> ## Documentation Index > Fetch the complete documentation index at: https://heygen-1fa696a7.mintlify.app/llms.txt > Use this file to discover all available pages before exploring further. # Send Message or Request Revision > Sends a follow-up message to an existing session. Use to answer agent questions, add context, or request edits to a generated video. Only valid for sessions created in chat mode. ## OpenAPI ````yaml /openapi/external-api.json post /v3/video-agents/{session_id} 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: 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) paths: /v3/video-agents/{session_id}: post: tags: - Video Agent summary: Send Message or Request Revision description: >- Sends a follow-up message to an existing session. Use to answer agent questions, add context, or request edits to a generated video. Only valid for sessions created in chat mode. operationId: sendVideoAgentMessage parameters: - name: session_id in: path required: true schema: type: string description: Session ID requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/SendMessageRequest' responses: '200': description: Successful response content: application/json: schema: type: object properties: data: $ref: '#/components/schemas/SendMessageResponse' '400': description: Invalid request parameters content: application/json: schema: type: object properties: error: $ref: '#/components/schemas/StandardAPIError' example: error: code: invalid_parameter message: '''message'' is required.' param: message 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' example: error: code: not_found message: Session not found. param: null doc_url: null '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: SendMessageRequest: additionalProperties: false description: >- Request body for sending a follow-up message, answering the agent's question, or requesting edits and revisions to a previously generated video. properties: message: description: Text message to the agent maxLength: 10000 minLength: 1 title: Message type: string avatar_id: anyOf: - type: string - type: 'null' default: null description: Override avatar for this message title: Avatar Id voice_id: anyOf: - type: string - type: 'null' default: null description: Override voice for this message title: Voice Id brand_kit_id: anyOf: - type: string - type: 'null' default: null description: Brand kit ID to apply for this message title: Brand Kit Id files: anyOf: - items: discriminator: mapping: asset_id: $ref: '#/components/schemas/AssetId' base64: $ref: '#/components/schemas/AssetBase64' url: $ref: '#/components/schemas/AssetUrl' propertyName: type oneOf: - $ref: '#/components/schemas/AssetUrl' - $ref: '#/components/schemas/AssetId' - $ref: '#/components/schemas/AssetBase64' maxItems: 20 type: array - type: 'null' default: null description: Optional file attachments (max 20 files) title: Files required: - message title: SendMessageRequest type: object SendMessageResponse: description: Response from sending a message to a session. properties: session_id: description: Session ID title: Session Id type: string run_id: description: Run ID for this message processing title: Run Id type: string title: anyOf: - type: string - type: 'null' default: null description: LLM-generated session title title: Title required: - session_id - run_id title: SendMessageResponse 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 AssetId: additionalProperties: false description: Asset input via HeyGen asset ID from the asset upload endpoint. properties: type: const: asset_id description: Input type discriminator title: Type type: string asset_id: description: HeyGen asset ID from the asset upload endpoint title: Asset Id type: string required: - type - asset_id title: AssetId type: object AssetBase64: additionalProperties: false description: Asset input via base64-encoded content. properties: type: const: base64 description: Input type discriminator title: Type type: string media_type: description: MIME type of the encoded content (e.g. "image/png") title: Media Type type: string data: description: Base64-encoded file content title: Data type: string required: - type - media_type - data title: AssetBase64 type: object x-mcp-visible: false AssetUrl: additionalProperties: false description: Asset input via publicly accessible HTTPS URL. properties: type: const: url description: Input type discriminator title: Type type: string url: description: Publicly accessible HTTPS URL for the asset title: Url type: string required: - type - url title: AssetUrl type: object 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. ````