Anthropic’s Python SDK 1.9.0 adds support for Claude Sonnet 5.5, a new betweentools thinking type and the option to run tool calls while a reply streams. The release also makes cache diagnostics generally available in the Messages API, giving developers several practical changes in one update.
Anthropic Watch analysis
What happened
Anthropic’s Python SDK 1.9.0 release notes, published on 28 September, list six feature changes, alongside bug fixes and documentation updates. The additions cover model support, tool use, workspace rate-limit fields, Managed Agents event filters and cache diagnostics.
The betweentools thinking type can be used with the new option to run tool calls while a reply is streaming. The release notes also say the SDK now accepts diagnostics in stream() and parse(), and that cache diagnostics are generally available on Message and MessageCreateParams.
Why it matters
For developers, the most immediate changes are new model support and more ways to build tool-using applications. Running tool calls during a streamed reply may help applications start work sooner; diagnostics give developers another way to inspect cache-related information through the Messages API.
This is a Python SDK update, not a claim that every Anthropic API client or model behaviour has changed. The release notes describe available interfaces, but don’t give comparative performance results. Useful plumbing, then, rather than a stopwatch-winning promise.
Our read
This is a solid developer release: it adds a supported model and several API capabilities that can make integrations more flexible. If you build with Anthropic’s Python SDK, check whether streaming tool calls or cache diagnostics fit your workflow before changing your implementation. The release notes are the practical checklist; the interesting test is what developers can do with the new options in working applications.
What to watch
- How developers use betweentools thinking alongside streaming tool calls.
- Whether Anthropic publishes further examples of cache diagnostics in practice.
- Which applications adopt the new SDK support for Claude Sonnet 5.5.
Discussion spark: When an assistant can start tool calls before its reply has finished streaming, is the speed gain worth the extra complexity for developers?
Sources and evidence
- v1.9.0 (28 September 2026, 18:03 UTC)
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