Step-3.5-Flash-MTP-GGUF

Step-3.5-Flash MTP GGUF

Experimental Step-3.5-Flash MTP GGUF builds based on stepfun-ai/Step-3.5-Flash.

These GGUFs were originally published for a same-GGUF experimental fork, but Step-3.5 MTP support has now landed in upstream llama.cpp via ggml-org/llama.cpp#23274. Current upstream llama.cpp uses a separate draft/MTP GGUF through --spec-type draft-mtp and -md.

Important packaging note: the main quants in this repo are combined target+MTP files from the experimental fork era. Upstream can load them as target models, but the embedded MTP tensors are not what upstream uses for speculative decoding once -md is supplied. For upstream llama.cpp, the cleaner native layout is a target-only GGUF plus a separate draft-only MTP GGUF. The combined files are kept here for compatibility and reproducibility, not because upstream needs the target and draft tensors bundled together.

The old tnhnyzc/llama.cpp fork remains useful as historical context for the same-GGUF implementation, but the recommended path is now upstream llama.cpp.

Files

The model files are split into folders. Choose one quant folder and load the first shard; the split-aware llama.cpp loader will find the sibling shards.

• Step-3.5-Flash-MTP-IQ4_XS-3.90BPW-Q8_MTP/
• Step-3.5-Flash-MTP-IQ3_S-3.64BPW-Q8_MTP/
• Step-3.5-Flash-MTP-IQ3_XXS-3.27BPW-Q8_MTP/

In these files, the MTP / nextn tensors are kept Q8_0. The public metadata reports step35.nextn_predict_layers = 1.

For upstream llama.cpp MTP, use the matching draft-only GGUF with -md. The draft-only file contains the MTP/draft tensors needed by upstream's separate draft-model path. If you make fresh upstream-native quants, you should not need to include the MTP tensors in the target GGUF.

The calibration imatrix is Bartowski's stepfun-ai_Step-3.5-Flash-imatrix.gguf from bartowski/stepfun-ai_Step-3.5-Flash-GGUF. The IQ4_XS variant follows the public AesSedai Step-3.5-Flash IQ4_XS expert layout. The IQ3_S and IQ3_XXS variants are smaller custom expert layouts.

Usage

Build current upstream llama.cpp, then run the target GGUF and the draft-only GGUF as separate files:

./build/bin/llama-server \
  --model /path/to/target/first-shard-or-gguf.gguf \
  -md /path/to/Step-3.5-Flash-*-draft-only.gguf \
  --spec-type draft-mtp \
  --spec-draft-n-max 3 \
  --spec-draft-p-min 0.65 \
  --ctx-size 131072 \
  -ctk q8_0 -ctv q8_0 \
  -ngl 99 \
  -np 1 \
  -b 4096 \
  -ub 1024 \
  -fa on \
  --cache-prompt \
  --cache-ram 8192

Add normal sampler/server args as needed.

The old fork flags were -mtp --draft 1; upstream uses --spec-type draft-mtp --spec-draft-n-max ... -md ... instead.

Notes

• Tested locally on Apple M3 Max / Metal with upstream llama.cpp b9490-3571fa543 after #23274 was merged.
• Recommended upstream starting point: --spec-type draft-mtp --spec-draft-n-max 3 --spec-draft-p-min 0.65 -md /path/to/draft-only.gguf.
• In one local upstream server smoke with the IQ3_XXS target and draft-only MTP GGUF, the server reported 32.86 t/s generation and 50/60 draft tokens accepted (83.3% acceptance) for a 128-token completion.
• The target KV cache worked with -ctk q8_0 -ctv q8_0. In this smoke, setting draft KV to Q8_0 with -ctkd q8_0 -ctvd q8_0 hit an upstream assert, so keep the draft KV cache at its default f16 unless you have tested otherwise.
• The published files report one trained nextn layer. Higher --spec-draft-n-max values reuse that layer recurrently.
• Treat the numbers as directional. Context length, sampler settings, cache reuse, memory pressure, and host load can move them.

See the GitHub README for current limitations and implementation details.