onnx_runtime_dart 0.4.2

A pure-Dart ONNX inference runtime for transformer-style models — no FFI, runs on every target including web and WebAssembly.

onnx_runtime_dart

A small, dependency-light pure-Dart ONNX inference runtime. No FFI and no native onnxruntime — the graph is interpreted in plain Dart, so the same code runs on every Dart/Flutter target, including the web and WebAssembly.

It implements the operator set used by transformer / attention models (BERT/XLM-R/MPNet/DeBERTa embedders and rerankers, RoPE / ALiBi variants, Qwen3 decoders), the convolution / pooling family (CNN classifiers, detectors, segmentation and diffusion VAEs), recurrent ops (LSTM/GRU/RNN), control flow (If/Loop/Scan with subgraph execution) and quantized models (QDQ, QOperator and int4 MatMulNBits). In practice this covers text embedders, rerankers, seq2seq translation with KV-cache, OCR, object detection, segmentation, diffusion VAEs, three ASR stacks, TTS and audio scoring — all verified against native ONNX Runtime (see Verified models). It is still not a complete ONNX runtime — see the operator list below for exactly what is covered.

Verified models

Every model below is run against native ONNX Runtime (onnxruntime, CPU provider) on deterministic inputs and checked op-for-op; the "parity" column is the metric the architecture actually admits (see Parity criteria). On top of that, every operator has per-op parity fixtures generated from ONNX Runtime (test/fixtures/, built by tool/gen_fixtures.py).

Unless noted, parity = cosine 1.0, max abs diff ≈ 1e-6–1e-5 (float32 rounding). Repos in the cstr/ namespace are the author's ONNX rehosts; † marks models tested from a local/pre-existing ONNX export (upstream repo linked).

Text embeddings & rerankers

Model	HF repo	Architecture	Parity
all-MiniLM-L6-v2	sentence-transformers/all-MiniLM-L6-v2	BERT 384d	1.0
all-MiniLM-L12-v2	sentence-transformers/all-MiniLM-L12-v2	BERT 384d	1.0
all-mpnet-base-v2	sentence-transformers/all-mpnet-base-v2	MPNet, relative-position buckets	1.0
bge-small-en-v1.5	BAAI/bge-small-en-v1.5	BERT 384d	1.0
bge-m3	BAAI/bge-m3	XLM-R 1024d, external-data Constant attrs	1.0
gte-small	Xenova/gte-small	BERT 384d	1.0
gte-base-en-v1.5	Alibaba-NLP/gte-base-en-v1.5	GTE: pre-LN + RoPE + GeGLU	1.0
gte-modernbert-base	Alibaba-NLP/gte-modernbert-base	ModernBERT, global/local sliding-window attn	1.0
nomic-embed-text-v1.5	nomic-ai/nomic-embed-text-v1.5	NomicBERT, RoPE	1.0
multilingual-e5-small	intfloat/multilingual-e5-small	XLM-R 384d	1.0
snowflake-arctic-embed-xs	Snowflake/snowflake-arctic-embed-xs	BERT CLS	1.0
granite-embedding-107m	ibm-granite/granite-embedding-107m-multilingual	XLM-R	1.0
jina-embeddings-v2-base-en †	jinaai/jina-embeddings-v2-base-en	BERT + ALiBi	1.0
Splade_PP_en_v1	Qdrant/Splade_PP_en_v1	SPLADE sparse-lexical	1.0
Octen-Embedding-0.6B †	cstr/Octen-Embedding-0.6B-ONNX	Qwen3 decoder, RoPE, external-data	1.0 (fp32)
ms-marco-MiniLM-L-6-v2 †	cross-encoder/ms-marco-MiniLM-L-6-v2	BERT cross-encoder	1.0
mxbai-rerank-xsmall-v1	mixedbread-ai/mxbai-rerank-xsmall-v1	DeBERTa-v2, disentangled attention	1.0
mxbai-rerank-base-v1	mixedbread-ai/mxbai-rerank-base-v1	DeBERTa-v2	1.0
awesome-align	cstr/awesome-align-onnx	multilingual BERT word aligner	1.0
embeddinggemma-300m	onnx-community/embeddinggemma-300m-ONNX	Gemma3; fused MultiHeadAttention + RotaryEmbedding + SimplifiedLayerNormalization	1.0 (fp16 export)
harrier-270m	onnx-community/harrier-oss-v1-270m-ONNX	Gemma3 decoder; fused GroupQueryAttention (causal, internal RoPE, GQA grouping)	1.0 (fp16 export)
F2LLM-v2-0.6B †	cstr/F2LLM-v2-0.6B-ONNX	Qwen3 decoder	int8 dynamic-quant band (0.997)
jina-embeddings-v5-small †	jinaai/jina-embeddings-v5-text-small-retrieval	Qwen3, IsNaN attention masking	1.0 (fp16 export)
partitura-jina †	jina variant (local export)	BERT/jina	1.0

Quantized language models (int4 / int8)

Model	HF repo	Format	Parity
Octen-0.6B int4 †	cstr/Octen-Embedding-0.6B-ONNX	MatMulNBits, 1.8 GB packed	cosine 1.0 (static block quant)
llama-nemotron-rerank-1B int4 †	cstr/llama-nemotron-rerank-1b-v2-ONNX	MatMulNBits	logit within 1.7e-5
llama-nemotron-rerank-1B int8 †	cstr/nemotron int8 (local)	dynamic quant	dynamic-int8 band
Octen-0.6B int8 †	cstr/Octen-Embedding-0.6B-ONNX	dynamic quant, 196 MatMulInteger	intrinsic-band (below)
zerank-1-small int4 †	cstr/zerank-1-small-ONNX	int4 + fp16-compute regions	fp16-caveat (below)
PIXIE-Rune-v1.0 int4	cstr/PIXIE-Rune-v1.0-ONNX	XLM-R, ONNX-native INT4 QDQ (packed 2/byte)	cosine 1.0

Generative LLMs (autoregressive decoding, KV cache)

Model	HF repo	Notes	Parity
SmolLM2-135M-Instruct	HuggingFaceTB/SmolLM2-135M-Instruct (ONNX export)	Llama-style decoder; 30× fused GroupQueryAttention with real past/present KV cache (9 query / 3 KV heads), external RotaryEmbedding	prefill + decode 1.0 on logits and present.*; 24-step greedy generation token-for-token identical to ORT
Qwen2.5-0.5B-Instruct	onnx-community/Qwen2.5-0.5B-Instruct (fp16)	fully decomposed decoder (no fused ops — RoPE / attention as primitives, 24 layers, 2 KV heads); graph-level KV cache via Concat/Slice	prefill logits cosine 0.99999885, decode 0.99999976 (fp16 band); coherent text generation

Full autoregressive text generation runs in pure Dart — both the fused-GroupQueryAttention export style and the fully-decomposed one — with present_key/present_value feeding straight back as the next step's past_key/past_value. OnnxModel.inputSpecs reports the decoder's cache shape (layers / KV heads / head size) so the empty first-step past can be sized generically. tool/llm_chat.dart is a complete text-in / text-out demo (byte-level BPE tokenizer loaded from tokenizer.json, ChatML prompt, greedy + temperature/top-k sampling); tool/smollm2_generate.dart is a greedy loop checked token-for-token against ORT.

Sequence-to-sequence & OCR

Model	HF repo	Notes	Parity
NLLB-200-600M (enc + dec + decoder_with_past) †	facebook/nllb-200-distilled-600M (Optimum ONNX export)	full translation loop, KV cache, 256k-vocab logits + all present-KV; Trilu, ScatterND, external-data Constant	1.0
TrOCR (ViT encoder + text decoder) †	microsoft/trocr-* (local ONNX export)	image → text	1.0
CrispTranslator (enc + dec) †	NLLB-family, int8 dynamic-quant (local export)	MatMulInteger+DynamicQuantizeLinear; 256k-vocab logits, KV outputs exact	dynamic-int8 band (enc 0.9999, dec 0.998)

Vision

Model	Source	Task	Parity
MobileNetV2	ONNX Model Zoo (mobilenetv2-7)	classification	1.0
ResNet18	ONNX Model Zoo (resnet18-v1-7)	classification	1.0
MobileNetV2 QDQ	ONNX Model Zoo (mobilenetv2-12-qdq)	quantized classification	same top-5 (below)
SSD-MobileNetV1	ONNX Model Zoo (ssd_mobilenet_v1_10)	detection: uint8 in, preprocessing Loop, per-class NMS (TopK/NonZero/NonMaxSuppression)	all 4 outputs bit-identical
UltraFace RFB-320	ONNX Model Zoo (version-RFB-320)	face detection	1.0
fast-neural-style candy	ONNX Model Zoo (candy-9)	style transfer, InstanceNorm + Upsample	1e-4 relative
sub-pixel CNN super-resolution	ONNX Model Zoo (super-resolution-10)	super-resolution	1.0
emotion-ferplus	ONNX Model Zoo (emotion-ferplus-8)	classification	1.0
SAM (ViT-H) mask decoder	Annotation-AI/sam-vit-h-decoder-onnx	segmentation prompting, 3 outputs	1.0
TAESD (SD VAE decoder)	julienkay/taesd	diffusion VAE, latent [1,4,64,64] → 512²	1.0 (max|Δ| 9e-6)
Depth-Anything-v2-small	onnx-community/depth-anything-v2-small	monocular depth (DINOv2 ViT + DPT head: Resize×6, ConvTranspose); dense [1,H,W] depth map	1.0 (max|Δ| 1.5e-5)

Speech — ASR, TTS, VAD, scoring, tokenization

Model	HF / source	Stack	Parity
Whisper-tiny (enc + dec + decoder_with_past)	onnx-community/whisper-tiny	Transformer ASR, KV cache	dec 1.0; enc 2e-4 rel (below)
Moonshine-tiny (enc + merged decoder)	UsefulSensors/moonshine	ASR; single top-level If decoder, 24-tensor KV cache	1.0 (encoder-KV exact)
Parakeet-TDT 0.6B †	NVIDIA Parakeet-TDT 0.6B v3 (ONNX export)	NeMo mel featurizer (STFT + float64 weights); RNN-T decoder/joint (LSTM+Split); int8 conformer encoder	featurizer/decoder 1.0; int8 enc intrinsic-band
Kokoro-82M TTS	onnx-community/Kokoro-82M-v1.0-ONNX	StyleTTS2 / iSTFT-Net: LSTMs, harmonic sine source, mid-graph STFT, 1-D conv/transposed-conv	log-mel cosine 0.995 (below)
Silero VAD	snakers4/silero-vad	Conv1D + LSTM + If + reflect-Pad	1.0
AECMOS (2 echo-MOS models)	microsoft/AEC-Challenge (AECMOS_local)	Conv + MaxPool + bidirectional GRU + ReduceMax; full scorer in example/aecmos/	1.0 (max|Δ| 1.2e-7)
CAM++ speaker embedding †	CosyVoice3 (campplus.onnx)	x-vector: 225 convs, ReduceProd	1.0
CosyVoice3 speech tokenizer †	CosyVoice3 (speech_tokenizer_v3.onnx)	discrete speech tokens	25/25 tokens exactly equal
FastConformer CTC †	NeMo FastConformer (local ONNX export)	Conformer + CTC, LogSoftmax	1.0
ECAPA-TDNN language-ID †	SpeechBrain ECAPA-TDNN (local ONNX export)	SE-Res2 TDNN, attentive stat pooling	1.0
Piper (VITS) TTS †	rhasspy/piper voices (e.g. en_US-libritts_r-medium)	end-to-end VITS, RandomNormalLike	1.0 (noise-scale 0 → deterministic)

Games

Model	HF repo	Notes	Parity
Maia3-5M	cstr/maia3-onnx-int32	chess transformer, policy (4352) + WDL value heads, Einsum attention	1.0

Parity criteria

Not every architecture admits bitwise (or even high-cosine) whole-output parity; the oracle defines what's achievable, so each model is judged by the right metric:

• Float models — cosine 1.0, max abs diff ~1e-5 (or ~2e-4 relative for very deep encoders like Whisper's, where summation-order alone drifts that far; confirmed by running with load-time fusion on vs off).
• QDQ classification (MobileNetV2) — no bitwise logit parity exists for any runtime: tiny float-ordering differences flip quantization buckets, and ORT's own optimized-vs-unoptimized execution self-agrees only to cosine ≈0.993. Criterion: same top-k, cosine inside that band (~0.991). Our classification is identical.
• Static int4 (MatMulNBits) — reproduces exactly (cosine 1.0): block quantization has no runtime quantization boundaries.
• Dynamic int8 (DynamicQuantizeLinear+MatMulInteger) — transcendental ulps amplify across layers, so no independent runtime matches ORT-int8 bitwise. Criterion: our deviation from ORT-int8 ≪ that export's own int8-vs-fp32 quantization error (e.g. Octen 0.96 vs 0.73 pooled; Parakeet conformer 0.997 vs 0.63).
• fp16-compute exports (e.g. zerank-1-small int4, 115 Cast-to-fp16 pairs) — we execute float32 between cast points (rounding through fp16 at each cast); ORT computes those regions in true half precision, so results agree only to the model's fp16 sensitivity (~2%). Ours is the more precise side, not bit-matching.
• TTS vocoders (Kokoro) — whole-waveform cosine is meaningless: LSTM recurrence and sine-phase integration chaos-amplify float ulps for any implementation. Verified instead by (a) components in isolation (feed-forward path bitwise, harmonic source cosine 0.99996, first 2000 samples 0.9998) and (b) log-mel spectrogram cosine 0.995 on the audio.

Execution uses a packed, register-tiled Float32x4 SIMD GEMM kernel on native targets (scalar fallback on web), im2col convolution, load-time constant folding and weight prepacking; see BENCHMARKS.md for current numbers vs native ONNX Runtime.

On native targets you can additionally spread large matmuls across an isolate worker pool — each worker permanently owns a column slice of the big weights, so per-run messages carry only activations:

await model.parallelize(workers: 4);        // once, after loading
final out = await model.runAsync(inputs, ['output']); // bitwise == run()
model.dispose();                            // shuts the workers down

Install

dependencies:
  onnx_runtime_dart: ^0.3.4

Usage

import 'dart:typed_data';
import 'package:onnx_runtime_dart/onnx_runtime_dart.dart';
import 'package:onnx_runtime_dart/onnx_runtime_dart_io.dart'; // native only (dart:io)

void main() {
  // Resolves companion external-data files (large models) automatically.
  final model = loadOnnxModel('model.onnx');

  final out = model.run(
    {'input': Tensor.float(Float32List.fromList([1, 2, 3, 4]), [1, 4])},
    ['output'],
  );

  print(out['output']!.asFloatList());
}

On the web (no dart:io), use OnnxModel.fromBytes(bytes) directly; for external-data models pass an externalData resolver.

Weights load from float32, float16, float64, int32, int64, bool, and int8 / uint8 (kept in compact 1-byte storage) tensors, plus 4-bit block-quantized MatMulNBits weights (kept packed) — inline or from a companion .onnx.data file (read on demand, so multi-GB models don't load into memory all at once).

See example/onnx_runtime_dart_example.dart for a self-contained, runnable graph built with the protobuf types.

Supported operators

• Math: Add, Sub, Mul, Div, Pow, Sqrt, Reciprocal, Abs, Neg, Exp, Log, Relu, Erf, Sigmoid, Tanh, Cos, Sin, Clip, Sign.
• Compare / logic: Equal, Greater, Less, GreaterOrEqual, LessOrEqual, And, Or, Not, Where, Max, Min.
• Shape / index: Shape, Reshape, Transpose, Squeeze, Unsqueeze, Concat, Gather, GatherND, GatherElements, Expand, Slice, Range, Cast, Constant, ConstantOfShape.
• Reduce / linalg: ReduceMean, ReduceSum, ReduceMax, ReduceMin, ReduceProd, CumSum, Softmax, LayerNormalization, MatMul, Gemm, Einsum (general 1/2-operand equations without ellipsis; the transformer- hot patterns keep specialized kernels), ArgMax, ArgMin, LogSoftmax.
• Fused transformer ops (com.microsoft): MultiHeadAttention, GroupQueryAttention (causal, grouped KV heads, internal RoPE, real past/present KV cache for autoregressive decoding — the fused op modern LLM exports use: Llama3 / Qwen2-3 / Gemma2-3 / Phi3 / Mistral), RotaryEmbedding (interleaved + rotate-half, partial-rotation), SimplifiedLayerNormalization / SkipSimplifiedLayerNormalization (RMSNorm) — so onnx-community / Optimum-optimized transformer exports (which fuse attention rather than emitting decomposed graphs) run directly.
• Convolution / pooling: Conv (1–3 spatial dims, strides / pads / dilations / groups / depthwise / auto_pad, im2col+GEMM fast path), ConvTranspose, MaxPool, AveragePool, GlobalAveragePool, GlobalMaxPool, BatchNormalization, InstanceNormalization, GroupNormalization, Resize (nearest + linear), GridSample (bilinear/nearest, zeros/border/reflection), RoiAlign (avg/max), Flatten.
• Activations: LeakyRelu, Elu, PRelu, HardSigmoid, HardSwish, Softplus, Gelu (erf + tanh forms).
• Recurrent: LSTM (incl. peepholes), GRU (incl. linear_before_reset), RNN — forward / reverse / bidirectional, sequence_lens, initial_h/initial_c; default activations only.
• Control flow / misc: If, Loop, Scan (subgraphs capture the outer scope; scan outputs, non-default axes and reverse directions supported), Identity, Pad (constant / reflect / edge), Size, Tile, Trilu, ScatterND, Split, NonZero, TopK, NonMaxSuppression, Upsample (deprecated pre-Resize form), Dropout (inference identity), STFT, Floor, Ceil, Round.
• Quantization: QDQ format — QuantizeLinear, DequantizeLinear (per-tensor + per-axis, uint8/int8), DynamicQuantizeLinear; weight DequantizeLinear nodes constant-fold at load, so QDQ models run at float speed. QOperator format — MatMulInteger (scalar / per-row / per-column zero points, batched), ConvInteger, QLinearMatMul, QLinearConv (per-channel weight scales + int32 bias) with exact int32 accumulation and round-half-to-even requantization. int8/uint8 tensors use compact 1-byte storage end to end — a 1 GB int8 model needs ~1 GB, not the 8 GB an int64 widening would cost. MatMulNBits (com.microsoft, 4-bit block-quantized weights, packed zero points, partial blocks) runs int4 exports with weights kept packed (0.5 bytes/weight); dequantization streams through the SIMD GEMM per call.
• Fusion (load-time, automatic): the erf-GELU chain (0.5·x·(1+Erf(x/√2))) fuses to a single pass, and the attention epilogue MatMul(Softmax(MatMul(Q,K)·s + mask), V) fuses so scale + mask + softmax happen in one sweep over the attention matrix. Fusions only fire when the intermediate values have no other consumers; results stay within float rounding of the unfused graph.

Tensors are float32 or int64 (int32 and bool are widened to int64), row-major (matching ONNX's own layout). An unimplemented op throws UnsupportedError naming the op.

How it works

ONNX graphs are stored in topological order, so execution is a single linear pass over the nodes with a name -> Tensor value cache — initializers (weights) are decoded up front, runtime inputs are supplied to run, and the requested outputs are returned.

To build or inspect models programmatically, import package:onnx_runtime_dart/onnx_proto.dart for the ONNX message types (ModelProto, GraphProto, NodeProto, …).

Licensing

The runtime code is MIT. The generated protobuf bindings (lib/src/onnx.pb*.dart) derive from the ONNX project's onnx.proto3 and are used under Apache-2.0 — see NOTICE.md.