Claude Fable 5.1 vs Claude Sonnet 5.5
| Benchmark | Fable 5.1 | Sonnet 5.5 |
|---|---|---|
| Terminal-Bench 4.0 | 44.9 | 43.9 |
| FrontierCode 1.1 | 49 · max | 44.3 |
| WebDev Arena | 1,743 · max | 1,710 |
| SWE-bench Pro (v2 full) | 99.1 · high | 95.8 · xhigh |
| SWE-bench Pro (v2 hard) | 92.2 · high | 70.6 · xhigh |
| SciCode | 56.4 | 53.7 |
| Terminal-Bench Science 0.1 | 43.3 · max | 34.3 |
| DeepSWE 1.1 | — | 70.5 · max |
| MirrorCode | 73.3 · high | — |
Specs
| Spec | Fable 5.1 | Sonnet 5.5 |
|---|---|---|
| Input, per 1M | $10 | $2 |
| Output, per 1M | $50 | $10 |
| Cost per task? | $2.98 | $0.88 |
| Speed, tok/s | 56 | 106 |
| Context | 1M | 1M |
| Weights | Closed | Closed |
| Released | Sep 2026 | Sep 2026 |
| See images | ✓ | ✓ |
| Read PDFs | Partial | ✓ |
| Hear audio | — | — |
| Watch video | — | — |
| Call tools | ✓ | ✓ |
| Force a tool call | Partial | — |
| Return strict JSON | ✓ | ✓ |
| Search the web | ? | ✓ |
| Run code | ? | ✓ |
| Use a computer | ? | ✓ |
What breaks when you switch
No forced tool use›
Forced tool_choice any or tool returns HTTP 400; remove the forced choice.
Replace disabled thinking›
Disabled thinking returns HTTP 400; use between_tools for the lowest thinking setting.
No manual thinking budgets›
Manual thinking budgets and extra between_tools fields return HTTP 400; remove manual budgets and send between_tools without display, budget_tokens or block_binding.
Remove sampling parameters›
Non-default temperature, top_p or top_k returns HTTP 400; remove these parameters.
No assistant prefill›
A prefilled final assistant turn returns HTTP 400; remove that prefill.
Old computer tool rejected · Claude API / Google Cloud›
The computer_20251124 tool returns HTTP 400 on the Claude API and Google Cloud; use computer_toolset_20260801 there.
Replace forced tool_choice›
Forced tool_choice returns HTTP 400; use auto with an explicit instruction and strict tools, or JSON outputs.
Adaptive thinking is always on›
Disabled thinking or manual budget_tokens returns HTTP 400; omit thinking or use adaptive thinking.
No non-default sampling values›
Non-default temperature, top_p or top_k returns HTTP 400; omit non-default sampling values.
Requires 30-day data retention · Claude API›
A Claude API organization or workspace without 30-day retention receives HTTP 400; configure workspace retention or contact the Anthropic account team.
Thinking blocks bind to conversations›
Where conversation binding is enforced, replaying an invalidated block returns HTTP 400; use the documented drop_block control to continue without it.
No assistant prefill›
Assistant prefill returns HTTP 400; use system prompt instructions instead.