Anthropic Messages API — POST /v1/messages - A2Agent -Unified API Gateway For LLMs

A2Agent uses the Anthropic Messages request and response shape at this endpoint. The examples below use Bearer Token authentication, as required by A2Agent. The model IDs are A2Agent model IDs and may be aliases controlled by A2Agent routing configuration.

Endpoint

POST https://api.a2agent.me/v1/messages

Request Headers

Header	Required	Value
`Authorization`	Yes	`Bearer YOUR_API_KEY`
`anthropic-version`	Yes	`2023-06-01`
`Content-Type`	Yes	`application/json`

Replace YOUR_API_KEY with the API key created in the A2Agent console.

A2Agent uses Authorization: Bearer for this compatibility endpoint. Do not replace it with x-api-key unless A2Agent explicitly instructs you to do so.

Request Body Parameters

string

required

The ID of the model to use. For example: deepseek-v4-pro, deepseek-v4-flash, qwen3.8-max. See List Models for the full list of available IDs.

array

required

An ordered array of message objects representing the conversation history. Each object must contain a role and content.

Show Message object fields

string

required

The role of the message author. Use "user" or "assistant".

string

required

The text content of the message, or an array of supported Anthropic content blocks.

integer

Maximum output tokens. This field is required; a low value may end generation with stop_reason: "max_tokens". The allowed maximum depends on the model and account.

number

Controls randomness: lower values are more deterministic and higher values more varied. The range and default depend on the model; use it or top_p, not both.

boolean

When true, returns an SSE stream; when omitted or false, returns one complete JSON response. Anthropic streams use message/content-block events.

number

Nucleus-sampling threshold. Lower values focus the output and higher values allow more candidates. The range and default depend on the model; use it or temperature, not both.

Message Object

Each item in messages must contain:

Field	Type	Required	Description
`role`	string	Yes	`user` or `assistant`.
`content`	string or array	Yes	Plain text or Anthropic content blocks.

For a basic text request, use a string:

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

Example Request

curl https://api.a2agent.me/v1/messages \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "deepseek-v4-pro",
    "max_tokens": 256,
    "messages": [
      {
        "role": "user",
        "content": "What is 2 + 2?"
      }
    ]
  }'

Use the exact model ID returned by GET /v1/models. Model IDs are case-sensitive.

Example Response

```json
{
  "id": "chatcmpl-90a02bbe-f7a5-9cc6-9046-5a1251672edc",
  "type": "message",
  "role": "assistant",
  "content": [
    {
      "type": "text",
      "text": "Let’s work through this step by step.  
       We start with:  2 + 2
       When you add 2 and 2 together, you combine their values:2, then count 2 more: 3, 4. 
       So:  2 + 2 = 4.
       Final answer:  [boxed{4}]"
    }
  ],
  "model": "deepseek-v4-pro",
  "stop_reason": "end_turn",
  "usage": {
    "input_tokens": 12,
    "output_tokens": 78,
    "cache_creation_input_tokens": 0,
    "cache_read_input_tokens": 0
  }
}

This is an observed response sample from A2Agent. The id, generated text, and token counts vary for each request. A2Agent may add, omit, or extend metadata fields over time.

Reading the Text Response

Unlike an OpenAI Chat Completions response, the text is not located at choices[0].message.content. For a basic text response, read:

content[0].text

For example:

const text = response.content[0].text;

If a response contains multiple content blocks, inspect each block’s type before reading its fields.

Response Fields

The response follows the Anthropic message shape. The field path content[] refers to each content block in the content array.

Field	Type	Description	How to use it
`id`	string	Unique identifier for the message. A2Agent may use an identifier such as `chatcmpl-...`.	Use for request tracing and support inquiries.
`type`	string	Message resource type, normally `"message"`.	Confirms that the response is a message object.
`role`	string	Role that produced the response, normally `"assistant"`.	Identifies the response as model output.
`content`	array	Ordered array of Anthropic content blocks.	Iterate through blocks to read text or other supported content.
`content[].type`	string	Content block type, such as `"text"`.	Check the block type before reading its fields.
`content[].text`	string	Text contained in a text content block.	Read this value for the model’s text response.
`model`	string	Model ID associated with the request or response.	Useful for tracing the requested A2Agent model ID. It is not by itself proof of the final upstream model when A2Agent uses aliases or routing rules.
`stop_reason`	string or null	Reason generation stopped, such as `end_turn` or `max_tokens`.	Distinguish a normal completion from a token-limit stop.
`stop_sequence`	string or null	Stop sequence that ended generation, when returned.	Optional metadata; it may be omitted when no stop sequence was used.
`usage.input_tokens`	integer	Number of input tokens processed.	Use for usage reporting and cost estimation.
`usage.output_tokens`	integer	Number of output tokens generated.	Use for usage reporting and cost estimation.
`usage.cache_creation_input_tokens`	integer or null	Input tokens used to create a prompt cache, when supported.	Optional usage metadata.
`usage.cache_read_input_tokens`	integer or null	Input tokens read from a prompt cache, when supported.	Optional usage metadata.

Streaming

Set "stream": true to request server-sent events (SSE), if streaming is enabled for the selected model and account. Anthropic streaming events are not the same as OpenAI chat.completion.chunk objects or the [DONE] marker. A standard Anthropic stream can contain events such as:

message_start
content_block_start
content_block_delta
content_block_stop
message_delta
message_stop

Text deltas are normally found in a content_block_delta event under a text delta object. Confirm the exact event sequence and fields against the A2Agent response before documenting streaming as generally available.

Overview

Endpoints

Anthropic Messages API — POST /v1/messages

Endpoint

Request Headers

Request Body Parameters

Message Object

Example Request

Example Response

Reading the Text Response

Response Fields

Streaming

​Endpoint

​Request Headers

​Request Body Parameters

​Message Object

​Example Request

​Example Response

​Reading the Text Response

​Response Fields

​Streaming

Endpoint

Request Headers

Request Body Parameters

Message Object

Example Request

Example Response

Reading the Text Response

Response Fields

Streaming