Messages - NanoGPT API Documentation

Overview

The /v1/messages endpoint provides full Anthropic API compatibility. Clients using the Anthropic SDK can use NanoGPT by simply changing the base URL — no code changes required. This endpoint supports:

Text generation (streaming and non-streaming)
Multi-turn conversations
Tool use (function calling)
Vision (images) and document/PDF processing
Extended thinking (reasoning models)
Prompt caching

Endpoint

POST https://nano-gpt.com/api/v1/messages

Authentication

Use either header:

Authorization: Bearer YOUR_API_KEY
x-api-key: YOUR_API_KEY

Request Format

Required Fields

Field	Type	Description
`model`	string	Model identifier (e.g., `claude-3-5-sonnet-20241022`)
`max_tokens`	number	Maximum tokens to generate (must be a finite number)
`messages`	array	Array of conversation messages

Optional Fields

Field	Type	Default	Description
`system`	string or array	—	System prompt (string or array of text blocks)
`stream`	boolean	`false`	Enable streaming responses
`temperature`	number	—	Sampling temperature
`top_p`	number	—	Nucleus sampling parameter
`top_k`	number	—	Top-k sampling parameter
`stop_sequences`	string[]	—	Custom stop sequences
`tools`	array	—	Tool definitions for function calling
`tool_choice`	string or object	—	Control tool selection behavior
`disable_parallel_tool_use`	boolean	—	Disable parallel tool calls
`thinking`	object	—	Enable extended thinking for supported models
`metadata`	object	—	Request metadata (`user` or `user_id`)

Message Format

Messages must have a role (user or assistant) and content:

{
  "role": "user",
  "content": "Hello!"
}

Or with structured content blocks:

{
  "role": "user",
  "content": [
    { "type": "text", "text": "What's in this image?" },
    {
      "type": "image",
      "source": {
        "type": "base64",
        "media_type": "image/jpeg",
        "data": "<base64-encoded-image>"
      }
    }
  ]
}

Content Block Types

Text Block

{ "type": "text", "text": "Your message here" }

Image Block (for vision-capable models)

{
  "type": "image",
  "source": {
    "type": "base64",
    "media_type": "image/jpeg",
    "data": "<base64-data>"
  }
}

Or with URL:

{
  "type": "image",
  "source": {
    "type": "url",
    "url": "https://example.com/image.jpg"
  }
}

Supported media types: image/jpeg, image/png, image/gif, image/webp

Document Block (for PDF-capable models)

{
  "type": "document",
  "source": {
    "type": "base64",
    "media_type": "application/pdf",
    "data": "<base64-data>"
  }
}

Tool Use Block (in assistant messages)

{
  "type": "tool_use",
  "id": "tool_abc123",
  "name": "get_weather",
  "input": { "city": "Paris" }
}

Tool Result Block (in user messages)

{
  "type": "tool_result",
  "tool_use_id": "tool_abc123",
  "content": "The weather in Paris is sunny, 22 C"
}

Tool Definitions

{
  "tools": [
    {
      "name": "get_weather",
      "description": "Get current weather for a city",
      "input_schema": {
        "type": "object",
        "properties": {
          "city": { "type": "string", "description": "City name" }
        },
        "required": ["city"]
      }
    }
  ]
}

Tool Choice Options

Value	Description
`"auto"`	Model may use tools if appropriate
`"none"`	Disable tool use for this request
`"any"`	Model must use at least one tool
`"required"`	Model must use at least one tool
`{"type": "tool", "name": "tool_name"}`	Force use of a specific tool

Extended Thinking

For models that support extended thinking (reasoning):

{
  "thinking": {
    "type": "enabled",
    "budget_tokens": 8192
  }
}

Requirements:

budget_tokens must be >= 1024
budget_tokens must be < max_tokens
Model must support thinking (e.g., models with -thinking suffix)

Response Format

Non-Streaming Response

{
  "id": "msg_abc123",
  "type": "message",
  "role": "assistant",
  "model": "claude-3-5-sonnet-20241022",
  "content": [
    { "type": "text", "text": "Hello! How can I help you today?" }
  ],
  "stop_reason": "end_turn",
  "stop_sequence": null,
  "usage": {
    "input_tokens": 10,
    "output_tokens": 12,
    "cache_creation_input_tokens": 0,
    "cache_read_input_tokens": 0,
    "cache_creation": {
      "ephemeral_5m_input_tokens": 0,
      "ephemeral_1h_input_tokens": 0
    },
    "service_tier": "standard"
  }
}

Stop Reasons

Stop Reason	Description
`end_turn`	Natural end of response
`max_tokens`	Hit token limit
`stop_sequence`	Hit a custom stop sequence
`tool_use`	Model wants to use a tool
`content_filter`	Content was filtered

Streaming Response (SSE)

When stream: true, the response is Server-Sent Events with named event types:

Event: message_start

event: message_start
data: {"type": "message_start", "message": {"id": "msg_abc", "type": "message", "role": "assistant", "model": "claude-3-5-sonnet-20241022", "content": [], "stop_reason": null, "usage": {"input_tokens": 10, "output_tokens": 0}}}

Event: content_block_start

event: content_block_start
data: {"type": "content_block_start", "index": 0, "content_block": {"type": "text", "text": ""}}

Event: content_block_delta

event: content_block_delta
data: {"type": "content_block_delta", "index": 0, "delta": {"type": "text_delta", "text": "Hello"}}

Event: content_block_stop

event: content_block_stop
data: {"type": "content_block_stop", "index": 0}

Event: message_delta

event: message_delta
data: {"type": "message_delta", "delta": {"stop_reason": "end_turn"}, "usage": {"output_tokens": 12}}

Event: message_stop

event: message_stop
data: {"type": "message_stop"}

Streaming Tool Use

When the model uses tools during streaming:

event: content_block_start
data: {"type": "content_block_start", "index": 0, "content_block": {"type": "tool_use", "id": "tool_abc", "name": "get_weather", "input": {}}}

event: content_block_delta
data: {"type": "content_block_delta", "index": 0, "delta": {"type": "input_json_delta", "partial_json": "{\"city\":"}}

event: content_block_delta
data: {"type": "content_block_delta", "index": 0, "delta": {"type": "input_json_delta", "partial_json": " \"Paris\"}"}}

event: content_block_stop
data: {"type": "content_block_stop", "index": 0}

Supported Models

Claude Models (Full Support)

Model	Streaming	Tools	Vision	Thinking
claude-sonnet-4-5-20250929	Yes	Yes	Yes	Yes*
claude-3-5-sonnet-20241022	Yes	Yes	Yes	—
claude-3-5-haiku-20241022	Yes	Yes	Yes	—

*Use the model with -thinking suffix for extended reasoning.

Other Models (Via Compatibility Layer)

The v1/messages endpoint also works with non-Anthropic models:

Model	Streaming	Tools	Vision
gpt-4o	Yes	Yes	Yes
gpt-4o-mini	Yes	Yes	Yes
gemini-2.0-flash	Yes	Yes	Yes
llama-3.3-70b	Yes	Yes	—
deepseek-chat	Yes	Yes	—

See the Models documentation for the full list.

Prompt Caching

Enable prompt caching to reduce costs on repeated prompts.

Enable via Header

anthropic-beta: prompt-caching-2024-07-31

TTL Options

Default: 5-minute cache TTL
Extended: Add extended-cache-ttl to request 1-hour TTL

Cache Control in Content

Add cache_control to content blocks:

{
  "type": "text",
  "text": "This is a long system prompt...",
  "cache_control": { "type": "ephemeral" }
}

Cache Usage in Response

{
  "usage": {
    "input_tokens": 100,
    "output_tokens": 50,
    "cache_creation_input_tokens": 80,
    "cache_read_input_tokens": 0
  }
}

Error Handling

Error Response Format

{
  "type": "error",
  "error": {
    "type": "invalid_request_error",
    "message": "max_tokens is required",
    "param": "max_tokens"
  }
}

Error Types

HTTP Status	Error Type	Description
400	`invalid_request_error`	Invalid request (missing fields, bad format)
401	`authentication_error`	Invalid or missing API key
403	`permission_error`	Insufficient permissions
404	`not_found_error`	Unknown model
429	`rate_limit_error`	Rate limit exceeded
500+	`api_error`	Server error

All error responses include an X-Request-ID header for support requests.

Headers

Request Headers

Header	Required	Description
`Authorization`	Yes*	Bearer token authentication
`x-api-key`	Yes*	Alternative API key header
`Content-Type`	Yes	Must be `application/json`
`anthropic-beta`	No	Enable beta features (e.g., prompt caching)
`anthropic-version`	No	API version (accepted but not required)

*One of Authorization or x-api-key is required.

BYOK Headers

For Bring Your Own Key:

Header	Description
`x-use-byok`	Set to `true` to use your own API key
`x-byok-provider`	Provider name for your key

Examples

Basic Request (cURL)

curl -X POST https://nano-gpt.com/api/v1/messages \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -d '{
    "model": "claude-3-5-sonnet-20241022",
    "max_tokens": 256,
    "messages": [
      { "role": "user", "content": "Hello!" }
    ]
  }'

Streaming Request (cURL)

curl -N -X POST https://nano-gpt.com/api/v1/messages \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -d '{
    "model": "claude-3-5-sonnet-20241022",
    "stream": true,
    "max_tokens": 256,
    "messages": [{ "role": "user", "content": "Hello!" }]
  }'

With Tools (cURL)

curl -X POST https://nano-gpt.com/api/v1/messages \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -d '{
    "model": "claude-3-5-sonnet-20241022",
    "max_tokens": 1024,
    "tools": [
      {
        "name": "get_weather",
        "description": "Get current weather",
        "input_schema": {
          "type": "object",
          "properties": {
            "city": { "type": "string" }
          },
          "required": ["city"]
        }
      }
    ],
    "messages": [
      { "role": "user", "content": "What is the weather in Tokyo?" }
    ]
  }'

Anthropic SDK (Node.js)

import Anthropic from "@anthropic-ai/sdk";

const anthropic = new Anthropic({
  apiKey: process.env.NANOGPT_API_KEY,
  baseURL: "https://nano-gpt.com/api"
});

const message = await anthropic.messages.create({
  model: "claude-3-5-sonnet-20241022",
  max_tokens: 256,
  messages: [
    { role: "user", content: "Hello!" }
  ]
});

console.log(message.content[0].text);

Anthropic SDK with Streaming (Node.js)

import Anthropic from "@anthropic-ai/sdk";

const anthropic = new Anthropic({
  apiKey: process.env.NANOGPT_API_KEY,
  baseURL: "https://nano-gpt.com/api"
});

const stream = await anthropic.messages.stream({
  model: "claude-3-5-sonnet-20241022",
  max_tokens: 256,
  messages: [
    { role: "user", content: "Tell me a story" }
  ]
});

for await (const event of stream) {
  if (event.type === "content_block_delta" && event.delta.type === "text_delta") {
    process.stdout.write(event.delta.text);
  }
}

Anthropic SDK with Prompt Caching (Node.js)

import Anthropic from "@anthropic-ai/sdk";

const anthropic = new Anthropic({
  apiKey: process.env.NANOGPT_API_KEY,
  baseURL: "https://nano-gpt.com/api"
});

const message = await anthropic.messages.create({
  model: "claude-3-5-sonnet-20241022",
  max_tokens: 256,
  system: [
    {
      type: "text",
      text: "You are a helpful assistant with expertise in...",
      cache_control: { type: "ephemeral" }
    }
  ],
  messages: [
    { role: "user", content: "Hello!" }
  ]
}, {
  headers: {
    "anthropic-beta": "prompt-caching-2024-07-31"
  }
});

Vision Example (Node.js)

import Anthropic from "@anthropic-ai/sdk";
import fs from "fs";

const anthropic = new Anthropic({
  apiKey: process.env.NANOGPT_API_KEY,
  baseURL: "https://nano-gpt.com/api"
});

const imageData = fs.readFileSync("image.jpg").toString("base64");

const message = await anthropic.messages.create({
  model: "claude-3-5-sonnet-20241022",
  max_tokens: 1024,
  messages: [
    {
      role: "user",
      content: [
        { type: "text", text: "What's in this image?" },
        {
          type: "image",
          source: {
            type: "base64",
            media_type: "image/jpeg",
            data: imageData
          }
        }
      ]
    }
  ]
});

Python SDK

import anthropic

client = anthropic.Anthropic(
    api_key="YOUR_NANOGPT_API_KEY",
    base_url="https://nano-gpt.com/api"
)

message = client.messages.create(
    model="claude-3-5-sonnet-20241022",
    max_tokens=256,
    messages=[
        {"role": "user", "content": "Hello!"}
    ]
)

print(message.content[0].text)

Limits

Limit	Value
Request timeout	800 seconds
Tool argument size	~100 KB per tool call
Image types	JPEG, PNG, GIF, WebP

Migration from Anthropic

To migrate from Anthropic’s API to NanoGPT:

Change the base URL:
- From: https://api.anthropic.com
- To: https://nano-gpt.com/api
The full endpoint will be: https://nano-gpt.com/api/v1/messages
Use your NanoGPT API key instead of your Anthropic key
No other code changes required — the API is fully compatible

Notes

The anthropic-version header is accepted but not required
Token usage numbers use NanoGPT’s token accounting (may differ slightly from Anthropic’s exact counts)
All Anthropic SDK features are supported, including streaming, tools, and caching

Get Started

Endpoint Examples

API Reference

Miscellaneous

Integrations

​Overview

​Endpoint

​Authentication

​Request Format

​Required Fields

​Optional Fields

​Message Format

​Content Block Types

​Text Block

​Image Block (for vision-capable models)

​Document Block (for PDF-capable models)

​Tool Use Block (in assistant messages)

​Tool Result Block (in user messages)

​Tool Definitions

​Tool Choice Options

​Extended Thinking

​Response Format

​Non-Streaming Response

​Stop Reasons

​Streaming Response (SSE)

​Event: message_start

​Event: content_block_start

​Event: content_block_delta

​Event: content_block_stop

​Event: message_delta

​Event: message_stop

​Streaming Tool Use

​Supported Models

​Claude Models (Full Support)

​Other Models (Via Compatibility Layer)

​Prompt Caching

​Enable via Header

​TTL Options

​Cache Control in Content

​Cache Usage in Response

​Error Handling

​Error Response Format

​Error Types

​Headers

​Request Headers

​BYOK Headers

​Examples

​Basic Request (cURL)

​Streaming Request (cURL)

​With Tools (cURL)

​Anthropic SDK (Node.js)

​Anthropic SDK with Streaming (Node.js)

​Anthropic SDK with Prompt Caching (Node.js)

​Vision Example (Node.js)

​Python SDK

​Limits

​Migration from Anthropic

​Notes