> ## Documentation Index > Fetch the complete documentation index at: https://heygen-1fa696a7.mintlify.site/llms.txt > Use this file to discover all available pages before exploring further. # Append Avatar Realtime Text > Append a text delta to a session created with `type: text_stream`. Send fragments as they become available (e.g. tokens streamed from an upstream LLM); the avatar speaks them in order. Set `final: true` on the last delta to close input — further appends to a closed stream return 410. Returns the total bytes buffered so far. ## OpenAPI ````yaml /openapi/external-api.json post /v3/avatar-realtime/{stream_id}/text openapi: 3.1.0 info: title: HeyGen External API version: 1.0.0 description: >- HeyGen's external API for programmatic AI video creation. See https://docs.heygen.com for full documentation. contact: name: HeyGen Product Infra url: https://heygen.com servers: - url: https://api.heygen.com description: Production security: - ApiKeyAuth: [] - BearerAuth: [] tags: - name: Video Agent description: Create videos from text prompts using AI - name: Videos description: Create, list, retrieve, and delete videos - name: Voices description: Text-to-speech and voice management - name: Audio description: Search the background-music and sound-effects catalog - name: Video Translate description: Translate videos into other languages - name: User description: Account information and billing - name: Avatars description: List and manage avatars and looks - name: Avatar Realtime description: >- Low-latency streaming avatar sessions — create a stream, poll for its HLS URL, push text, consume per-word timestamps - name: Assets description: Upload files for use in video creation - name: Webhooks description: Manage webhook endpoints and events - name: Lipsync description: Dub or replace audio on existing videos - name: Brand description: >- Brand-related resources — brand kits (colors, fonts, logos) and brand glossaries (custom term translations) - name: HyperFrames description: Render HyperFrames composition zips into video — separate from /v3/videos - name: AI Clipping description: Turn long-form videos into ready-to-share short clips with captions paths: /v3/avatar-realtime/{stream_id}/text: post: tags: - Avatar Realtime summary: Append Avatar Realtime Text description: >- Append a text delta to a session created with `type: text_stream`. Send fragments as they become available (e.g. tokens streamed from an upstream LLM); the avatar speaks them in order. Set `final: true` on the last delta to close input — further appends to a closed stream return 410. Returns the total bytes buffered so far. operationId: appendAvatarRealtimeText parameters: - name: stream_id in: path required: true schema: type: string description: Streaming session identifier returned by POST /v3/avatar-realtime. requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/AppendTextDeltaRequest' responses: '202': description: Accepted — submission acknowledged; poll for completion. content: application/json: schema: type: object properties: data: $ref: '#/components/schemas/AppendTextDeltaResponse' '400': description: Invalid request parameters content: application/json: schema: type: object properties: error: $ref: '#/components/schemas/StandardAPIError' example: error: code: invalid_parameter message: '''delta'' must be non-empty unless final=true.' param: delta doc_url: null '401': description: Authentication failed content: application/json: schema: type: object properties: error: $ref: '#/components/schemas/StandardAPIError' example: error: code: authentication_failed message: Invalid or expired API key. Verify your x-api-key header. param: null doc_url: null '404': description: Resource not found content: application/json: schema: type: object properties: error: $ref: '#/components/schemas/StandardAPIError' '429': description: Rate limit exceeded content: application/json: schema: type: object properties: error: $ref: '#/components/schemas/StandardAPIError' example: error: code: rate_limit_exceeded message: >- Too many requests. Retry after the duration specified in the Retry-After header. param: null doc_url: null headers: Retry-After: description: Seconds to wait before retrying schema: type: integer security: - ApiKeyAuth: [] - BearerAuth: [] components: schemas: AppendTextDeltaRequest: additionalProperties: false description: >- Append one text delta to a streaming session created with type='text_stream'. properties: delta: description: >- Text fragment to append. May be a single token or a coalesced batch of tokens. Must be non-empty unless `final: true` (which allows an empty delta to close the stream). title: Delta type: string final: default: false description: >- If true, this is the last delta — input is closed after this request. Further POSTs to this stream return 410. title: Final type: boolean required: - delta title: AppendTextDeltaRequest type: object AppendTextDeltaResponse: description: Response data for POST /v3/avatar-realtime/{stream_id}/text. properties: ok: default: true description: Always true on success. title: Ok type: boolean buffered_bytes: description: Total bytes of text buffered for this session so far. title: Buffered Bytes type: integer required: - buffered_bytes title: AppendTextDeltaResponse type: object StandardAPIError: type: object properties: code: type: string description: Machine-readable error code example: invalid_parameter message: type: string description: Human-readable error message example: Video not found param: type: - string - 'null' description: Which request field caused the error doc_url: type: - string - 'null' description: Link to error documentation required: - code - message securitySchemes: ApiKeyAuth: type: apiKey in: header name: x-api-key description: HeyGen API key. Obtain from your HeyGen dashboard. BearerAuth: type: http scheme: bearer description: OAuth2 bearer token. ````