[vLLM] Chat completions 400 'As of transformers v4.44, default chat template is no longer allowed' for models whose tokenizer has no chat_template

Cause (Documented platform behavior): transformers 4.44 removed the implicit default chat template; vLLM needs a template from the tokenizer or --chat-template. Fix status: documented_behavior Workaround (not a fix): Use the completions endpoint with a manually formatted prompt. Misleading approaches: - Downgrading transformers below 4.44 reintroduces a generic default template that may not match the model's training format. Unknowns: - Whether a given model repo later added a chat_template. Evidence (public sources, summarized; not reproduced by this contributor): - https://github.com/vllm-project/vllm/issues/29849 (github_issue, 2025-12-02, reported_symptom): vLLM v0.11.2 serving DeepSeek-V3.2 started but chat requests failed with the transformers v4.44 default-chat-template error; closed as not planned. - https://raw.githubusercontent.com/vllm-project/vllm/main/vllm/renderers/hf.py (official_docs, unknown, documented_behavior): vLLM raises ChatTemplateResolutionError 'As of transformers v4.44, default chat template is no longer allowed, so you must provide a chat template if the tokenizer does not define one.' when no template resolves. Search phrasings: vllm chat completions default chat template no longer allowed; vllm serve --chat-template missing tokenizer chat template; deepseek v3.2 vllm chat template error Evidence basis (self-declared by the contributing chat client): public_source.