June 9, 2026 · Anthropic

Claude Fable 5 and Claude Mythos 5 launched with 1M-token context window

We’ve launched Claude Fable 5 (claude-fable-5), our most capable model open to all customers, alongside Claude Mythos 5 (claude-mythos-5) for Project Glasswing participants. Both models support a 1M token context window by default, 128k max output tokens, and always-on adaptive thinking. See Claude Fable 5 for specifications, pricing, and availability.
Claude Fable 5 and Claude Mythos 5 use the tokenizer introduced with Claude Opus 4.7. Compared to models before Claude Opus 4.7, the same text produces roughly 30% more tokens. The exact increase depends on the content and workload shape. Use the token counting API with model: "claude-fable-5" to measure your prompts under the new tokenizer.
Claude Fable 5 runs safety classifiers on requests and during response generation. When a classifier declines a request, the Messages API returns stop_reason: "refusal". You are not billed for a request refused before any output is generated. An opt-in fallbacks parameter (in beta on the Claude API and Claude Platform on AWS; not supported on the Message Batches API) re-runs refused requests on another model, billed at the fallback model’s rates. See Handling stop reasons.
The stop_details.category field on refusal responses now includes "reasoning_extraction" on Claude Fable 5, returned when a request is blocked under Anthropic’s Terms of Service restrictions on reverse engineering or duplicating model outputs. The existing "cyber" and "bio" categories are unchanged. No beta header is required.
On Claude Fable 5 and Claude Mythos 5, adaptive thinking is the only thinking mode: thinking: {"type": "disabled"} is not supported, and manual extended thinking budgets and assistant prefill are not supported (both return a 400 error).
On Claude Fable 5 and Claude Mythos 5, thinking.display defaults to "omitted", the same as Claude Opus 4.8, Claude Opus 4.7, and Claude Mythos Preview; set display: "summarized" to receive readable thinking summaries. The raw chain of thought is never returned; pass thinking blocks back unchanged in multi-turn conversations on the same model. See Thinking output on Claude Fable 5 and Claude Mythos 5.
Claude Fable 5 requires 30-day data retention and is not available under zero data retention. See Model-specific data retention requirements.
Claude Managed Agents now supports scheduled deployments, letting you run sessions on a cron schedule without managing your own scheduler.
Claude Managed Agents vaults now support environment variable credentials, so you can securely inject secrets into the agent’s sandbox for CLIs, SDKs, and other services that authenticate through environment variables.
The Compliance API Activity Feed (GET /v1/compliance/activities) is now available on Claude Platform on AWS. See Compliance IAM actions for Claude Platform on AWS for the ListComplianceActivities action that authorizes it.
The session.thread_* webhook events now include a session_thread_id field identifying the multiagent thread that triggered the event.
We’ve released a Swift package in beta that adds Claude as a server-side LanguageModel in Apple’s Foundation Models framework. Call Claude through the same LanguageModelSession API as Apple’s on-device model on iOS 27, macOS 27, visionOS 27, and watchOS 27 (beta).

Read the Anthropic changelog