LocalAI/backend/go/vllm-cpp

vllm-cpp backend

LocalAI text-generation backend for vllm.cpp, the LocalAI-team C++20 port of vLLM (paged KV cache, continuous batching, safetensors + GGUF loading, CUDA / CPU / Metal / Vulkan) with no Python at inference time.

The backend dlopens the engine's stable C ABI (libvllm, include/vllm.h, ABI v2) through purego:

• Load -> vllm_engine_load: accepts a .gguf file or a HF-style model directory (config.json + safetensors). context_size maps to max_model_len; options: ["block_size:<n>", "num_blocks:<n>", "max_num_seqs:<n>"] size the KV cache and scheduler admission.
• Predict -> vllm_complete (blocking).
• PredictStream -> vllm_complete_stream; concurrent gRPC requests batch continuously in the engine's shared AsyncLLM scheduler.
• Chat / tool calling rides the SAME code path as the llama.cpp autoparser: with use_tokenizer_template: true the backend implements PredictRich/PredictStreamRich over the ABI v3 chat entry points (vllm_chat / vllm_chat_stream). The ENGINE applies the model's chat template (GGUF tokenizer.chat_template or tokenizer_config.json), decides when a tool call engages (tool_choice: auto lowers to a LAZY structural-tag decode constraint; required/named force one), parses tool calls with its streaming Hermes-style parser, and the backend maps each chat.completion.chunk onto ChatDelta/ToolCallDelta protos.
• Without structured messages the plain path applies: PredictOptions.Grammar -> the ABI's structured_grammar (GBNF) for LocalAI's Go-side grammar-constrained tool calling; JSON-schema / regex / choice constraints are also exposed by the ABI.

Model config example:

name: qwen3-vllm
backend: vllm-cpp
context_size: 8192
parameters:
  model: Qwen3-4B   # model dir (safetensors) or .gguf file
options:
- max_num_seqs:16

Apple Silicon: the MLX GEMM provider (ON by default, gated to prefill)

BUILD_TYPE=metal builds vllm.cpp's MLX provider for the dense GEMM (VLLM_CPP_MLX=on, the default here). It is on because upstream now SHAPE-GATES it to prefill; it was briefly off in this branch's history, and that was correct at the time for an ungated provider.

The gate matters more than the flag. MLX's steel GEMM wins prefill but loses decode, because the provider pays an mx::eval synchronisation plus an output memcpy on every call and decode makes ~112 calls per token. Measured on an Apple M4, Qwen3-1.7B-bf16 warm at p=512 g=128:

configuration	prefill TTFT	warm throughput
MLX gated to prefill (pin >= 89c46aeb)	524.5 ms	24.37 tok/s, 97.6% of MLX-LM
MLX ungated (older pins)	537 ms	12.7 tok/s
MLX off	602 ms	23.9 tok/s, 95.9%

Ratios are against an MLX-LM baseline measured INTERLEAVED with ours over four ABBA blocks (its spread 0.34%, ours 0.12%). An earlier revision of this file claimed 99.1%; that used a two-run MLX-LM baseline containing an outlier and overstated us by about 1.5 points.

VLLM_CPP_VERSION and this flag are coupled. Moving the pin back before 89c46aeb while leaving VLLM_CPP_MLX=on would take the middle row — roughly half throughput. If you roll the pin back, roll the default back with it.

One caveat: MLX's GEMM is not bit-identical to the native kernel, so an MLX build produces a different greedy sequence than a non-MLX one. That is a property of the provider, not of the gate, and it predates this packaging. Full disposition in vllm.cpp docs/BENCHMARKS.md.

Build knobs:

• VLLM_CPP_MLX=off builds Metal without the provider: ~124 MB smaller, and 96.4% of MLX-LM instead of 99.1%.
• MLX_VERSION pins the wheel (default 0.29.3). MLX is consumed as the prebuilt pip wheel because building it from source needs xcrun metal, i.e. a full Xcode the macOS runners do not have.

Packaging vendors libmlx.dylib, mlx.metallib and MLX's MIT license into package/lib/, and rewrites libvllm.dylib's rpath to @loader_path/lib (re-signing it, since install_name_tool invalidates the signature). The metallib must stay beside libmlx.dylib: MLX looks for it there.

Testing: make test runs the unit specs; export VLLM_CPP_MODEL=<model> (and optionally VLLM_CPP_LIBRARY=<libvllm path>) to enable the e2e specs.