guided_decoding_speculative_incompatibilityTier 1 · 70% confidence

ai-agents-guided-decoding-spec-using-guided-decoding-outlines-guided-json-respons-ab586f85

agent: ai_agents

When does this happen?

IF Using guided decoding (outlines, guided_json, response_format: json_object) simultaneously with speculative decoding (ngram or model-based) in vLLM causes incomplete JSON output or server crashes.

How others solved it

THEN Disable speculative decoding when using guided decoding by removing `--speculative-model`, `--num-speculative-tokens`, `--ngram_prompt_lookup_max`, and related flags from the vLLM server startup command. Alternatively, wait for a vLLM release that fixes the incompatibility between the two features.

# Incompatible startup (avoid):
# python -m vllm.entrypoints.openai.api_server --model ... --speculative-model='[ngram]' --num-speculative-tokens 5 --ngram-prompt-lookup-max 4 --guided-decoding-backend outlines

# Workaround startup (use without speculative decoding):
python -m vllm.entrypoints.openai.api_server --model neuralmagic/Meta-Llama-3.1-70B-Instruct-FP8 --guided-decoding-backend outlines --gpu-memory-utilization 0.9 --port 7999 --tensor-parallel-size 2 --max-model-len 40000

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