> ## Documentation Index > Fetch the complete documentation index at: https://impossibl.com/docs/llms.txt > Use this file to discover all available pages before exploring further. # Create a message (Anthropic Messages API) > Drop-in compatible with Anthropic's Messages API. Authenticate with `x-api-key` (or `Authorization: Bearer`). A bare model id like `claude-haiku-4-5` routes to Anthropic; use `provider/model` for others. Set `stream: true` for SSE. ## OpenAPI ````yaml /openapi.yaml post /v1/messages openapi: 3.1.0 info: title: Impossibl AI API version: 0.0.1 description: > The Impossibl AI API is one API for all AI providers, OpenAI-compatible across OpenAI, Anthropic, Google, xAI, and more (`GET /v1/models` lists the full catalog), billed from a prepaid credit balance. Provider usage is billed at provider list prices with no usage markup. Prepaid credit purchases charge a separate fixed 5% platform fee before tax. **Agent quickstart** 1. `POST /v1/accounts` — create a funded account; you get an API key and a signup-bonus balance in one call (no human step required). 2. Call `POST /v1/chat/completions`, `POST /v1/responses`, or `POST /v1/audio/transcriptions` with `Authorization: Bearer ` and a `provider/model` id from `GET /v1/models`. 3. Credits are deducted per request by token or audio-duration usage; check `GET /v1/account`. servers: - url: https://api.impossibl.com description: Production - url: http://localhost:8787 description: Local dev security: - BearerKey: [] paths: /v1/messages: post: summary: Create a message (Anthropic Messages API) description: >- Drop-in compatible with Anthropic's Messages API. Authenticate with `x-api-key` (or `Authorization: Bearer`). A bare model id like `claude-haiku-4-5` routes to Anthropic; use `provider/model` for others. Set `stream: true` for SSE. operationId: createMessage requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/MessagesRequest' responses: '200': description: > A `message` object. When `stream: true`, the body is an SSE stream of Anthropic events (`message_start`, `content_block_delta`, …, `message_stop`). content: application/json: schema: $ref: '#/components/schemas/AnthropicMessage' '400': $ref: '#/components/responses/BadRequest' '401': $ref: '#/components/responses/Unauthorized' '402': $ref: '#/components/responses/InsufficientCredits' '404': $ref: '#/components/responses/UnknownModel' '502': $ref: '#/components/responses/UpstreamError' '503': $ref: '#/components/responses/ProviderNotConfigured' security: - BearerKey: [] - ApiKeyHeader: [] components: schemas: MessagesRequest: type: object required: - model - max_tokens - messages properties: model: type: string example: claude-haiku-4-5 max_tokens: type: integer description: Required by the Anthropic API. messages: type: array items: type: object properties: role: type: string enum: - user - assistant content: description: >- A text string, or an array of content blocks (text, image, tool_use, tool_result). oneOf: - type: string - type: array items: $ref: '#/components/schemas/AnthropicContentBlock' system: type: string description: Top-level system prompt. stream: type: boolean default: false temperature: type: number top_p: type: number top_k: type: integer stop_sequences: type: array items: type: string thinking: description: >- Anthropic thinking control, preserved on native Anthropic and Bedrock routes. oneOf: - type: object required: - type - budget_tokens properties: type: type: string const: enabled budget_tokens: type: integer minimum: 1024 - type: object required: - type properties: type: type: string const: adaptive display: type: string enum: - omitted - summarized - type: object required: - type properties: type: type: string const: disabled output_config: type: object properties: effort: type: string enum: - low - medium - high - xhigh - max tools: type: array description: >- Tools the model may call (relay mode — the gateway does not execute them). items: type: object required: - name - input_schema properties: name: type: string description: type: string input_schema: type: object description: JSON Schema for the tool input. tool_choice: description: >- `{ type: auto }`, `{ type: any }`, `{ type: tool, name }`, or `{ type: none }`. type: object properties: type: type: string enum: - auto - any - tool - none name: type: string AnthropicMessage: type: object description: Non-streaming Anthropic `message` response. properties: id: type: string example: msg_... type: type: string const: message role: type: string const: assistant model: type: string content: type: array description: Text blocks and, when the model calls a tool, tool_use blocks. items: type: object properties: type: type: string enum: - text - tool_use text: type: string description: Present on `text` blocks. id: type: string description: Present on `tool_use` blocks. name: type: string description: Present on `tool_use` blocks. input: type: object description: Parsed tool input on `tool_use` blocks. stop_reason: type: string enum: - end_turn - max_tokens - stop_sequence - tool_use nullable: true stop_sequence: type: string nullable: true usage: type: object properties: input_tokens: type: integer output_tokens: type: integer AnthropicContentBlock: type: object description: A content block on a message — text, image, tool_use, or tool_result. properties: type: type: string enum: - text - image - tool_use - tool_result text: type: string description: Present when `type` is `text`. source: type: object description: Present when `type` is `image`. properties: type: type: string enum: - base64 - url media_type: type: string description: For base64, e.g. `image/png`. data: type: string description: For base64 the raw base64 image data.: null url: type: string description: For url an http(s) URL.: null id: type: string description: tool_use block id. name: type: string description: tool_use tool name. input: type: object description: tool_use parsed input. tool_use_id: type: string description: tool_result — the tool_use id it answers. content: description: tool_result content — a string or nested blocks. oneOf: - type: string - type: array items: type: object is_error: type: boolean Error: type: object properties: error: type: object properties: message: type: string type: type: string description: >- One of authentication_error, invalid_request_error, insufficient_quota, upstream_error, provider_not_configured. responses: BadRequest: description: Malformed request (bad JSON, or missing required fields). content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: '`model` and `messages` are required' type: invalid_request_error Unauthorized: description: Missing or invalid API key. content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: Invalid API key type: authentication_error InsufficientCredits: description: The account's credit balance is empty. Top up to continue. content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: Insufficient credits. Top up to continue. type: insufficient_quota UnknownModel: description: The requested `model` id is not in the registry (see GET /v1/models). content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: 'Unknown model: foo/bar' type: invalid_request_error UpstreamError: description: The upstream provider returned an error. content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: 'permission-denied: ...' type: upstream_error ProviderNotConfigured: description: The provider for this model has no API key configured on the gateway. content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: 'Provider not configured: missing OPENAI_API_KEY' type: provider_not_configured securitySchemes: BearerKey: type: http scheme: bearer description: Gateway API key (`imp-rt-...`) from POST /v1/accounts or /v1/keys. ApiKeyHeader: type: apiKey in: header name: x-api-key description: >- Gateway API key via the `x-api-key` header (what the Anthropic SDK sends). ````