Model comparison

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Claude Fable 5.1 vs Claude Opus 5.5

Two Anthropic models, compared on listed price, real workload cost, context and capabilities.

The short answer

  • On Terminal-Bench 4.0, as each maker reports it, Claude Fable 5.1 scores 55.8% and Claude Opus 5.5 66.4%.
  • Claude Opus 5.5 lists lower input and output prices.
  • Claude Opus 5.5 reads cached input for less ($0.2/M against Claude Fable 5.1's $0.25/M), which decides the bill for agents that resend long context.

Side-by-side specs

SpecClaude Fable 5.1Claude Opus 5.5
ProviderAnthropicAnthropic
Input price$10/M$4/M
Output price$50/M$20/M
Cached input$0.25/M$0.2/M
Blended price (3:1)$20/M$8/M
Context window1M tokens1M tokens
Max output128k tokens128k tokens
InputText, ImagesText, Images
Structured outputYesYes
WeightsClosedClosed
ReleasedSeptember 1, 2026September 22, 2026
Terminal-Bench 4.0 (maker-reported)55.8%66.4%
Humanity's Last Exam (with tools) (maker-reported)65.0%67.7%

Same work, both models

WorkloadClaude Fable 5.1Claude Opus 5.5
One chat reply3,000 tokens in, 500 outClaude Fable 5.1 costs 2.5× as much as Claude Opus 5.5$0.06$0.02
Summarise a 100-page report50,000 tokens in, 1,500 out, one request$0.58$0.23
1,000 classification calls500 tokens in, 20 out, per call$6.00$2.40
Coding-agent session2M tokens in across ~60 turns, 60,000 out, no cache$23.00$9.20
Same session, cached85% of input read from the prompt cache, the rest written to itClaude Fable 5.1 costs 2.4× as much as Claude Opus 5.5$7.18$3.04

At listed rates. Token counts are typical sizes, not measurements.

Claude Fable 5.1 pricing notes

  • Cache hits bill at $0.25 per 1M tokens (0.025x input), a quarter of Fable 5's $1; cache writes at $12.50 (5-minute tier).
  • The full 1M-token window bills at the standard rate, with no long-context surcharge.
  • Adaptive thinking is always on and cannot be disabled; the effort setting (default high) steers its depth.

Claude Opus 5.5 pricing notes

  • The full 1M-token window bills at the standard rate, with no long-context surcharge.
  • Cache writes bill at $5 per 1M tokens (5-minute tier; $8 for 1-hour); cache hits at $0.20, 0.05x input.
  • Adaptive thinking is always on and cannot be disabled; the effort parameter (default medium) sets its depth.

When to pick which

Claude Fable 5.1

Anthropic's Claude model for demanding reasoning and long-horizon agentic work, released as the successor to Fable 5. Same $10 / $50 rate with cache reads at a quarter of the cost; Anthropic points most workloads to the cheaper Opus 5.5.

Pick it if

  • Opus 5.5 at higher effort still falls short on your evals
  • you run long agent sessions; cache hits bill at 2.5% of input
  • you run hours-long agentic coding or multistep research over a 1M window

Skip it if

  • Opus 5.5 covers your workload at 40% of the per-token price
  • your code forces tool use, prefills replies or sets temperature; all return errors

Claude Opus 5.5

Anthropic's Opus-tier Claude model, built for long-running agentic coding and knowledge work. It replaced Opus 5 at a lower $4 / $20 rate with the same 1M-token context, and sits between Sonnet 5.5 and Fable 5.1.

Pick it if

  • you want Opus-tier Claude at 20% below Opus 5's per-token rates
  • you resend long prompts; cache hits bill at 5% of input
  • you run long agentic coding sessions across a 1M-token window

Skip it if

  • Sonnet 5.5 at half the per-token price covers your workload
  • your code forces tool use or turns thinking off; both are unsupported

FAQ

Which is better, Claude Fable 5.1 or Claude Opus 5.5?

On Terminal-Bench 4.0, as each maker reports it, Claude Fable 5.1 scores 55.8% and Claude Opus 5.5 66.4%. These are each maker's own numbers, not a controlled head-to-head; the spec table lists every benchmark both publish.

Which is cheaper, Claude Fable 5.1 or Claude Opus 5.5?

Claude Opus 5.5 lists lower input and output prices. On a coding-agent session (2M tokens in, 60,000 out) Claude Fable 5.1 costs $23.00 and Claude Opus 5.5 $9.20.

Can I switch between Claude Fable 5.1 and Claude Opus 5.5 mid-task?

Yes. In an Octomind session, /model octohub:claude-fable-5-1 and /model octohub:claude-opus-5-5 switch models without losing the conversation, so you can draft on one and review on the other.

Try either model

Switch between models inside the same Octomind session.

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Prices from the catalog of 2026-10-31