The official vLLM guide recommends serving with --chat-template examples/tool_chat_template_gemma4.jinja for reasoning and tool use(Gemma 4 Usage Guide - vLLM Recipes). However, I’ve noticed a bug where reasoning is omitted during tool calls when this template is applied. Removing the option fixes the issue(v0.20.0). Is it safe to serve without this specific chat template, or is it strictly required for proper model behavior?
The official vLLM documentation recommends using --chat-template examples/tool_chat_template_gemma4.jinja for Gemma 4 to ensure correct reasoning and tool calling behavior, as this template is optimized for vLLM’s reasoning and tool call parsers. Serving without this template may work in some cases, but can lead to subtle bugs or degraded multi-turn tool use, since the default HuggingFace template may not handle all vLLM-specific requirements for reasoning and tool call boundaries, especially in multi-turn or streaming scenarios. See Gemma 4 vLLM Recipe and Gemma 4 Reasoning/Tool Use.
If you observe reasoning being omitted during tool calls with the recommended template, this is a known issue that has been discussed and patched in recent vLLM/Gemma4 updates (PR #39027, issue #39885). Using the latest vLLM main branch and ensuring your model’s chat template is up to date is advised. Serving without the template is not strictly required, but may result in unpredictable or unsupported behavior, especially for advanced features. Would you like a step-by-step breakdown of the bug and template interactions?
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