Claude Fable 5 vs Claude Sonnet 5.5
| Benchmark | Fable 5 | Sonnet 5.5 |
|---|---|---|
| Humanity's Last Exam | 55.5 | 45.8 |
| Terminal-Bench 4.0 | 42.4 | 43.9 |
| AutomationBench-AA | 54.1 | 59.4 |
Specs
| Spec | Fable 5 | Sonnet 5.5 |
|---|---|---|
| Input, per 1M | $10 | $2 |
| Output, per 1M | $50 | $10 |
| Cost per task? | $8.75 | $0.88 |
| Speed, tok/s | 67 | 106 |
| Context | 1M | 1M |
| Weights | Closed | Closed |
| Released | Jun 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
Fable 5 → Sonnet 5.5
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.
Sonnet 5.5 → Fable 5
Nothing known.