## Issue
Closes#4796
## Change
Current vLLM returns the reasoning text of its OpenAI-compatible chat
responses in a `reasoning` field rather than `reasoning_content`, and
it's not just vLLM. OpenAI's own guidance for serving gpt-oss recommends
`reasoning` on chat completions responses, which is what prompted the
vLLM rename (vllm-project/vllm#27755). OpenRouter and Groq already use
`reasoning`, and SGLang is planning the same migration
(sgl-project/sglang#18219). `reasoning_content` came from DeepSeek and
is still what their API, llama.cpp and others return, so clients
realistically need to read both names for the foreseeable future.
Right now the module only parses `reasoning_content`, so
`returnThinking(true)` quietly returns null thinking against any of the
`reasoning` backends. Setting `thinkingFieldName` doesn't help because
it only affects how thinking is serialized back into request messages,
not response parsing.
This adds `@JsonAlias("reasoning")` on the `reasoningContent` builder
setters of `AssistantMessage` and `Delta`, so both field names
deserialize into `reasoningContent`. Serialization is untouched,
requests on the wire don't change, and `reasoning_content` keeps working
as before. Same approach that was suggested in the review of #5018
before that PR was closed.
Besides the unit tests I verified both models manually against a live
vLLM v0.25.1 server running a Qwen3.5 reasoning model: before the change
`thinking()` is null for `OpenAiChatModel` and `onPartialThinking` never
fires for the streaming model, after the change both come through.
## General checklist
- [x] There are no breaking changes (API, behaviour)
- [x] I have added unit and/or integration tests for my change
- [x] The tests cover both positive and negative cases
- [x] I have manually run all the unit and integration tests in the
module I have added/changed, and they are all green
- [x] I have manually run all the unit and integration tests in the
[core](https://github.com/langchain4j/langchain4j/tree/main/langchain4j-core)
and
[main](https://github.com/langchain4j/langchain4j/tree/main/langchain4j)
modules, and they are all green
- [ ] I have added/updated the
[documentation](https://github.com/langchain4j/langchain4j/tree/main/docs/docs)
- [ ] I have added an example in the [examples
repo](https://github.com/langchain4j/langchain4j-examples) (only for
"big" features)
- [ ] I have added/updated [Spring Boot
starter(s)](https://github.com/langchain4j/langchain4j-spring) (if
applicable)
---------
Co-authored-by: Andrea Moccia <brokenlander@users.noreply.github.com>
4b5b6675c1