[Feature]: Enable Partial Guided Decoding / Structured Output Support

[HollowMan6]

🚀 The feature, motivation and pitch

In some use cases, we don’t need the entire output to strictly follow a defined grammar, but only a specific segment of the generation. It would be great to support partial guided decoding, where structured output is enforced only within defined token boundaries.

For example:

• Guided decoding begins at <json> and ends at </json>
• Or for markdown, it starts at ```json and ends at ```

This would make guided decoding more flexible and broadly applicable to tasks where only a partial defined structure is needed (e.g., code generation or agent tool).

Alternatives

Currently, vLLM supports a guided_grammar option that allows context-free grammar-based customization. However, using it for partial guided decoding introduces additional complexity and isn’t as intuitive for cases where a pre-defined structure (such as JSON) is only required in segments of the output.

Additional context

No response

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