LFM2.5-Embedding-350M

LiteRT is Google's on-device runtime, the new name for TensorFlow Lite (Android: com.google.ai.edge.litert:litert), and litert-torch, the renamed ai-edge-torch, is its PyTorch converter: a PyTorch model converted unmodified with litert_torch.convert matched the original to 4e-7 on a Galaxy S26 (measured, LiteRT 2.2.0, Android 16, 2026-09-05).

LFM2.5-Embedding-350M — LiteRT

LiquidAI/LFM2.5-Embedding-350M converted to LiteRT (.tflite) for on-device inference. A multilingual (11 languages) dense bi-encoder on the LFM2 hybrid backbone (gated short-convolutions + grouped-query attention) — one 1024-d vector per text, for retrieval, semantic search and RAG, fully offline on CPU.

Since litert-lm 0.17.0 the same weights are also published as LiteRT-LM EmbeddingEngine bundles (.litertlm) that the runtime loads without any host-side tokenization or pooling code — see the section below.

Signatures

All signatures take batch-1, right-padded static shapes: input_ids int32 [1, S], attention_mask int32 [1, S] (1 = real token, 0 = pad).

Pick the smallest signature that fits your token count. The vector does not depend on which one you pick: padding is zeroed inside the graph before the short-convolution and masked in attention, so the same text through embed_64 and through embed_512 returns a bitwise identical vector — verified on Mac and again on-device. Model max length is 512 tokens.

Contract

Two things must match the upstream model or results silently degrade:

• Asymmetric prompts, with the trailing space. Prefix queries with query: and passages with document: , exactly as stored in the base repo's config_sentence_transformers.json.
• CLS pooling and normalization are already in the graph. The output is the position-0 token vector, L2-normalized, so a plain dot product is cosine similarity. Do not pool or normalize again.

LiteRT-LM EmbeddingEngine bundles (.litertlm, litert-lm ≥ 0.17.0)

Since litert-lm 0.17.0 the runtime hosts embedding models directly through EmbeddingEngine (Python, C and Kotlin), so the same weights are also published as bundles that the engine loads without any host-side tokenization or pooling code:

The vectors are identical to the .tflite path (cosine 1.000000 on the check set below; CLS pooling and L2 normalization are inside the graph). Keep insert_special_tokens at its default (True): the bundle declares <|startoftext|> as the token the CLS position reads, and the engine inserts it — the runtime's tokenizer does not run the tokenizer's own post-processor, so turning the option off silently returns wrong vectors.

Python (pip install litert-lm>=0.17.0):

import litert_lm
from litert_lm.embedding_engine import EmbeddingEngine, EmbeddingOptions

engine = EmbeddingEngine("LFM2.5-Embedding-350M_wi8fc.litertlm", backend=litert_lm.Backend.CPU())
vec = engine.compute_embedding("query: What is the tallest mountain in Japan?").embedding   # 1024 floats, L2-normalized
batch = engine.compute_embedding_batch(["document: first text", "document: second text"])                  # list of responses

Use the model's prefixes as plain text (query: for queries, document: for passages). Inputs longer than 512 tokens are an error by default; pass EmbeddingOptions(input_overflow_strategy=InputOverflowStrategy.TRUNCATE) or CHUNK_AND_AVERAGE to choose. EmbeddingOptions(output_size=256) keeps the first 256 dimensions.

Kotlin (com.google.ai.edge.litertlm:litertlm-android:0.17.0, needs a Kotlin 2.4 project):

val engine = EmbeddingEngine(EmbeddingEngineConfig(modelPath = "/data/local/tmp/LFM2.5-Embedding-350M_wi8fc.litertlm", backend = Backend.CPU()))
engine.initialize()
val vec = engine.computeEmbedding(listOf(InputData.Text("query: What is the tallest mountain in Japan?"))).embedding
engine.close()

Measured (CPU, single text, 10-text check set of 6–70 tokens, median):

Quality

Measured against the PyTorch fp32 reference (Lfm2BidirectionalModel + CLS + normalize) on the same inputs. fp16 is bitwise identical to the reference on every task metric below.

int8 costs 0.005 nDCG@10 and is a rounding difference on STS17. Per-vector agreement with the reference is cosine 0.9945–0.9955 for int8 and ≥0.9999999 for fp16.

Graph mechanics on the shipped artifacts, all three variants: cross-signature max|diff| 0.0 at 64/128/256/512, pad-content invariance 0.0 (garbage in the padded region cannot move the vector), and a live attention_mask (shortening it moves the output by 4.8e-02).

Speed

Apple-silicon Mac, XNNPACK, 16 threads, 20 iterations after warmup:

fp16 is slower than fp32 on CPU: XNNPACK unpacks fp16 weights to fp32 at run time, so the smaller file buys disk, not latency.

iPhone 17 Pro, XNNPACK, 6 threads (warm, after the first call on each signature):

The int8 model reproduces the Mac outputs bit-exactly on device across all tested cases (en/ja/de/ar/hi, short and long, both prompt forms). The fp16 model does run on an iPhone 17 Pro, but its 5.8 GiB peak is a function of that device's memory — treat fp16 as the desktop artifact. Its first invocation on each signature costs 355–538 ms of one-time XNNPACK fp16→fp32 unpacking, excluded from the warm figures above.

GPU (2026-08-13 re-export)

The re-export respells the one idiom mobile GPU delegates refuse — transformers' rank-5 repeat_kv expand — into an equivalent rank-4 matmul (outputs bitwise-identical on CPU), and the int8 file now fully delegates on mobile GPUs. Measured with the LiteRT CompiledModel API (fp32 GPU precision, real inputs, best of 3 warm runs): Pixel 8a OpenCL embed_512 20.1 ms, cosine 0.9992 vs the fp32 desktop reference; iPhone 17 Pro Metal embed_512 171–194 ms (eight fixture cases, cosine ≥ 0.9995). Set the GPU precision to fp32 — at fp16 GPU precision this family's norm reductions overflow and every output is NaN; CompiledModel timings are not comparable to classic-delegate benchmark_model timings.

Snapdragon NPU (Hexagon)

• LFM2.5-Embedding-350M_fp16.tflite — the NPU runs it at 182.6 ms. The GPU does not — LiteRtException: Failed to compile model.
• LFM2.5-Embedding-350M_wi8fc.tflite — the GPU runs it at 81.10 ms. The NPU does not — LiteRtException: Failed to compile model.

Measured on a Samsung Galaxy S26 (Snapdragon 8 Elite Gen 5 / SM8850, Hexagon v81, Android 16) with LiteRT CompiledModel 2.2.0, one accelerator per process, 5 warm-up runs then N=50 timed runs, median reported. Every run held thermal status NONE throughout. Headroom 0.75–0.78, where 1.0 is the throttling threshold.

The NPU row marked AOT ran an artifact compiled ahead of time for SM8850 (ai-edge-litert 2.2.0 + QAIRT 2.47.0), not the published file. That artifact is not distributed here; the compile is one command in the NPU guide.

GPU wiring: GPU guide.

Usage (Python)

import numpy as np
from ai_edge_litert.interpreter import Interpreter
from tokenizers import Tokenizer

tok = Tokenizer.from_file("tokenizer.json")
it = Interpreter(model_path="LFM2.5-Embedding-350M_wi8fc.tflite")
embed = it.get_signature_runner("embed_128")

def encode(text, prompt):           # prompt = "query: " or "document: "
    ids = tok.encode(prompt + text).ids[:128]
    x = np.zeros((1, 128), np.int32)
    m = np.zeros((1, 128), np.int32)
    x[0, :len(ids)] = ids
    m[0, :len(ids)] = 1
    return list(embed(input_ids=x, attention_mask=m).values())[0][0]  # [1024]

q = encode("What is the capital of France?", "query: ")
d = encode("Paris is the capital and largest city of France.", "document: ")
print(float(q @ d))                 # already normalized -> dot product is cosine

On Android/iOS use the LiteRT runtime's SignatureRunner APIs with the same signature names; the tokenizer is the standard Hugging Face tokenizer.json (works with the tokenizers libraries for Rust/Swift/Kotlin).

License

LFM Open License v1.0 (see LICENSE, unchanged from the base model). Note the license's commercial-use threshold (Section 5). This repository redistributes converted Derivative Works of LiquidAI/LFM2.5-Embedding-350M with modification notices per Section 4; all credit for the model to Liquid AI.