# Anthropic Messages Format ## Create Message (Anthropic Format) > Creates a message using Anthropic's native Messages API format. Supports all Claude models through FastRouter with full feature parity including streaming, tool use, extended thinking, and vision.\ > \ > \*\*Available at both:\*\*\ > \- \`POST > \- \`POST > \ > \*\*Authentication:\*\* Use your FastRouter API key via either:\ > \- \`x-api-key: YOUR\_API\_KEY\` (Anthropic style)\ > \- \`Authorization: Bearer YOUR\_API\_KEY\` (OpenAI style)\ > \ > \*\*Streaming:\*\* Set \`stream: true\` to receive Server-Sent Events (SSE) with Anthropic's native streaming format (\`message\_start\`, \`content\_block\_start\`, \`content\_block\_delta\`, \`message\_delta\`, etc.).\ > \ > \*\*Cost tracking:\*\* The response \`usage\` object includes a \`cost\` field showing credits consumed.\ > \ > \*\*Drop-in replacement:\*\* This endpoint is a drop-in replacement for Anthropic's \`/v1/messages\` API. Simply change the base URL to \` and use your FastRouter API key. ```json {"openapi":"3.1.0","info":{"title":"FastRouter API Reference","version":"1.0.0"},"tags":[{"name":"Anthropic Messages","description":"Anthropic-native Messages API. Send requests in Anthropic's native format with FastRouter authentication. Supports streaming, tool use, extended thinking, and all Claude model features."}],"servers":[{"url":"https://api.fastrouter.ai","description":"Production API"}],"security":[{"bearerAuth":[]}],"components":{"securitySchemes":{"bearerAuth":{"type":"http","scheme":"bearer","bearerFormat":"API Key","description":"FastRouter API Key. Get yours at https://fastrouter.ai\n\nFormat: `Authorization: Bearer YOUR_API_KEY`"}},"responses":{"BadRequestError":{"description":"Bad Request - The request is malformed. This could be due to missing parameters, invalid formats, or CORS issues.","content":{"application/json":{"schema":{"type":"object","properties":{"error":{"type":"object","properties":{"message":{"type":"string"},"type":{"type":"string"},"code":{"type":"string"},"status":{"type":"integer"}}}}}}}},"UnauthorizedError":{"description":"Invalid Credentials - Your API key is invalid, disabled, or your OAuth session has expired. Check your credentials.","content":{"application/json":{"schema":{"type":"object","properties":{"error":{"type":"object","properties":{"message":{"type":"string"},"type":{"type":"string"},"code":{"type":"string"},"status":{"type":"integer"}}}}}}}},"RateLimitError":{"description":"Rate Limited - You have exceeded your request limits (TPM/RPM). Slow down or increase your limits.","content":{"application/json":{"schema":{"type":"object","properties":{"error":{"type":"object","properties":{"message":{"type":"string"},"type":{"type":"string"},"code":{"type":"string"},"status":{"type":"integer"}}}}}}}},"InternalServerError":{"description":"Internal Error - Something went wrong on our side. Retry the request, and contact support if the issue persists.","content":{"application/json":{"schema":{"type":"object","properties":{"error":{"type":"object","properties":{"message":{"type":"string"},"type":{"type":"string"},"code":{"type":"string"},"status":{"type":"integer"}}}}}}}}}},"paths":{"/api/v1/messages":{"post":{"operationId":"createAnthropicMessage","tags":["Anthropic Messages"],"summary":"Create Message (Anthropic Format)","description":"Creates a message using Anthropic's native Messages API format. Supports all Claude models through FastRouter with full feature parity including streaming, tool use, extended thinking, and vision.\n\n**Available at both:**\n- `POST https://api.fastrouter.ai/api/v1/messages`\n- `POST https://api.fastrouter.ai/v1/messages`\n\n**Authentication:** Use your FastRouter API key via either:\n- `x-api-key: YOUR_API_KEY` (Anthropic style)\n- `Authorization: Bearer YOUR_API_KEY` (OpenAI style)\n\n**Streaming:** Set `stream: true` to receive Server-Sent Events (SSE) with Anthropic's native streaming format (`message_start`, `content_block_start`, `content_block_delta`, `message_delta`, etc.).\n\n**Cost tracking:** The response `usage` object includes a `cost` field showing credits consumed.\n\n**Drop-in replacement:** This endpoint is a drop-in replacement for Anthropic's `/v1/messages` API. Simply change the base URL to `https://api.fastrouter.ai` and use your FastRouter API key.","requestBody":{"required":true,"content":{"application/json":{"schema":{"type":"object","required":["model","messages","max_tokens"],"properties":{"model":{"type":"string","description":"Model ID. Use `anthropic/` prefix or bare Anthropic model name. Examples: `anthropic/claude-sonnet-4-20250514`, `claude-sonnet-4-20250514`, `anthropic/claude-opus-4-20250514`"},"messages":{"type":"array","description":"Array of input messages. Each message has a role (user or assistant) and content.","minItems":1,"items":{"type":"object","required":["role","content"],"properties":{"role":{"type":"string","enum":["user","assistant"],"description":"The role of the message author. System messages are passed via the top-level system parameter."},"content":{"description":"Message content. Can be a string or an array of content blocks.","oneOf":[{"type":"string"},{"type":"array","items":{"type":"object","properties":{"type":{"type":"string","enum":["text","image","tool_use","tool_result"]},"text":{"type":"string"}}}}]}}}},"max_tokens":{"type":"integer","description":"The maximum number of tokens to generate. Required for all requests."},"system":{"type":"string","description":"System prompt. Provides instructions or context to the model."},"temperature":{"type":"number","minimum":0,"maximum":1,"description":"Randomness. Ranges from 0.0 to 1.0."},"top_p":{"type":"number","description":"Nucleus sampling parameter."},"top_k":{"type":"integer","description":"Only sample from the top K options for each subsequent token."},"stop_sequences":{"type":"array","items":{"type":"string"},"description":"Custom text sequences that will cause the model to stop generating."},"stream":{"type":"boolean","default":false,"description":"Whether to stream the response using SSE."},"tools":{"type":"array","description":"Definitions of tools that the model may use.","items":{"type":"object","properties":{"name":{"type":"string","description":"Name of the tool."},"description":{"type":"string","description":"Description of what the tool does."},"input_schema":{"type":"object","description":"JSON Schema for the tool input."}}}},"tool_choice":{"type":"object","description":"How the model should use the provided tools.","properties":{"type":{"type":"string","enum":["auto","any","tool"]},"name":{"type":"string","description":"Tool name (when type is tool)."}}},"thinking":{"type":"object","description":"Extended thinking configuration for Claude models.","properties":{"type":{"type":"string","enum":["enabled","disabled"]},"budget_tokens":{"type":"integer","description":"Max tokens for thinking."}}}}}}}},"responses":{"200":{"description":"Message created successfully","content":{"application/json":{"schema":{"type":"object","properties":{"id":{"type":"string","description":"Unique message identifier"},"type":{"type":"string","enum":["message"]},"role":{"type":"string","enum":["assistant"]},"content":{"type":"array","description":"Response content blocks (text, tool_use, thinking).","items":{"type":"object","properties":{"type":{"type":"string","enum":["text","tool_use","thinking"]},"text":{"type":"string"},"id":{"type":"string"},"name":{"type":"string"},"input":{"type":"object"}}}},"model":{"type":"string"},"stop_reason":{"type":"string","enum":["end_turn","max_tokens","stop_sequence","tool_use"]},"usage":{"type":"object","properties":{"input_tokens":{"type":"integer"},"output_tokens":{"type":"integer"},"cost":{"type":"number","description":"Credits consumed"}}}}}}}},"400":{"$ref":"#/components/responses/BadRequestError"},"401":{"$ref":"#/components/responses/UnauthorizedError"},"429":{"$ref":"#/components/responses/RateLimitError"},"500":{"$ref":"#/components/responses/InternalServerError"}}}}}} ```