1. What is Claude Fable 5.1?

Released by Anthropic on September 1, 2026, Claude Fable 5.1 represents the cutting edge of Constitutional AI. Designed specifically to compete with OpenAI's GPT-6 Astra, Fable 5.1 brings:

• 1,000,000 Token Context Window: Ingest entire software libraries, full books, or year-long financial reports in a single prompt.
• Extended Thinking: Transparent reasoning blocks where Fable validates internal logic, analyzes edge cases, and writes formal proofs before emitting final text.
• 128,000 Token Maximum Output: Capable of writing complete multi-file modules in one single generation turn.
• Exceptional Code Synthesis: Highly ranked on SWE-bench for architectural refactoring and bug patching.

2. The Free Promotional Tier on Experiential Labs

Just like GPT-6 Astra, Experiential Labs is currently sponsoring a daily free promotional quota for Claude Fable 5.1. The exact specifications captured from the gateway API are:

3. The Prompt Caching Superpower: Stretch Your Free Tier 10x

Here is a critical secret that most developers miss: Prompt Caching tokens do not count against your 375,000 daily free token quota!

When using Claude Fable 5.1 through Experiential Labs, cached prompt prefixes benefit from a massive 97.5% price reduction on paid tiers and are completely exempt from free tier token counters.

If you are working on a large codebase (e.g. 100,000 tokens of code), by placing your repository content at the beginning of the prompt and keeping it unchanged between turns, only the first request consumes input tokens. Subsequent turns read directly from the high-speed cache, allowing you to have extended, multi-hour coding sessions completely for free.

4. How to Get Started: Step-by-Step

1. Register on Experiential Labs: Go to platform.experientiallabs.ai.
2. Complete the $1 Card Check: Just as with GPT-6 Astra, an identity check (saved card + settled $1 charge) is required to unlock promotional frontier models. This is a one-time anti-abuse step.
3. Generate your API Key: Visit Settings → API Keys and copy your xpl_... key.
4. Open Web Playground or Configure Your Code: You can immediately select claude-fable-5.1 in the browser playground or use the code samples below.

A. Python Integration (Using OpenAI SDK)

import os
from openai import OpenAI

client = OpenAI(
    base_url="https://api.experientiallabs.ai/v1",
    api_key=os.environ.get("EXPLABS_API_KEY")
)

# Crucial: Model slug must use DOT syntax, not dashed
response = client.chat.completions.create(
    model="claude-fable-5.1",
    messages=[
        {"role": "user", "content": "Analyze potential concurrency race conditions in Raft consensus leader election."}
    ],
    # Note: Temperature is pinned to 1.0 on Fable 5.1
    temperature=1.0
)

print(response.choices[0].message.content)

B. Setting up with Claude Code CLI

Experiential Labs provides a native Anthropic Messages bridge. You can run Anthropic's official Claude Code CLI directly on the free tier:

# In your terminal:
export ANTHROPIC_BASE_URL="https://api.experientiallabs.ai"
export ANTHROPIC_API_KEY="xpl_YOUR_API_KEY_HERE"
export ANTHROPIC_MODEL="claude-fable-5.1"
export CLAUDE_CODE_MAX_CONTEXT_TOKENS=1000000

# Launch Claude Code
claude

⚠️ Warning: Never run /login inside Claude Code when using this gateway setup, as that triggers OAuth that will override your free gateway key.

5. Critical Gotchas to Avoid

• Slug Naming Mismatch: Anthropic internal wire formats use dashes (claude-fable-5-1). If you send this to the gateway, you will receive an HTTP 403 model_not_granted error. You MUST use the dot format: claude-fable-5.1.
• Temperature Setting: Unlike GPT-6 Astra which rejects temperature, Claude Fable 5.1 expects temperature: 1.0 (or none). Do not set temperature to 0.0 or 0.7.