Fable 5.1 was Anthropic’s top model until Opus 5.5 arrived on 22 September 2026 with a claim that is unusual for a cheaper model: roughly Fable-level results on most work. The launch table backs it up on every benchmark the two share, so the real question is narrower — which tasks, if any, still justify paying about two and a half times more.
The widest gaps are in agentic coding and automation (Terminal-Bench 4.0 +10.6 points, AutomationBench +8.6). On chart reading and hard exams the two are within about two points, which is where Fable 5.1’s deeper default reasoning keeps it competitive.
| Benchmark | Opus 5.5 | Fable 5.1 | Opus 5 |
|---|---|---|---|
| Terminal-Bench 4.0 | 66.4% | 55.8% | 52.3% |
| FrontierCode 1.1 | 54.4% | 50.3% | 48.0% |
| CursorBench 4.0 | 57.8% | 51.8% | 46.6% |
| GDPval-AA v2.1 | 1846 | 1735 | 1708 |
| AutomationBench | 40.0% | 31.4% | 26.9% |
| Chartography | 89.0% | 88.4% | 83.4% |
| Humanity’s Last Exam | 67.7% | 65.6% | 63.6% |
| Terminal-Bench-Science | 58.7% | 52.6% | 29.0% |
Price: Fable 5.1 lists at $10 / $50 per million tokens, Opus 5.5 at $4 / $20. On apimodels.app they are $5 / $25 and $2.40 / $12, so a job that costs $1 on Opus 5.5 costs about $2.08 on Fable 5.1 before counting that Fable also tends to write more reasoning.
Default effort: Opus 5.5 defaults to medium effort, Fable 5.1 to high. Part of Fable’s edge on the hardest problems is simply that it thinks longer by default; raising Opus 5.5 to high or xhigh closes much of that gap for less money. Opus 5.5 cannot turn thinking off.
Thinking blocks across models: Fable 5.1 can read Opus 5.5 thinking blocks, but not the other way round. If you escalate a conversation from Opus 5.5 to Fable 5.1 it works; going back down means dropping Fable’s thinking blocks.
cURL
curl https://api.apimodels.app/v1/messages \
-H "x-api-key: $APIMODELS_API_KEY" \
-H "anthropic-version: 2023-06-01" \
-H "Content-Type: application/json" \
-d '{
"model": "claude-opus-5-5",
"max_tokens": 16000,
"messages": [{"role": "user", "content": "Review this function for race conditions: ..."}]
}'Python
from openai import OpenAI
client = OpenAI(base_url="https://api.apimodels.app/v1", api_key="YOUR_APIMODELS_KEY")
# Same prompt, two models, one key: compare them on your own task before you commit.
for model in ["claude-opus-5-5", "claude-fable-5-1"]:
r = client.chat.completions.create(
model=model,
messages=[{"role": "user", "content": "Refactor this module and explain the risky parts: ..."}],
)
print(model, r.usage.prompt_tokens, r.usage.completion_tokens)
print(r.choices[0].message.content[:400])On every benchmark in Anthropic’s Opus 5.5 launch table, yes, by margins from 0.6 points (Chartography) to 10.6 points (Terminal-Bench 4.0). Anthropic’s own wording is that Opus 5.5 performs at the level of Fable 5.1 on most tasks.
When you have measured it winning on your own hardest reasoning tasks and the extra cost is small next to the cost of a wrong answer. Try Opus 5.5 at high or xhigh effort first; much of Fable’s edge comes from its higher default effort.
Opus 5.5 is $2.40 / $12 per million tokens (Anthropic list $4 / $20); Fable 5.1 is $5 / $25 (list $10 / $50). Both are billed per token on the same key, and failed calls are free.
Yes. Both support a 1M-token context window and up to 128,000 output tokens per response, with a June 2026 knowledge cutoff.
