ClassCastException: MessageDeltaEvent cannot be cast to ToolUseAggregationEvent in StreamHelper.mergeToolUseEvents when using Anthropic streaming with tools

[magnusaxelqvist]

Bug Description

When using Spring AI with Anthropic Claude and MCP tools in streaming mode, a ClassCastException occurs preventing tool execution.

Stack Trace

java.lang.ClassCastException: class org.springframework.ai.anthropic.api.AnthropicApi$MessageDeltaEvent cannot be cast to class org.springframework.ai.anthropic.api.AnthropicApi$ToolUseAggregationEvent
at org.springframework.ai.anthropic.api.StreamHelper.mergeToolUseEvents(StreamHelper.java:86)
at reactor.core.publisher.MonoReduceSeed$ReduceSeedSubscriber.onNext(MonoReduceSeed.java:116)
...

Environment

• Spring AI: 1.0.2, 1.1.0-M1 (both affected)
• Spring Boot: 3.5.6
• MCP SDK: 0.13.0
• Tools: MCP Gmail integration

Reproduction

1. Configure Anthropic ChatClient with MCP tools
2. Call .stream() on ChatClient with tool-enabled prompt
3. ClassCastException occurs when tool execution is attempted

Expected Behavior

Tools should execute without ClassCastException during streaming

Actual Behavior

ClassCastException prevents all tool execution in streaming mode

Workaround

None found. Switching to .call() (non-streaming) might work but defeats the purpose.

[jonathannaguin]

@ilayaperumalg I also encountered the same problem while using Bedrock. It does not happen every single time but occasionally I see this:

java.lang.ClassCastException: class software.amazon.awssdk.services.bedrockruntime.model.conversestreamoutput.DefaultContentBlockDelta cannot be cast to class org.springframework.ai.bedrock.converse.api.ConverseApiUtils$ToolUseAggregationEvent (software.amazon.awssdk.services.bedrockruntime.model.conversestreamoutput.DefaultContentBlockDelta and org.springframework.ai.bedrock.converse.api.ConverseApiUtils$ToolUseAggregationEvent are in unnamed module of loader 'app')
	at org.springframework.ai.bedrock.converse.api.ConverseApiUtils.mergeToolUseEvents(ConverseApiUtils.java:263)
	at reactor.core.publisher.MonoReduceSeed$ReduceSeedSubscriber.onNext(MonoReduceSeed.java:116)
	at reactor.core.publisher.FluxWindowPredicate$WindowFlux.drainRegular(FluxWindowPredicate.java:670)
	at reactor.core.publisher.FluxWindowPredicate$WindowFlux.drain(FluxWindowPredicate.java:748)
	at reactor.core.publisher.FluxWindowPredicate$WindowFlux.onNext(FluxWindowPredicate.java:790)
	at reactor.core.publisher.FluxWindowPredicate$WindowPredicateMain.onNext(FluxWindowPredicate.java:268)
	at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:122)
	at reactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.onNext(FluxContextWriteRestoringThreadLocals.java:118)
	at reactor.core.publisher.SinkManyEmitterProcessor.drain(SinkManyEmitterProcessor.java:476)
	at reactor.core.publisher.SinkManyEmitterProcessor.tryEmitNext(SinkManyEmitterProcessor.java:273)
	at reactor.core.publisher.SinkManySerialized.tryEmitNext(SinkManySerialized.java:100)
	at reactor.core.publisher.InternalManySink.emitNext(InternalManySink.java:27)
	at org.springframework.ai.bedrock.converse.BedrockProxyChatModel.lambda$converseStream$17(BedrockProxyChatModel.java:735)

[jonathannaguin]

I am getting this issue while using Bedrock with some models that use <thinking> while streaming.

Example of one invocation that failed with this error, from a breakpoint:

previousEvent:

ContentBlockDeltaEvent(Delta=ContentBlockDelta(Text=<thinking), ContentBlockIndex=2)

event:

ContentBlockDeltaEvent(Delta=ContentBlockDelta(Text=>), ContentBlockIndex=2)