this comment is downvoted but it is correct. while fill-in-the-middle is less obvious in the decoder-only paradigm, it is still possible. one example is code-llama https://ai.meta.com/blog/code-llama-large-language-model-cod... it is a variant of llama 2 (GPT-style decoder-only) but it supports infilling

> [W]e split training documents at the character level into a prefix, a middle part[,] and a suffix with the splitting locations sampled independently from a uniform distribution over the document length. We apply this transformation with a probability of 0.9 and to documents that are not cut across multiple model contexts only. We randomly format half of the splits in the prefix-suffix-middle (PSM) format and the other half in the compatible suffix-prefix-middle (SPM) format described in Bavarian et al. (2022, App. D). We extend Llama 2’s tokenizer with four special tokens that mark the beginning of the prefix, the middle part or the suffix, and the end of the infilling span

Tokens are super powerful :)