AzureOpenAIChatModel reordering bug when streaming

[robinmayerhofer]

Bug description

Streamed chunks might end up in a wrong order:

data: <A>
data: <B>

can end up as <B> => <A>

Environment

• Spring AI 1.0.1
• Java 21

Steps to reproduce
It's enough to inject the default AzureOpenAIChatModel, build a ChatClient and call

val chatClient = ChatClient.builder(chatModel).build()

chatClient
    .prompt(createPrompt(simpleResponseDeploymentName))
    .stream()
    .content()
    .collectList()
    .block()

On a local machine I need many sequential runs to trigger the issue, thus, a setup with WireMock is advisable.

Expected behavior
The response chunks should be ordered correctly.

Minimal Complete Reproducible example
Repo with test setup: https://github.com/robinmayerhofer/spring-ai-azure-openai-streaming-issue

[robinmayerhofer]

I have a 99% suspicion that these two flatMap operations cause this:

• spring-ai/models/spring-ai-azure-openai/src/main/java/org/springframework/ai/azure/openai/AzureOpenAiChatModel.java

  Line 354 in e5e2f45

  .flatMap(mono -> mono);

• spring-ai/models/spring-ai-azure-openai/src/main/java/org/springframework/ai/azure/openai/AzureOpenAiChatModel.java

  Line 380 in e5e2f45

  return chatResponseFlux.flatMap(chatResponse -> {

flatMap does not guarantee ordering and by evaluating the thread and I can see that this runs on boundedElastic by checking the thread name.
Moreover, AzureOpenAiChatModel also explicitly uses Schedulers.boundedElastic() for tool calls:

spring-ai/models/spring-ai-azure-openai/src/main/java/org/springframework/ai/azure/openai/AzureOpenAiChatModel.java

Line 406 in e5e2f45

}).subscribeOn(Schedulers.boundedElastic());

Using flatMapSequential fixes this behavior, at least I cannot trigger it anymore with my setup.

[robinmayerhofer]

The GitHub Action run on my reproduction repo shows that the issue can be triggered: https://github.com/robinmayerhofer/spring-ai-azure-openai-streaming-issue/actions/runs/16973297866/job/48115646716

> Task :test

SpringAiAzureOpenaiStreamingIssueApplicationTests > tool call response scenario - order should not be messed up(RepetitionInfo) > repetition 21 of 20000 FAILED
    org.opentest4j.AssertionFailedError at SpringAiAzureOpenaiStreamingIssueApplicationTests.kt:106

SpringAiAzureOpenaiStreamingIssueApplicationTests > simple response - order should not be messed up(RepetitionInfo) > repetition 18863 of 20000 FAILED
    org.opentest4j.AssertionFailedError at SpringAiAzureOpenaiStreamingIssueApplicationTests.kt:84

PR with the fix: #4156