Claude Fable 5.1
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