nanoE5.c

A blazing-fast, dependency-free CPU engine for multilingual-e5-small text embeddings.

A tiny C core (the .c is the whole point) packaged for one-command use: pip install nanoe5 from Python, or a single self-contained server binary.

Two 4-bit models are bundled — there is nothing to download or configure:

• original → intfloat/multilingual-e5-small
• enpt → cnmoro/portuguese-multilingual-e5-small

Use it from Python in two lines, or run an OpenAI-compatible server from a single self-contained binary.

pip install nanoe5

import nanoe5
q = nanoe5.query("how much protein per day")     # 384-dim, L2-normalized
P = nanoe5.passage(["doc a", "doc b"])           # (2, 384)
scores = P @ q                                   # cosine similarity

…or run an OpenAI-compatible server (works with the official openai client):

nanoe5-serve --port 8000           # OpenAI-compatible embeddings API

No PyTorch. No transformers. No ONNX. No BLAS. Just C, libm, and OpenMP.

---

Why

• One file to deploy. Both bundled 4-bit models are linked inside the ./e5 binary. Copy it to a server and run — nothing to download, install, or mount.
• Fast where it counts. ~2 ms to embed a single query on a desktop CPU — about 7× faster than sentence-transformers for one-at-a-time serving.
• Tiny. 72 MB 4-bit model vs 471 MB fp32. Instant startup (mmap).
• Faithful. Real XLM-RoBERTa SentencePiece tokenizer + exact BERT forward pass; cosine 0.98–0.99 vs the fp32 reference, retrieval rankings preserved.
• Handles long text. Inputs over 512 tokens are windowed automatically and transparently, in bounded memory.

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Install

From PyPI (Python)

pip install nanoe5

That's it — the 4-bit model is inside the package. The tiny C engine compiles on install (needs a C compiler with OpenMP, e.g. gcc), then everything runs with no ML dependencies (just NumPy). Requires an x86-64 CPU with AVX2 for the fast path; other CPUs fall back to a portable scalar build automatically.

From source (server binary + CLI)

# 1. download + quantize the original model -> e5-small-q4.bin  (one-time, ~72 MB)
make convert        # pip install torch transformers safetensors tokenizers numpy

# optional: quantize the bundled EN+PT-pruned model -> e5-small-enpt-q4.bin
make convert-enpt

# 2a. build the self-contained server/CLI binary  ->  ./e5
make server

# 2b. (optional) build the Python shared library   ->  libe5.so
make lib

make convert / make convert-enpt are the only steps that touch the Python ML stack. After that, the binary runs with no ML dependencies at all.

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Use it: the OpenAI-compatible server

Start a server with one command — works with the official openai Python client out of the box (verified against openai>=1.0):

pip install nanoe5
nanoe5-serve --port 8000 --variant original   # or --variant enpt

from openai import OpenAI                       # the official OpenAI client

client = OpenAI(base_url="http://localhost:8000/v1", api_key="not-needed")

resp = client.embeddings.create(
    model="e5-query",                           # see "Query vs passage" below
    input=["how much protein per day", "best protein sources"],
)
embeddings = [d.embedding for d in resp.data]   # two 384-dim vectors

Both encoding_format="float" and the client's default "base64" path are supported, so nothing in your existing OpenAI code needs to change — just point base_url at the server.

Prefer a single dependency-free binary? make server builds ./e5, which embeds the model and serves the same API with zero Python: ./e5 --server --port 8000.

…or hit it with plain curl:

curl http://localhost:8000/v1/embeddings \
  -H 'Content-Type: application/json' \
  -d '{"input": ["doc one", "doc two"], "input_type": "passage"}'

{
  "object": "list",
  "data": [
    {"object": "embedding", "index": 0, "embedding": [0.031, -0.044, ...]},
    {"object": "embedding", "index": 1, "embedding": [0.018,  0.007, ...]}
  ],
  "model": "multilingual-e5-small-q4",
  "usage": {"prompt_tokens": 8, "total_tokens": 8}
}

Endpoints

Method & path	Purpose
POST /v1/embeddings	Create embeddings (string or array of strings).
GET /v1/models	List the served model.
GET /health	Liveness check → {"status":"ok"}.

Request fields

Field	Values	Default
input	a string or an array of strings	required
encoding_format	"float" or "base64"	"float"
input_type	"query", "passage" (alias "document")	server default
model	any string; if it contains query/passage/doc it sets the modality	—

encoding_format: "base64" returns each embedding as base64-encoded little-endian float32 — this is what the official OpenAI Python client requests by default, and it's fully supported.

Server flags

Both server forms take the same flags:

nanoe5-serve  [--host H] [--port P] [--threads N] [--default-type query|passage] [--variant original|enpt] [--model FILE]
./e5 --server [--host H] [--port P] [--threads N] [--default-type query|passage] [--variant original|enpt] [--model FILE]

• --threads N caps OpenMP threads (default: all cores).
• --default-type sets the modality when a request doesn't specify one (default query).
• --variant selects which bundled model to serve:
  • original → multilingual-e5-small-q4
  • enpt → portuguese-multilingual-e5-small-q4
• --model FILE loads an external e5-small-q4.bin (the binary otherwise uses the selected bundled copy; the pip server uses the bundled one).

---

Use it: from Python

The simplest form uses module-level helpers backed by a shared, hot model (loaded once, reused for every call):

import nanoe5

q = nanoe5.query("how much protein per day")     # (384,)
docs = nanoe5.passage([                            # (N, 384)
    "The recommended protein intake for adult women is about 46 g/day.",
    "Mount Everest is the highest mountain above sea level.",
])

scores = docs @ q          # already L2-normalized -> dot product = cosine
print(scores.argmax())     # -> 0

Or hold an explicit handle (e.g. to cap threads):

from nanoe5 import E5
model = E5(num_threads=8, variant="enpt")      # or variant="original"
model.query("...");  model.passage(["...", "..."])

You can also select the variant through the module helpers:

import nanoe5

q = nanoe5.query("quanta proteína por dia", variant="enpt")

That's the whole API:

Call	Prefix added	Returns
nanoe5.query(text | list) / model.query(...)	query:	(384,) or (N, 384) float32
nanoe5.passage(text | list) / model.passage(...)	passage:	(384,) or (N, 384) float32
nanoe5.encode(x, is_query=False) / model.encode(...)	either	generic form

A single text is parallelized across all CPU cores (low latency); a list is parallelized across texts (high throughput).

Bundled variants

• variant="original" keeps the full multilingual tokenizer and weights from intfloat/multilingual-e5-small.
• variant="enpt" uses the bundled pruned tokenizer/weights from cnmoro/portuguese-multilingual-e5-small, keeping English + Portuguese tokens.

If model_path=... is given, it overrides variant.

---

Query vs passage

multilingual-e5-small is trained with two prefixes, and you should use the right one:

• query: — short search queries / questions.
• passage: — documents you want to retrieve.

Embed your documents with passage, your search queries with query, then rank documents by cosine similarity (a plain dot product, since outputs are normalized).

• Python: model.query(...) vs model.passage(...).
• Server: set "input_type": "query" or "passage" per request (or name the model e5-query / e5-passage), otherwise the server's --default-type is used.

---

Long inputs (automatic)

The base model maxes out at 512 tokens. Instead of truncating, nanoE5.c slides a window over longer text: it splits into ≤510-token windows, embeds each, and returns the token-count-weighted average (then re-normalizes). This is mathematically equivalent to mean-pooling over the whole document and needs no API change — just pass a long string. Memory stays bounded (~350 MB) even for million-token inputs.

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Sparse "latent terms" & hybrid retrieval (optional)

A single dense vector has a fixed capacity; a high‑dimensional sparse vector can encode complementary lexical signal and improves recall. Inspired by mixedbread's latent terms, nanoE5.c can attach a sparse head: a TopK sparse autoencoder (sae.bin, 3.6 MB) trained on e5 token embeddings that maps each token to a 16,384‑dim sparse code, max‑pooled over the document. It was trained on Portuguese + English only.

from nanoe5 import E5
m = E5()                          # auto-loads the bundled sae.bin
m.has_sparse                      # True
v = m.sparse("quanta proteína por dia")          # dense (16384,) float32 (default)
S = m.sparse(docs, fmt="scipy")                  # (N, 16384) scipy.sparse.csr_matrix
i, w = m.sparse(text, fmt="indices")             # raw (feature_id, weight) arrays

m.sparse(...) returns a numpy array by default — (sparse_dim,) for one text, (N, sparse_dim) for a list — or a scipy.sparse.csr_matrix with fmt="scipy" (use this to index large corpora).

On standard benchmarks, hybrid (dense + sparse) beats dense alone — small but consistent across both languages (best at dense‑weight ≈ 0.8):

	nDCG@10	Recall@100
scifact (EN) dense	0.654	0.917
scifact (EN) hybrid	0.668	0.930
quati (PT‑BR) dense	0.387	0.796
quati (PT‑BR) hybrid	0.392	0.816

How to use it in a retrieval pipeline

Two patterns — pick based on whether you want better ranking or better recall:

1. Hybrid retrieval (recommended — improves recall). Index both representations and fuse at query time. Sparse catches exact/rare‑term matches the dense vector structurally cannot.

import numpy as np

# --- index time ---
D = m.passage(docs)                  # (N, 384)  dense
S = m.sparse(docs, fmt="scipy")      # (N, 16384) sparse  (use "numpy" for small corpora)

# --- query time ---
qd, qs = m.query(query), m.sparse(query)
dense  = D @ qd                      # cosine (vectors are normalized)
sparse = np.asarray(S @ qs).ravel()  # sparse dot
def mm(x): return (x - x.min()) / (np.ptp(x) + 1e-9)
score  = 0.8 * mm(dense) + 0.2 * mm(sparse)       # or Reciprocal Rank Fusion

At scale, put dense in an ANN index (HNSW/FAISS) and the sparse vectors in an inverted index (feature_id → postings); the SAE feature ids behave like terms. Both are first‑stage retrievers whose candidate sets you union, then fuse.

2. Dense‑retrieve → sparse rerank (cheaper, improves ranking only). Take the dense top‑K, re‑score those K with 0.8·dense + 0.2·sparse, reorder. This is what you proposed and it's the lightest option — but note a reranker can only reorder what dense already found, so it improves ordering, not recall. Most of the measured gain above is in Recall@100, which needs pattern 1.

The sparse head is optional: without sae.bin, m.has_sparse is False and everything else works unchanged. Retrain it with python sae_train.py (uses a GPU; PT+EN corpus only) and evaluate with python sae_eval.py.

---

CLI

The same binary is also a quick CLI:

./e5 query   "how much protein should a female eat"
./e5 --variant enpt query "quanta proteína devo comer por dia"
./e5 passage "a document to index"
./e5 --model e5-small-q4.bin query "use an external model file"

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How it works (short version)

• 4-bit weights (Q4_0). Every large matrix is stored in 32-weight blocks with an fp16 scale (~4.5 bits/weight) — ~10× less memory traffic than fp32.
• int8 × int4 matmul. Activations are quantized to int8 and multiplied against the 4-bit weights with AVX2 integer MACs — no fp32 dequant in the hot loop. Scalar fallback included for non-AVX CPUs.
• One pass per batch. All tokens of a batch share a single matmul per layer, so weights stream once; attention runs per text.
• OpenMP across matrix rows / texts; deterministic regardless of thread count.
• Faithful tokenizer. XLM-RoBERTa SentencePiece-unigram (Viterbi) with the real Precompiled normalizer baked in as a per-codepoint table.

The model is packed into one binary blob by convert.py; e5.c is the entire engine (loader, tokenizer, BERT, quantized matmul); server.c adds the HTTP server and CLI; e5.py is the ctypes wrapper.

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Performance

On a Ryzen 7 5800X3D (8 cores / 16 threads, AVX2):

	nanoE5.c (4-bit)	sentence-transformers (fp32)
single-query latency (hot)	~2 ms	~13 ms
batch throughput	~190–340 texts/s	~280 texts/s
model size	72 MB	471 MB
dependencies	libc, libm, OpenMP	torch + transformers
cold start	instant (mmap)	seconds

For online serving (one query at a time, model hot) nanoE5.c is ~7× faster per call. For huge offline batch jobs, PyTorch's oneDNN GEMM edges ahead on raw throughput — but at 1/6th the footprint and zero dependencies.

---

Validate & stress

make test                                   # original parity vs HF reference + speed
E5_SRC=hf_src_enpt E5_MODEL=e5-small-enpt-q4.bin E5_VARIANT=enpt E5_TEXTSET=enpt python3 test_parity.py
python3 test_variants.py                    # constructor/server variant wiring
python3 bench_variants.py                   # original vs enpt speed
env E5_VARIANT=original python3 stress_test.py
env E5_VARIANT=enpt python3 stress_test.py

make stress throws adversarial inputs at every layer and asserts: no crashes, no hangs, finite & unit-norm outputs, determinism, batch == single (exact), server == binding parity, base64 == float parity, real OpenAI-client compatibility, correct 4xx handling for malformed requests, survival of a raw garbage barrage, and concurrent requests with zero errors or races.

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Files

e5.c / e5.h      the entire inference engine
server.c         OpenAI-compatible HTTP server + CLI
convert.py       build e5-small-q4.bin / e5-small-enpt-q4.bin from HF checkpoints
sae_train.py     train the sparse "latent terms" head -> sae.bin (PT+EN, GPU)
sae_eval.py      dense vs sparse vs hybrid retrieval eval (scifact + quati)
nanoe5/          the pip package (engine + both 4-bit models + sae.bin bundled)
pyproject.toml   / setup.py   packaging (compiles the engine, bundles the model)
e5.py            standalone ctypes wrapper (repo-local use)
test_parity.py   parity vs HF reference + benchmark
stress_test.py   hard stress / edge-case suite
Makefile

License

The code here is yours to use. The bundled model weights are intfloat/multilingual-e5-small (MIT) and cnmoro/portuguese-multilingual-e5-small (MIT-compatible derivative of the same base) — see the model cards for details.