SentencePiece

SentencePiece is a fast, lightweight, and unsupervised text tokenizer and detokenizer designed for neural network-based text generation systems (such as Large Language Models) where the vocabulary size is fixed prior to training.

It implements subword units—including Byte-Pair-Encoding (BPE) [Sennrich et al.] and the unigram language model [Kudo.]—with the ability to train directly from raw sentences. By treating input text as a raw sequence of Unicode characters, SentencePiece enables a purely end-to-end, language-independent pipeline that completely eliminates the need for language-specific pre- or post-processing.

This is not an official Google product.

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Quick Start (Python)

SentencePiece provides an easy-to-use Python module. Install it via pip:

pip install sentencepiece

Basic Example

Here is how to train a model, encode text into tokens/IDs, and decode them back to the original string:

import sentencepiece as spm

# 1. Train a model directly from a raw text file.
# (No pre-tokenization or language-specific preprocessing required!)
spm.SentencePieceTrainer.train(
    input='data/botchan.txt', 
    model_prefix='m', 
    vocab_size=1000
)

# 2. Load the trained model.
sp = spm.SentencePieceProcessor(model_file='m.model')

# 3. Encode raw text into subword pieces (strings) or vocabulary IDs (integers).
text = "I saw a girl with a telescope."
pieces = sp.encode(text, out_type=str)
ids = sp.encode(text, out_type=int)

print(f"Pieces: {pieces}")
# Output: ['▁I', '▁saw', '▁a', '▁girl', '▁with', '▁a', '▁', 'te', 'le', 's', 'c', 'o', 'pe', '.']

print(f"IDs:    {ids}")
# Output: [9, 459, 11, 939, 44, 11, 4, 142, 82, 8, 28, 21, 132, 6]

# 4. Decode IDs or pieces back into the original text.
# The reconstruction is completely lossless and reversible!
print(sp.decode(ids))
# Output: "I saw a girl with a telescope."

print(sp.decode(pieces))
# Output: "I saw a girl with a telescope."

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Why SentencePiece?

1. Reversible & Lossless Tokenization (Whitespace as a Basic Symbol)

Traditional tokenizers drop whitespace information (e.g., treating Tokenize("World.") identically to Tokenize("World .")), making detokenization ambiguous and language-dependent.

SentencePiece treats the input text as a raw sequence of Unicode characters. It escapes whitespaces with a meta-symbol ▁ (U+2581) and includes it in the tokenization. This design ensures that detokenization is a simple, lossless string join operation, entirely independent of the language:

# Lossless detokenization
original_text = "".join(pieces).replace("▁", " ")

2. Purely Data-Driven & Language-Independent

SentencePiece trains tokenization and detokenization models directly from raw sentences. It does not require language-specific pre-tokenizers (such as Moses, MeCab, or KyTea). This makes it highly effective for languages without explicit word boundaries, such as Chinese, Japanese, and Korean.

3. Subword Regularization & BPE-Dropout

To improve the robustness and accuracy of translation and language models, SentencePiece supports on-the-fly subword sampling during training. By sampling different segmentations for the same input text (Subword Regularization for Unigram, BPE-Dropout for BPE), it virtually augments your training data and makes the model more resilient to spelling variations and noise.

# Sample different segmentations on-the-fly
for _ in range(3):
    print(sp.encode('New York', out_type=str, enable_sampling=True, alpha=0.1, nbest_size=-1))
# May output:
# ['▁', 'N', 'e', 'w', '▁York']
# ['▁New', '▁York']
# ['▁New', '▁Y', 'o', 'r', 'k']

4. Fast, Lightweight, and Self-Contained

• Performance: Written in highly optimized C++. Segmentation speed is around 50,000 sentences per second, with a memory footprint of only ~6MB.
• Self-Contained: The generated .model file contains the entire normalization rules, vocabulary mapping, and segmentation model. You are guaranteed to get the exact same tokenization results in any environment (C++, Python, Go, etc.) as long as you use the same model file.

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Performance Benchmark (SentencePiece vs. Hugging Face Fast)

Benchmark Setup

• Environment: 24-core CPU, Python 3.13.
• Dataset: Balanced raw multilingual text from FLORES-200 (parallel sentences in English, Chinese, Japanese, and Thai; 11.29 MB, 60,720 lines). CJK and Thai texts are raw and do not contain artificial space delimiters.
• Batch Request Size: The entire dataset (60,720 sentences) is fed as a single batch request (a single Python list[str]) in one call.
• Metric: Encoding throughput in MB/s (higher is better).

1. Unigram Model: T5-base (32k vocab)

Tokenizer	1 Thread	2 Threads	4 Threads	8 Threads	16 Threads	24 Threads
SentencePiece	27.41	43.83	71.62	102.08	123.33	127.60
Hugging Face Fast	3.78	7.15	12.45	20.33	27.00	31.49

2. BPE Model: Gemma 3 (256k vocab)

Tokenizer	1 Thread	2 Threads	4 Threads	8 Threads	16 Threads	24 Threads
SentencePiece	7.44	12.82	23.03	36.66	48.65	52.43
Hugging Face Fast	3.66	6.37	10.45	15.54	21.05	20.48

Why performance does not scale linearly:

While the core tokenization (C++ or Rust) runs in parallel, the final step of converting the native results (C++ vector of vectors or Rust vector of vectors) into Python objects (list[list[int]] or list[Encoding]) is sequential and must be done on Python's main thread (GIL-locked). At high thread counts, this single-threaded serialization step becomes the dominant bottleneck, capping the scaling performance.

For the detailed analysis and single-thread reference comparison, see Performance Benchmark Details.

To run these benchmarks yourself, see the reproduction instructions and scripts.

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Documentation & Resources

For detailed guides, API references, and advanced usage, please refer to the following resources:

• Command Line Interface (CLI) & Build Guide
• C++ API Reference
• Python API Reference & Python Module Directory
• Python Tokenizer Comparison Cheat Sheet
• Performance Benchmark Details
• Performance Benchmark Code & Reproduction Guide
• Training Options Reference
• Text Normalization & Custom Rules
• Special Symbols & Control Tokens
• Vocabulary Piece Constraints
• Model Protobuf Schema
• Docker Deployment Guide
• NLCodec BPE Trainer (Contrib)

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License

SentencePiece is licensed under the Apache 2.0 License.