Build an Anthropic Messages API request

Constructs an httr2 request object for Anthropic's Messages API. Handles message formatting, system prompts, and optional JSON schema for structured outputs. When using structured outputs you must select the correct model.

Usage

ant_build_messages_request(
  input,
  endpointr_id = NULL,
  model = .ANT_DEFAULT_MODEL,
  temperature = 0,
  max_tokens = 500L,
  schema = NULL,
  system_prompt = NULL,
  key_name = "ANTHROPIC_API_KEY",
  endpoint_url = .ANT_MESSAGES_ENDPOINT,
  timeout = 30L,
  max_retries = 5L
)

Arguments

input

Text input to send to the model

endpointr_id

An id that will persist through to response

model

Anthropic model to use (default: "claude-haiku-4.5")

temperature

Sampling temperature (0-2), higher values = more randomness

max_tokens

Maximum tokens in response

schema

Optional JSON schema for structured output (json_schema object or list)

system_prompt

Optional system prompt

key_name

Environment variable name for API key

endpoint_url

Anthropic API endpoint URL

timeout

Request timeout in seconds

max_retries

Maximum number of retry attempts for failed requests

Value

An httr2 request object

Details

This function creates the HTTP request but does not execute it. For structured outputs, you must use a supported model (Claude Sonnet 4.5 or Opus 4.1) and the request will automatically include the required beta header.

The schema parameter accepts either:

• A json_schema S7 object created with create_json_schema()

• A raw list in Anthropic's output_format structure

Unlike OpenAI, Anthropic uses output_format (not response_format) and the schema structure differs slightly.

See also

https://platform.claude.com/docs/en/build-with-claude/structured-outputs

Examples

if (FALSE) { # \dontrun{
  # simple request
  req <- ant_build_messages_request(
    input = "What is the capital of France?",
    max_tokens = 100
  )

  # with structured output
  schema <- create_json_schema(
    name = "capital_response",
    schema = schema_object(
      country = schema_string(),
      capital = schema_string(),
      required = c("country", "capital")
    )
  )
  req <- ant_build_messages_request(
    input = "What is the capital of France?",
    schema = schema,
    max_tokens = 100,
    model = "sonnet-4-5"
  )
} # }