[Feature]: Support non-standard GGUF quant type prefixes (e.g. Unsloth Dynamic UD-IQ1_S )

[injaeryou]

🚀 The feature, motivation and pitch

GGUF models with non-standard quant type prefixes like Unsloth Dynamic 2.0 (UD-) cannot be loaded via repo_id:quant_type format.

vllm serve unsloth/Qwen3-0.6B-GGUF:UD-IQ1_S --tokenizer Qwen/Qwen3-0.6B

huggingface_hub.errors.HFValidationError: Repo id must use alphanumeric chars, '-', '_' or '.'. The name cannot start or end with '-' or '.' and the maximum length is 96: 'unsloth/Qwen3-0.6B-GGUF:UD-IQ1_S'.

Currently, is_remote_gguf() validates quant types against GGMLQuantizationType members and a hardcoded suffix list (_M, _S, _L, etc.). Prefixed types like UD-IQ1_S are rejected, and the model string falls through to HuggingFace Hub as a plain repo ID.

Since quant_type is only used for glob file matching (*-{quant_type}.gguf) and not for actual quantization logic (read from GGUF binary headers), accepting non-standard prefixed names is safe.

Alternatives

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Additional context

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[ianliuy]

Hey! Thanks for filing this it's a real issue that affects anyone using Unsloth Dynamic or other vendor-prefixed GGUF quants.

Just a heads-up: this is the same underlying problem as #39198, where UD-Q4_K_XL hits the exact same HFValidationError. I have a fix up in PR #39470 that should cover your case too.

What's happening:
When you run unsloth/Qwen3-0.6B-GGUF:UD-IQ1_S, vLLM tries to validate UD-IQ1_S as a GGUF quant type. It doesn't recognize it (because of the UD- prefix), so it never strips the : from the model string. The full unsloth/Qwen3-0.6B-GGUF:UD-IQ1_S then gets passed to HuggingFace as a repo ID, which rejects the : character.

How #39470 fixes it:
GGML quant type names never contain hyphens only underscores and alphanumerics. So a - is a reliable signal of a vendor prefix. The fix updates is_valid_gguf_quant_type() to strip the prefix and validate the remainder:

UD-IQ1_S strip UD- IQ1_S strip suffix _S IQ1 valid

Because the fix is inside is_valid_gguf_quant_type() itself, all downstream functions (is_remote_gguf(), split_remote_gguf(), etc.) automatically support any vendor-prefixed quant type not just UD-.