Claude Opus 5.5 vs Fable 5.1: Deep Technical Differences, Benchmarks and When to Use Each

Claude Opus 5.5 and Claude Fable 5.1 sit at a similar capability level, and the decisive difference between them is price and API behaviour rather than raw intelligence: Opus 5.5 costs 4 dollars per million input tokens and 20 dollars for output, while Fable 5.1 costs 10 and 50 — roughly 60% more expensive. Opus 5.5 leads Fable 5.1 on most of Anthropic's published benchmarks and generates tokens about 30% faster. The rule Anthropic itself recommends: start with Opus 5.5, and move to Fable 5.1 only when Opus leaves a long problem unresolved or repeatedly misses an important condition.

The difference, one line per item

Item	Claude Opus 5.5	Claude Fable 5.1
API model ID	claude-opus-5-5	claude-fable-5-1
Release date	22 September 2026	Earlier in the same generation
Price (input / output per million tokens)	4 / 20 dollars	10 / 50 dollars
Cache read	0.20 dollars	0.25 dollars
Context window	1M tokens	1M tokens (also the default)
Max output	128K tokens (300K via Batch with a beta header)	128K tokens
Default effort level	medium	high
Knowledge cutoff	June 2026	Earlier
Priority Tier	Not supported	Not supported

Pricing in detail, and what the token price hides

Anthropic cut Opus 5.5 pricing by 20% against Opus 5 on almost every line, and cut cache reads by 60%:

Item (per million tokens)	Opus 5.5	Opus 5	Change
Input	4 dollars	5 dollars	-20%
Output	20 dollars	25 dollars	-20%
Cache write (5 minutes)	5 dollars	6.25 dollars	-20%
Cache write (1 hour)	8 dollars	10 dollars	-20%
Cache read	0.20 dollars	0.50 dollars	-60%
Batch API (input / output)	2 / 10 dollars	2.50 / 12.50 dollars	-20%

A worked example: a request with 20,000 input tokens and 5,000 output tokens costs 0.18 dollars on Opus 5.5, 0.225 dollars on Opus 5, and 0.45 dollars on Fable 5.1 — roughly two and a half times more. In Saudi riyals: one million input tokens is 15 SAR on Opus 5.5 against 37.5 SAR on Fable 5.1.

But the token price is not the cost. Anthropic says a typical workload runs about 40% cheaper on Opus 5.5 than on Opus 5, because the model finishes tasks using fewer tokens. In the opposite direction, independent analysis found Opus 5.5 at maximum effort consumes around 119,000 output tokens per task — a high figure offset by the lower price. The conclusion: measure the cost of completing the task, not the price per million tokens.

Performance: what the published numbers say

Benchmark	Opus 5.5	Fable 5.1	Opus 5	GPT-6 Astra
Terminal-Bench 4.0 — terminal work	66.4%	55.8%	52.3%	57.9%
FrontierCode v1.1 — integrating code into an existing project	54.4%	50.3%	48.0%	53.3%
CursorBench 4.0 — editing inside the code editor	57.8%	51.8%	—	—
OSWorld 2.0 — computer and interface use	81.8%	80.7%	74.0%	73.5%
AutomationBench — business workflow automation	40.0%	31.4%	26.9%	41.4%
Terminal-Bench-Science — scientific tasks	58.7%	52.6%	29.0%	64.6%

And here is the caveat Anthropic published alongside its own table, which matters more than the table: at these levels of capability we have found that benchmark margins have become a less reliable guide to real-world differences. In our own use, the gap between Opus 5.5 and Claude Fable 5.1 is narrower than these scores suggest. The company selling both models is telling you not to read the table literally.

Note too that the lead is not absolute: GPT-6 Astra is ahead of Opus 5.5 on AutomationBench (41.4% against 40.0%) and clearly ahead on Terminal-Bench-Science (64.6% against 58.7%), while Opus 5.5 leads on terminal work and computer use.

And what about the difference between Opus 5.5 and Opus 5?

This is the in-family upgrade question, and it is different from the Fable 5.1 comparison: here you are not choosing between two families, you are moving within one. The gap is wider than the version number suggests:

Item	Opus 5	Opus 5.5	Change
Price (input / output)	5 / 25 dollars	4 / 20 dollars	-20%
Cache read	0.50 dollars	0.20 dollars	-60%
Default effort level	high	medium	A cheaper default setting
Output generation speed	Baseline	More than 30% faster	+30%
Terminal-Bench 4.0	52.3%	66.4%	+14.1 points
AutomationBench	26.9%	40.0%	+13.1 points
Terminal-Bench-Science	29.0%	58.7%	Roughly double
OSWorld 2.0	74.0%	81.8%	+7.8 points
GDPval-AA v2.1 (knowledge work, Elo)	1708	1846	+138

The sentence that matters most if you are costing this out: Anthropic states that Opus 5.5 at medium effort matches or exceeds Opus 5 at high effort on coding and knowledge-work evaluations. You reach the expensive setting's result with a cheaper and faster one — and that, on top of the 20% price cut, is where the claim of 40% savings on a typical workload comes from.

The clearest example from Anthropic's announcement: auditing and repairing a 200,000-line codebase took Opus 5.5 under three hours, while the same task took Opus 5 more than twenty.

On the other side, two changes deserve attention before you move production: disabling thinking was accepted on Opus 5 at effort levels up to high and is no longer possible at all on 5.5, and the older computer-use tool is now deprecated. Opus 5 is still served, and Anthropic's guidance for existing systems is to test 5.5 against saved examples of your own before switching, rather than switching and finding out.

The deep technical differences — what actually changes your code

Most comparisons stop at the scores. What stops your system in production is API behaviour, and here the differences are real and expensive to miss:

1. Thinking is no longer something you switch off

On Fable 5.1 thinking is always on: sending thinking as disabled returns a 400 error, and so does the legacy budget_tokens parameter. On Opus 5.5 thinking can no longer be disabled either. The correct lever is depth control through output_config.effort across its five levels from low to max. In practice, code written for 2025-era models that disables thinking or sets a thinking token budget breaks immediately on upgrade.

2. The raw chain of thought is never returned

What you receive is thinking blocks that are either empty (the default, omitted) or summarised if you set display to summarised. Anyone streaming reasoning to users while leaving the default in place will show a long silence followed by the whole answer at once. Fable 5.1 alone has a third, beta mode that returns short progress notes between tool calls.

3. Forcing a tool call is now rejected on Fable 5.1

The values any and tool in tool_choice return a 400 on Fable 5.1, and forced tool use is also among the breaking changes in Opus 5.5. The alternative: leave tool_choice on auto with an explicit instruction naming the tool, enable strict on the tool to guarantee schema-valid arguments, or use structured outputs if forcing the call was only ever your way of getting JSON back.

4. Thinking blocks are bound to conversation state

This is the point that surprises teams most: thinking blocks are bound to the model that produced them and to the conversation sequence. Editing an earlier turn in the history invalidates later thinking blocks, and for accounts created after 31 August 2026 the server returns a 400 instead of silently ignoring them. The engineering implication: your orchestrator must append to history, never rewrite it — any logic that compacts the conversation, deletes old messages, or edits a previous turn needs review before you upgrade.

5. A new stop reason called refusal

The model may return HTTP 200 with a stop reason of refusal and a category for it, rather than raising an error. Code that reads the response content without checking the stop reason first will get empty text that surfaces to the user as a fault. The supported remedy is the server-side fallbacks parameter, which re-runs the same request on a fallback model inside the same call.

6. Data retention — a compliance point, not a technical one

Fable 5.1 requires a 30-day data retention policy and is not available under zero data retention unless expressly authorised by Anthropic; otherwise the request fails with a 400. That is a real constraint for any Saudi organisation processing personal data under a no-retention requirement. Opus 5.5, by contrast, shipped on day one across the Anthropic API, Amazon Bedrock, Claude Platform on AWS, Google Cloud and Microsoft Foundry.

7. Small details that break an upgrade

• The older computer-use tool computer_20251124 is deprecated on Opus 5.5; migrate to computer_toolset_20260801.
• Assistant message prefill is rejected on both models.
• Sampling parameters such as temperature and top_p have been removed on this class of models.
• The default effort level differs: medium on Opus 5.5 and high on Fable 5.1 — so comparing the two at their defaults is not a fair comparison to begin with.

The real-world evidence Anthropic published

• A 680,000-line code migration completed by one tester in under a day.
• Auditing and repairing a 200,000-line codebase in under three hours, where the same task took Opus 5 more than twenty hours.
• Translating HAProxy from C to Rust: 9.5 hours on Opus 5.5 against 12 hours on Fable 5.1, at 51% lower cost.

And one change no benchmark measures: Anthropic addressed what users call the Claudish writing style — padding, preambles, surplus jargon. The model now leads with what matters and follows user instructions more closely. Anyone using it for customer-facing content will feel that more than any number in the tables.

When to use each

Situation	Model	Why
The default for most work: writing, analysis, coding	Opus 5.5	Higher on most benchmarks at 60% lower price
Reviewing documents, comparing versions, reading screenshots	Opus 5.5	Anthropic's own recommended starting point
A coding agent working for hours on a large codebase	Opus 5.5	Clear lead on terminal and editor work, lower cost per task
A long problem Opus leaves unresolved even at higher effort	Fable 5.1	This is the only case where the price gap earns itself
A task where Opus repeatedly misses an important condition	Fable 5.1	Deeper reasoning over interlocking constraints
Stable existing systems on Opus 5	Opus 5, then test Opus 5.5	Benchmark against saved examples before moving production
An environment requiring zero data retention	Opus 5.5	Fable 5.1 needs express authorisation to waive the 30-day requirement

Pre-upgrade checklist

• Search your code for any setting that disables thinking or sets a thinking token budget, and replace it with an effort level.
• Search for tool_choice set to any or tool and convert it to auto with strict.
• Confirm your conversation-management logic appends to history and never edits earlier turns.
• Check the stop reason before reading content, and enable a fallback model on refusal.
• Re-tune effort after upgrading: the default changed, and what was tuned for an older model no longer fits.
• Measure on 20 to 50 real tasks from your own operation, not a single example: first-attempt success rate, elapsed time, tokens, and how often a human had to intervene.

What this means if you are building in Saudi Arabia

Three practical points. First, the price gap between the two models is large enough to change the viability of a whole project: a system consuming 200 million input and 30 million output tokens a month costs 1,400 dollars on Opus 5.5 against 3,500 dollars on Fable 5.1 — about 5,250 SAR against 13,125 SAR. Second, the data retention requirement on Fable 5.1 deserves legal review before any contractual commitment if the data is personal data under the Personal Data Protection Law. Third, the breaking changes in this generation prove the rule we keep repeating: the model is a configuration value, not a decision carved into the code.

That insulation layer between your application and the model provider is what we build for clients at Origami, so switching models is a five-minute operational decision rather than a rebuild project.

Conclusion

Opus 5.5 is the rational default today: ahead on most published benchmarks, about 30% faster at generating tokens, and roughly 60% cheaper than Fable 5.1. Fable 5.1 remains a reserve card for specific problems your own testing proves exceed Opus. And the decision should not come from a benchmark table — Anthropic itself says the gap is narrower than it looks — but from measurement on your own tasks.

Sources: Anthropic's Claude Opus 5.5 announcement dated 22 September 2026 and coverage by VentureBeat and The Decoder for figures and quotes, with API details from Anthropic's developer documentation at https://docs.anthropic.com. Prices and figures are as published on 23 September 2026 and are subject to change.