Short answer
For most production workloads, start by evaluating Claude Opus 5.5: Anthropic’s model guidance recommends it for most work. Consider Claude Fable 5.1 for demanding reasoning or long-horizon agent work if Opus 5.5 does not meet your requirements. Standard input and output token rates listed in Anthropic’s documentation are 2.5 times higher for Fable 5.1.
This is a version-specific comparison of Anthropic documentation entries for Fable 5.1 and Opus 5.5; it does not establish that these are the latest versions or guarantee API access for every account. Confirm availability and applicable rates before deployment.
Claude Fable 5.1 vs. Opus 5.5 at a glance
| Production API detail | Claude Fable 5.1 | Claude Opus 5.5 |
|---|---|---|
| Anthropic API model ID | claude-fable-5-1 |
claude-opus-5-5 |
| Best-fit description in Anthropic’s docs | Demanding reasoning and long-horizon agentic work | Long-running agentic coding and knowledge work |
| Context window / maximum output | 1 million / 128,000 tokens | 1 million / 128,000 tokens |
| Relative latency in Anthropic’s comparison | Slower | Moderate |
| Standard input price | $10 per million tokens | $4 per million tokens |
| Standard output price | $50 per million tokens | $20 per million tokens |
These are Anthropic-listed API IDs, specifications, relative-latency labels, and standard prices—not results from an independent benchmark. Anthropic’s guidance recommends Opus 5.5 for most workloads and suggests considering Fable 5.1 for demanding reasoning or long-horizon work.
Which model should you choose?
Evaluate Opus 5.5 first for typical production tasks such as agentic coding and knowledge work. The documented context window and maximum output match Fable 5.1’s, while its listed standard token rates are lower. Anthropic categorizes its relative latency as “moderate,” compared with “slower” for Fable 5.1. These labels are not measurements of latency in your application.
Test Fable 5.1 for demanding reasoning or long-horizon agent work—for example, tasks spanning many steps or extended coding and research workflows—when Opus 5.5 falls short of your quality bar. Anthropic describes Fable 5.1 as intended for demanding reasoning and long-horizon agentic work. That positioning is a reason to evaluate it, not a guarantee that it will outperform Opus on a particular task.
The practical question is whether Fable’s additional cost produces a measurable improvement on your workload. The available documentation does not establish a universal winner or a guaranteed quality advantage for Fable.
API pricing and an example
Anthropic lists standard API prices in USD per million tokens. For uncached input and output, Fable 5.1 is listed at $10 per million input tokens and $50 per million output tokens; Opus 5.5 is listed at $4 per million input tokens and $20 per million output tokens. For equal uncached input and output volumes, Fable’s listed token charges are 2.5 times Opus’s.
Illustrative example: Assume a request uses 100,000 uncached input tokens and generates 10,000 output tokens, with no other pricing adjustments:
- Fable 5.1: (0.1 × $10) + (0.01 × $50) = $1.50
- Opus 5.5: (0.1 × $4) + (0.01 × $20) = $0.60
This is an arithmetic illustration, not a measured workload cost. Actual charges depend on token use and applicable pricing terms. If your workload uses prompt caching or batch processing, check Anthropic’s current pricing documentation for the applicable terms before estimating costs.
How to decide in production
Run a prospective, controlled comparison before routing important traffic:
- Choose representative tasks. Use real workload examples, including difficult cases—not only easy prompts.
- Keep the comparison fair. Use the same inputs, tools, success criteria, and relevant settings for both models. Record the model IDs so results stay tied to the versions tested.
- Set quality criteria in advance. For coding, these might include passing tests and handling edge cases; for research, coverage and factual support. Review consequential outputs rather than judging by answer length alone.
- Measure operational outcomes. Track task success, latency, input and output tokens, retries or tool calls, and API cost. Compare any quality gain with Fable’s additional cost.
- Route by evidence. Keep Opus 5.5 where it meets your acceptance bar; use Fable 5.1 for task segments where your own results justify the higher listed price.
Anthropic’s model guidance and specifications can help narrow the choice, but they do not replace a version-matched test on your workload. The documented relative-latency categories do not predict your application’s latency, success rate, or cost per completed task.
Sources
Claude Fable 5.1 — Claude Platform Docs