Claude Fable 5 vs Opus 4.8 vs Sonnet 4.6: Full Anthropic Model Tier Comparison 2026
In June 2026, Claude Fable 5's complete system prompt was leaked, accidentally revealing Anthropic's new Mythos-class model hierarchy. According to the leaked prompt's own description, the current model lineup is:
Claude Fable 5 > Claude Opus 4.8 > Claude Sonnet 4.6 > Claude Haiku 4.5
This is the first time we've received official confirmation of the model tier order directly from the prompt. In this article, we compare all four models based on the leaked system prompt and publicly available data, helping you choose the right model for your needs.
1. The Full Model Landscape: What Is Mythos-class?
Anthropic's traditional product line followed a three-tier structure: Opus > Sonnet > Haiku. The leaked Claude Fable 5 prompt reveals a new tier above Opus:
"This iteration of Claude is Claude Fable 5, the first model in Anthropic's new Claude 5 family and part of a new Mythos-class model tier that sits above Claude Opus in capability."
The Mythos-class includes two variants:
Model | Positioning | Availability |
|---|---|---|
Claude Fable 5 | Most intelligent general model, full safety restrictions | Public |
Claude Mythos 5 | Same underlying model as Fable 5, fewer restrictions | Approved organizations only |
We had previously noticed during testing that Mythos 5 appeared to have more lenient safety restrictions — the leaked prompt now confirms its existence as an uncensored variant for enterprise/research use.
2. Model Capability Comparison
2.1 Claude Fable 5 (Mythos-class Flagship)
Positioning: Anthropic's intelligence ceiling
Based on the prompt, Fable 5 features:
Most powerful reasoning and analysis capabilities
Full feature support (Code Execution, Artifacts, Web Search, Deep Research)
Best suited for: complex reasoning, code generation, academic research, deep analysis
API model string:
claude-fable-5
Safety restrictions: Most comprehensive. The prompt includes complete mental health safeguards, refusal mechanisms, political neutrality, and financial/legal advice protocols — stricter than any other model.
Our testing: In multi-step reasoning and long-context tasks, Fable 5 significantly outperforms Opus 4.8. It demonstrates notably clearer reasoning chains in academic reading and code review scenarios.
2.2 Claude Opus 4.8
Positioning: Previous flagship, still capable
As the successor to the Opus series, Opus 4.8 remains top-tier for most professional tasks:
API model string:
claude-opus-4-8Best for: daily professional writing, code development, mid-complexity reasoning
Pricing: more cost-effective than Fable 5
Gap from Fable 5: Noticeable on extremely complex tasks (multi-step math, cross-domain knowledge synthesis), but for most users, Opus 4.8 offers more than enough performance.
2.3 Claude Sonnet 4.6
Positioning: Best value
The Sonnet series remains Anthropic's most balanced option:
API model string:
claude-sonnet-4-6Best for: content creation, translation, summarization, mid-complexity programming
Pricing: significantly lower than Opus and Fable 5
Our testing: Sonnet 4.6 performs admirably on creative writing and dialogue tasks. For users who don't need cutting-edge reasoning, it's the most efficient choice.
2.4 Claude Haiku 4.5
Positioning: Speed and cost priority
The Haiku series focuses on low latency and high throughput:
API model string:
claude-haiku-4-5-20251001Best for: simple classification, data extraction, customer service, lightweight tasks
Speed: fastest of all models
3. Safety Strategy Comparison: Fable 5 Is Strictest
Based on the leaked prompt, safety restrictions vary significantly across models:
Dimension | Fable 5 / Opus 4.8 | Sonnet 4.6 / Haiku 4.5 |
|---|---|---|
Mental Health | Full (suicide intervention, self-harm red lines) | Simplified |
Refusal Rules | Detailed multi-tier (weapons, drugs, malware) | Basic refusal rules |
Political Neutrality | Strict (must present both sides) | More relaxed |
MCP Recommendations | Full recommendation logic | Basic |
Artifacts Storage | Full support | Supported |
Key finding: Since Fable 5 is "the most intelligent generally available model," it also has the strictest safety restrictions of any model. In Anthropic's own words — "sits above Claude Opus in capability" — greater capability means tighter constraints.
4. Mythos 5: The Real Uncensored Option
One of the most important revelations from the leaked prompt is the existence of Claude Mythos 5:
"Claude Fable 5 and Claude Mythos 5 share the same underlying model. Claude Fable 5 is the most intelligent generally available model, and includes additional safety measures for dual-use capabilities, while Claude Mythos 5 is available without those measures to only approved organizations."
This means:
Same underlying capability — Mythos 5 matches Fable 5's intelligence
Mythos 5 removes dual-use safety measures — can handle a broader range of queries
Approved organizations only — not available to general users
For users seeking a truly uncensored AI experience, check out our Claude Mythos 5 jailbreak guide, or use a platform like HackAIGC that offers native uncensored access.
5. Mid-Conversation Model Switching
One interesting detail from the prompt:
"The person is able to switch models mid-conversation, so previous messages claiming to be from a different model or to have a different knowledge cutoff may be accurate."
Users can seamlessly switch models mid-conversation — for example, starting with Haiku and switching to Fable 5 for a complex subtask. This flexible design lets users dynamically adjust based on the complexity needs of different conversation phases.
6. Pricing and Model Selection Guide
Based on model capability rankings and our hands-on testing:
When to Choose Claude Fable 5
Extreme multi-step reasoning tasks
High-precision code review and architecture design
Deep academic analysis or cross-domain knowledge synthesis
Budget-unlimited, maximum performance needed
When to Choose Claude Opus 4.8
Professional-grade programming and code generation
Formal document writing and deep content creation
Most professional task scenarios
Better value than Fable 5
When to Choose Claude Sonnet 4.6
Daily writing and content translation
Mid-complexity programming assistance
Budget-conscious but quality-reliable scenarios
When to Choose Claude Haiku 4.5
Large-scale batch processing
Low-latency requirements (customer service, real-time)
Simple classification and data extraction
Lowest cost option
7. Uncensored Options Comparison
If Claude Fable 5 / Opus 4.8's safety restrictions feel too restrictive:
Option | Type | Availability |
|---|---|---|
Claude Mythos 5 | Official uncensored version | Organization approval only |
HackAIGC Chat | Native uncensored AI chat | Open registration |
ChatGPT Jailbreak | Bypassing ChatGPT restrictions | Manual setup |
We've tested uncensored AI on our platform and found that uncensored mode offers irreplaceable advantages in creative content generation, deep roleplay, and boundary-free discussion.
FAQ
Q: How much better is Claude Fable 5 than Opus 4.8? A: According to the prompt, Fable 5 belongs to the new Mythos-class tier above Opus. In our testing, Fable 5 significantly outperforms Opus 4.8 on extremely complex reasoning tasks, though the difference is minimal for daily tasks.
Q: Is Claude Sonnet 4.6 still worth using? A: Absolutely. Sonnet 4.6 remains the best value option. For users who don't need cutting-edge reasoning, it fully meets requirements at a fraction of the cost.
Q: Can I use Claude Fable 5 for uncensored conversations? A: No. Fable 5 has the strictest safety restrictions of any Claude model. For truly uncensored access, try platforms like HackAIGC.
Q: What's the API string for Claude Mythos 5? A: The leaked prompt doesn't specify Mythos 5's API string, only Fable 5's (claude-fable-5). Mythos 5 may be accessible through a separate approval channel.
Q: Is there a big price difference between models? A: Following Anthropic's typical pricing structure: Fable 5 (Mythos-class) > Opus 4.8 > Sonnet 4.6 > Haiku 4.5. Higher tier = higher cost. Refer to Anthropic's official pricing page for current rates.