About Claude Fable 5.1
Claude Fable 5.1 is the successor to Claude Fable 5 in Anthropic's Mythos-class tier, built for demanding reasoning and long-horizon agentic work. It keeps Claude Fable 5's input and output prices and brings stronger long-running agentic coding, multistep research, and document, spreadsheet, and slide work. Anthropic positions Claude Opus 5 as the starting point for most workloads and Claude Fable 5.1 for tasks where Opus 5 at higher effort still falls short.
Adaptive thinking is always on and steered by the effort parameter (low through max); the raw chain of thought is never returned. Safety classifiers may decline requests in sensitive domains such as cybersecurity and biology, returning a refusal stop reason that can be routed to another Claude model via server-side fallback. Compared with Claude Fable 5, forced tool use is no longer supported, thinking blocks are readable only by the model that produced them, and editing earlier turns invalidates thinking blocks, so conversation histories should be append-only. Prompt cache reads cost a quarter of Claude Fable 5's rate. It accepts text and images and has a reliable knowledge cutoff of June 2026.
| Metric | Value |
|---|---|
| Context Length | 1,000,000 tokens |
| Max Output | 128,000 tokens |
| Multilingual | Yes |
Ready to build with Claude Fable 5.1?
Try Claude Fable 5.1 in the Workbench to prompt it, compare outputs, and iterate on prompts without writing any code. When you're ready to ship, call the same model from our API and build your own apps on top of it.
curl -sSf -X POST https://hub.oxen.ai/api/ai/chat/completions \
-H "Content-Type: application/json" \
-H "Authorization: Bearer $OXEN_API_KEY" \
-d '{
"model": "claude-fable-5-1",
"messages": [
{
"role": "user",
"content": "Try sending a message."
}
]
}'