zai-org/GLM-5.3-Flash

Overview

GLM-5.3-Flash is a multimodal mixture-of-experts model with approximately 321B total parameters and 18B active parameters per token. Its 45-layer language model combines KDA linear-attention layers with NoPE sparse MLA layers, routes each token through 8 of 288 experts, and supports image and video inputs. The checkpoint declares a maximum context length of 1,048,576 tokens and includes one MTP draft layer.

The implementation supports NVIDIA Hopper and newer GPUs, and AMD Instinct gfx950 via ROCm. It can scale with tensor, pipeline, expert, or data+expert parallelism.

Prerequisites

• Weights: about 306 GiB for the default native FP8 checkpoint before runtime and KV-cache overhead; the BF16 variant requires roughly twice the weight memory
• vLLM: use docker before the integration is included in the public repo
• FlashInfer: 0.6.17 or newer is required for NoPE sparse MLA

The default model ID, zai-org/GLM-5.3-Flash, is the FP8 checkpoint. Select the BF16 variant to serve zai-org/GLM-5.3-Flash-BF16, or the NVFP4 variant to serve RedHatAI/GLM-5.3-Flash-NVFP4. Its MoE experts are quantized to 4-bit (NVFP4), and it requires NVIDIA Blackwell GPUs.

Launching the server

FP8 with TP4 and MTP on one GB200 tray

vllm serve zai-org/GLM-5.3-Flash \
  --tensor-parallel-size 4 \
  --kv-cache-dtype fp8 \
  --speculative-config '{"method":"mtp","num_speculative_tokens":5}' \
  --tool-call-parser glm47 \
  --reasoning-parser glm45 \
  --enable-auto-tool-choice \
  --served-model-name zai-org/GLM-5.3-Flash

Prefill/Decode disaggregation (single node, TP4 + TP4)

Split one 8-GPU node into a prefill pool (GPUs 0-3) and a decode pool (GPUs 4-7), bridged by NIXL KV transfer. The KDA conv-state and KV-cache layouts must be pinned identically on both pools, and MTP drafts run on both sides. On Blackwell you can add --kv-cache-dtype fp8 to both pools; Hopper does not support FP8 KV cache for this model and must run BF16 KV.

Note: num_speculative_tokens must be the same in prefill and decode instance

# Prefill pool
CUDA_VISIBLE_DEVICES=0,1,2,3 \
VLLM_SSM_CONV_STATE_LAYOUT=DS \
VLLM_KV_CACHE_LAYOUT=HND \
UCX_NET_DEVICES=all \
VLLM_NIXL_SIDE_CHANNEL_PORT=5557 \
vllm serve zai-org/GLM-5.3-Flash \
  --port 8001 \
  --tensor-parallel-size 4 \
  --kv-transfer-config '{"kv_connector":"NixlConnector","kv_role":"kv_producer"}' \
  --speculative-config '{"method":"mtp","num_speculative_tokens":5}' \
  --compilation-config '{"cudagraph_mm_encoder": true}' \
  --enforce-eager \
  --no-disable-hybrid-kv-cache-manager \
  --served-model-name zai-org/GLM-5.3-Flash

# Decode pool
CUDA_VISIBLE_DEVICES=4,5,6,7 \
VLLM_SSM_CONV_STATE_LAYOUT=DS \
VLLM_KV_CACHE_LAYOUT=HND \
UCX_NET_DEVICES=all \
VLLM_NIXL_SIDE_CHANNEL_PORT=5558 \
vllm serve zai-org/GLM-5.3-Flash \
  --port 8002 \
  --tensor-parallel-size 4 \
  --kv-transfer-config '{"kv_connector":"NixlConnector","kv_role":"kv_consumer"}' \
  --speculative-config '{"method":"mtp","num_speculative_tokens":5}' \
  --max-num-seqs 512 \
  --no-disable-hybrid-kv-cache-manager \
  --served-model-name zai-org/GLM-5.3-Flash

# Router
vllm-router --policy round_robin --vllm-pd-disaggregation \
  --prefill http://127.0.0.1:8001 \
  --decode http://127.0.0.1:8002 \
  --host 0.0.0.0 --port 8000 \
  --intra-node-data-parallel-size 1

Reasoning modes

Thinking is always on — the generation prompt opens a <think> block unconditionally. GLM-5.3-Flash offers three reasoning effort levels driven by the reasoning_effort field; the default is max:

Mode	How to request	Behavior
Think Max (default)	omit reasoning_effort, or set "max"	Deepest reasoning — hard math, multi-step planning, agentic tasks. Highest token cost.
Think High	"reasoning_effort": "high"	Balanced depth and latency.
Think Low	"reasoning_effort": "low"	Lightest reasoning — simple Q&A, lowest latency and token cost.

The chat template resolves effort to max unless reasoning_effort is explicitly "low" or "high" (any other value falls back to max), then injects Reasoning Effort: Low|High|Max into the system prompt. Pass it through chat_template_kwargs or the top-level OpenAI reasoning_effort field.

from openai import OpenAI

client = OpenAI(api_key="EMPTY", base_url="http://localhost:8000/v1")
msgs = [{"role": "user", "content": "Summarize sparse attention in one sentence."}]

# Think Max (default) — just omit reasoning_effort
client.chat.completions.create(model="zai-org/GLM-5.3-Flash", messages=msgs, max_tokens=4096)

# Think High / Think Low — explicitly request the effort level
client.chat.completions.create(
    model="zai-org/GLM-5.3-Flash",
    messages=msgs,
    max_tokens=4096,
    extra_body={"chat_template_kwargs": {"reasoning_effort": "low"}},
)

curl http://localhost:8000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "zai-org/GLM-5.3-Flash",
    "messages": [{"role": "user", "content": "Summarize sparse attention in one sentence."}],
    "temperature": 1,
    "max_tokens": 4096,
    "chat_template_kwargs": {"reasoning_effort": "high"}
  }'

Client usage

from openai import OpenAI

client = OpenAI(api_key="EMPTY", base_url="http://localhost:8000/v1")
response = client.chat.completions.create(
    model="zai-org/GLM-5.3-Flash",
    messages=[{"role": "user", "content": "Summarize sparse attention in one sentence."}],
    temperature=1.0,
    max_tokens=256,
)
print(response.choices[0].message.content)

Multimodal input (image / video)

Standard OpenAI multi-part content — list image_url / video_url items alongside the text. The chat template expands each into its <|begin_of_image|>… / <|begin_of_video|>… placeholder tokens, and the same image token is reused for video frames (frame spans are delimited by the video start/end tokens).

from openai import OpenAI
client = OpenAI(api_key="EMPTY", base_url="http://localhost:8000/v1")

# Image
client.chat.completions.create(
    model="zai-org/GLM-5.3-Flash",
    messages=[{
        "role": "user",
        "content": [
            {"type": "image_url", "image_url": {"url": "https://example.com/image.png"}},
            {"type": "text", "text": "Describe the image."},
        ],
    }],
    max_tokens=512,
)

from openai import OpenAI
client = OpenAI(api_key="EMPTY", base_url="http://localhost:8000/v1")

# Video
client.chat.completions.create(
    model="zai-org/GLM-5.3-Flash",
    messages=[{
        "role": "user",
        "content": [
            {"type": "video_url", "video_url": {"url": "https://example.com/clip.mp4"}},
            {"type": "text", "text": "What happens in this video?"},
        ],
    }],
    max_tokens=512,
)

# Image
curl http://localhost:8000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "zai-org/GLM-5.3-Flash",
    "messages": [{
      "role": "user",
      "content": [
        {"type": "image_url", "image_url": {"url": "https://example.com/image.png"}},
        {"type": "text", "text": "Describe the image."}
      ]
    }],
    "max_tokens": 512
  }'

# Video
curl http://localhost:8000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "zai-org/GLM-5.3-Flash",
    "messages": [{
      "role": "user",
      "content": [
        {"type": "video_url", "video_url": {"url": "https://example.com/clip.mp4"}},
        {"type": "text", "text": "What happens in this video?"}
      ]
    }],
    "max_tokens": 512
  }'

Running on MI355X (gfx950)

The vllm/vllm-openai-rocm:glm53-flash docker image gates on gfx950 only. https://github.com/vllm-project/vllm/pull/53906 will add support for mi300x/mi325x.

export VLLM_ROCM_USE_AITER=1
vllm serve zai-org/GLM-5.3-Flash \
  --served-model-name zai-org/GLM-5.3-Flash \
  --tensor-parallel-size 4 \
  --max-num-seqs 512 \
  --tool-call-parser glm47 \
  --reasoning-parser glm45 \
  --enable-auto-tool-choice \
  --attention-backend ROCM_AITER_MLA_SPARSE

At TP=4 the FP8 checkpoint reports a 14.92M-token KV pool and ~113.81x max concurrency at 128K context; the BF16 checkpoint reports an 8.87M-token KV pool at the same TP and context.

Note: MTP speculative decoding is not supported on ROCm with this image.

Benchmarking

vllm bench serve \
  --backend vllm \
  --model zai-org/GLM-5.3-Flash \
  --served-model-name zai-org/GLM-5.3-Flash \
  --dataset-name random \
  --random-input-len 8192 \
  --random-output-len 1024 \
  --max-concurrency 16 \
  --num-prompts 64

Troubleshooting

• Sparse-MLA initialization error: verify that the image contains FlashInfer 0.6.18 or newer.

References

• Model card
• vLLM