Anthropic Python SDK 1.9.0 Adds Reference to Unreleased 'claude-sonnet-5-5' Model ID

Anthropic SDK v1.9.0 Ships With Unannounced Model Reference

Anthropic released version 1.9.0 of its anthropic-sdk-python library on September 28, 2026, adding a code-level reference to a model ID called claude-sonnet-5-5 — a name that has not been formally announced or documented by Anthropic elsewhere. The release also introduces a new between_tools thinking type and lets developers run tool calls while a reply is still streaming.

This is a software development kit update, not a model launch. No context window size, pricing, benchmark scores, or parameter count for claude-sonnet-5-5 accompanies this release. The string appears only as an addition to the SDK's list of known model IDs, alongside a separate chore that reorders how known model IDs are displayed in the SDK's type definitions.

What changed in v1.9.0

Features:

• A new between_tools thinking type, letting models reason between sequential tool calls
• Addition of the claude-sonnet-5-5 model identifier to the SDK's type list
• New parameters (include_inherited, source) for workspace rate-limit objects
• Typed event filters for the Managed Agents events list
• General availability of cache diagnostics on Message and MessageCreateParams objects
• Optional execution of tool calls while a streamed reply is still being generated

Bug fixes:

• The client no longer sends a placeholder filename for unnamed file uploads
• between_tools thinking now degrades to disabled during fallback hops, fixing a helper-level bug (#952)
• stream() and parse() methods now correctly accept diagnostics data (#929)

Chores and documentation:

• Known model IDs are now listed first in the SDK's Model type definitions
• CI runner selection now varies by repository
• The beta header is no longer sent from parse() and the tool runner
• Documentation clarifies that stream: true returns the raw event stream
• Managed Agents actor descriptions were made resource-neutral
• A research-preview notice was restored on the Dream type

Full commit history is available in Anthropic's GitHub compare view.

What this means

The appearance of claude-sonnet-5-5 in the SDK's type definitions is the most notable detail here, but it stops well short of confirming a model release. Companies routinely stage model IDs in client libraries ahead of public announcements to reduce integration lag once a model does ship — this happened with earlier Claude versions too. Until Anthropic publishes specs, pricing, or a model card for claude-sonnet-5-5, treat the reference as a signal of an upcoming release, not evidence one has occurred.

The more concrete changes in this release — streaming tool execution and the between_tools thinking type — are developer-facing improvements to how Claude models handle multi-step tool use, letting agents act on partial output instead of waiting for a full response. These are meaningful for latency-sensitive agent applications, but they modify how existing models are accessed, not the models themselves.