Tiktoken

One of the fastest BPE tokenizers in any language — the fastest in .NET, competitive with pure Rust implementations. Zero-allocation token counting, built-in multilingual cache, and up to 41x faster than other .NET tokenizers. We will be happy to accept any PR.

Implemented encodings

• o200k_base
• cl100k_base
• r50k_base
• p50k_base
• p50k_edit

Usage

using Tiktoken;

var encoder = TikTokenEncoder.CreateForModel(Models.Gpt4o);
var tokens = encoder.Encode("hello world"); // [15339, 1917]
var text = encoder.Decode(tokens); // hello world
var numberOfTokens = encoder.CountTokens(text); // 2
var stringTokens = encoder.Explore(text); // ["hello", " world"]

// Alternative APIs:
var encoder = ModelToEncoder.For("gpt-4o");
var encoder = new Encoder(new O200KBase());

Load from HuggingFace tokenizer.json

The Tiktoken.Encodings.Tokenizer package enables loading any HuggingFace-format tokenizer.json file — supporting GPT-2, Llama 3, Qwen2, DeepSeek, and other BPE-based models.

using Tiktoken;
using Tiktoken.Encodings;

// From a local file
var encoding = TokenizerJsonLoader.FromFile("path/to/tokenizer.json");
var encoder = new Encoder(encoding);

// From a stream (HTTP responses, embedded resources)
using var stream = File.OpenRead("tokenizer.json");
var encoding = TokenizerJsonLoader.FromStream(stream);

// From a URL (e.g., HuggingFace Hub)
using var httpClient = new HttpClient();
var encoding = await TokenizerJsonLoader.FromUrlAsync(
    new Uri("https://huggingface.co/openai-community/gpt2/raw/main/tokenizer.json"),
    httpClient,
    name: "gpt2");

// Custom regex patterns (optional — auto-detected by default)
var encoding = TokenizerJsonLoader.FromFile("tokenizer.json", patterns: myPatterns);

Supported pre-tokenizer types:

• ByteLevel — GPT-2 and similar models
• Split with regex pattern — direct regex-based splitting
• Sequence[Split, ByteLevel] — Llama 3, Qwen2, DeepSeek, and other modern models

Count message tokens (OpenAI chat format)

Count tokens for chat messages using OpenAI's official formula, including support for function/tool definitions:

using Tiktoken;

// Simple message counting
var messages = new List<ChatMessage>
{
    new("system", "You are a helpful assistant."),
    new("user", "What is the weather in Paris?"),
};
int count = TikTokenEncoder.CountMessageTokens("gpt-4o", messages);

// With tool/function definitions
var tools = new List<ChatFunction>
{
    new("get_weather", "Get the current weather", new List<FunctionParameter>
    {
        new("location", "string", "The city name", isRequired: true),
        new("unit", "string", "Temperature unit",
            enumValues: new[] { "celsius", "fahrenheit" }),
    }),
};
int countWithTools = TikTokenEncoder.CountMessageTokens("gpt-4o", messages, tools);

// Or use the Encoder instance directly
var encoder = ModelToEncoder.For("gpt-4o");
int toolTokens = encoder.CountToolTokens(tools);

Nested object parameters and array types are also supported:

new FunctionParameter("address", "object", "Mailing address", properties: new List<FunctionParameter>
{
    new("street", "string", "Street address", isRequired: true),
    new("city", "string", "City name", isRequired: true),
});

Custom encodings

Load encoding data from .tiktoken text files or .ttkb binary files:

using Tiktoken.Encodings;

// Load from file (auto-detects format by extension)
var ranks = EncodingLoader.LoadEncodingFromFile("my_encoding.ttkb");
var ranks = EncodingLoader.LoadEncodingFromFile("my_encoding.tiktoken");

// Load from binary byte array (e.g., from embedded resource or network)
byte[] binaryData = File.ReadAllBytes("my_encoding.ttkb");
var ranks = EncodingLoader.LoadEncodingFromBinaryData(binaryData);

// Convert text format to binary for faster loading
var textRanks = EncodingLoader.LoadEncodingFromFile("my_encoding.tiktoken");
using var output = File.Create("my_encoding.ttkb");
EncodingLoader.WriteEncodingToBinaryStream(output, textRanks);

The .ttkb binary format loads ~30% faster than .tiktoken text (no base64 decoding) and is 34% smaller. See data/README.md for the format specification and conversion tools.

Benchmarks

Benchmarked on Apple M4 Max, .NET 10.0, o200k_base encoding. Tested with diverse inputs: short ASCII, multilingual (12 scripts + emoji), CJK-heavy, Python code, and long documents.

CountTokens — zero allocation, fastest in class

Input	SharpToken	TiktokenSharp	Microsoft.ML	Tiktoken	Throughput	Speedup
Hello, World! (13 B)	226 ns	168 ns	317 ns	104 ns	119 MiB/s	1.6-3.0x
Multilingual (382 B, 12 scripts)	15.3 us	9.8 us	5.2 us	1.2 us	304 MiB/s	4.4-12.9x
CJK-heavy (1,676 B, 6 scripts)	113.0 us	68.9 us	40.1 us	2.8 us	571 MiB/s	14.5-40.8x
Python code (879 B)	13.5 us	10.2 us	22.5 us	6.6 us	127 MiB/s	1.5-3.4x
Multilingual long (4,312 B)	306.9 us	170.3 us	77.8 us	9.7 us	424 MiB/s	8.0-31.6x
Bitcoin whitepaper (19,884 B)	425.9 us	269.8 us	343.8 us	132.4 us	143 MiB/s	2.0-3.2x

Zero allocation across all inputs (0 B). Tiktoken's advantage is most pronounced on multilingual/CJK text — up to 41x faster than competitors. Throughput on cached multilingual text reaches 571 MiB/s, competitive with the fastest Rust tokenizers.

Cache effect on CountTokens

Built-in token cache dramatically accelerates repeated non-ASCII patterns:

Input	No cache	Cached	Cache speedup
Hello, World! (13 B)	108 ns	109 ns	—
Multilingual (382 B)	7.7 us	1.2 us	6.6x
CJK-heavy (1,676 B)	58.1 us	2.7 us	21.3x
Python code (879 B)	6.8 us	7.2 us	—
Multilingual long (4,312 B)	165.8 us	10.7 us	15.5x
Bitcoin whitepaper (19,884 B)	188.5 us	169.6 us	1.1x

Cache has no effect on ASCII-dominant inputs (already on fast path). On multilingual/CJK text, cache provides 6-21x speedup by skipping UTF-8 conversion and BPE on subsequent calls.

Encode — returns token IDs

Input	SharpToken	TiktokenSharp	Microsoft.ML	Tiktoken	Throughput	Speedup
Hello, World! (13 B)	218 ns	168 ns	322 ns	128 ns	97 MiB/s	1.3-2.5x
Multilingual (382 B, 12 scripts)	14.7 us	9.6 us	5.4 us	1.4 us	260 MiB/s	3.9-10.5x
CJK-heavy (1,676 B, 6 scripts)	111.6 us	65.7 us	37.9 us	3.4 us	470 MiB/s	11.1-32.6x
Python code (879 B)	13.5 us	9.8 us	22.3 us	7.0 us	120 MiB/s	1.4-3.2x
Multilingual long (4,312 B)	294.2 us	156.4 us	72.5 us	12.5 us	329 MiB/s	5.8-23.5x
Bitcoin whitepaper (19,884 B)	399.8 us	257.0 us	335.0 us	142.4 us	133 MiB/s	1.8-2.8x

Same performance characteristics as CountTokens, with additional allocation for the output int[] array.

Construction — encoder initialization

Encoding	Time	Description
o200k_base	0.78 ms	GPT-4o (200K vocab, pre-computed hash table, lazy FastEncoder)
cl100k_base	0.46 ms	GPT-3.5/4 (100K vocab)

Encoder construction includes loading embedded binary data, building hash tables, and compiling regex. FastEncoder and Decoder dictionaries are lazy-initialized on first use only. Reuse Encoder instances across calls for best performance.

Cross-language context

All numbers below measured on Apple M4 Max with identical inputs and o200k_base encoding. See benchmarks/cross-language/results/ for full reports.

Implementation	Language	Encode Throughput	CountTokens Throughput	Notes
Tiktoken (cached)	.NET/C#	97-467 MiB/s	119-577 MiB/s	Zero-alloc counting; cache gives 6-21x on multilingual
Tiktoken (no cache)	.NET/C#	97-133 MiB/s	28-123 MiB/s	Cold/first-call — fairest comparison to Rust/Python
tiktoken v3	Rust	34-88 MiB/s	—	Pure Rust, arena-based
GitHub bpe v0.3	Rust	33-64 MiB/s	29-66 MiB/s	Aho-Corasick, O(n) worst case
OpenAI tiktoken 0.12	Python	7-20 MiB/s	—	Rust core, but Python FFI overhead

.NET Tiktoken's token cache makes it dramatically faster than native Rust on repeated/multilingual text. Even without the cache, .NET is competitive with or faster than both Rust crates on most inputs. Contributions welcome to further close the gap on cold-path multilingual text!

You can view the full raw BenchmarkDotNet reports for each version here.

Support

Priority place for bugs: https://github.com/tryAGI/LangChain/issues
Priority place for ideas and general questions: https://github.com/tryAGI/LangChain/discussions
Discord: https://discord.gg/Ca2xhfBf3v