Providers

provider = Belay::AnthropicProvider.new(
  api_key: Rails.application.credentials.anthropic_api_key, # or ENV["ANTHROPIC_API_KEY"] by default
  pricing: { "claude-sonnet-5-5" => { input_per_million: 3.0, output_per_million: 15.0 } } # optional; cost_cents is nil without it
)
Belay.configure { |config| config.provider = provider }

Belay isn’t Anthropic-only. Belay::Provider is a pure interface - #call(model:, prompt:, schema: nil) returning a Belay::ProviderResponse or raising. Belay::AnthropicProvider (and Belay::FakeProvider, used throughout this gem’s own test suite) are just two implementations of it; nothing in Context#llm, caching, replay’s model_override, or cost tracking knows or cares which provider is configured.

Writing your own

class OpenAIProvider < Belay::Provider
  def call(model:, prompt:, schema: nil)
    # translate to the provider's own request shape, call its API, parse the response
    Belay::ProviderResponse.new(content: ..., model: model, input_tokens: ..., output_tokens: ...,
                                cost_cents: ..., latency_ms: ...)
  end
end

content and model are required; the token/cost/latency fields default to nil (not every provider call has them - a tool_call step has no token usage at all, and only the provider itself knows its own pricing well enough to fill in cost_cents).

What AnthropicProvider adds

InvoiceSchema must set additionalProperties: false on every object in the schema, if it’s ever run against AnthropicProvider - Anthropic’s own requirement for structured outputs. pricing: is deliberately not pre-filled with Anthropic’s current rates - they change, and a stale hardcoded number is worse than an honest nil. response.cost_cents is nil for any model you haven’t priced.

ctx.llm itself also validates response.content against schema: (via json_schemer), provider-agnostically - AnthropicProvider’s own schema: already asks Anthropic’s API to constrain its output, but FakeProvider enforces nothing at all, so this is the one place that’s true no matter which provider is configured.