> ## Documentation Index > Fetch the complete documentation index at: https://www.cometchat.com/docs/llms.txt > Use this file to discover all available pages before exploring further. > Sends Message on behalf of a user. # Send Message ## Constraints | Item | Constraint | Notes | | --------------------------------------------------------------------------- | ------------------------------------------------------- | ------------------------------------------------------------------ | | Maximum message size | 65 KB (\~65,536 characters including metadata) | Total size limit for the entire message payload | | File size per message | 100 MB | Applies to the uploaded file associated with the message | | Message data object | 10 KB max (UTF8mb4) | Keys with special meaning: text, attachments, customData, metadata | | Tags | Up to 25 tags, 100 characters each (UTF8mb4) | Tags can be in any language | | User mentions | Up to 10 distinct users | Format: `<@uid:{uid of the user}>` | | Reactions | Max 25 distinct reactions, 45 characters each (UTF8mb4) | Emoji supported (may use multiple code points) | | Unread message count (groups) | Up to 300 members | Not updated for larger groups | | Message receipts (groups) | Up to 300 online users | Delivery/read receipts disabled in large groups | | For the complete error reference, see [Error Guide](/articles/error-guide). | | | ## OpenAPI ````yaml post /messages openapi: 3.0.0 info: title: Chat APIs description: Manage messages, users, groups for a particular app using our Chat API. version: '3.0' servers: - url: https://{appId}.api-{region}.cometchat.io/v3 variables: appId: default: appId description: (Required) App ID region: enum: - us - eu - in default: us description: Select Region security: [] tags: - name: API Keys description: The API keys are used to authorise the APIs - name: Roles description: The roles are used to give user access rights - name: Users description: The REST collection for users. - name: Auth Tokens description: The auth tokens are used to login end users using client SDKs. - name: Blocked Users description: The REST collections for blocked users. - name: Friends description: List,add and remove friends by passing UID in path variables - name: Groups description: The REST collections for groups. - name: Banned Users description: Ban and Unban user by passing other UID in path variables. - name: Group Members description: The REST collections for group members. - name: Messages description: The REST collections for messages. - name: Conversations description: The REST collections for conversations. - name: Restrict Features description: Allows Restricting Features - name: Metrics description: Allows accessing Data Metrics - name: Triggers description: Allows adding triggers to a webhook. - name: Webhooks description: Allows accessing Webhooks. - name: Notifications description: Allows configuring Notifications core. paths: /messages: post: tags: - Messages summary: Send Message description: Sends Message on behalf of a user. operationId: send-message parameters: - $ref: '#/components/parameters/onBehalfOf' requestBody: content: application/json: schema: discriminator: propertyName: category mapping: message: $ref: '#/components/schemas/messageCategorySchema' custom: $ref: '#/components/schemas/customCategorySchema' oneOf: - $ref: '#/components/schemas/messageCategorySchema' title: Message - $ref: '#/components/schemas/customCategorySchema' title: Custom examples: {} responses: '200': description: Create Message content: application/json: schema: properties: data: type: object type: object example: data: id: '1' conversationId: cometchat-uid-3_user_cometchat-uid-5 sender: cometchat-uid-3 receiverType: user receiver: cometchat-uid-5 category: message type: text data: text: test hello metadata: key1: val1 entities: sender: entity: uid: cometchat-uid-3 name: Nancy Grace avatar: >- https://assets.cometchat.io/sampleapp/v2/users/cometchat-uid-3.webp status: offline role: default createdAt: 1638361550 entityType: user receiver: entity: uid: cometchat-uid-5 name: John Paul avatar: >- https://assets.cometchat.io/sampleapp/v2/users/cometchat-uid-5.webp status: offline role: default createdAt: 1638361550 conversationId: cometchat-uid-3_user_cometchat-uid-5 entityType: user sentAt: 1638423490 updatedAt: 1638423490 security: - apiKey: [] components: parameters: onBehalfOf: name: onBehalfOf in: header description: UID of the user on whose behalf the action is performed. schema: type: string schemas: messageCategorySchema: properties: receiver: description: The receiver of the message. type: string muid: description: The muid will be a unique Identifier of the message. type: string receiverType: description: The receiverType of the message. either user or group type: string enum: - user - group category: description: >- Category of the message. The available categories are message and custom. type: string default: message enum: - message type: description: >- Type of the message. The available values are text, image, file, audio, video. type: string default: text enum: - text - image - file - audio - video data: description: JSON containing message attributes. properties: text: type: string metadata: type: object attachments: description: >- For the messages with image, video, audio or file type (i.e. category=="message" && type !="text"), the property contains an array of attachment objects. type: array items: properties: url: description: Contains the URL of the attachment. type: string name: description: Name of the attachment. type: string mimeType: description: Mime Type of attachment. type: string extension: description: The extension of the attachment. type: string size: description: The size of the attachment(in bytes). type: string type: object type: object multipleReceivers: description: >- JSON containing array of UIDs and GUID for whom the message must be sent. Format for multiple receivers - {"uids": ["uid1","uid2"], "guids":["guid1"]} properties: uids: type: array items: type: string guids: type: array items: type: string type: object tags: description: A list of tags to identify specific messages. type: array items: type: string type: object customCategorySchema: properties: receiver: description: The receiver of the message. type: string muid: description: The muid will be a unique Identifier of the message. type: string receiverType: description: The receiverType of the message. either user or group type: string enum: - user - group category: description: Category of the message. The available categories is custom. type: string default: custom enum: - custom type: description: Type of the message. type: string data: description: JSON containing message attributes. properties: customData: description: when category==”custom”, this property can have any JSON object. type: object type: object multipleReceivers: description: >- JSON containing array of UIDs and GUID for whom the message must be sent. Format for multiple receivers - {"uids": ["uid1","uid2"], "guids":["guid1"]} properties: uids: type: array items: type: string guids: type: array items: type: string type: object tags: description: A list of tags to identify specific messages. type: array items: type: string type: object securitySchemes: apiKey: type: apiKey description: API Key with fullAccess scope(i.e. Rest API Key from the Dashboard). name: apikey in: header ````