Vllm Configuration

vLLM configuration

Target audience: operators deploying vLLM in production — datacenter GPUs, containerized, often inside networks that can't reach huggingface.co directly and need to use internal mirrors or fully offline caches.

Why this matters

vLLM's config surface is deceptively layered: CLI flags, a YAML --config file, VLLM_* env vars, and the HuggingFace / Transformers env vars it inherits transparently. The same setting can exist in three places, and the precedence ordering is not intuitive. Getting this wrong produces three classic failure modes:

1. First-boot network errors — operator pre-downloaded weights to a local path, but vLLM still hits huggingface.co for a revision check, a missing tokenizer file, or usage stats. The "local path" illusion is incomplete.
2. Env-var namespace collisions — a Kubernetes Service named vllm injects VLLM_SERVICE_HOST / VLLM_SERVICE_PORT into every pod, which silently overrides VLLM_HOST_IP / VLLM_PORT. vLLM's internal distributed init then uses the k8s cluster IP and breaks.
3. VLLM_HOST_IP as an API host — operators alias --host $VLLM_HOST_IP assuming symmetry with the API server. VLLM_HOST_IP is the internal inter-worker bind address, not the OpenAI-compat server host. Using it as the API host breaks TP/PP distributed init.

The fix in every case is understanding the layering. This skill teaches that layering, then gives the operator-facing knobs, then the air-gapped recipe.

Precedence, in one sentence

CLI arg > --config FILE.yaml > VLLM_* env var > library default.

So vllm serve /models/llama --config prod.yaml uses /models/llama even if prod.yaml sets model: meta-llama/Llama-3.1-8B. And VLLM_LOGGING_LEVEL=DEBUG is overridden by --log-level=INFO.

One non-obvious case: env vars that vLLM reads directly (like HF_HUB_OFFLINE) are consumed by the library layer, not the arg parser, so they aren't subject to this precedence — they gate behaviour unconditionally.

The YAML config file

Every CLI flag has a YAML equivalent. Keys use the same name, hyphens allowed or underscored. Booleans must be explicit (true/false, not YAML's loose yes/on).

# prod.yaml
model: /mnt/models/Llama-3.1-70B-Instruct
tensor-parallel-size: 4
gpu-memory-utilization: 0.9
max-model-len: 32768
dtype: bfloat16
enable-prefix-caching: true
served-model-name: llama-70b

# Nested composite configs — YAML dict becomes a JSON string on the CLI
speculative-config:
  model: nvidia/Llama-3.1-70B-Instruct-Eagle3
  num_speculative_tokens: 3
compilation-config:
  pass_config:
    fuse_allreduce_rms: true

vllm serve --config prod.yaml

Gotchas:

• list args become YAML sequences (allowed-origins: ["http://a", "http://b"]).
• Dict args (--kv-transfer-config, --speculative-config, --compilation-config) can be written as YAML dicts; the parser serializes to JSON before handing to the CLI layer.
• Older versions had a key-order bug (issue #8947) where served-model-name placed last could break parsing. Fixed in v0.10.1; only pre-v0.10.1 images are affected.
• trust_remote_code in YAML: trust-remote-code: true (explicit boolean).

For the full per-section catalog (ModelConfig, CacheConfig, ParallelConfig, SchedulerConfig, LoadConfig, LoRAConfig, SpeculativeConfig, ObservabilityConfig, FrontendArgs), see references/config-file.md.

Environment variables — the operator-facing subset

Full catalog in references/env-vars.md. The ones that matter most in production:

Storage / cache (persist these):

• VLLM_CACHE_ROOT (default ~/.cache/vllm) — Torch compile, Triton, XLA, assets. Mount on PVC; otherwise the compile tax is paid every pod restart.
• HF_HOME (default ~/.cache/huggingface) — HuggingFace hub cache. Preferred over the deprecated TRANSFORMERS_CACHE.
• HF_HUB_CACHE — subdir under HF_HOME, rarely set directly.

Air-gap control:

• HF_HUB_OFFLINE=1 — required in air-gapped mode. Without it, vLLM hits HF on every startup to check for newer revisions, even with a warm cache. (vLLM CI itself adopted HF_HUB_OFFLINE=1 to avoid this — issue #23451, closed 2025-11-26.)
• TRANSFORMERS_OFFLINE=1 — set both; some transformers-layer code paths honour only this one.
• HF_ENDPOINT=https://hf-mirror.com — redirect all HF traffic to a mirror. No trailing slash or it breaks.
• VLLM_USE_MODELSCOPE=true — route base-model downloads to ModelScope. Known gap: LoRA adapters still try HuggingFace. PR #13220 attempted the fix but was closed unmerged (2025-06-20); no upstream fix has landed. Workaround: download LoRA adapters manually, pass --lora-modules name=/local/path.
• HF_TOKEN — still required offline for gated repos (meta-llama/, google/gemma). vLLM validates access through the hub config layer before weight loading even when weights are local (issue #9255).

Telemetry (disable in air-gap):

• VLLM_NO_USAGE_STATS=1 or VLLM_DO_NOT_TRACK=1 or DO_NOT_TRACK=1 or touch $HOME/.config/vllm/do_not_track. Default endpoint is https://stats.vllm.ai. In air-gap, connection errors in logs result otherwise.

Networking (internal — read the warning below):

• VLLM_HOST_IP — internal inter-worker bind IP for distributed (TP/PP/DP) init. NOT the API server host. Use --host on the CLI for the API server.
• VLLM_PORT — internal base port for distributed; auto-increments for each worker. NOT the API server port. Use --port.

Server auth:

• VLLM_API_KEY — Bearer token for the OpenAI-compat server. Equivalent to --api-key on the CLI.

Long-context / safety overrides:

• VLLM_ALLOW_LONG_MAX_MODEL_LEN=1 — bypasses the model's max_position_embeddings sanity check. Footgun — usually means the rope scaling config doesn't match the served weights.
• VLLM_ALLOW_INSECURE_SERIALIZATION=1 — permits unsafe serialization formats over the wire. Never enable on multi-tenant.

Logging:

• VLLM_LOGGING_LEVEL=DEBUG|INFO|WARN|ERROR, VLLM_LOGGING_PREFIX="rank0: ", VLLM_CONFIGURE_LOGGING=0 (let the host app own config).

Air-gapped operation — the short recipe

Full recipe in references/air-gapped.md. The essentials:

1. Pre-stage on a connected host:

   # new hf CLI
   HF_HOME=/export/hf hf download meta-llama/Llama-3.1-70B-Instruct
   # or python
   python -c "from huggingface_hub import snapshot_download; \
     snapshot_download('meta-llama/Llama-3.1-70B-Instruct', local_dir='/export/models/llama-70b')"
   

2. Transfer the directory into the enclave (rsync, physical media, MinIO, whatever the security posture allows).

3. Pick one of three patterns:

   A. Fully offline with local path (simplest, recommended):

   export HF_HOME=/mnt/hf-cache
   export HF_HUB_OFFLINE=1
   export TRANSFORMERS_OFFLINE=1
   export VLLM_NO_USAGE_STATS=1
   vllm serve /mnt/models/llama-70b --tensor-parallel-size 8
   

   B. Internal HF mirror (reverse proxy):

   export HF_ENDPOINT=https://hf.internal.example.com   # NO trailing slash
   export HF_TOKEN=<internal-bot-token>                  # mirror may still gate
   vllm serve meta-llama/Llama-3.1-70B-Instruct --tensor-parallel-size 8
   

   C. ModelScope (Chinese / restricted networks):

   export VLLM_USE_MODELSCOPE=true
   vllm serve qwen/Qwen2-72B-Instruct --trust-remote-code --tensor-parallel-size 8
   

   Caveat: LoRA adapters historically still fetched from HF even with this flag.

4. For gated models, HF_TOKEN must be in the pod environment even when weights are local — it gates the config-validation path.

Critical pitfalls

1. Kubernetes Service named vllm poisons env vars. Kubernetes injects <SERVICE>_SERVICE_HOST / <SERVICE>_SERVICE_PORT into every pod in the namespace. A Service named vllm collides with vLLM's VLLM_ namespace and in some versions interferes. The vLLM docs explicitly warn against naming the service vllm. Name it vllm-api or inference instead.

2. HF_HUB_OFFLINE=1 needs the cache fully populated or first-request fails. Include config.json, tokenizer*, special_tokens_map.json, generation_config.json, and any modeling_*.py referenced by auto_map — not just weights.

3. Gated models offline still need HF_TOKEN. The token is consulted during hub-config validation before weight load. Putting it only on the staging host isn't enough; bake it into the runtime env.

4. trust_remote_code executes arbitrary Python from the model repo. Any model not in vLLM's hard-coded architecture registry falls through to AutoConfig + auto_map, which only runs with the flag. In air-gap this runs pre-staged code — treat model directory provenance as equivalent to running arbitrary binaries. Verify checksums.

5. TRANSFORMERS_CACHE is deprecated. Rename legacy scripts to use HF_HOME (and let HF_HUB_CACHE default). Setting the old var still works but emits FutureWarning.

6. YAML CLI positional beats config model:. vllm serve /local/path --config prod.yaml uses /local/path regardless of what prod.yaml says. Intentional but surprising.

7. Revision pinning: --revision applies to model weights; --tokenizer-revision is separate. Pinning the model to a commit but leaving the tokenizer floating lets it drift, and token counts change subtly. Pin both.

8. load-format dummy for profiling. Skips weight download entirely, materializes random weights. Useful for measuring startup / attention kernel perf in air-gap before weights arrive. Don't ship with this. Sibling flag --load-format fastsafetensors accelerates safetensors load via batched pread + NUMA-aware buffers (requires fastsafetensors>=0.2.2 + libnuma-dev; no env-var toggle exists in vLLM source).

9. Torch compile cache misses cost minutes per startup. Persist VLLM_CACHE_ROOT across pod restarts (PVC, hostPath, or a shared NFS mount). First warmup of a new model config rebuilds CUDA graphs and torch.compile artifacts; subsequent starts hit cache.

For more failure modes and the fuller troubleshooting flow (first-boot hang, revision-check failure detection, tokenizer mismatch diagnosis), see references/troubleshooting.md.

Verifying a configuration

Before shipping, sanity-check the effective config:

# Grep the startup log — vLLM prints the resolved EngineConfig in the first 200 lines
kubectl logs <pod> --tail=400 | grep -A 40 'EngineConfig\|EngineArgs'

# Hit /metrics to confirm prefix caching, block size, KV dtype are live
curl -s http://localhost:8000/metrics | grep -E 'prefix_cache|gpu_cache|kv_cache'

# Confirm the model path that was actually loaded
curl -s http://localhost:8000/v1/models | jq '.data[].id'

${CLAUDE_SKILL_DIR}/scripts/check-config.sh bundles these checks — run it against a staged server to verify the config landed as intended.

External references

• Env vars (authoritative): https://docs.vllm.ai/en/stable/configuration/env_vars/
• Engine args: https://docs.vllm.ai/en/latest/configuration/engine_args/
• Serve args + YAML config: https://docs.vllm.ai/en/latest/configuration/serve_args/
• CLI: https://docs.vllm.ai/en/stable/cli/
• HF integration design: https://github.com/vllm-project/vllm/blob/main/docs/design/huggingface_integration.md
• Air-gapped thread: https://discuss.vllm.ai/t/setting-up-vllm-in-an-airgapped-environment/916
• Offline discussion: https://github.com/vllm-project/vllm/discussions/1405
• Blog — Anatomy of vLLM: https://vllm.ai/blog/anatomy-of-vllm
• Sibling skills: vllm-caching (KV tiering + offload config), vllm-benchmarking (bench methodology + output JSON)