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Claude Fable 5.1

AnthropicProprietaryPending Human Review

Claude Fable 5.1 is the independent update to Fable 5 for long-running coding, knowledge work, and research. It improves grounding, root-cause debugging, and efficiency at lower effort. Claude Mythos 5.1 is the same model with more permissive safeguards for verified cybersecurity and life sciences work, represented as a variant. Adaptive thinking is always enabled, and a million-token context supports extended tool-assisted tasks.

2026-09-01
Unreleased
Undisclosed proprietary architecture
Proprietary

Specifications

Parameters
Unreleased
Architecture
Undisclosed proprietary architecture
License
Proprietary
Context Window
1,000,000 tokens
Max Output
128,000 tokens
Training Data Cutoff
2026-06
Type
multimodal
Modalities
textimage

Benchmark Scores

Advanced Specifications

Model Family
Claude
API Access
Available
Chat Interface
Available
Multilingual Support
Yes
Variants
claude-fable-5-1Claude Mythos 5.1 (same underlying model; trusted access)

Capabilities & Limitations

Capabilities
deep reasoningagentic codingadaptive thinkingvisioncomputer useresearch
Known Limitations
Can make factual or coding errors; tool results and important outputs require verification.Longer reasoning and agent work increase latency and token consumption.Sensitive requests may be blocked or routed to another model; Mythos safeguards require trusted access.
Notable Use Cases
difficult engineeringlong professional workflowsscientific research assistance
Function Calling Support
Yes
Tool Use Support
Yes

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