> ## Documentation Index > Fetch the complete documentation index at: https://docs.inworld.ai/llms.txt > Use this file to discover all available pages before exploring further. # Synthesize speech (async) > Submit a synthesis job that runs in the background and immediately receive a [long-running operation](https://google.aip.dev/151). Poll the operation via the Get operation endpoint until `done` is `true`, then download the results from the time-limited signed URLs in its `response`. The request body is identical to synchronous synthesis. Unlike the synchronous endpoints, a job never returns audio with requested timestamps silently missing: `timestampType` in a language without alignment support is rejected at submit, and an alignment failure during synthesis fails the operation. ## OpenAPI ````yaml post /tts/v1/voice:synthesizeAsync openapi: 3.0.0 info: title: Inworld Text-to-Speech API version: v1 contact: name: Inworld AI url: https://inworld.ai email: support@inworld.ai servers: - url: https://api.inworld.ai security: - inworld_basic: [] tags: - name: TextToSpeech - name: AudioPromptPreparationService - name: SpeechToPhonemesService paths: /tts/v1/voice:synthesizeAsync: post: tags: - TextToSpeech summary: Synthesize speech (async) description: >- Submit a synthesis job that runs in the background and immediately receive a [long-running operation](https://google.aip.dev/151). Poll the operation via the Get operation endpoint until `done` is `true`, then download the results from the time-limited signed URLs in its `response`. The request body is identical to synchronous synthesis. Unlike the synchronous endpoints, a job never returns audio with requested timestamps silently missing: `timestampType` in a language without alignment support is rejected at submit, and an alignment failure during synthesis fails the operation. operationId: TextToSpeech_SynthesizeSpeechAsync requestBody: $ref: '#/components/requestBodies/ttsv1SynthesizeSpeechRequest' responses: '200': description: >- The job was accepted. The returned operation is not yet done; poll it to track progress. content: application/json: schema: $ref: '#/components/schemas/longrunningOperation' examples: submitted: summary: Job accepted value: name: >- workspaces/{workspace}/ttsAsyncJobs/8f14e45f-ceea-4673-93d8-04f724c8a1b2/operations/1784837936461-p0sEhU metadata: null done: false 4XX: description: An unexpected error response. content: application/json: schema: $ref: '#/components/schemas/rpcStatus' x-codeSamples: - lang: bash label: cURL source: >- curl --location 'https://api.inworld.ai/tts/v1/voice:synthesizeAsync' \ --header "Authorization: Basic $INWORLD_API_KEY" \ --header 'Content-Type: application/json' \ --data '{ "text": "Hello, world! What a wonderful day to be a text-to-speech model!", "voiceId": "Dennis", "modelId": "inworld-tts-2", "audioConfig": { "audioEncoding": "MP3" }, "timestampType": "WORD" }' - lang: python label: Python source: |- import requests url = "https://api.inworld.ai/tts/v1/voice:synthesizeAsync" headers = { "Authorization": "Basic ", "Content-Type": "application/json" } payload = { "text": "Hello, world! What a wonderful day to be a text-to-speech model!", "voiceId": "Dennis", "modelId": "inworld-tts-2", "audioConfig": {"audioEncoding": "MP3"}, "timestampType": "WORD" } operation = requests.post(url, json=payload, headers=headers).json() print(operation["name"]) # poll this via GET /lro/v1alpha/{name} - lang: javascript label: JavaScript source: >- const url = 'https://api.inworld.ai/tts/v1/voice:synthesizeAsync'; const response = await fetch(url, { method: 'POST', headers: { 'Authorization': 'Basic ', 'Content-Type': 'application/json', }, body: JSON.stringify({ text: 'Hello, world! What a wonderful day to be a text-to-speech model!', voiceId: 'Dennis', modelId: 'inworld-tts-2', audioConfig: { audioEncoding: 'MP3' }, timestampType: 'WORD', }), }); const operation = await response.json(); console.log(operation.name); // poll this via GET /lro/v1alpha/{name} components: requestBodies: ttsv1SynthesizeSpeechRequest: content: application/json: schema: $ref: '#/components/schemas/ttsv1SynthesizeSpeechRequest' example: text: Hello, world! What a wonderful day to be a text-to-speech model! voiceId: Dennis modelId: inworld-tts-2 audioConfig: audioEncoding: LINEAR16 sampleRateHertz: 22050 deliveryMode: BALANCED applyTextNormalization: 'ON' required: true schemas: longrunningOperation: type: object description: >- A [google.longrunning.Operation](https://google.aip.dev/151) tracking an asynchronous or batch synthesis job. properties: name: type: string description: >- Server-assigned operation resource name, in the format `workspaces/{workspace}/ttsAsyncJobs/{job}/operations/{operation}` for async jobs or `workspaces/{workspace}/ttsBatchJobs/{batch}/operations/{operation}` for batch jobs. Pass it verbatim as the path of the Get operation endpoint to poll for completion. example: >- workspaces/{workspace}/ttsAsyncJobs/8f14e45f-ceea-4673-93d8-04f724c8a1b2/operations/1784837936461-p0sEhU metadata: description: >- Service-specific metadata associated with the operation. Not populated for TTS jobs yet; job metadata is planned for a later release, so do not write code that depends on this field staying absent. nullable: true allOf: - $ref: '#/components/schemas/protobufAny' done: type: boolean description: >- If `false`, the job is still running. If `true`, the job has finished and exactly one of `error` or `response` is set. error: $ref: '#/components/schemas/rpcStatus' response: description: >- Set when the job succeeded. Its `@type` identifies which kind: an async job carries SynthesizeSpeechAsyncResponse, a batch job SynthesizeSpeechBatchResponse. oneOf: - $ref: '#/components/schemas/ttsv1SynthesizeSpeechAsyncResponse' - $ref: '#/components/schemas/ttsv1SynthesizeSpeechBatchResponse' rpcStatus: type: object properties: code: type: integer format: int32 description: >- The error code, as specified by [gRPC status codes](https://grpc.io/docs/guides/status-codes/). example: 5 message: type: string description: A short description of the error. example: 'Unknown voice: John not found!' details: type: array items: $ref: '#/components/schemas/protobufAny' example: [] ttsv1SynthesizeSpeechRequest: type: object properties: text: type: string description: >- The text to be synthesized into speech. Maximum input of 2,000 characters. voiceId: type: string description: The ID of the voice to use for synthesizing speech. audioConfig: $ref: '#/components/schemas/ttsv1AudioConfig' modelId: type: string description: >- The ID of the model to use for synthesizing speech. See [Models](../../../tts/tts-models) for available models. language: type: string description: >- BCP-47 language tag (e.g., `en-US`, `fr-FR`, `ja-JP`) specifying the language that the given voice should speak the text in. Matching is case- and separator-insensitive for standard two-part tags (`en-gb`, `EN_GB`, and `en-GB` are equivalent); longer tags with extension subtags must match a catalog entry exactly. If a localized voice prompt exists for the language, it will be used. When omitted, the original voice prompt will be used and the language will be auto-detected from the input text. If an invalid language code is provided, an error will be returned. See [Languages](../../../tts/capabilities/multilingual) for more details. deliveryMode: type: string enum: - DELIVERY_MODE_UNSPECIFIED - STABLE - BALANCED - CREATIVE default: DELIVERY_MODE_UNSPECIFIED description: >- *Only supported by `inworld-tts-2`. The field is ignored on other models.* Controls how varied the output is. - `DELIVERY_MODE_UNSPECIFIED`: Defaults to `BALANCED` behavior. - `STABLE`: Optimizes for more consistent, predictable output. - `BALANCED`: Balanced between stability and diversity. - `CREATIVE`: Optimizes for increased emotional range and variation. instruction: type: string description: >- *Only supported by `inworld-tts-2`. The field is ignored on other models.* Speaking-style instruction for this request — for example `speak loudly and urgently` or `sound out of breath`. Applies to the whole request. Write it in English, even when `text` is in another language. An empty string means unset. You can also change the instruction mid-text with inline `[bracket]` tags. A tag applies from where it appears until you change it, so it overrides this field from that point on; `[reset]` removes the instruction for the rest of the text. Prefer one approach or the other rather than combining them. See [Steering](../../../tts/capabilities/steering) for the full guide. temperature: type: number format: float default: 1 description: >- *Ignored on `inworld-tts-2`. Use [`deliveryMode`](#body-delivery-mode) instead.* Determines the degree of randomness when sampling audio tokens to generate the response. Defaults to 1.0. Accepts values between 0 (exclusive) and 2 (inclusive). Higher values will make the output more random and can lead to more expressive results. Lower values will make it more deterministic. If 0 is provided, the default value will be used. For the most stable results, we recommend using the default value. timestampType: type: string enum: - TIMESTAMP_TYPE_UNSPECIFIED - WORD - CHARACTER default: TIMESTAMP_TYPE_UNSPECIFIED description: >- Controls timestamp metadata returned with the audio. When enabled, the response includes timing arrays, which can be useful for word-highlighting, karaoke-style captions, and lipsync. - WORD: Output arrays under `timestampInfo.wordAlignment` (words, wordStartTimeSeconds, wordEndTimeSeconds). - CHARACTER: Output arrays under `timestampInfo.characterAlignment` (characters, characterStartTimeSeconds, characterEndTimeSeconds). - TIMESTAMP_TYPE_UNSPECIFIED: Do not compute alignment; timestamp arrays will be empty or omitted. **Phonetic details:** `phoneticDetails` is currently only returned for **WORD** alignment (not CHARACTER). **Latency note:** Alignment adds additional computation. Enabling alignment can increase latency. applyTextNormalization: type: string enum: - APPLY_TEXT_NORMALIZATION_UNSPECIFIED - 'ON' - 'OFF' default: APPLY_TEXT_NORMALIZATION_UNSPECIFIED description: >- When enabled, text normalization automatically expands and standardizes things like numbers, dates, times, and abbreviations before converting them to speech. For example, Dr. Smith becomes Doctor Smith, and 3/10/25 is spoken as March tenth, twenty twenty-five. Turning this off may reduce latency, but the speech output will read the text exactly as written. Defaults to automatically deciding whether to apply text normalization. enhanceGeneration: type: boolean default: false description: >- When `true`, applies denoising to the synthesized audio to reduce background noise and artifacts, improving the overall audio quality of the generation. Defaults to `false` (no denoising). synthesisContext: $ref: '#/components/schemas/ttsv1SynthesisContext' required: - text - voiceId - modelId protobufAny: type: object properties: '@type': type: string additionalProperties: {} ttsv1SynthesizeSpeechAsyncResponse: type: object description: >- Result of a completed asynchronous synthesis job, packed into the operation's `response` field. The download URLs are pre-signed and time-limited: fetch them without an `Authorization` header, and before `expireTime`. properties: '@type': type: string description: >- Type of the serialized message, always `type.googleapis.com/ai.inworld.tts.v1.SynthesizeSpeechAsyncResponse`. example: type.googleapis.com/ai.inworld.tts.v1.SynthesizeSpeechAsyncResponse audioUri: type: string description: >- Pre-signed download URL for the synthesized audio, in the format requested via `audioConfig.audioEncoding`. example: https://storage.googleapis.com/…/audio.mp3?X-Goog-Signature=… timestampsUri: type: string description: >- Pre-signed download URL for the timestamp alignment JSON. Only present when `timestampType` was set in the request. example: https://storage.googleapis.com/…/timestamps.json?X-Goog-Signature=… expireTime: type: string format: date-time description: >- Time at which the download URLs expire, approximately 7 days after job completion. example: '2026-07-30T12:00:00Z' ttsv1SynthesizeSpeechBatchResponse: type: object description: >- Result of a completed batch, packed into the operation's `response` field. A batch's per-item URLs do not fit in one response, so this names a results *file* instead. The URL is pre-signed and time-limited: fetch it without an `Authorization` header, and before `expireTime`. properties: '@type': type: string description: >- Type of the serialized message, always `type.googleapis.com/ai.inworld.tts.v1.SynthesizeSpeechBatchResponse`. example: type.googleapis.com/ai.inworld.tts.v1.SynthesizeSpeechBatchResponse resultsUri: type: string description: >- Pre-signed download URL for the results file, whose contents are described by SynthesizeSpeechBatchResults. example: https://storage.googleapis.com/…/results.json?X-Goog-Signature=… expireTime: type: string format: date-time description: >- Time at which the results file and every audio URL inside it expire, approximately 7 days after the batch completes. example: '2026-08-13T18:13:47Z' ttsv1AudioConfig: type: object properties: audioEncoding: $ref: '#/components/schemas/v1AudioConfigAudioEncoding' bitRate: type: integer format: int32 description: >- Bits per second of the audio. Only for compressed audio formats (`MP3`, `OGG_OPUS`). The default is 128,000. sampleRateHertz: type: integer format: int32 description: >- The synthesis sample rate (in hertz) for this audio. Accepts values within the range [8000, 48000]. Supported sample rates are: 8000, 16000, 22050, 24000, 32000, 44100, 48000. When this is specified, if this is different from the voice's natural sample rate, then the audio will be converted to the desired sample rate (which might result in worse audio quality), unless the specified sample rate is not supported for the encoding chosen, in which case it will fail the request and return an error. The default is 48,000. speakingRate: type: number format: double description: >- Speaking rate/speed, in the range [0.5, 1.5]. The default is 1.0, which is the normal native speed supported by the specific voice. We recommend using values above 0.8 to ensure high quality. description: Configurations to use when synthesizing speech. ttsv1SynthesisContext: type: object description: >- Context for the current synthesis request. Supplying the text of earlier requests from the same session or conversation gives the model additional context and can improve the quality of the generation, especially for short or ambiguous input text. properties: previousRequests: type: array description: >- Previous requests from the same session or conversation, in the order they were synthesized. items: type: object properties: text: type: string description: The text that was synthesized in the previous request. v1AudioConfigAudioEncoding: type: string enum: - LINEAR16 - MP3 - OGG_OPUS - ALAW - MULAW - FLAC - PCM - WAV default: MP3 description: |- The desired output format of the synthesized audio. Defaults to `MP3`. - `LINEAR16`: Uncompressed 16-bit signed little-endian samples (Linear PCM). For non-streaming, the WAV header is included in the response. For streaming, the WAV header is included in every audio chunk. - `MP3`: MP3 audio. - `OGG_OPUS`: Opus encoded audio wrapped in an ogg container. The result will be a file which can be played natively on Android, and in browsers (at least Chrome and Firefox). The quality of the encoding is considerably higher than MP3 while using approximately the same bitrate. - `ALAW`: ALAW encoded audio. 8-bit companded PCM. - `MULAW`: MULAW encoded audio. 8-bit companded PCM. - `FLAC`: FLAC encoded audio. Lossless audio format. - `PCM`: PCM audio. Uncompressed 16-bit signed little-endian samples with no WAV header. - `WAV`: WAV audio. Uncompressed 16-bit signed little-endian samples. For non-streaming, the WAV header is included in the response. For streaming, the WAV header is included in the first audio chunk only. securitySchemes: inworld_basic: type: apiKey in: header name: Authorization description: >- Your [authentication](../../../api-reference/introduction) credentials. For Basic authentication, please populate `Basic $INWORLD_API_KEY`. You can create a key in one command with the [Inworld CLI](../../../tts/resources/inworld-cli): `inworld workspace add-key`. ````