Claude Fable 5 API: Pricing, Context and Setup

Claude Fable 5 is Anthropic's highest-capability generally available model. Its API model ID is claude-fable-5, its context window is 1 million tokens, and standard pricing is $10 per 1M input tokens and $50 per 1M output tokens.

Claude Fable 5 is live on RouterPlex through an OpenAI-compatible chat-completions endpoint, so the same SDK configuration can call Fable, GPT-5.6, Kimi K3 and other providers.

Claude Fable 5 API pricing #

Fable is expensive relative to general-purpose models. Its economic case depends on the value of the task: a difficult engineering decision, deep research synthesis or long-running agent can justify the rate when a weaker result would cost more in review time or failure risk.

See the live Claude Fable 5 API pricing page.

Claude Fable 5 specifications #

Anthropic says Fable uses the newer tokenizer introduced with later Claude models. Token counts may differ from older Claude versions for the same text, so estimate costs from actual API usage rather than assuming an old prompt has the same token count.

Call Claude Fable 5 with RouterPlex #

Use the regular chat-completions shape:

curl https://api.routerplex.com/v1/chat/completions   -H "Authorization: Bearer $ROUTERPLEX_KEY"   -H "Content-Type: application/json"   -d '{
    "model": "claude-fable-5",
    "messages": [
      {"role": "user", "content": "Review this architecture and identify the most expensive wrong assumption."}
    ]
  }'

Python uses the same RouterPlex base URL as every other chat model:

import os
from openai import OpenAI
 
client = OpenAI(
    api_key=os.environ["ROUTERPLEX_KEY"],
    base_url="https://api.routerplex.com/v1",
)
 
response = client.chat.completions.create(
    model="claude-fable-5",
    messages=[{"role": "user", "content": "Audit this rollout plan for irreversible steps."}],
)
 
print(response.choices[0].message.content)

Claude Fable 5 vs Claude Sonnet 5 #

Sonnet 5's introductory pricing is scheduled through August 31, 2026. If Fable does not produce a measurable improvement on your workload, Sonnet is the more economical default.

Use Fable selectively: escalation after another model fails, final review of high-risk output, difficult repository-wide reasoning, or the small number of requests whose quality has outsized value.

Control Fable spend before the first request #

At $50 per 1M output tokens, an unconstrained agent can consume a balance quickly. Give the Fable key its own hard budget and use explicit completion limits where the client supports them. Separate keys also make it easy to compare Fable revenue or task value against its cost.

Prompt caching matters for repeated long prefixes. A cache read costs one tenth of normal input, while cache writes cost more than normal input. Reuse stable system instructions and tool definitions when the workflow naturally supports it; do not add complexity only to chase a cache hit.

Test Fable against a real task #

Choose a task where a better result has clear value. Run the same input through Fable, Sonnet 5 and one non-Claude model, then compare correctness, review time, latency and final billed cost.

Create a RouterPlex key with a $5 hard budget, or compare Fable with GPT-5.6 and Kimi K3.