> ## Documentation Index > Fetch the complete documentation index at: https://docs.modellix.ai/llms.txt > Use this file to discover all available pages before exploring further. # Messages > Anthropic Messages–compatible endpoint on the Modellix LLM gateway. Accepts a synchronous request with `model` (provider/name format, typically `anthropic/...`), Anthropic-style `messages`, and required `max_tokens`; optional `system` prompt is supported. Returns a Messages JSON object by default, or SSE (`text/event-stream`) when `stream=true`. Auth accepts `Authorization: Bearer` or `x-api-key` (same Modellix API Key). Use this path for the Anthropic SDK, Claude Code, and other Anthropic-native clients; optional `anthropic-version` header is forwarded when present. Do not send OpenAI Chat Completions or Responses field shapes on this endpoint—use `/chat/completions` or `/responses` instead. Optional fields follow [Anthropic Messages](https://docs.anthropic.com/en/api/messages); support depends on the selected model. ## OpenAPI ````yaml /llm/llm.json post /messages openapi: 3.1.0 info: title: Modellix LLM API description: >- Public API of the Modellix LLM Gateway (llm-bff): OpenAI-compatible Chat Completions and Responses, plus Anthropic-compatible Messages. Optional fields follow the corresponding official protocols; this document lists commonly used core fields only. See `docs/api/llm.md` for the human-readable guide. version: '2026-08-04' contact: name: Modellix servers: - url: https://llm.modellix.ai/v1 description: Production security: - BearerAuth: [] - ApiKeyAuth: [] tags: - name: Chat Completions description: OpenAI-compatible Chat Completions - name: Responses description: OpenAI-compatible Responses - name: Messages description: Anthropic-compatible Messages paths: /messages: post: tags: - Messages summary: Messages description: >- Anthropic Messages–compatible endpoint on the Modellix LLM gateway. Accepts a synchronous request with `model` (provider/name format, typically `anthropic/...`), Anthropic-style `messages`, and required `max_tokens`; optional `system` prompt is supported. Returns a Messages JSON object by default, or SSE (`text/event-stream`) when `stream=true`. Auth accepts `Authorization: Bearer` or `x-api-key` (same Modellix API Key). Use this path for the Anthropic SDK, Claude Code, and other Anthropic-native clients; optional `anthropic-version` header is forwarded when present. Do not send OpenAI Chat Completions or Responses field shapes on this endpoint—use `/chat/completions` or `/responses` instead. Optional fields follow [Anthropic Messages](https://docs.anthropic.com/en/api/messages); support depends on the selected model. operationId: createMessage parameters: - $ref: '#/components/parameters/SessionIdHeader' - name: anthropic-version in: header required: false description: >- Anthropic API version header (optional; often set automatically by native Anthropic clients; forwarded upstream when present) schema: type: string example: '2023-06-01' requestBody: required: true description: >- Anthropic Messages request body. Requires model, messages, and max_tokens; optional system and stream. Message roles are user or assistant only. content: application/json: schema: $ref: '#/components/schemas/MessagesRequest' example: model: anthropic/claude-sonnet-5 max_tokens: 256 messages: - role: user content: ping responses: '200': description: Success. Non-streaming returns JSON; streaming returns SSE. content: application/json: schema: $ref: '#/components/schemas/MessagesResponse' text/event-stream: schema: type: string description: 'SSE: Anthropic Messages event stream' '400': $ref: '#/components/responses/BadRequest' '401': $ref: '#/components/responses/Unauthorized' '402': $ref: '#/components/responses/PaymentRequired' '404': $ref: '#/components/responses/NotFound' '429': $ref: '#/components/responses/RateLimited' 5XX: $ref: '#/components/responses/UpstreamError' components: parameters: SessionIdHeader: name: X-Mdlx-Session-Id in: header required: false description: >- Optional session affinity header. Length 8–128; alphanumeric, `-`, and `_` only. schema: type: string minLength: 8 maxLength: 128 pattern: ^[A-Za-z0-9_-]+$ example: my-conversation-001 schemas: MessagesRequest: type: object required: - model - messages - max_tokens additionalProperties: true description: Core fields below; other fields follow Anthropic Messages. properties: model: $ref: '#/components/schemas/ModelId' messages: type: array minItems: 1 description: >- Anthropic Messages list (min 1). Roles are user or assistant only; put system text in system, not as a message role. items: type: object required: - role - content additionalProperties: true description: A single Anthropic Messages turn with role and content. properties: role: type: string enum: - user - assistant description: >- Anthropic message role. Only user or assistant; system belongs in the system field. content: description: Text string or content-block array oneOf: - type: string - type: array items: type: object additionalProperties: true description: Anthropic Messages content block max_tokens: type: integer minimum: 1 description: Required. Maximum number of tokens to generate. stream: type: boolean default: false description: >- Default false. When true, returns an Anthropic Messages SSE event stream. system: description: System prompt (string or content-block array) oneOf: - type: string - type: array items: type: object additionalProperties: true description: Anthropic system content block MessagesResponse: type: object additionalProperties: true description: Shape follows Anthropic message properties: id: type: string description: Unique identifier for this Anthropic message. type: type: string example: message description: Object type. Typically message for Anthropic Messages. role: type: string example: assistant description: Role of the response message. Typically assistant. model: type: string description: Model ID that produced this Anthropic message. content: type: array description: Array of Anthropic content blocks in the assistant reply. items: type: object additionalProperties: true description: A single Anthropic Messages content block in the response. stop_reason: type: - string - 'null' description: Why generation stopped, or null. usage: type: object additionalProperties: true description: >- Token usage object for the Anthropic Messages API. Shape follows Anthropic usage (for example input and output tokens), not Chat Completions prompt_tokens fields. ModelId: type: string description: Model ID in `provider/name` form examples: - openai/gpt-5.5 - openai/gpt-5.6-sol - anthropic/claude-sonnet-5 - google/gemini-3.6-flash ErrorBody: type: object required: - error description: >- Gateway error envelope shared by Chat Completions, Responses, and Messages. properties: error: type: object required: - message - type additionalProperties: true description: Error details object. properties: message: type: string description: Human-readable error message. type: type: string description: >- Error type string (for example invalid_request_error, rate_limit_exceeded, insufficient_quota). examples: - invalid_request_error - api_error - rate_limit_exceeded - request_limited - insufficient_quota code: type: - string - 'null' description: Optional machine-readable error code, or null. param: type: - string - 'null' description: Optional parameter name related to the error, or null. responses: BadRequest: description: Invalid request parameters content: application/json: schema: $ref: '#/components/schemas/ErrorBody' Unauthorized: description: >- Missing or invalid API key, or conflicting Authorization and x-api-key values content: application/json: schema: $ref: '#/components/schemas/ErrorBody' example: error: message: invalid API key type: invalid_request_error PaymentRequired: description: Insufficient account balance content: application/json: schema: $ref: '#/components/schemas/ErrorBody' example: error: message: insufficient balance, please recharge your account type: insufficient_quota NotFound: description: Unknown path, or the selected model is unavailable content: application/json: schema: $ref: '#/components/schemas/ErrorBody' examples: modelUnavailable: summary: Model unavailable value: error: message: the selected model is unavailable type: invalid_request_error RateLimited: description: >- Rate limited (RPM or other request limits), or the selected model is temporarily unavailable (e.g. upstream deployment cooldown) headers: X-RateLimit-Limit: description: Maximum number of requests allowed in the current rate-limit window. schema: type: integer X-RateLimit-Remaining: description: Number of requests remaining in the current rate-limit window. schema: type: integer X-RateLimit-Reset: description: Unix timestamp (seconds) when the current rate-limit window resets. schema: type: integer Retry-After: schema: type: integer description: >- May be returned when the model is temporarily unavailable; value in seconds content: application/json: schema: $ref: '#/components/schemas/ErrorBody' examples: rpm: summary: RPM exceeded value: error: message: rate limit exceeded type: rate_limit_exceeded modelCooldown: summary: Model temporarily unavailable value: error: message: >- the selected model is temporarily unavailable, please retry later or try another model type: api_error UpstreamError: description: >- Temporary service unavailability or upstream error (client-facing message may be sanitized) content: application/json: schema: $ref: '#/components/schemas/ErrorBody' securitySchemes: BearerAuth: type: http scheme: bearer bearerFormat: API Key description: >- Authorization: Bearer (OpenAI SDK, Codex, OpenCode, etc.) ApiKeyAuth: type: apiKey in: header name: x-api-key description: >- x-api-key: (Anthropic SDK, Claude Code, etc.). Equivalent to Bearer; if both are present they must be the same key. ````