Anthropic::FallbackParam

One entry in the top-level fallbacks: array on a message request.

Each entry names a model to try if the primary model refuses. The four override fields (max_tokens, output_config, speed, thinking) replace the top-level field for that attempt only.

Requires the server-side-fallback-2026-07-01 beta header (auto-attached when fallbacks: is set). The older server-side-fallback-2026-06-01 header is still accepted by the API.

message = client.beta.messages.create(
  model: Anthropic::Model::CLAUDE_FABLE_5,
  max_tokens: 1024,
  fallbacks: [
    Anthropic::FallbackParam.new(model: Anthropic::Model::CLAUDE_OPUS_4_8),
    Anthropic::FallbackParam.new(model: Anthropic::Model::CLAUDE_SONNET_5),
  ],
  messages: [{role: "user", content: "..."}]
)

# Or use the server-defined default chain for the primary model:
message = client.beta.messages.create(
  model: Anthropic::Model::CLAUDE_FABLE_5,
  max_tokens: 1024,
  fallbacks: "default",
  messages: [{role: "user", content: "..."}]
)