Claude Fable 5.1 vs Claude Opus 5
MMMU-Pro (vision) MMMU-Pro (vision): College-level questions that require understanding images: charts, diagrams, photos
Fable 5.1—
Opus 581.6
Chartography Chartography: Expert chart reasoning over professional scientific, medical, financial and engineering charts
Fable 5.1 · max46.2
Opus 5 · max27.3
GDP.pdf · AA all-pass GDP.pdf · AA all-pass: Models reason over professional PDF documents; an attempt passes only when every atomic criterion passes
Fable 5.126.8
Opus 520
Arena Document · overall Arena Document · overall: Human pairwise preferences in the stated document vote pool
Fable 5.1 · max1,512.6
Opus 5 · high1,516.3
Arena Vision · overall Arena Vision · overall: Human pairwise preferences in the stated vision vote pool
Fable 5.1 · max1,319.5
Opus 5 · high1,319.6
| Benchmark | Fable 5.1 | Opus 5 |
|---|---|---|
| MMMU-Pro (vision) | — | 81.6 |
| Chartography | 46.2 · max | 27.3 · max |
| GDP.pdf · AA all-pass | 26.8 | 20 |
| Arena Document · overall | 1,512.6 · max | 1,516.3 · high |
| Arena Vision · overall | 1,319.5 · max | 1,319.6 · high |
Specs
| Spec | Fable 5.1 | Opus 5 |
|---|---|---|
| Input, per 1M | $10 | $5 |
| Output, per 1M | $50 | $25 |
| Cost per task? | $2.98 | $2.19 |
| Speed, tok/s | 56 | 55 |
| Context | 1M | 1M |
| Weights | Closed | Closed |
| Released | Sep 2026 | Jul 2026 |
| See images | ✓ | ✓ |
| Read PDFs | Partial | Partial |
| Hear audio | — | — |
| Watch video | — | — |
| Call tools | ✓ | ✓ |
| Force a tool call | Partial | Partial |
| Return strict JSON | ✓ | ✓ |
| Search the web | ? | ? |
| Run code | ? | ? |
| Use a computer | ? | ? |
What breaks when you switch
Fable 5.1 → Opus 5
Nothing known.
Opus 5 → Fable 5.1
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.