Spring AI 2.1.0-RC2: DecisionModel API, Streaming Tool Arguments, Modern MCP Server

Spring AI 2.1.0-RC2, released November 9, is the final release candidate before 2.1.0 GA. It targets Spring Boot 4.2.0-RC2 and the MCP Java SDK 2.2.0-M2. The main new abstraction is the DecisionModel API: a DecisionPrompt goes in and a typed DecisionResponse comes out, built from question types such as YesNoQuestion, ChoiceQuestion and ScoreQuestion. It gives classification, routing and LLM-as-judge code a structured contract instead of hand-parsed text. Implementations ship for OpenAI, Ollama and System One.

Other additions:

  • Tool-call argument streaming: tool arguments now stream as partial ToolCallParts, so UIs can show tool calls as they are built.
  • Pre-computed embedding search: added to the Neo4j, MongoDB Atlas, Typesense, MariaDB and Chroma vector stores.
  • Modern MCP server: support for the 2026-07-28 MCP specification, enabled with spring.ai.mcp.server.protocol=MODERN.
  • Anthropic Java SDK 3.4.0: a major upgrade from 2.72.0.

Check before upgrading. The Anthropic SDK 3.x defaults to API version 2026-10-21, which rejects temperature, topP and topK, in line with how newer Claude models such as Fable 5.2 handle sampling parameters. Spring AI now sends Anthropic API keys as Bearer tokens, and the way cached-token prompt usage is counted has changed, which can affect dashboards and cost tracking.

Read more — Spring Blog

Spring for Apache Pulsar Moves to the Apache Pulsar Project

On November 10, the Spring team announced that Spring for Apache Pulsar is leaving the Spring portfolio and becoming "Apache Pulsar support for Spring" under the Apache Pulsar project. Spring for Apache Pulsar 2.0.8 is the last release under the Spring umbrella, and Spring will no longer publish maintenance releases or fixes for it.

For users, the Spring Boot starter for Pulsar stays at 2.0.8 with Spring Boot 4.2.0, and the Spring Cloud Stream Pulsar binder moves into Spring Cloud Stream 5.1.0. To get future updates, applications must switch to the new Apache Maven coordinates and update package imports when the ASF releases them. Teams running Pulsar with Spring should plan that migration alongside their Boot 4.2 upgrade.

Read more — Spring Blog