Claude Opus 5.5 vs Claude Fable 5.1
Opus 5.5 is 60% cheaper, faster and scores higher on all seven Anthropic benchmarks — reserve Fable 5.1 only when your own evals prove the premium pays off.
Both models share a 1M-token context window and adaptive always-on thinking, but differ sharply on price, speed and default effort. This Claude model comparison breaks down specifications, benchmarks and practical trade-offs — including thinking-block portability — so you can decide whether Fable 5.1's premium is justified for your workload.
Last updated 2026-09-30 · All data sourced from Anthropic's official documentation
At a Glance: Opus 5.5 or Fable 5.1?
Choose Claude Opus 5.5 when…
You want the strongest published benchmark results at $4 / $20, need fast mode or 300K Batch output, or are starting a new project — Anthropic recommends Opus as the default for most workloads.
Choose Claude Fable 5.1 when…
Your own evals show Opus 5.5 at higher effort still falls short on demanding reasoning or long-horizon agentic tasks, and the 2.5x price premium is justified by measurable gains on your workload.
Opus 5.5 vs Fable 5.1: Key Specifications
API identifiers, pricing, context limits, thinking behavior and more — sourced from Anthropic's documentation. The models match on context window, output limit and modalities but differ on price, latency, default effort and retirement date.
| Spec | Claude Opus 5.5 | Claude Fable 5.1 |
|---|---|---|
| API model ID | claude-opus-5-5 | claude-fable-5-1 |
| Released | Sep 22, 2026 | Sep 1, 2026 |
| Tagline | For long-running agentic coding and knowledge work | For demanding reasoning and long-horizon agentic work |
| Latency | Moderate | Slower |
| Input price (per MTok) | $4 | $10 |
| Output price (per MTok) | $20 | $50 |
| Context window | 1M tokens | 1M tokens |
| Max output | 128K tokens (300K on Batch API) | 128K tokens |
| Thinking | Adaptive, always on, cannot be disabled | Adaptive, always on, cannot be disabled |
| Default effort | medium | high |
| Knowledge cutoff | Jun 2026 | Jun 2026 |
| Input modalities | Text + images | Text + images |
| Retirement | Not sooner than Sep 22, 2027 | Not sooner than Sep 1, 2027 |
Pricing: Fable 5.1 Costs 2.5x More per Token
At list price, Opus 5.5 is $4 / $20 vs Fable 5.1 at $10 / $50 per million tokens (MTok) for input / output — 60% cheaper on both. A typical 100K-input, 20K-output request costs $0.80 on Opus and $2.00 on Fable.
| Pricing Tier | Claude Opus 5.5 | Claude Fable 5.1 |
|---|---|---|
| Standard API (per MTok) | $4 / $20 | $10 / $50 |
| Batch API (50% off) | $2 / $10 | $5 / $25 |
| Cache write (5 min) | $5 | $12.50 |
| Cache write (1 hour) | $8 | $20 |
| Cache read | $0.20 (0.05x base) | $0.25 (0.025x base) |
Fable 5.1 prices cache reads at just 2.5% of its base input price, a quarter of what Fable 5 charged. Even so, its $0.25 per MTok cache read is still higher than Opus 5.5's $0.20, and Fable's cache writes ($12.50 for 5 minutes, $20 for 1 hour) cost 2.5x Opus's ($5 and $8). Opus is cheaper on every line of the price list.
Only Opus 5.5 offers a fast mode (research preview, API only) at $8 / $40 per MTok, up to 2.5x speed. Even that premium tier costs less per token than Fable 5.1's standard $10 / $50.
Benchmark Results: Opus 5.5 Leads All 7
Scores published by Anthropic in the Opus 5.5 announcement. Effort level varies by benchmark, so check the footnotes in the original table before comparing individual rows.
| Benchmark | Claude Opus 5.5 | Claude Fable 5.1 | Note |
|---|---|---|---|
| Terminal-Bench 4.0 | 66.4% | 55.8% | Agentic coding |
| FrontierCode 1.1 Main | 54.4% | 50.3% | — |
| CursorBench 4.0 | 57.8% | 51.8% | Real Cursor sessions |
| GDPval-AA v2.1 (Elo) | 1846 | 1735 | Knowledge work across 44 occupations |
| Humanity's Last Exam | 67.7% | 65.6% | With tools |
| OSWorld 2.1 | 81.8% | 80.7% | Computer use |
| Chartography | 89.0% | 88.4% | With tools |
Takeaway: the widest gaps are on agentic coding (Terminal-Bench 4.0, +10.6 pp) and knowledge work (GDPval-AA, +111 Elo). On OSWorld 2.1 and Chartography the models are within about one point.
Independent Evaluations & Third-Party Data
Third-party aggregators use their own test setups, so their numbers will not match Anthropic's table exactly.
BenchLM category averages (Sep 29, 2026):
| Category | Claude Opus 5.5 | Claude Fable 5.1 |
|---|---|---|
| Overall | 86.94 | 82.5 |
| Agentic | 87.9 | 78.9 |
| Coding | 83.6 | 80 |
| Reasoning | 82.4 | 81.4 |
| Knowledge | 88.5 | 85.4 |
BenchLM's largest gap is again agentic work (87.9 vs 78.9). View the full BenchLM comparison
Further reading and hands-on comparisons:
Fable 5.1 vs Opus 5.5: Which to Choose by Use Case
Not every task needs the premium model. Use this table to match your workload to the model that gives you the best balance of quality, speed and cost.
| Scenario | Recommended | Why |
|---|---|---|
| Agentic coding (Terminal-Bench style) | Opus 5.5 | 66.4% vs 55.8% — the widest coding gap (+10.6 pp) on Anthropic's table |
| Long-running agents | Opus 5.5 | Higher benchmark scores across the board and 60% cheaper per token for sustained runs |
| Large refactors and migrations | Opus 5.5 | CursorBench 4.0 lead (57.8% vs 51.8%) and lower cost per multi-file session |
| Code review | Opus 5.5 | Scores higher on coding benchmarks; fast mode available for time-boxed review pipelines |
| Deep research and multistep search | Fable 5.1 (eval first) | The one scenario Anthropic explicitly positions Fable for — but only if Opus at high effort still falls short on your tasks |
| Documents, spreadsheets and slides | Opus 5.5 | Adequate quality for content generation at 60% less cost; speed advantage on moderate latency |
| Computer use | Opus 5.5 | OSWorld 2.1 gap is narrow (81.8% vs 80.7%) but Opus is much cheaper for long GUI sessions |
| High-volume batch processing | Opus 5.5 | $2 / $10 batch pricing vs $5 / $25 — half the cost at higher or equal benchmark scores |
| Latency-sensitive applications | Opus 5.5 | Listed as moderate vs Fable's slower latency; fast mode adds a further speed boost |
Switching Between Opus 5.5 and Fable 5.1: What to Know
Thinking carries over in one direction only. Fable 5.1 reads thinking blocks from Opus 5.5, so a conversation that moves up from Opus to Fable keeps its reasoning. Opus 5.5 cannot read Fable thinking blocks: moving down drops the earlier reasoning for the turns that run on Opus. The request still succeeds, and dropped blocks are not billed.
Keep the conversation append-only. On both models the API checks whether the system prompt, tools or an earlier message changed after a thinking block was produced. On accounts created on or after August 31, 2026, replaying a block after such an edit returns a 400 error.
Thinking is always on, and the default effort differs. Both models reject thinking: disabled with a 400 error. Opus 5.5 defaults to medium effort and Fable 5.1 to high, so set effort explicitly when comparing the two.
No forced tool use on either model. Both return a 400 error when tool_choice is set to "any" or a specific tool name. Use auto with strict tool use or structured outputs instead.
Data retention differs by model. Fable 5.1 carries 30-day data retention and is not available under zero data retention unless Anthropic expressly authorizes it. Check the model-specific retention requirements before choosing it for regulated workloads.
The Bottom Line
Opus 5.5 is the better default for nearly every workload: it is 60% cheaper per token, faster and scores higher on all seven benchmarks Anthropic published. Anthropic's own guidance is to start with Opus and increase effort before moving to Fable. Reserve Fable 5.1 for the demanding reasoning and long-horizon agentic tasks where your own evals show Opus 5.5 — even at high or max effort — still falls short. If Fable does not measurably outperform Opus on your workload, there is no cost or speed reason to choose it.
Opus 5.5 vs Fable 5.1: FAQ
What are the differences between Claude Opus 5.5 and Fable 5.1?
Both have a 1M-token context window, 128K max output, adaptive always-on thinking and a June 2026 knowledge cutoff. The key differences are price ($4/$20 vs $10/$50 per million input/output tokens), speed (Opus is moderate, Fable is slower), default effort (medium vs high), fast mode (Opus only) and cache-read pricing ($0.20 vs $0.25 per million tokens). On Anthropic's published benchmark table, Opus 5.5 scores higher on all seven listed benchmarks.
Is Claude Opus 5.5 better than Fable 5.1?
On Anthropic's published benchmarks, Opus 5.5 scores higher than Fable 5.1 on all seven listed tests, including Terminal-Bench 4.0 (66.4% vs 55.8%) and CursorBench 4.0 (57.8% vs 51.8%), and an independent BenchLM comparison also ranks Opus 5.5 higher overall. Anthropic still positions Fable 5.1 for demanding reasoning and long-horizon agentic work, so the best choice depends on your own evals.
How much cheaper is Opus 5.5 than Fable 5.1?
60% cheaper per token: $4 / $20 (Opus) vs $10 / $50 (Fable) per million input / output tokens. A typical 100K-input, 20K-output request costs $0.80 on Opus and $2.00 on Fable. Opus is also cheaper on Batch API ($2/$10 vs $5/$25) and on every prompt-caching tier. Per-task cost can differ from per-token cost because the models use different amounts of thinking.
Is Fable 5.1 worth the extra cost?
For most workloads, no. Fable 5.1 costs 2.5x more per token and is slower, yet scores lower than Opus 5.5 on every benchmark in Anthropic's published table. Anthropic's own guidance is to start with Opus and increase effort before considering Fable. The extra cost is justified only when your own evals on Opus at high or max effort still fall short on demanding reasoning or long-horizon agentic tasks — and that gap should be measurable, not assumed.
Is Fable 5.1 available on the free plan?
No. As of September 2026, Fable 5.1 is not available on the free plan at claude.ai and is not the default on any plan. It requires a Pro, Max, Team or Enterprise subscription. Opus 5.5 is the default model for Pro subscribers and also requires a paid plan. Sonnet 5.5 is the free-plan default.
Which is better for coding, Opus 5.5 or Fable 5.1?
On Anthropic's published table Opus 5.5 leads on all three coding benchmarks: Terminal-Bench 4.0 (66.4% vs 55.8%), FrontierCode 1.1 (54.4% vs 50.3%) and CursorBench 4.0 (57.8% vs 51.8%). BenchLM's coding average also favors Opus (83.6 vs 80). Fable 5.1 is positioned by Anthropic for long-running agentic coding and multistep research, so test both on your own repository before committing.
Do Opus 5.5 and Fable 5.1 have the same context window?
Yes. Both support a 1M-token context window at standard per-token pricing and up to 128K output tokens on the Messages API. Opus 5.5 additionally supports up to 300K output tokens on the Batch API with a beta header.
Can I switch between Opus 5.5 and Fable 5.1 mid-conversation?
Yes, but reasoning only carries over in one direction. Fable 5.1 can read Opus 5.5 thinking blocks, so moving a conversation from Opus up to Fable keeps its reasoning. Opus 5.5 cannot read Fable thinking blocks, so moving down drops earlier reasoning for the turns that run on Opus. The request still succeeds and dropped blocks are not billed.
Does Fable 5.1 have a fast mode like Opus 5.5?
No. Fast mode (a research preview, Claude API only) is listed for Opus 5.5 at $8 / $40 per million tokens with up to 2.5x speed, according to Anthropic. Fable 5.1 is not listed among fast-mode models. Opus 5.5 fast mode is priced below Fable 5.1's standard $10 / $50.
When were Opus 5.5 and Fable 5.1 released?
Fable 5.1 launched on September 1, 2026 and Opus 5.5 on September 22, 2026. Both share a June 2026 knowledge cutoff. Anthropic guarantees availability on its own platforms until at least September 1, 2027 for Fable 5.1 and September 22, 2027 for Opus 5.5.