Enhanced Qwen 3.8 Chat Templates for vLLM

⚠️ vLLM only. These templates rely on the from_json filter, which vLLM provides but plain Jinja2 and minijinja (llama.cpp, LM Studio, MLX) don't. If you need something engine-agnostic, use froggeric/Qwen-Fixed-Chat-Templates instead.

Quick start

• --tool-call-parser qwen3_xml matches the XML format these templates emit (<tool_call> → <function=name> → <parameter=x>). Stuck on an older vLLM build? Fall back to qwen3_coder.
• --reasoning-parser qwen3 is required. The generation prompt pre-fills <think>\n, so the model's output starts mid-thought with no opening tag. This parser handles that case and splits reasoning_content from content, which is also what the template needs fed back to it on later turns.
• qwen3_xml leans on your tool JSON schemas for type coercion, so keep those parameters schemas honest. Loose schemas mean arguments come back as strings.

What gets fixed

Ported from allanchan339/vLLM-Qwen3-3.5-3.6-chat-template-fix (qwen3.6-enhanced.jinja)

1. JSON-string tool arguments get parsed instead of crashing the server. The official template assumes tool_call.arguments is a mapping and dies with Can only get item pairs from a mapping when an OpenAI-style client echoes back stringified arguments, which most of them do. JSON strings are parsed here into proper <parameter> blocks, nested objects included, so multi-turn history always re-renders in the trained format.
2. Self-healing </think> closure. If echoed history contains a dangling <think> before a <tool_call>, the closing tag gets inserted where it belongs. Tool calls never end up trapped inside reasoning.
3. Inline <think>...</think> extraction. Some clients stuff reasoning into message.content instead of reasoning_content. Rather than double-wrapping it, the template pulls it out and re-renders it canonically.
4. tojson(ensure_ascii=False) for tool definitions and argument values, so CJK text stays readable instead of turning into \uXXXX escapes.
5. Extra agentic rules, appended after the official <IMPORTANT> block rather than replacing it: close </think> before tool calls, prefer tools over answering from memory, reuse ids with update_* tools, keep user-provided strings verbatim, write valid JSON for object and array parameters. There's a worked follow-up-update example too.

Merged from unsloth/Qwen3.8-27B-NVFP4's shipped template

1. developer role support. Leading system and developer messages are accepted and merged into a single system block. Harnesses like Codex won't work without this.
2. reasoning_effort: "high" aliased to xhigh instead of throwing. OpenAI proxies routinely send "high".
3. The no-user-query exception is gone. System-only and tool-continuation prompts from agentic harnesses no longer crash the render.
4. A clear exception when a tool call is missing its function name, rather than a cryptic failure three layers down.

Kept as-is from the official Qwen 3.8 template

• reasoning_effort steering (xhigh default, medium, low) with the official instruction wording, word for word.
• preserve_thinking defaults to true. Qwen 3.8 was trained with retained reasoning context; stripping it by default would fight the model.
• The official tool-instruction wording as the skeleton, since that is the exact phrasing the model was trained against.
• Official </tool_response><|im_end|> spacing and scalar argument rendering (true, null, 5 via tojson; strings verbatim).
• Always-closed <think> blocks, even when reasoning is empty. That's the trained format, empty or not.

What the -extra variant adds

1. Tolerant think-tag extraction. Beyond the canonical tags, it recognizes <thinking>...</thinking>, </ think>, </think >, content that starts with </think>, and bare reasoning followed by </think>. These are the artifacts you get when a client echoes raw output from the prefilled <think>\n generation prompt. Every variant gets normalized back to a clean <think>...</think> on re-render.
2. message.thinking accepted as an alias of message.reasoning_content. A handful of clients use it; reasoning_content still wins when both are present.
3. preserve_reasoning accepted as an alias of preserve_thinking, matching llama.cpp's --reasoning-preserve naming. Takes precedence if both are passed.

What was deliberately left out

• Rewritten tool instructions forbidding conversational text before tool calls. The official template, and the training behind it, explicitly allows it.
• Tool-response mutation. Injecting "⚠️ SYSTEM WARNING" messages into tool results based on keyword heuristics false-positives easily (a legitimate result containing error: 0 trips it) and puts the whole conversation off-distribution.
• In-template payload truncation (max_tool_arg_chars / max_tool_response_chars). History truncation belongs in the client, where it can happen without silently corrupting replayed turns.
• Raw string-argument passthrough. Dumping unparsed JSON blobs into <function> blocks teaches the model to imitate malformed history. These templates parse instead.
• Omitting empty <think> blocks. The official format always emits the wrapper for preserved turns, so these templates do too.
• tool_call_format="json". Qwen 3.8 is trained on the XML format. Use qwen3_xml.

Template kwargs reference

Credits

License