Claude Fable 5
Claude Fable 5 is Anthropic's most capable widely released model, built for the most demanding reasoning and long-horizon agentic work. Claude Fable 5 runs long, complex, and asynchronous tasks with fewer check-ins and is available on AI Gateway via anthropic, vertexAnthropic with adaptive thinking and configurable effort.
import { streamText } from 'ai'
const result = streamText({ model: 'anthropic/claude-fable-5', prompt: 'Why is the sky blue?'})About Claude Fable 5
Claude Fable 5 became available on AI Gateway on July 1, 2026. Access was briefly suspended under US export controls and restored in July 2026 once the controls were lifted; the restored model is the same model, with updated and more robust safety classifiers. Anthropic describes Claude Fable 5 as a Mythos-class model made safe for general availability, with capabilities that exceed any model Anthropic had widely released before. The launch materials report leading results across nearly every benchmark Anthropic tested, spanning software engineering, knowledge work, vision, and scientific research.
Long-horizon agentic work is the design target. Claude Fable 5 handles long-running, complex, and asynchronous tasks where previous models needed more frequent check-ins. At launch, Claude Fable 5 supports the memory tool, code execution, programmatic tool calling, compaction, tool result clearing through context editing, and vision input, plus task budgets in beta. Those are the building blocks for agents that run for hours without losing state.
Adaptive thinking is always on. Claude Fable 5 decides when and how deeply to reason, and thinking: { type: 'disabled' } is not supported. Use the effort parameter to control thinking depth and cost per request. Raw chain of thought is never returned: set thinking.display to summarized for a readable summary, or keep the default omitted.
Safety classifiers ship as part of Claude Fable 5 and can decline certain requests, including occasional false positives on routine tasks such as coding and debugging. A refusal returns stop_reason: "refusal" as a successful response, not an error, and reports which classifier declined. Configure fallback models through the models array in providerOptions.gateway so AI Gateway retries refused requests on the next Claude model in your list, such as Claude Opus 4.8 or Claude Sonnet 5. Fallbacks work on every API format.
Through AI Gateway, Claude Fable 5 is available with the standard unified API, observability, retries, and failover across anthropic, vertexAnthropic. Set the model to anthropic/claude-fable-5 in the AI SDK, Chat Completions API, Responses API, Messages API, or other API formats, from TypeScript or Python. AI Gateway mirrors provider pricing with no markup and adds no platform fee on inference, including for Bring Your Own Key requests.