Stream Speech (SSE)

The text to convert to speech. Max 8000 characters after trim; whitespace-only strings are rejected.

TTS model to route the request to. Controls which model pool serves this synthesis.

• lightning_v3.1 (default) — standard Lightning v3.1.
• lightning_v3.1_pro — Lightning v3.1 Pro pool. Improved audio quality and naturalness, with a curated voice catalog. See the Lightning v3.1 Pro model card for supported voice IDs.

Same concurrency and latency profile across both. Other request parameters behave identically.

Language code for synthesis. Influences pronunciation, number/date normalization, and phoneme selection.

Default on lightning_v3.1_pro: when language is omitted, the Pro pool defaults to en + hi (mixed Indian + Western English coverage, auto-detected from the input text).

Each voice has its own tags.language set in the voice catalog — query GET /waves/v1/lightning-v3.1/get_voices. Pass a language the voice was trained on; passing other codes is accepted by the API but produces English-pronounced output.

auto (recommended for cross-language use cases): routes internally based on the input text. Any English or Hindi voice can be used across all supported languages when auto is set; the platform handles language-appropriate routing without needing a code per call.

On lightning_v3.1 — 20 supported languages:

• 10 European: English, Spanish, French, German, Italian, Dutch, Swedish, Portuguese, Polish, Russian
• 10 Indic: Hindi, Marathi, Gujarati, Punjabi, Bengali, Odia, Tamil, Telugu, Kannada, Malayalam

On lightning_v3.1_pro — 31 supported languages (adds 11 over base):

• 13 European: base 10 plus Greek, Finnish, Norwegian
• 8 Asian & Middle Eastern: Chinese, Japanese, Korean, Indonesian, Malay, Vietnamese, Turkish, Arabic
• 10 Indic: same as base
• Pass en → UK + American accented English.
• Pass hi → Indian accented English + Hindi (code-switching).
• Omit language → defaults to en + hi (mixed Indian + Western English coverage, auto-detected from input text).

Optional. Sets the language used to read out numeric content — numbers, currency amounts, times, and the numeric parts of dates and years — independently of the synthesis voice. Ordinary words are not translated.

• If you omit language, this value also becomes the synthesis language: model selection and voice routing follow it.
• If you set language explicitly, language always wins for synthesis and number_pronunciation_language only changes how numeric content is normalized. It works both ways — read numbers in Hindi under an English voice, or in English under a Hindi voice (tuned for Indian, often mixed-script, use cases).
• Omit this field to keep the existing behaviour — normalization follows language.

Note: only numeric tokens are re-spoken; the words around them stay in the text language. On a cross-language request names may also render in the target script (e.g. “Smith” → “स्मिथ”), which is generally the desired reading for native-language voices.

Accepts the same language codes as language (including auto, nl, sv).

Optional. Controls how all-numeric dates (e.g. 05/03/2024) are read out, independent of the synthesis voice:

• EU — day/month order (DD/MM): 05/03/2024 → “fifth March, twenty twenty four”.
• US — month/day order (MM/DD): 05/03/2024 → “May third, twenty twenty four”.

Omit this field to keep the existing behaviour: English dates default to US (MM/DD). Applies across all languages, so you can force day-first (DD/MM) reading — e.g. the Indian convention — on any voice.

Opt-in flag that reads digit-flanked math operators (5 x 3, 2 ^ 10, 6 ÷ 2) as words instead of leaving them for the default number reader. Off by default because in real traffic digit-flanked NxN is more often a product dimension, the 24x7 idiom, or a vehicle-registration code than an actual multiplication.

When true, the normalizer replaces the operator with the spoken word matched to number_pronunciation_language:

Localized only for hi and mr; every other language falls back to the English words. The operator word follows number_pronunciation_language, not the synthesis language, so language=en, number_pronunciation_language=hi reads 6 x 7 as “छः गुणा सात”.

Matching rules: unambiguous glyphs (× ÷ * ^ ** = + and the wrong-glyph x/X) fire glued or spaced (5x3, 5 x 3). The ambiguous - – − and / fire only when space-padded, so 5-3 stays a range and 1/2 stays a fraction. See Math notation for the full lexicon, known limitations (product dimensions, 24x7 idiom, vehicle-reg codes), and EU-language localizations.

Opt-in profanity filter for the submitted text. Off by default; passing this object is the only way to turn it on. The filter rewrites your text only with action: redact; otherwise it either lets the request through or rejects it before synthesis.

action: "reject" returns HTTP 400 with error_code: "CONTENT_FILTER_BLOCKED", the language checked and a match_count; the matched terms are never returned or logged. action: "flag" synthesizes normally and records the match.

Matching is whole-word, not substring. If no verdict is returned the request fails open and the audio is synthesized unfiltered.

See Content filter.

Format of the returned audio. pcm is the lowest-latency option but requires a decoder to play; mp3 and wav are directly playable in browsers and most media players. The server default is pcm when the field is omitted — the API playground uses mp3 so the generated audio is directly playable.

The IDs of the pronunciation dictionaries to use for speech generation. Available on both lightning_v3.1 and lightning_v3.1_pro.

WebSocket-only feature. Accepted on this endpoint but ignored — no per-word timing information is returned in the sync HTTP or SSE response shape. To receive status: "word_timestamp" frames with per-word { id, word, start, end } data, use the WebSocket endpoint wss://api.smallest.ai/waves/v1/tts/live. See Word-level timestamps.

Optional client-provided session identifier for correlation. Only alphanumeric characters, hyphens, underscores, and dots are allowed. Max 128 characters. Echoed back in response headers as X-External-Session-Id.

Optional client-provided request identifier for correlation. Only alphanumeric characters, hyphens, underscores, and dots are allowed. Max 128 characters. Echoed back in response headers as X-External-Request-Id.