> ## Documentation Index > Fetch the complete documentation index at: https://docs.canary.effectiveai.app/llms.txt > Use this file to discover all available pages before exploring further. # Send a message to a session > Send a follow-up to the agent. It returns at once with the message ID; the agent works through messages one at a time, in the order sent. Follow the message with GET /sessions/{id}/messages/{messageId}, or subscribe to session.message.completed. Repeating POST sends the message again. ## OpenAPI ````yaml /openapi.json post /sessions/{id}/messages openapi: 3.0.3 info: title: Effective API version: 2.0.0 description: The public Effective REST API. Authenticate using an Effective API key. servers: - url: https://api.canary.effectiveai.app/api/v2 description: Canary security: - bearerAuth: [] paths: /sessions/{id}/messages: post: tags: - Sessions summary: Send a message to a session description: >- Send a follow-up to the agent. It returns at once with the message ID; the agent works through messages one at a time, in the order sent. Follow the message with GET /sessions/{id}/messages/{messageId}, or subscribe to session.message.completed. Repeating POST sends the message again. operationId: sendSessionMessage parameters: - schema: type: string format: uuid description: Session UUID, e.g. 00000000-0000-4000-8000-000000000001. example: 00000000-0000-4000-8000-000000000001 required: true description: Session UUID, e.g. 00000000-0000-4000-8000-000000000001. name: id in: path requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/V2SendSessionMessageRequest' responses: '202': description: Message queued. headers: Location: schema: type: string required: true content: application/json: schema: $ref: '#/components/schemas/V2QueuedSessionMessage' '400': description: invalid_request or invalid_cursor. content: application/json: schema: $ref: '#/components/schemas/V2CodedError' '401': description: 'unauthenticated: valid credentials are required.' content: application/json: schema: $ref: '#/components/schemas/V2CodedError' '403': description: >- forbidden: readonly members, app credentials, members who cannot spend credits and members over their spending limit cannot run the agent. content: application/json: schema: $ref: '#/components/schemas/V2CodedError' '404': description: 'not_found: the session does not exist or is not visible to you.' content: application/json: schema: $ref: '#/components/schemas/V2CodedError' '409': description: >- agent_not_mentioned: the session is set in the web app to answer only messages that mention @agent; include @agent and resend. content: application/json: schema: $ref: '#/components/schemas/V2CodedError' '413': description: 'request_too_large: the request exceeds 64 KiB.' content: application/json: schema: $ref: '#/components/schemas/V2CodedError' '415': description: 'unsupported_media_type: use application/json.' content: application/json: schema: $ref: '#/components/schemas/V2CodedError' '500': description: 'internal_error: unexpected server error.' content: application/json: schema: $ref: '#/components/schemas/V2CodedError' '503': description: 'temporarily_unavailable: retry later.' content: application/json: schema: $ref: '#/components/schemas/V2CodedError' security: - bearerAuth: [] components: schemas: V2SendSessionMessageRequest: type: object properties: message: type: string minLength: 1 maxLength: 20000 description: What to ask or tell the agent. 1–20,000 characters after trimming. required: - message additionalProperties: false V2QueuedSessionMessage: type: object properties: id: type: string minLength: 1 maxLength: 200 description: >- Message ID returned when the message was sent, or listed in the conversation. example: 00000000-0000-4000-8000-000000000002 status: type: string enum: - queued required: - id - status additionalProperties: false V2CodedError: allOf: - $ref: '#/components/schemas/V2Error' - type: object properties: code: type: string minLength: 1 description: Stable error code. Clients must tolerate unknown future codes. details: type: array items: type: object properties: location: type: string enum: - path - query - header - body path: type: array items: type: string maxLength: 64 maxItems: 12 code: type: string enum: - required - invalid_type - invalid_value - unknown_field message: type: string minLength: 1 maxLength: 200 required: - location - path - code - message minItems: 1 maxItems: 20 description: Bounded safe validation issues; may omit some invalid fields. requestId: type: string minLength: 1 maxLength: 128 description: >- Existing server correlation for this HTTP attempt, when available. dataset: type: string enum: - naic - osha description: Dataset needed for dataset_not_enabled. businessCode: type: string description: The rater’s own refusal code, for business_rejection. violations: type: array items: type: object properties: code: type: string description: Authored or built-in constraint code message: type: string description: Authored message for the failed constraint path: type: string description: Input path of the offending value; empty for the request value: nullable: true description: Offending input value, size-bounded by the runtime constraint: type: string description: The constraint the value failed, as text cite: type: string nullable: true description: >- Filing citation of the constraint, when the source cites one required: - code - message - path - value - constraint - cite additionalProperties: false maxItems: 256 description: >- Each input constraint the submission failed, for invalid_inputs and business_rejection. violationsTruncated: type: boolean description: >- True when the rater reported more violations than violations holds. accessUrl: type: string nullable: true format: uri description: >- Where to add the product to your account, for product_access_required. required: - code V2Error: type: object properties: error: type: string minLength: 1 description: Human-readable explanation of the failure. required: - error securitySchemes: bearerAuth: type: http scheme: bearer description: 'Effective API key. Send Authorization: Bearer sk-eai-...' ```` This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.