Kokoro Tts

Kokoro TTS (open-weight, self-hosted)

Kokoro is a small, fast, permissively licensed text-to-speech model. Its entire value proposition is that you run it yourself: no API key, no per-character billing, no audio leaving your machine. This skill helps an agent decide whether Kokoro fits a job, run it through the right runtime, and get acceptable output — and, just as importantly, recognize the jobs where Kokoro will disappoint the user and something else is the correct answer.

All version, license, voice-count, ranking, and performance facts below were verified on 2026-07-10 against the sources listed at the end. Treat them as volatile.

What Kokoro is (documented facts)

• Model. ~82 million parameters. Architecture is StyleTTS 2 (arXiv 2306.07691) with an ISTFTNet decoder (arXiv 2203.02395). The card describes it as "Decoder only: no diffusion, no encoder release." Source: hexgrad/Kokoro-82M model card.
• License. Apache 2.0, including the weights. v0.19 weights were released in full fp32 on 2024-12-25; v1.0 released 2025-01-27 and is the current default. Because weights are Apache-2.0 you may deploy commercially, redistribute, and fine-tune (subject to attribution). Source: model card.
• Training data & provenance. "Few hundred hrs" for v1.0, trained exclusively on permissive / non-copyrighted material: public-domain audio, Apache/MIT-licensed content, and synthetic audio generated by closed TTS models, plus <1 hr from Koniwa (CC BY 3.0) and <11 hrs from SIWIS (CC BY 4.0). Reported training cost ≈ $1000 (~1000 A100-80GB GPU-hours). The heavy reliance on synthetic data is the root cause of Kokoro's flat prosody and its uneven non-English quality — keep it in mind. Source: model card.
• Output. 24 kHz mono audio. Sample rate is fixed at 24000 Hz. Source: model card.
• Coverage. v1.0 ships 8 languages and 54 voices (American + British English count as one language). Sources: model card, VOICES.md.

When to use Kokoro vs. when not to

Reach for Kokoro when:

• You need cost-free, high-volume English narration (audiobooks, course/video voiceover, batch document-to-speech, screen readers). Marginal cost is electricity.
• The pipeline is offline or privacy-constrained — medical, legal, on-device, air-gapped — and no text may be sent to a cloud API.
• You want in-browser / on-device TTS with no server (kokoro-js + WebGPU/WASM).
• You are prototyping and want a good-enough voice today without provisioning a paid provider.

Do NOT use Kokoro (and tell the user so) when the job needs:

• Voice cloning / a specific person's voice. Kokoro has no speaker encoder and no zero-shot cloning. The encoder was deliberately not released. You cannot clone a reference voice. Route to a cloning-capable provider.
• Emotional or character performance — laughter, crying, shouting, sarcasm, dynamic delivery. Kokoro has no emotion/style tokens and a narrow prosodic range; output is competent but flat. Fine for a neutral narrator, wrong for a video-game character.
• High-fidelity non-English or many languages. Non-English voices are mostly C/D-graded and trained on little data (see quality grades below); several languages also truncate long text. English is the only tier-1 language.
• Conversational agents that need real-time bidirectional dialogue with barge-in and personality. Kokoro is a batch/streaming synthesizer, not a dialog voice.

If the request is voice cloning or emotional VO, do not try to fake it with blending or prompt tricks — state the limitation plainly and suggest a cloning/expressive provider.

Voices, language codes, and quality grades

Voice IDs follow [langprefix][gender]_[name], e.g. af_heart = American Female "Heart", bm_george = British Male "George", if_sara = Italian Female "Sara".

Language codes (pass as lang_code in Python; aliases en-us→a, en-gb→b):

Source: VOICES.md.

Quality is not uniform — pick by grade, not by name. VOICES.md assigns each voice an "Overall" grade combining a target-quality letter and how much training audio it received (more audio = higher grade). Documented highlights (verified 2026-07-10):

• Best English female: af_heart (grade A, the card's default), af_bella (A−), af_nicole (B−, breathy/ASMR), bf_emma (B−, British).
• Solid English: af_aoede, af_kore, af_sarah, am_fenrir, am_michael, am_puck (all C+). bm_fable, bm_george (C, British male).
• Avoid unless you have a reason: many voices grade C or below; e.g. am_adam (F+), af_jessica / af_river (D). Most non-English voices are C/D, trained on ~minutes of synthetic data.

Production heuristic: default to af_heart (lang a). For a project the user should audition 3–4 A/B-graded voices before committing — grades predict, they do not guarantee, per-sentence quality.

Text length, chunking, and long-form synthesis

Documented limit: Kokoro processes at most 510 phonemized tokens per forward pass (512 with boundary tokens). VOICES.md notes voices "perform best on a goldilocks range of 100–200 tokens," are weak on very short utterances (<10–20 tokens)**, and **rush on long ones (>400). Source: model card / VOICES.md.

Consequences for production:

• Never feed a whole chapter as one string. Split into sentences/paragraphs and stitch.
• The Python KPipeline splits automatically; its split_pattern defaults to r'\n+' for English and returns one (graphemes, phonemes, audio) result per chunk, which you concatenate. Source: pipeline.py.
• Non-English chunking is not fully implemented. Long non-English text can be truncated unless you pre-split it yourself (insert \n at sentence boundaries). This is a common silent-failure trap — verify non-English output length.
• Heuristic for clean long-form: chunk to roughly 100–250 tokens at sentence boundaries (≈ one to three sentences), synthesize each, and concatenate with a short silence pad. Kokoro-FastAPI's defaults (~175 target / 250 / 450 absolute max tokens) are a reasonable starting point if you build your own splitter.

Pronunciation control (misaki + espeak-ng)

Kokoro does not read graphemes directly — text is converted to phonemes by misaki, hexgrad's G2P library, then fed to the model. English uses misaki's dictionary (spaCy + num2words). Out-of-dictionary words fall back to espeak-ng (EspeakFallback, on by default); espeak-ng is also the backbone for non-English G2P. Install espeak-ng as a system dependency or OOV words degrade to letter-by-letter spelling. Documented example: with fallback, eBook → ˈi bˈʊk; without it, → ˈiː bˈi ˈoʊ ˈoʊ kˈeɪ (spelled out). Source: misaki README.

To fix a mispronounced word (proper noun, brand, acronym, number read wrong):

1. Inline phoneme override — misaki accepts a markdown-like syntax [word](/phonemes/), e.g. [Misaki](/misˈɑki/) or [Kokoro](/kˈOkəɹO/). Put the IPA/Kokoro phonemes between the slashes; stress marks like ˈ matter.
2. Phonemize once, reuse — generate phonemes with misaki, hand-correct, and pass phonemes directly to the model so a batch job stays consistent.
3. Spell it out in text — reword ("A-P-I", "twenty twenty-six") when phonemes are overkill.

Heuristic: always dry-run domain jargon, names, and numbers before a long batch — these are Kokoro's most common error class, and each is a one-line phoneme fix.

Voice blending (mixing)

A Kokoro "voice" is a style vector (voicepack tensor). Blending is a weighted average of two style vectors, which produces a new usable voice. The documented mechanism is a weighted numpy add, style1*(w0/100) + style2*(w1/100), with weights normalized if they don't sum to 100; several tools cap blending at exactly two voices. Source: nazdridoy/kokoro-tts voice-blending docs.

• In Kokoro-FastAPI, request a blend by combining voice IDs: voice="af_sky+af_bella" (equal), or weighted per that server's syntax. Source: remsky/Kokoro-FastAPI.
• In native Python you can load two voicepack tensors and average them yourself for full control (any ratio; nothing forces a 2-voice cap if you write the math).

Use blending to: nudge timbre/pitch between two graded voices, or build a house voice that isn't any single shipped one. It does not add emotion, create a new speaker identity from a reference, or rescue a low-grade voice — averaging two C-grade voices yields a C-grade blend.

Runtimes — pick by deployment target

1. Python kokoro package (default for servers/batch)

Best for narration pipelines, audiobooks, and anything on your own box. PyTorch backend; GPU optional. Example (labeled example — adapt paths/voices):

# pip install kokoro>=0.9.2 soundfile   ; plus system espeak-ng
from kokoro import KPipeline
import soundfile as sf
import numpy as np

pipeline = KPipeline(lang_code='a')          # 'a' = American English
text = "The quarterly report is ready.\nRevenue rose twelve percent."

chunks = []
for graphemes, phonemes, audio in pipeline(text, voice='af_heart', speed=1.0):
    chunks.append(audio)                     # one result per split (default r'\n+')

sf.write('out.wav', np.concatenate(chunks), 24000)   # 24 kHz mono

Why structured this way: KPipeline does G2P + chunking + inference; iterating yields per-chunk audio you concatenate, which is exactly the long-form pattern above. speed (~0.8–1.3) trades pace for naturalness. For lower-level control, KModel runs a single already-phonemized chunk. Source: model card, Python API.

2. kokoro-js / ONNX (browser, on-device, Node)

Runs 100% client-side via Transformers.js — no server, nothing uploaded. Model id onnx-community/Kokoro-82M-v1.0-ONNX. Example (labeled example):

// npm i kokoro-js
import { KokoroTTS, TextSplitterStream } from "kokoro-js";

const tts = await KokoroTTS.from_pretrained("onnx-community/Kokoro-82M-v1.0-ONNX", {
  dtype: "q8",        // "fp32" | "fp16" | "q8" | "q4" | "q4f16"
  device: "webgpu",   // "wasm" | "webgpu" in-browser, "cpu" in Node ; use fp32 with webgpu
});

const audio = await tts.generate("Hello from the browser.", { voice: "af_heart" });
audio.save("audio.wav");                 // tts.list_voices() lists all IDs

// Streaming: push tokens, get audio incrementally
const splitter = new TextSplitterStream();
const stream = tts.stream(splitter);
(async () => { for await (const { text, phonemes, audio } of stream) audio.save("chunk.wav"); })();

Quantization trade-off: q8/q4 shrink download and speed WASM at some quality cost; fp32 is highest quality and is recommended with WebGPU. Source: kokoro-js README, onnx-community/Kokoro-82M-v1.0-ONNX.

For non-JS ONNX use, kokoro-onnx (Python) runs on onnxruntime (CPU) or onnxruntime-gpu (CUDA) and is what enables Raspberry-Pi / edge deployments.

3. Kokoro-FastAPI (drop-in OpenAI-compatible server)

The fastest way to give an existing app a local TTS backend: a Dockerized wrapper exposing an OpenAI-compatible /v1/audio/speech endpoint, so any client written for OpenAI TTS works by changing the base URL. Supports voice mixing (af_sky+af_bella), MP3/WAV/Opus/FLAC/M4A/PCM, streaming, per-word timestamps, a phoneme endpoint, and CPU/NVIDIA/AMD images. Example (labeled example):

from openai import OpenAI
client = OpenAI(base_url="http://localhost:8880/v1", api_key="not-needed")

client.audio.speech.create(
    model="kokoro", voice="af_heart", input="Local TTS, OpenAI-shaped API.",
    response_format="mp3",
).stream_to_file("out.mp3")

Source: remsky/Kokoro-FastAPI. Note it is a third-party wrapper (Apache-2.0-licensed model, separate project) — pin a version and verify its endpoint/voice-mixing syntax against its current README, as it evolves.

Hosted endpoints. Several inference platforms (e.g. Replicate, Baseten, and others) host Kokoro if you want the model's economics without self-hosting; those reintroduce a per-use cost and send text off-box, so they undercut the two main reasons to choose Kokoro. Prefer them only for burst capacity or when you can't run the model locally.

Performance expectations (secondary, dated evidence)

RTF (real-time factor) definitions differ between sources — some report audio-seconds-per-compute-second (higher = faster), others the inverse. Read the units.

• GPU is dozens of times faster than real-time. One benchmark (PyTorch, ~16k chars, chunked ≤510 tokens) reports ~96× RTF on an A10G, ~81× on L4, ~36× on T4; ONNX ran lower (20–37×). Source: Kokoro v1 benchmark gist, retrieved 2026-07-10.
• CPU is still comfortably faster than real-time on many cores — the same benchmark shows ~5× RTF on a 32-vCPU instance; another reports RTF ≈ 0.45–0.51 (i.e. ~2× real-time) on 4 cores. Source: gist above and a 4-core AMD EPYC run, retrieved 2026-07-10.
• Footprint: 82M params is tiny; the model loads in well under a GB, and 4 GB RAM suffices for inference (8 GB+ for comfortable batching). Runs on modest hardware and Raspberry-Pi-class devices via ONNX.

Heuristic: for real-time or streaming UX, prefer GPU or a strong multi-core CPU; low-core/edge CPUs work for batch/offline but may fall near or below real-time on long text. Quantized ONNX (q8/q4) helps on constrained CPU/WASM at a quality cost.

Benchmark / ranking standing (dated, mixed evidence)

• First-party claim (2024-12): the card states Kokoro v0.19 was "#1 ranked" in the TTS Spaces Arena in the weeks around its release. This was a limited-model / single-voice Arena setting — strong signal for its size, not a claim of beating all commercial models. Source: model card.
• Broader arenas (2026): on wider TTS leaderboards Kokoro sits mid-pack among open-weight models. As of ~2026-03, one aggregated leaderboard placed Kokoro-82M v1.0 ~4th among open-weight models (Elo ≈ 1060), with newer/larger open models (e.g. Step Audio EditX, Elo ≈ 1118) ahead. Secondary source, retrieved 2026-07-10: TTS Arena / Artificial Analysis.

Honest framing for a user: Kokoro is exceptional for 82M parameters and $0 marginal cost, competitive with far larger models on neutral English narration, and clearly behind frontier commercial and larger open models on expressiveness, cloning, and multilingual fidelity. Sell it on economics, privacy, and footprint — not on being the highest-quality voice available.

Output review checklist

Before shipping Kokoro audio, listen for:

• Mispronounced names / jargon / numbers — the top failure. Fix with a phoneme override.
• Rushed or clipped delivery on chunks over ~400 tokens — re-split shorter.
• Truncated non-English long text — verify duration; pre-split with \n.
• Artifacts on very short lines (one or two words) — pad with context or a trailing period.
• Wrong-accent voice for the language — match lang_code to the voice's prefix (a/b voices with English text, etc.); mismatches sound off.
• Flat affect where the script needed emotion — if it reads wrong, the fix is a different model, not more retries.

Safety, licensing, and rights

• Weights are Apache-2.0 — commercial use, redistribution, and fine-tuning are permitted with attribution. Verify the license of any wrapper (e.g. Kokoro-FastAPI) separately; they are distinct projects. (Verified 2026-07-10.)
• Synthetic-data provenance: training included synthetic audio from closed TTS models. This is documented and the released data (Koniwa CC BY 3.0, SIWIS CC BY 4.0) is permissively licensed, but if a client has strict provenance requirements, disclose it.
• No cloning ≠ no misuse risk. Even without cloning, generated speech can be used to impersonate a style or produce misleading audio. Don't generate audio that impersonates a real, identifiable person or is designed to deceive; disclose synthetic voice where the audience could reasonably assume it's human.
• Voices are model artifacts, not real people — the names (Heart, Emma, George) are labels, not consenting individuals, so there's no per-speaker consent issue; the general synthetic-media disclosure norm still applies.

Sources (verified 2026-07-10)

• hexgrad/Kokoro-82M model card — https://huggingface.co/hexgrad/Kokoro-82M
• VOICES.md — https://huggingface.co/hexgrad/Kokoro-82M/blob/main/VOICES.md
• kokoro Python package / repo — https://github.com/hexgrad/kokoro , https://pypi.org/project/kokoro/
• misaki G2P — https://github.com/hexgrad/misaki
• kokoro-js — https://github.com/hexgrad/kokoro/tree/main/kokoro.js , https://www.npmjs.com/package/kokoro-js
• ONNX build — https://huggingface.co/onnx-community/Kokoro-82M-v1.0-ONNX
• Kokoro-FastAPI — https://github.com/remsky/Kokoro-FastAPI
• Voice-blending mechanism (secondary) — https://deepwiki.com/nazdridoy/kokoro-tts/3.2-voice-blending
• Performance benchmark (secondary) — https://gist.github.com/efemaer/23d9a3b949b751dde315192b4dcf0653
• TTS ranking (secondary) — https://artificialanalysis.ai/text-to-speech/model-families/kokoro