Claude Fable 5.1

Claude Fable 5.1 is Anthropic's flagship Mythos-level model, above the Opus, Sonnet, and Haiku tiers, built for long-running agentic and whole-codebase work.

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input · usd / 1m tok

$10.00

output · usd / 1m tok

$50.00

context window

1mtokens

modalities · in → out

pricing

base api ratesusd per 1m tokens

input: $10.00
output: $50.00
cache read: $0.25
cache write · default ttl: $12.50
cache write · 1h ttl: $20.00

providers

Anthropic hosts Claude Fable 5.1. Impossibl routes your request intelligently and handles failover automatically when a provider is down.

Anthropic

specifications

takes text and image in, gives text outanthropic/claude-fable-5-1

creatorAnthropic
model typeText generation
context window1,000,000 tokens
maximum outputnot published
input
takes text and image in, gives text out
outputtext
tool callingnot verified
releasedsep 1, 2026
endpoint/v1/chat/completions

unknown means the public catalog does not establish that fact. a model's documented capabilities can differ from what a particular api supports. see the model documentation for integration details.

try it · your prompt, this model, live

# responses API
import os
from openai import OpenAI

client = OpenAI(
    api_key=os.environ["IMPOSSIBL_API_KEY"],
    base_url="https://api.impossibl.com/v1",
)

resp = client.responses.create(
    model="anthropic/claude-fable-5-1",
    input="▌",
)
print(resp.output_text)

edit the prompt, then run

# responses API
import os
from openai import OpenAI

client = OpenAI(
    api_key=os.environ["IMPOSSIBL_API_KEY"],
    base_url="https://api.impossibl.com/v1",
)

resp = client.responses.create(
    model="anthropic/claude-fable-5-1",
    input="▌",
)
print(resp.output_text)

edit the prompt, then run

benchmarks

intelligence index

53.4#1 of 90 models on the gateway

coding index81.6#1 of 60
agentic index58.0#1 of 49
omniscience43.5#1 of 86
gpqa93.7%#7 of 88
humanity’s last exam59.1%#1 of 89
terminal-bench v2.191.4%#1 of 57

GPQA93.7%
Harvey LAB93.0%
Terminal-Bench v2.191.4%
AA-LCR85.3%
Multilingual LCR71.1%
SciCode63.1%
AutomationBench Partial59.4%
Humanity’s Last Exam59.1%
AnalystAgent57.5%
Terminal-Bench v4.052.0%
Tau Banking47.2%
CritPt29.7%
GDP.pdf All-pass26.2%

Intelligence Index53.37
Omniscience Index43.45
GDPval Elo1763.64
coding index (API)81.6
agentic index (API)58
intelligence index (API)53.4

view source ↗all source data and breakdowns (json) ↗

each configuration is listed separately; these evaluations describe the tested model configuration, and performance and costs can differ on impossibl. unmeasured scores are omitted and api values keep their source units. last checked 2026-09-11.

compare Claude Fable 5.1 with other models

frontier labs and open weights, best-ranked firstusd / 1m tok · in / out

openai/gpt-6-astra$10.00 / $50.00compare →
meta/muse-spark-1.3$1.25 / $4.25compare →
zai/glm-5.3$1.40 / $4.40compare →
xai/grok-4.6$2.00 / $6.00compare →

impossibl models --all →

frequently asked questions

What is Claude Fable 5.1?

Claude Fable 5.1 is Anthropic's flagship Mythos-level model, above the Opus, Sonnet, and Haiku tiers, built for long-running agentic and whole-codebase work. Against Claude Fable 5 it runs faster, completes more tasks, and uses fewer tokens per task.

How much does Claude Fable 5.1 cost?

Claude Fable 5.1 costs $10.00 per million input tokens and $50.00 per million output tokens through the Impossibl API. Cache reads cost $0.25 per million tokens; cache writes cost $12.50 per million tokens; cache writes with one-hour retention cost $20.00 per million tokens. All rates are in USD.

view the pricing table ↑

What is the context length of Claude Fable 5.1?

Claude Fable 5.1 has a 1,000,000-token context window through the Impossibl API.

What inputs and outputs does Claude Fable 5.1 support?

Claude Fable 5.1 accepts text and image as input. Claude Fable 5.1 returns text.

How do I use Claude Fable 5.1 through an API?

Claude Fable 5.1 is available through the Impossibl API. Use anthropic/claude-fable-5-1 as the model identifier with /v1/chat/completions. The quickstart explains how to create an API key and send your first request.

follow the api quickstart →

When was Claude Fable 5.1 released?

Anthropic released Claude Fable 5.1 on September 1, 2026.

Creator release announcement ↗

this page describes Claude Fable 5.1 as listed by the impossibl public api. prices and availability come from that catalog, refreshed here every five minutes. this is an impossibl offering, not a comparison of independent hosting-provider quotes. model descriptions and release dates, where available, use the creator sources linked under the answers above.

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