new model constant for Claude Opus 5.5. The change updates the README and Swift source files, adding the opus5_5 identifier with capabilities including low, medium, high, xhigh, and max effort levels, adaptive thinking, structured output, and image input. Notably, Opus 5.5 inherits the Opus 4.8 request surface but rejects forced tool use.
Opus 5 and Fable 5 keep it too, except that they also reject forced tool use (`tool_choice` `any` / `tool`) — a `.required` tool-calling mode is a contract, so it is sent as-is and the API error names the field.
- tomhow
Comments moved to https://news.ycombinator.com/item?id=49803892.
- sidrag22
Cool maybe this makes the 20$ sub less of a joke. I deemed 5.0 unworthy of spending time fighting with and personally considered it by far the worst release of 2026 by either of the two major labs(promising model, but obviously not even close to ready for the general public).
So for that 20$ tier for the entire summer and into fall, i was on their 2nd class public model(4.8) released in May. Not surprisingly it became my grunt model, doing the simple work. By far the least I've used Anthropic models in the last 2 years.