> ## Documentation Index > Fetch the complete documentation index at: https://docs.megallm.io/llms.txt > Use this file to discover all available pages before exploring further. # Create message > Create conversational messages using the Anthropic-compatible Messages API ## OpenAPI ````yaml /cn/api-reference/openapi.json post /messages openapi: 3.1.0 info: title: MegaLLM API description: >- Unified API for accessing 70+ AI models through OpenAI-compatible and Anthropic-compatible endpoints version: 1.0.0 contact: name: MegaLLM Support email: hi@megallm.io url: https://megallm.io license: name: Proprietary url: https://megallm.io/terms-of-service servers: - url: https://ai.megallm.io/v1 description: Production API Server security: - bearerAuth: [] - apiKeyAuth: [] tags: - name: Chat description: OpenAI-compatible chat completions API - name: Messages description: Anthropic-compatible messages API - name: Models description: List available AI models - name: Authentication description: API authentication endpoints paths: /messages: post: tags: - Messages summary: Create message description: >- Create conversational messages using the Anthropic-compatible Messages API operationId: createMessage parameters: - name: anthropic-version in: header required: true schema: type: string example: '2023-06-01' description: >- The version of the Anthropic API you want to use. Use '2023-06-01' for the current stable version. - name: anthropic-beta in: header required: false schema: type: string example: max-tokens-3-5-sonnet-2022-07-15 description: >- Optional header for accessing beta features. Format: comma-separated list of beta feature names. requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/MessageRequest' examples: basic: summary: Basic message value: model: claude-3.5-sonnet max_tokens: 100 messages: - role: user content: What are the primary colors? with_system: summary: Message with system prompt value: model: claude-3.5-sonnet max_tokens: 200 system: You are a helpful coding assistant. messages: - role: user content: Explain list comprehensions with_thinking: summary: Extended thinking enabled value: model: claude-3.5-sonnet max_tokens: 1000 thinking: type: enabled budget_tokens: 500 messages: - role: user content: 'Solve this complex problem: A train leaves station A...' with_tools: summary: Tool use (function calling) value: model: claude-3.5-sonnet max_tokens: 500 tools: - name: get_weather description: Get the current weather in a given location input_schema: type: object properties: location: type: string description: The city and state, e.g. San Francisco, CA required: - location messages: - role: user content: What's the weather like in London? multimodal: summary: Image understanding value: model: claude-3.5-sonnet max_tokens: 300 messages: - role: user content: - type: image source: type: base64 media_type: image/jpeg data: >- iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg== - type: text text: What's in this image? responses: '200': description: Successful response content: application/json: schema: $ref: '#/components/schemas/MessageResponse' '400': $ref: '#/components/responses/BadRequest' '401': $ref: '#/components/responses/Unauthorized' '429': $ref: '#/components/responses/RateLimit' '500': $ref: '#/components/responses/ServerError' security: - apiKeyAuth: [] components: schemas: MessageRequest: type: object required: - model - messages - max_tokens properties: model: type: string description: >- The model that will complete your prompt. See the list of Claude models at https://docs.claude.com/en/docs/about-claude/models example: claude-3.5-sonnet messages: type: array description: >- Input messages. Alternating user and assistant conversational turns. The first message must use the user role. Text content is the only format supported for the first user message. items: $ref: '#/components/schemas/AnthropicMessage' minItems: 1 max_tokens: type: integer description: >- The maximum number of tokens to generate before stopping. Note that Claude may stop before reaching this maximum. This parameter only specifies the absolute maximum number of tokens to generate. minimum: 1 system: oneOf: - type: string description: System prompt as plain text - type: array description: Array of system content blocks items: type: object properties: type: type: string enum: - text text: type: string cache_control: type: object properties: type: type: string enum: - ephemeral required: - type - text description: >- System prompt. A system prompt is a way of providing context and instructions to Claude, such as specifying a particular goal or role. temperature: type: number minimum: 0 maximum: 1 default: 1 description: >- Amount of randomness injected into the response. Defaults to 1.0. Ranges from 0.0 to 1.0. Use temperature closer to 0.0 for analytical / multiple choice, and closer to 1.0 for creative and generative tasks. top_p: type: number minimum: 0 maximum: 1 description: >- Use nucleus sampling. In nucleus sampling, Claude computes the cumulative distribution over all the options for each subsequent token in decreasing probability order and cuts it off once it reaches a particular probability specified by top_p. top_k: type: integer minimum: 0 description: >- Only sample from the top K options for each subsequent token. Used to remove "long tail" low probability responses. Recommended for advanced use cases only. You usually only need to use temperature. stream: type: boolean default: false description: >- Whether to incrementally stream the response using server-sent events. See streaming documentation for details. stop_sequences: type: array items: type: string description: >- Custom text sequences that will cause the model to stop generating. Claude will stop when it encounters any of these strings. tools: type: array description: >- Definitions of tools that the model may use. If you include tools in your API request, the model may return tool_use content blocks that represent the model's use of those tools. items: $ref: '#/components/schemas/AnthropicTool' tool_choice: oneOf: - type: object properties: type: type: string enum: - auto required: - type description: >- allows Claude to decide whether to call any provided tools or not - type: object properties: type: type: string enum: - any required: - type description: tells Claude that it must use one of the provided tools - type: object properties: type: type: string enum: - tool name: type: string required: - type - name description: allows Claude to use only the specified tool description: >- How the model should use the provided tools. The model can use a specific tool, any available tool, or decide by itself. thinking: type: object properties: type: type: string enum: - enabled budget_tokens: type: integer description: Maximum number of tokens to spend on thinking description: >- Enable extended thinking by Claude. When enabled, Claude will think through the problem before responding. service_tier: type: string enum: - auto - standard_only description: >- The service tier to use for the request. 'auto' lets us choose the tier, 'standard_only' restricts to standard tier. metadata: type: object properties: user_id: type: string description: >- An external identifier for the user who is associated with the request description: An object describing metadata about the request MessageResponse: type: object required: - id - type - role - content - model - stop_reason - usage properties: id: type: string description: >- Unique object identifier. The format and length of IDs may change over time. example: msg_abc123 type: type: string enum: - message description: Object type. For Messages, this is always 'message'. role: type: string enum: - assistant description: >- Conversational role of the generated message. This is always 'assistant'. content: type: array description: >- Content generated by the model. This is an array of content blocks, each of which has a type that determines its shape. items: $ref: '#/components/schemas/AnthropicResponseContentBlock' model: type: string description: The model that handled the request. example: claude-3.5-sonnet stop_reason: type: string enum: - end_turn - max_tokens - stop_sequence - tool_use description: >- The reason that we stopped. This may be one of the following values: end_turn (the model reached a natural stopping point), max_tokens (we exceeded the requested max_tokens or the model's maximum), stop_sequence (one of your provided custom stop_sequences was generated), or tool_use (the model invoked one or more tools). stop_sequence: type: string description: >- Which custom stop sequence was generated, if any. This value will be a non-null string if one of your custom stop sequences was generated. nullable: true usage: type: object description: >- Billing and rate-limit usage. Anthropic's API bills and rate-limits by token counts, as tokens represent the underlying cost to our systems. required: - input_tokens - output_tokens properties: input_tokens: type: integer description: The number of input tokens which were used. output_tokens: type: integer description: The number of output tokens which were used. cache_creation_input_tokens: type: integer description: The number of input tokens used to create the cache entry. cache_read_input_tokens: type: integer description: The number of input tokens read from the cache. AnthropicMessage: type: object required: - role - content properties: role: type: string enum: - user - assistant description: The role of the messages author content: oneOf: - type: string description: Plain text content - type: array description: Array of content blocks (text, image, tool_use, tool_result) items: $ref: '#/components/schemas/AnthropicContentBlock' AnthropicTool: type: object required: - name - input_schema properties: name: type: string description: >- The name of the tool. Must be a-z, A-Z, 0-9, or contain underscores and dashes, with a maximum length of 64. description: type: string description: >- Description of what this tool does. Tool descriptions should be as detailed as possible. input_schema: type: object description: >- JSON schema for the tool input shape that the model will produce in tool_use output content blocks. properties: type: type: string enum: - object properties: type: object required: type: array items: type: string required: - type - properties cache_control: type: object properties: type: type: string enum: - ephemeral AnthropicResponseContentBlock: oneOf: - type: object description: Text content block in response required: - type - text properties: type: type: string enum: - text text: type: string description: The text content - type: object description: Thinking content block showing Claude's reasoning process required: - type - thinking properties: type: type: string enum: - thinking thinking: type: string description: Claude's internal thinking process - type: object description: Tool use content block required: - type - id - name - input properties: type: type: string enum: - tool_use id: type: string description: A unique identifier for this particular tool use block name: type: string description: The name of the tool being used input: type: object description: An object containing the input being passed to the tool Error: type: object properties: error: type: object properties: message: type: string type: type: string code: type: string AnthropicContentBlock: oneOf: - type: object description: Text content block required: - type - text properties: type: type: string enum: - text text: type: string cache_control: type: object properties: type: type: string enum: - ephemeral - type: object description: Image content block required: - type - source properties: type: type: string enum: - image source: type: object required: - type - media_type - data properties: type: type: string enum: - base64 media_type: type: string enum: - image/jpeg - image/png - image/gif - image/webp data: type: string description: Base64-encoded image data cache_control: type: object properties: type: type: string enum: - ephemeral - type: object description: Tool use content block required: - type - id - name - input properties: type: type: string enum: - tool_use id: type: string description: A unique identifier for this particular tool use block name: type: string description: The name of the tool being used input: type: object description: An object containing the input being passed to the tool - type: object description: Tool result content block required: - type - tool_use_id - content properties: type: type: string enum: - tool_result tool_use_id: type: string description: The id of the tool use request this is a result for content: oneOf: - type: string - type: array items: type: object description: The result of the tool use is_error: type: boolean description: Set to true if the tool execution resulted in an error cache_control: type: object properties: type: type: string enum: - ephemeral responses: BadRequest: description: Bad request - Invalid parameters content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: Invalid request parameters type: invalid_request_error code: invalid_parameters Unauthorized: description: Unauthorized - Invalid or missing API key content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: Invalid authentication credentials type: invalid_request_error code: invalid_api_key RateLimit: description: Rate limit exceeded content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: Rate limit exceeded. Please retry after some time. type: rate_limit_error code: rate_limit_exceeded ServerError: description: Internal server error content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: Internal server error type: api_error code: internal_error securitySchemes: bearerAuth: type: http scheme: bearer bearerFormat: API Key description: Bearer token authentication (OpenAI-compatible) apiKeyAuth: type: apiKey in: header name: x-api-key description: API key authentication (Anthropic-compatible) ````