Technology comparison

Claude Fable 5.1 vs Claude Opus 5.5: models, context & API pricing

Choose Claude Fable 5.1 when you need a very large context window or long outputs—it supports up to 1M tokens of context and 128K output tokens, at a documented base price of $10 per million input tokens and $50 per million output tokens, with cache reads at $0.25 per million. Choose Claude Opus 5.5 when cost efficiency at scale is the priority and its smaller context is sufficient: documented pricing is $4 per million input and $20 per million output tokens, with cache reads at $0.20 per million. Confirm current availability and limits in Anthropic's official documentation before committing.

Technical context and trade-offs

These differences are drawn directly from the comparison criteria below.

Context window and maximum output

Claude Fable 5.1: Claude Fable 5.1 supports a 1M-token context window and up to 128K output tokens, per Anthropic's model documentation. That makes it suitable for very long documents, large codebases, and multi-step outputs that would otherwise require chunking; exact effective limits can depend on the API tier and request configuration.

Claude Opus 5.5: Claude Opus 5.5's context window and maximum output should be verified against Anthropic's current model overview, because published limits are version- and date-sensitive. In practice, treat Opus 5.5 as the option to choose only after confirming that its documented context and output limits fit your workload, especially for long-document or large-repository tasks.

Published benchmark performance (MMLU, HumanEval)

Claude Fable 5.1: Benchmark results for Claude Fable 5.1 on MMLU and HumanEval are not assumed here; any figures should be taken from Anthropic's official model documentation or model card for the exact version. Compare only results published under comparable settings, and avoid extrapolating scores across versions or evaluation harnesses.

Claude Opus 5.5: Similarly, benchmark results for Claude Opus 5.5 on MMLU and HumanEval should be verified in Anthropic's official materials before drawing conclusions. Where comparable published results are unavailable, base the decision on qualitative capability descriptions and on your own task-specific evaluation rather than on unverified numeric claims.

Quick verdictThe bottom line

Claude Fable 5.1Claude Opus 5.5Explore the detailed comparison

Scores by criterion

Performance50–50 (2 criteria)

Ease of useNo direct evidence in the criteria

Ecosystem50–50 (1 criteria)

PopularityNo direct evidence in the criteria

Claude Fable 5.1Claude Opus 5.5

Scores reflect wins in matching criteria; ties count as half a point.

Technical feature matrix

Criterion	Claude Fable 5.1	Claude Opus 5.5	Edge
Context window and maximum output	Claude Fable 5.1 supports a 1M-token context window and up to 128K output tokens, per Anthropic's model documentation. That makes it suitable for very long documents, large codebases, and multi-step outputs that would otherwise require chunking; exact effective limits can depend on the API tier and request configuration.	Claude Opus 5.5's context window and maximum output should be verified against Anthropic's current model overview, because published limits are version- and date-sensitive. In practice, treat Opus 5.5 as the option to choose only after confirming that its documented context and output limits fit your workload, especially for long-document or large-repository tasks.	Claude Fable 5.1
Published benchmark performance (MMLU, HumanEval)	Benchmark results for Claude Fable 5.1 on MMLU and HumanEval are not assumed here; any figures should be taken from Anthropic's official model documentation or model card for the exact version. Compare only results published under comparable settings, and avoid extrapolating scores across versions or evaluation harnesses.	Similarly, benchmark results for Claude Opus 5.5 on MMLU and HumanEval should be verified in Anthropic's official materials before drawing conclusions. Where comparable published results are unavailable, base the decision on qualitative capability descriptions and on your own task-specific evaluation rather than on unverified numeric claims.	Comparable
API pricing per 1M input/output tokens	Anthropic's documented base API pricing for Claude Fable 5.1 is $10 per million input tokens and $50 per million output tokens. Prompt-cache reads are documented at $0.25 per million tokens, which is 97.5% below the base input price; prices and availability may change, so confirm against the official pricing page before budgeting.	Anthropic's documented pricing for Claude Opus 5.5 is $4 per million input tokens and $20 per million output tokens, with cache reads at $0.20 per million tokens. These rates make Opus 5.5 materially cheaper per token than Fable 5.1 at list prices, though total cost still depends on prompt caching, output length, and workload mix.	Claude Opus 5.5
Latency and throughput	Anthropic does not publish a single latency figure that applies to all deployments of Claude Fable 5.1; actual latency depends on region, load, request size, streaming, and output length. Long-context requests and 128K-token outputs can increase time to completion, so measure with your own representative prompts rather than relying on generalized claims.	Latency for Claude Opus 5.5 likewise depends on deployment and request characteristics, and any specific number should be verified or measured rather than assumed. Because Opus 5.5 is positioned as the lower-cost option, it may be preferable for high-volume, latency-tolerant workloads, but comparative latency should be confirmed empirically for your use case.	Comparable
Model access and open/closed availability	Claude Fable 5.1 is an Anthropic-hosted model accessed through the Claude API and related platform surfaces, subject to account eligibility and regional availability. It is not an open-weight release, so access terms, rate limits, and supported regions should be checked in Anthropic's official documentation.	Claude Opus 5.5 is likewise an Anthropic-hosted, closed model rather than an open-weight distribution. Availability may differ by plan, region, and API tier, so verify that your account and deployment environment can access the specific model version before designing around it.	Comparable
Practical use cases: long-context and output-heavy work	With a 1M-token context window and 128K maximum output, Claude Fable 5.1 is a strong fit for whole-repository analysis, lengthy contract or research review, and generation of extensive structured outputs. The higher per-token price means these workloads should be scoped carefully, and prompt caching can reduce repeated-input costs substantially.	Claude Opus 5.5 is better suited to cost-sensitive, high-throughput tasks such as classification, extraction, summarization of moderate documents, and conversational agents where its context limit is sufficient. Its lower documented token prices make it the more economical default for many production pipelines, provided its verified limits meet your requirements.	Claude Opus 5.5
Cost optimization with prompt caching	For Claude Fable 5.1, cache reads are documented at $0.25 per million tokens versus $10 per million for base input, a 97.5% reduction on cached input. Workloads with large, repeatedly reused system prompts or documents can benefit significantly, but cache writes and misses are billed differently and should be modeled against your traffic pattern.	For Claude Opus 5.5, cache reads are documented at $0.20 per million tokens versus $4 per million for base input. The absolute savings per cached token are smaller than for Fable 5.1, but the lower base price means Opus 5.5 is often cheaper overall for mixed workloads; evaluate cache hit rates and output volume together.	Claude Opus 5.5
Ecosystem, tooling, and integration fit	Claude Fable 5.1 is used through the same Anthropic API conventions as other Claude models, so existing SDKs, streaming, and tool-use integrations generally apply, subject to model-specific parameters and limits. Teams should confirm feature support for the exact model version, since capabilities such as extended thinking or specific beta features can vary by model.	Claude Opus 5.5 similarly integrates through Anthropic's API and tooling, and its lower price can make it easier to justify in existing pipelines. As with any model version, verify supported parameters, feature flags, and deprecation timelines in Anthropic's documentation before standardizing on it.	Comparable
Verified Claude Fable 5.1 specifications and pricing	Anthropic documents a 1M-token context window, up to 128K output tokens, and API rates of $10 per million input tokens and $50 per million output tokens for Claude Fable 5.1. Prompt-cache reads cost $0.25 per million tokens, 97.5% below the base input rate; verify current pricing before deployment.	Anthropic documents a 1M-token context window, up to 128K output tokens, and API rates of $4 per million input tokens and $20 per million output tokens for Claude Opus 5.5. Prompt-cache reads cost $0.20 per million tokens; prices may change.	Comparable

When to choose each option

Choose this option if… Claude Fable 5.1

Choose Claude Fable 5.1 when the strengths described here match your project requirements. Context window and maximum output: Claude Fable 5.1 supports a 1M-token context window and up to 128K output tokens, per Anthropic's model documentation. That makes it suitable for very long documents, large codebases, and multi-step outputs that would otherwise require chunking; exact effective limits can depend on the API tier and request configuration.

Choose this option if… Claude Opus 5.5

Choose Claude Opus 5.5 when the strengths described here match your project requirements. API pricing per 1M input/output tokens: Anthropic's documented pricing for Claude Opus 5.5 is $4 per million input tokens and $20 per million output tokens, with cache reads at $0.20 per million tokens. These rates make Opus 5.5 materially cheaper per token than Fable 5.1 at list prices, though total cost still depends on prompt caching, output length, and workload mix. Practical use cases: long-context and output-heavy work: Claude Opus 5.5 is better suited to cost-sensitive, high-throughput tasks such as classification, extraction, summarization of moderate documents, and conversational agents where its context limit is sufficient. Its lower documented token prices make it the more economical default for many production pipelines, provided its verified limits meet your requirements. Cost optimization with prompt caching: For Claude Opus 5.5, cache reads are documented at $0.20 per million tokens versus $4 per million for base input. The absolute savings per cached token are smaller than for Fable 5.1, but the lower base price means Opus 5.5 is often cheaper overall for mixed workloads; evaluate cache hit rates and output volume together.

Technology role diagram

Option A

Claude Fable 5.1

Comparison

Option B

Claude Opus 5.5

Comparison

This diagram shows broad areas of use, not a complete deployment architecture.

Code examples

Claude Fable 5.1

import anthropic

client = anthropic.Anthropic()

response = client.messages.create(
    model="claude-fable-5-1",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Hello, world!"}],
)

print(response.content[0].text)

Frequently asked questions

Which model should I choose for long-document analysis?

Claude Fable 5.1 is the better fit when your documents exceed the context limits of smaller models, because Anthropic documents a 1M-token context window and up to 128K output tokens for it. Claude Opus 5.5 can handle long-document work too, but you should verify its current context limit in Anthropic's official documentation and confirm that your documents fit before committing.

Is Claude Fable 5.1 more expensive than Claude Opus 5.5?

Yes, at documented base API list prices: Claude Fable 5.1 is $10 per million input tokens and $50 per million output tokens, while Claude Opus 5.5 is $4 per million input and $20 per million output tokens. Prices and availability may change, so confirm the current rates on Anthropic's pricing page and factor in prompt caching, which is $0.25 per million cached input tokens for Fable 5.1 and $0.20 for Opus 5.5.

Are MMLU and HumanEval scores available for a direct comparison?

Only compare MMLU or HumanEval scores when both models have published results from comparable evaluation settings, and treat any figures as version- and date-sensitive. For Claude Fable 5.1 and Claude Opus 5.5, check Anthropic's official model documentation for the exact versions rather than relying on third-party or outdated numbers.

How much can prompt caching reduce my costs?

For Claude Fable 5.1, cache reads are documented at $0.25 per million tokens versus a $10 base input price, which is 97.5% below the base input rate. For Claude Opus 5.5, cache reads are $0.20 per million versus $4 base input, so the percentage saving is similar but the absolute amount depends on how much of your prompt is cached and how often it is reused.

Which model is better for high-volume production workloads?

Claude Opus 5.5 is generally the more economical default for high-volume workloads because its documented token prices are lower than Claude Fable 5.1's. However, you should benchmark both models on your actual tasks and verify context limits, latency, and feature support, since the cheapest per-token option is not always the lowest total cost when retries or longer outputs are involved.

Can I access these models as open-weight downloads?

No, both Claude Fable 5.1 and Claude Opus 5.5 are Anthropic-hosted, closed models accessed through the Claude API and related platform surfaces rather than open-weight releases. Access depends on your account, plan, and region, so check Anthropic's official documentation for eligibility and availability before designing your integration.

How should I handle model version changes and deprecations?

Treat model identifiers and specifications as version-sensitive and pin explicit model names in your code rather than relying on aliases that may move. Review Anthropic's official model overview and deprecation notices regularly, and keep a fallback model configured so that version retirements do not break production traffic.

What is the best way to decide between the two models?

Start with hard requirements—context window, output length, latency, and compliance—then compare documented prices for your expected input and output token mix, including cache hit rates. Run a small evaluation on representative tasks for both models, and choose Claude Fable 5.1 for very long context or output-heavy work and Claude Opus 5.5 when cost efficiency at scale matters more.