Claude Fable 5.1 — deep reasoning for long, hard tasks
Claude Fable 5.1 takes on the multistep research and agent work other models give up on. $8.00 / 1M input, $40.00 / 1M output per 1M tokens.
- Adaptive thinking
- 1M-token context
- Images and PDFs
- Prompt caching
- Streaming
Try Claude Fable 5.1
Chat with Claude Fable 5.1, attach images or PDFs, and see the tokens and cost of every answer.
claude-fable-5-1$8.00 in · $40.00 out per 1M tokens
Try one of these prompts
Messages are billed from your balance at the rates below, like an API call. Failed requests are free.
Real Claude Fable 5.1 output
One prompt, sent to Claude Fable 5.1 at low effort. Unedited.
Explain the difference between optimistic and pessimistic locking in a database, in two short paragraphs, and say when to use each.
**Pessimistic locking** assumes conflicts are likely, so it prevents them up front: when a transaction reads a row it intends to modify, it acquires a lock (e.g., `SELECT ... FOR UPDATE`) that blocks other transactions from writing (and sometimes reading) that row until the first transaction commits or rolls back. This guarantees no one else can change the data underneath you, but it holds resources for the duration of the transaction, which can reduce concurrency and risks deadlocks if locks are taken in inconsistent orders. **Optimistic locking** assumes conflicts are rare, so it doesn't lock at all. Instead, each row carries a version number (or timestamp), which the transaction reads along with the data. When writing back, it issues an update conditioned on that version still matching (`UPDATE ... WHERE id = ? AND version = ?`); if zero rows are affected, someone else changed the data first, and the transaction must retry or report the conflict. This maximizes throughput and avoids held locks, at the cost of occasional wasted work and the need for retry logic. **When to use each:** Use pessimistic locking when contention is high, conflicts are expensive to redo, or correctness demands that a value not change mid-operation (e.g., decrementing inventory or account balances in a busy system, long-running batch jobs touching hot rows). Use optimistic locking when contention is low and reads dominate writes, transactions are short, or the workload spans user think-time or disconnected clients (e.g., a web form where a user edits a record and submits it minutes later), where holding a database lock would be impractical.
Generated 2026-09-27. Answers vary from run to run.
Claude Fable 5.1 pricing
Claude Fable 5.1 is billed per token from your balance. Rates below are live, the same ones your account is charged.
Example sizes are for illustration. Your bill follows the tokens each request reports.
| Item | Counted as | Rate | Billing |
|---|---|---|---|
| Input | Prompt tokens not read from cache | $8.00 / 1M | Per 1M tokens |
| Output | Answer and thinking tokens | $40.00 / 1M | Per 1M tokens |
| Cache read | Prompt tokens served from the prompt cache | $0.20 / 1M | Per 1M tokens |
| Cache write, 5 minutes | Tokens written to a 5-minute cache | $10.00 / 1M | Per 1M tokens |
| Cache write, 1 hour | Tokens written to a 1-hour cache | $16.00 / 1M | Per 1M tokens |
| Failed request | Any request that returns an error | Free | Not charged |
What is Claude Fable 5.1?
Claude Fable 5.1 is Anthropic's current Fable model for demanding reasoning and long-horizon agentic work. It improves on Claude Fable 5 in long-running coding, multistep research and document, spreadsheet and slide work, at the same input and output prices.
Claude Fable 5.1 reads up to 1M tokens and writes up to 128K per request. It thinks adaptively at the effort you set, with high as the default, and cached prompt reads cost a quarter of what they did on Claude Fable 5.
Model ID: claude-fable-5-1 · Input: text, images, PDFs · Output: text · Formats: Anthropic Messages, OpenAI Chat Completions, OpenAI Responses.
Two ways to use Claude Fable 5.1
Call it from your code, or let your coding agent do it for you.
Call it from your backend
Point the official Anthropic SDK at SeedRouter and keep your code. Claude Fable 5.1 also answers the OpenAI formats.
- 1Create an API key
- 2Set the base URL to https://api.seedrouter.ai
- 3Send claude-fable-5-1 as the model
Hand it to your coding agent
Copy a ready prompt. Your agent shows the request and cost before anything is spent.
- 1Copy the prompt
- 2Paste it into your agent
- 3Approve the request
What Claude Fable 5.1 is good at
Claude Fable 5.1 is for the tasks where getting it right matters more than getting it fast.
Hard reasoning
Claude Fable 5.1 works through problems that need many careful steps.
Long-running agents
Keep a Claude Fable 5.1 agent on one task for hours without it drifting.
Multistep research
Gather, compare and synthesize across long sources in one context.
Documents, sheets, slides
Claude Fable 5.1 is stronger at document, spreadsheet and slide work than Claude Fable 5.
Cheap cached context
Cached reads cost 2.5% of the input rate, so long reused prompts stay affordable.
Effort you control
Drop to low effort for simple steps and save the thinking for hard ones.
What teams build with Claude Fable 5.1
Large code migrations
Let Claude Fable 5.1 plan and carry out changes across many files.
Research assistants
Produce sourced reports from long document sets with Claude Fable 5.1.
Spreadsheet and model review
Let Claude Fable 5.1 check formulas and assumptions in complex workbooks.
Start using Claude Fable 5.1 in four steps
- 01
Create a key
Sign in and create an API key. Add credit whenever you need it; there is no subscription.
- 02
Change the base URL
Point the Anthropic or OpenAI SDK at https://api.seedrouter.ai. Your code stays the same.
- 03
Pick Claude Fable 5.1
Send claude-fable-5-1 as the model. Set effort to trade depth for speed and cost.
- 04
Check the bill
Every request shows its tokens and charge in your usage records.
Claude Fable 5.1 on SeedRouter at a glance
What you can use today.
| Feature | Claude Fable 5.1 |
|---|---|
| Anthropic Messages API | Yes, official format |
| OpenAI Chat Completions | Yes |
| OpenAI Responses | Yes |
| Image and PDF input | Yes, by URL or base64 |
| Prompt caching | 5-minute and 1-hour |
| Forced tool use | No, per the official model notes |
| Failed requests | Not charged |
Batch processing and fast mode are not offered.
Claude Fable 5.1 limits
1M-token context
On Claude Fable 5.1, prompt, files and history share one 1M-token window.
128K tokens out
One Claude Fable 5.1 request can return up to 128K tokens, thinking included.
Thinking stays on
Adaptive thinking cannot be turned off. Lower the effort to make it think less.
Sampling settings
Claude Fable 5.1 follows the official notes on temperature, top_p and top_k; test before relying on them.
Why call Claude Fable 5.1 through SeedRouter
One key, three formats
Call Claude Fable 5.1 with Anthropic Messages, OpenAI Chat Completions or Responses, all with the same key.
Pay per token
Top up once and spend it on any model. No plan, no monthly fee.
Failures are free
A request that errors is not charged.
Live prices
The rates on this page are the rates your account pays.
Prompt caching
Mark long, reused context and later Claude Fable 5.1 requests read it at the cache rate.
Images and PDFs
Send charts, screenshots and documents with your prompt.
Streaming
Stream Claude Fable 5.1 tokens as they arrive with stream: true.
Related models
Other models you can call with the same key.
Call Claude Fable 5 with the Anthropic or OpenAI SDK. Deep reasoning, 1M-token context, forced tool use, live per-token pricing and a playground to test it.
View pricingCall Claude Opus 5.5 with the Anthropic or OpenAI SDK. 1M-token context, adaptive thinking, live per-token pricing and a playground you can try right now.
View pricingClaude Fable 5.1 guides
Walkthroughs and comparisons for this model.
Claude Fable 5.1, Fable 5 and Opus 5.5 API pricing per million tokens: input, output and prompt-cache rates, how a request is billed, and API vs Claude plans.
Call the Claude Fable 5.1 API: get an API key, use the claude-fable-5-1 model ID, send a first request in Python or Node.js, and switch to Opus 5.5.
Run Claude Fable 5.1, Fable 5 or Opus 5.5 in Claude Code with a SeedRouter API key: the environment variables, model selection, and what each session costs.
Claude Fable 5.1 vs Fable 5 vs Opus 5.5 compared: specs, prices, official benchmark scores, what changed from Fable 5 and Opus 5, and which model to pick.
Is Claude Fable 5 still available? Why it was suspended on June 12, 2026, when it came back, its status today, and how to call it through the API right now.
Is Claude Fable 5 free? What the Claude Free, Pro and Max plans include, whether a free Fable 5 API key exists, and how to test it with free sign-up credit.
Claude Fable 5.1 FAQ
What is Claude Fable 5.1?+
Claude Fable 5.1 is Anthropic's current Fable model for demanding reasoning and long-horizon agent work, released September 1, 2026. It has a 1M-token context window, writes up to 128K tokens per request and thinks adaptively.
Is Claude Fable 5.1 worth it?+
Claude Fable 5.1 is worth it for tasks that need the deepest reasoning: long agent runs, multistep research, complex documents. For most everyday work, Claude Opus 5.5 costs less per token and is where Anthropic suggests starting.
What changed from Claude Fable 5 to Claude Fable 5.1?+
Claude Fable 5.1 keeps the input and output prices of Claude Fable 5, cuts cached reads to a quarter of the cost, and is stronger at long-running coding, research and office documents. Forcing a tool call is no longer supported.
What is the Claude Fable 5.1 model ID and how do I call it?+
The model ID is claude-fable-5-1. Create a SeedRouter API key, point the Anthropic SDK at https://api.seedrouter.ai and send claude-fable-5-1 as the model; the request and response keep the official Messages format.
What is the Claude Fable 5.1 context window and knowledge cutoff?+
Claude Fable 5.1 reads up to 1M tokens and writes up to 128K per request. Anthropic gives June 2026 as its reliable knowledge cutoff.
What is the difference between Claude Fable 5.1 and Claude Mythos 5.1?+
Claude Mythos 5.1 has the same capabilities and pricing but is offered by invitation only, for defensive cybersecurity work. Claude Fable 5.1 is the version anyone can call.
What effort levels does Claude Fable 5.1 support?+
Claude Fable 5.1 accepts low, medium, high, xhigh and max, with high as the default. Lower effort means fewer thinking tokens and a smaller bill.
How much does Claude Fable 5.1 cost?+
Claude Fable 5.1 is billed per token: $8.00 / 1M for input and $40.00 / 1M for output per 1M tokens. Cached prompt reads are the cheapest line. A request that fails is not charged.
Does Claude Fable 5.1 work with the OpenAI SDK?+
Yes. Claude Fable 5.1 also answers the OpenAI Chat Completions and Responses formats at https://api.seedrouter.ai/v1, with the same key and the model ID claude-fable-5-1.
Does SeedRouter store my Claude Fable 5.1 prompts? What about zero data retention?+
No. SeedRouter does not retain the prompts or replies of your Claude Fable 5.1 API requests: nothing you send or receive is written to our logs or storage. Each request leaves only a billing record with the model, token counts, cost, time, IP address and request ID.
Put Claude Fable 5.1 on your hardest task
Open the playground or send a Claude Fable 5.1 request from your code in minutes.