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Claude API — Ruby

Note: The Ruby SDK supports the Claude API. A tool runner is available in beta via client.beta.messages.tool_runner(). Agent SDK is not yet available for Ruby.

Installation

gem install anthropic

Client Initialization

require "anthropic"

# Default (uses ANTHROPIC_API_KEY env var)
client = Anthropic::Client.new

# Explicit API key
client = Anthropic::Client.new(api_key: "your-api-key")

Basic Message Request

message = client.messages.create(
  model: :"{{OPUS_ID}}",
  max_tokens: 16000,
  messages: [
    { role: "user", content: "What is the capital of France?" }
  ]
)
# content is an array of polymorphic block objects (TextBlock, ThinkingBlock,
# ToolUseBlock, ...). .type is a Symbol — compare with :text, not "text".
# .text raises NoMethodError on non-TextBlock entries.
message.content.each do |block|
  puts block.text if block.type == :text
end

Streaming

stream = client.messages.stream(
  model: :"{{OPUS_ID}}",
  max_tokens: 64000,
  messages: [{ role: "user", content: "Write a haiku" }]
)

stream.text.each { |text| print(text) }

Tool Use

The Ruby SDK supports tool use via raw JSON schema definitions and also provides a beta tool runner for automatic tool execution.

Tool Runner (Beta)

class GetWeatherInput < Anthropic::BaseModel
  required :location, String, doc: "City and state, e.g. San Francisco, CA"
end

class GetWeather < Anthropic::BaseTool
  doc "Get the current weather for a location"

  input_schema GetWeatherInput

  def call(input)
    "The weather in #{input.location} is sunny and 72°F."
  end
end

client.beta.messages.tool_runner(
  model: :"{{OPUS_ID}}",
  max_tokens: 16000,
  tools: [GetWeather.new],
  messages: [{ role: "user", content: "What's the weather in San Francisco?" }]
).each_message do |message|
  puts message.content
end

Manual Loop

See the shared tool use concepts for the tool definition format and agentic loop pattern.

Prompt Caching

system_: (trailing underscore — avoids shadowing Kernel#system) takes an array of text blocks; set cache_control on the last block. Plain hashes work via the OrHash type alias. For placement patterns and the silent-invalidator audit checklist, see shared/prompt-caching.md.

message = client.messages.create(
  model: :"{{OPUS_ID}}",
  max_tokens: 16000,
  system_: [
    { type: "text", text: long_system_prompt, cache_control: { type: "ephemeral" } }
  ],
  messages: [{ role: "user", content: "Summarize the key points" }]
)

For 1-hour TTL: cache_control: { type: "ephemeral", ttl: "1h" }. There's also a top-level cache_control: on messages.create that auto-places on the last cacheable block.

Verify hits via message.usage.cache_creation_input_tokens / message.usage.cache_read_input_tokens.