This isn’t entirely surprising. Language-model “reasoning” is basically the model internally exploring possibilities in token-space. These models are trained on enormous multilingual datasets and optimized purely for next-token prediction, not language purity. When reasoning traces or scratchpads are revealed directly (as OpenAI occasionally does with o-series models or DeepSeek-R1-zero), it’s common to see models slip into code-switching or even random language fragments, simply because it’s more token-efficient in their latent space.

For example, the DeepSeek team explicitly reported this behavior in their R1-zero paper, noting that purely unsupervised reasoning emerges naturally but brings some “language mixing” along. Interestingly, they found a small supervised fine-tuning (SFT) step with language-consistency rewards slightly improved readability, though it came with trade-offs (DeepSeek blog post).

My guess is OpenAI has typically used a smaller summarizer model to sanitize reasoning outputs before display (they mentioned summarization/filtering briefly at Dev Day), but perhaps lately they’ve started relaxing that step, causing more multilingual slips to leak through. It’d be great to get clarity from them directly on whether this is intentional experimentation or just a side-effect.

[1] DeepSeek-R1 paper that talks about poor readability and language mixing in R1-zero’s raw reasoning https://arxiv.org/abs/2501.12948

[2] OpenAI “Detecting misbehavior in frontier reasoning models” — explains use of a separate CoT “summarizer or sanitizer” before showing traces to end-users https://openai.com/index/chain-of-thought-monitoring/