Qwen3.8-27B-4bit

Qwen3.8-27B, 4-bit, with its draft head

Qwen3.8-27B quantized to four bits for MLX, carrying the model's own multi-token prediction head in mtp/. Speculative decoding therefore works from this repository alone -- nothing else to fetch, no environment variable pointing somewhere else.

Generation runs 1.8-2.3x faster with the head on, and says the same thing. Every token it proposes is checked by the model itself, so the reply is the model's own either way; the head only saves passes over the weights.

Speed

Measured on a 48 GB MacBook Pro (M4 Pro), greedy decoding, 96 tokens, with gbx_lm. Decode is timed from the first token, so prefill is not in it.

Requirements

These weights are held resident, not paged: on a 512 GB Mac Studio the model settles at about 16 GB, and a machine needs room for that much plus the conversation's cache.

Install

# upgrading? clear the previous version's unpack directory first
rm -rf ~/.libra/cache/onefile/gbx_lm

curl -fL -o gbx_lm-darwin-arm64.tar.gz 'https://github.com/GreenBitAI/gbx-lm/releases/latest/download/gbx_lm-darwin-arm64.tar.gz' \
  && tar -xzf gbx_lm-darwin-arm64.tar.gz gbx_lm \
  && mkdir -p "$HOME/.local/bin" \
  && mv gbx_lm "$HOME/.local/bin/gbx_lm" \
  && chmod +x "$HOME/.local/bin/gbx_lm"

gbx_lm -h

The build is signed with a Developer ID and notarised, so macOS runs it without the usual detour for a downloaded binary.

command not found -- $HOME/.local/bin is not on your PATH:

echo 'export PATH="$HOME/.local/bin:$PATH"' >> ~/.zshrc && source ~/.zshrc     # zsh
echo 'export PATH="$HOME/.local/bin:$PATH"' >> ~/.bash_profile && source ~/.bash_profile   # bash

Killed: 9 -- a previous version's files are still in the unpack directory, and macOS refuses to mix two builds. Run the rm -rf line above, then try again.

Run

gbx_lm --model GreenBitAI/Qwen3.8-27B-4bit

# the draft head is off unless asked for, and found in `mtp/` without a path
GBX_QWEN35_MTP=on gbx_lm --model GreenBitAI/Qwen3.8-27B-4bit

That serves an OpenAI-compatible API on port 11688, which is its default. The weights download on first use into ~/.libra/cache/models; set HF_HOME to put them elsewhere, and HF_TOKEN if you meet the Hub's rate limits for anonymous downloads.

curl http://127.0.0.1:11688/v1/chat/completions \
  -H 'Content-Type: application/json' \
  -d '{"model":"GreenBitAI/Qwen3.8-27B-4bit","messages":[{"role":"user","content":"Hello"}]}'

From the command line

From gbx_lm v0.7.1, the binary also runs the model without a server -- one prompt, or an interactive chat:

GBX_QWEN35_MTP=on gbx_lm generate --model GreenBitAI/Qwen3.8-27B-4bit --prompt "Hello" --max-tokens 2048
GBX_QWEN35_MTP=on gbx_lm chat --model GreenBitAI/Qwen3.8-27B-4bit --max-tokens 2048

gbx_lm generate -h and gbx_lm chat -h list their options. Keep --max-tokens generous: the default is 100 for generate and 256 for chat, and the model's reasoning before it answers counts against it. Leave out GBX_QWEN35_MTP=on to run without the draft head.

Coding agents

The server speaks three wire protocols on the same port, so the tools that expect a hosted API can be pointed at this one:

Codex -- a provider in ~/.codex/config.toml:

[model_providers.gbx]
name = "gbx-lm"
base_url = "http://127.0.0.1:11688/v1"
wire_api = "responses"

and a profile in ~/.codex/gbx.config.toml:

model_provider = "gbx"
model = "GreenBitAI/Qwen3.8-27B-4bit"
model_context_window = 262144

Claude Code -- ~/.claude/gbx.settings.json:

{
  "env": {
    "ANTHROPIC_BASE_URL": "http://127.0.0.1:11688",
    "ANTHROPIC_AUTH_TOKEN": "local",
    "ANTHROPIC_MODEL": "GreenBitAI/Qwen3.8-27B-4bit",
    "ANTHROPIC_DEFAULT_HAIKU_MODEL": "GreenBitAI/Qwen3.8-27B-4bit"
  }
}

Both clients ask for a small model for their own background work, so every name in the settings has to be one this server is serving.

What is in here

mtp/mtp.safetensors is built from the draft head Qwen/Qwen3.8-27B ships under mtp., quantized to match these weights. Apache 2.0, as the original is.