Decision summary
- New Claude default -> Claude Opus 5.5
- Already pinned to claude-fable-5-1 and evals pass -> stay until you replay
- Lower list price and cache reads -> Claude Opus 5.5
- Need Mythos-permissive cyber -> Project Glasswing, not this page
- Cheaper still -> evaluate Claude Sonnet 5 before either flagship
Claude family comparison
Frontier comparisonClaude Opus 5.5 vs Claude Fable 5.1
Claude Opus 5.5 is Anthropic's September 22, 2026 Opus model.
Featured · Updated today · Last verified: September 2026
Choose Claude Opus 5.5 when
New Claude long-running coding and knowledge work at the September 22 price.
Choose Claude Fable 5.1 when
Teams already pinned to claude-fable-5-1, or workloads that still clear only on that ID.
- New Claude default: Claude Opus 5.5
- Already pinned to claude-fable-5-1 and evals pass: stay until you replay
- Lower list price and cache reads: Claude Opus 5.5
Decision axes: Best for · Access and platforms · Context window · Input and output
Reasoning
Opus 5.5 defaults to medium. Fable 5.1 defaults to high in Claude Code and medium on Claude.ai.
Coding
Anthropic says the real-use gap is narrower than the benchmark margins.
Multimodal
Same text and image in, text out shape.
Speed
Opus 5.5 Fast mode is $8/$40. Compare live cards before assuming Fable speed.
Enterprise
Both are generally available Claude IDs. Mythos stays invite-only.
Curated matrix for Claude Opus 5.5 vs Claude Fable 5.1 — confirm live limits and pricing on each vendor’s official pages.
How they compare
Criterion-by-criterion notes from the catalog—not a ranking. Validate on your own gold set.
| Criterion | Claude Opus 5.5 | Claude Fable 5.1 |
|---|---|---|
| Best for | New Claude long-running coding and knowledge work at the September 22 price. | Teams already pinned to claude-fable-5-1, or workloads that still clear only on that ID. |
| Access and platforms | claude-opus-5-5 on Claude API, Bedrock, Google Cloud, Microsoft Foundry, and Claude Platform on AWS. | claude-fable-5-1, generally available on the same major clouds. |
| Context window | 1M context; 128K synchronous output. Knowledge cutoff June 2026. | 1M context; 128K max output. Knowledge cutoff June 2026. |
| Input and output | Text and image in; text out. | Text and image in; text out. |
| Reasoning control | Adaptive thinking always on. Default effort medium. | Adaptive thinking always on. Default effort is high in Claude Code and medium on Claude.ai. |
| Coding and agents | Anthropic's launch table reports higher Terminal-Bench 4.0, FrontierCode, and CursorBench scores than Fable 5.1, on Anthropic's harness. | September 1 long-horizon flagship. Anthropic says the real-use gap versus Opus 5.5 is narrower than the benchmark margins. |
| Cost posture | $4 / $20 per 1M. Cache read $0.20. Fast mode $8 / $40. | $10 / $50 list. Cache reads $0.25 per 1M. |
| Operational risk | Same class of breaking changes as Fable 5.1: thinking cannot be disabled and forced tool use errors. | Higher list price. Do not assume Opus 5.5 prompt settings transfer without a replay. |
Key insights
Concrete technical or product signals.
- Anthropic's own caveat: benchmark margins overstate the real-use gap versus Fable 5.1.
- Opus 5.5 defaults to medium. Fable 5.1 defaults to high in Claude Code and medium on Claude.ai. Untouched defaults will not spend the same tokens.
- Listed token prices are not total task cost. Measure cache-hit rate on your agent loop.
Use cases
Where this shines in production.
- Replacing a Fable 5.1 pin
- Choosing a Claude default for new agents
- Checking whether the cheaper Opus ID holds quality
Limitations & trade-offs
What to watch for.
- Not a Mythos 5.1 comparison. Mythos stays invite-only.
- Vendor scores are not a substitute for your repo eval.
- Sonnet 5.5 and Haiku 5.5 are announced as coming later, not in this catalog yet.
Watch-outs
Who should pass, and what to validate before rollout.
- Do not keep Fable 5.1 as the default only because the name says flagship.
- Do not switch pinned Fable clients without replaying thinking-block behavior.
- Do not use this page to choose Mythos 5.1.
Final recommendation
What to do after reading the table.
Default new Claude work to Opus 5.5 and keep Fable 5.1 only where your replay still needs it.
FAQ
Is Claude Opus 5.5 better than Claude Fable 5.1?
Default new Claude work to Opus 5.5 and keep Fable 5.1 only where your replay still needs it.
Which is better for coding: Claude Opus 5.5 or Claude Fable 5.1?
Claude Opus 5.5: Anthropic's launch table reports higher Terminal-Bench 4.0, FrontierCode, and CursorBench scores than Fable 5.1, on Anthropic's harness. Claude Fable 5.1: September 1 long-horizon flagship. Anthropic says the real-use gap versus Opus 5.5 is narrower than the benchmark margins. The catalog does not declare a winner on coding and agents.
Which is cheaper: Claude Opus 5.5 or Claude Fable 5.1?
Listed unit prices: Claude Opus 5.5 $4 / $20 per 1M. Cache read $0.20. Fast mode $8 / $40; Claude Fable 5.1 $10 / $50 list. Cache reads $0.25 per 1M. A lower listed unit price is not the same as a lower total workload cost—tokens, session minutes, caching, introductory rates, and add-on modes can reverse the ranking. Confirm live rates on each vendor’s official page.
Can I use both Claude Opus 5.5 and Claude Fable 5.1?
Yes. Many teams keep both and route by constraint. Use Claude Opus 5.5 for New Claude long-running coding and knowledge work at the September 22 price, and Claude Fable 5.1 for Teams already pinned to claude-fable-5-1, or workloads that still clear only on that ID. Confirm vendor terms before mixing them in one production workflow.