The Claude Fable 5 API is our default for the hard problems — top-tier reasoning, ships multi-file PRs that pass review. The 5% RouterBase discount is pure margin.
The Claude Fable 5 API is Anthropic's top-tier Claude model — a 1M-token context window, adaptive thinking, support for mid-conversation system messages, prompt caching, and structured JSON outputs, built for the hardest reasoning, coding, and agentic tasks. The Claude Fable 5 API is OpenAI-compatible: point any existing SDK at RouterBase and set the model to claude-fable-5. Input at $9.50 / 1M tokens, output at $47.50 / 1M tokens, cache reads at $0.95 / 1M, cache writes at $19.00 / 1M — 5% below the official rate, all through one REST endpoint.
Input
Claude Fable 5
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Claude Fable 5 API: Chat
Use the Claude Fable 5 API to run Anthropic's top-tier Claude model — a 1M-token context, adaptive thinking, mid-conversation system messages, and prompt caching.
The Claude Fable 5 API is Anthropic's top-tier Claude model — a 1M-token context window, adaptive thinking that scales reasoning effort to the task, support for mid-conversation system messages, structured outputs, and prompt caching, built for the hardest reasoning, coding, and agentic work. Routed through RouterBase, the Claude Fable 5 API is fully OpenAI-compatible: point any existing SDK at RouterBase's base URL, set the model to claude-fable-5, and your first Claude Fable 5 API call goes through immediately.
Pricing for the Claude Fable 5 API is $9.50 / 1M input tokens, $47.50 / 1M output tokens, $0.95 / 1M cached-input reads, and $19.00 / 1M cache writes — 5% below the official published rate. One RouterBase key — no Anthropic account required.
Six reasons teams ship on the Claude Fable 5 API
From top-tier reasoning to 5%-off pricing — what makes the Claude Fable 5 API stand out.
Top-tier Claude reasoning
The Claude Fable 5 API is Anthropic’s most capable model — frontier reasoning, coding, and analysis for the hardest problems your product can throw at it.
1M-token context
Accepts up to 1 million tokens per request — long documents, multi-file codebases, or full conversation history fit in a single Claude Fable 5 API call without chunking.
Adaptive thinking
The Claude Fable 5 API scales its reasoning effort to the task — deep step-by-step thinking on hard problems, fast answers on simple ones, controllable per request.
Mid-conversation system messages
Unlike many chat APIs, the Claude Fable 5 API accepts system messages mid-thread — re-steer tone, role, or constraints at any turn without restarting the conversation.
Prompt caching
Cache stable context once and the Claude Fable 5 API serves repeated reads at $0.95 / 1M — a fraction of the input rate — cutting cost on long system prompts and RAG context.
OpenAI-compatible, one key
The Claude Fable 5 API speaks the OpenAI chat-completions format; the same RouterBase key also routes to GPT-5.5, Gemini 3 Pro, and 200+ other models.
Quickstart
Get started with the Claude Fable 5 API in 3 steps
From sign-up to your first response in under 5 minutes.
Create a RouterBase API key
Sign up and generate an API key — one key reaches the Claude Fable 5 API and every other model in the catalog.
Send your first message
Point any OpenAI-compatible SDK at routerbase.com/v1 and set the model to claude-fable-5. The Claude Fable 5 API response follows the standard chat-completions schema — streaming and tool use supported.
Inspect usage
Every Claude Fable 5 API response includes a detailed token breakdown — input, output, and cache-read tokens — so cost and cache-hit rate are visible on every call.
Pay only for what you use
RouterBase passes through partner-tier pricing. Compared against the model's official published API rate.
What teams build with the Claude Fable 5 API
Real production loads running on the Claude Fable 5 API and the RouterBase model catalog.
Adaptive thinking on the Claude Fable 5 API gives us depth on the tricky tickets and speed on the rest — without us micromanaging the model.
Prompt caching on the Claude Fable 5 API cut our long-context bill hard — cached reads at $0.95 / 1M instead of the full input rate add up fast.
Mid-conversation system messages on the Claude Fable 5 API let us re-steer the agent mid-thread. RouterBase makes it 5% cheaper and routes around outages automatically.
1M of context means I drop a whole repo in. The Claude Fable 5 API reasons across files without losing the thread.
RouterBase puts the Claude Fable 5 API and 200+ other models behind one key. Our team stopped filing requests for new provider accounts.
We A/B the Claude Fable 5 API against GPT-5.5 per request — same SDK, one model field. The hardest jobs go to Fable 5.
We routed our toughest tasks to the Claude Fable 5 API over a weekend. The only PR comment was 'wait, that's all?'.
At $9.50 / 1M input minus 5%, the Claude Fable 5 API gave us frontier-grade reasoning at a price we could ship. The ROI math was instant.
Frequently Asked Questions
Common questions about the Claude Fable 5 API.
It is RouterBase's pass-through to Anthropic's Claude Fable 5 — Anthropic's top-tier Claude model, with a 1M-token context, adaptive thinking, mid-conversation system messages, prompt caching, and structured outputs, served via an OpenAI-compatible REST interface.