Claude Fable 5.2 vs Opus 5.5: Pricing, API Limits and When to Switch

Anthropic released Claude Fable 5.2 and Mythos 5.2 on November 9. Both cost $8 per million input tokens and $40 per million output tokens. That is 20% below their 5.1 predecessors, but twice Opus 5.5's base rates. Moving an existing agent requires more thought than changing its model name.

The useful question is whether Fable completes difficult work with less total effort. A cheaper token is valuable, but an agent that repeatedly explores the wrong fix can consume plenty of them. Equally, a stronger model that reviews everything through extra subagents can make a simple task unnecessarily expensive.

Fable 5.2 pricing compared with Opus 5.5

These are standard Claude API prices in USD per million tokens, from Anthropic's pricing documentation. Cloud-provider billing and deployment options can differ.

Model	Uncached input	Output	Cache reads	Five-minute cache writes
Fable 5.2	$8	$40	$0.20	$10
Mythos 5.2	$8	$40	$0.20	$10
Fable 5.1 / Mythos 5.1	$10	$50	$0.25	$12.50
Opus 5.5	$4	$20	$0.20	$5
Sonnet 5.5	$2	$10	$0.10	$2.50

The detail worth noticing is the cache-read column. Fable 5.2 and Opus 5.5 charge the same amount to read cached input. Their price difference sits in new input, cache creation and output, so a long conversation with a large reusable prefix has different economics from a stream of unrelated requests.

Fable and Mythos also offer fast mode at $16 input and $80 output per million tokens. Anthropic describes up to 2.5 times faster token generation. That is a premium option, not a promise that an entire task finishes 2.5 times faster: tool execution and other waiting still take time.

A worked cost example

Consider an illustrative agent run with 100,000 uncached input tokens, 900,000 cache-read tokens and 20,000 billable output tokens. Assume its cache already exists, so this example excludes cache creation, tools, infrastructure and other charges. These are calculations, not results from a hands-on test.

Fable 5.2 costs $0.80 for new input, $0.18 for cached input and $0.80 for output: $1.78 in total. The same token mix on Opus 5.5 costs $0.40 + $0.18 + $0.40, or $0.98. Fable is about 82% more expensive here, rather than exactly twice as expensive, because the cached portion costs the same.

Now suppose Opus needs two equivalent attempts to reach an accepted result while Fable needs one. Opus would cost $1.96 against Fable's $1.78. Reverse the success pattern and Fable becomes the worse choice. Neither outcome follows from the price list; the example shows what your evaluation needs to measure.

Count retries, abandoned branches and reviewer agents against the final accepted result. Comparing only the successful last request hides the expensive part of an unreliable workflow.

What changed in performance

In its launch announcement, Anthropic reports a FrontierCode 2.0 score of 49.1% for Fable 5.2 at Xhigh effort, compared with 32.4% for Fable 5.1 and 45.5% for Opus 5.5. More relevant to a budget decision, it says Fable 5.2 at High effort reached roughly Opus 5.5's best score at about half the cost per task on that evaluation.

Those are vendor evaluation results, not a guarantee for your repository. They justify testing a different model-and-effort combination, rather than assuming the model with the lower per-token rate is always cheaper. Anthropic still recommends Opus 5.5 as the starting point for most coding and knowledge work.

For a useful comparison, select a fixed set of previously difficult tasks and decide what counts as success before running either model. For a code migration, that might mean passing existing tests, preserving public behavior and getting through human review without another repair cycle. Keep the harness and available tools consistent, then compare accepted outcomes, elapsed time and total cost.

Fable 5.2 API access and context window

The Fable model reference lists claude-fable-5-2 for the Claude API, a one-million-token context window and up to 128,000 output tokens. It accepts text and images and produces text. The same context and output limits are listed for Opus 5.5, so a larger context window is not a reason to switch between these two models.

Fable's API default effort is high; Opus 5.5 defaults to medium. Set effort deliberately when comparing them. Leaving the field out changes two variables at once, and a successful request will not warn you that the new default may affect latency and cost.

Adaptive thinking is always on. Forced tool calls using any or a named tool return a 400 error, and Anthropic advises omitting temperature, top_p and top_k. Audit shared request-building code before routing traffic from an older model, especially if it adds these settings automatically.

Mythos 5.2 is an access decision

Fable and Mythos use the same underlying model, with different safeguards and access requirements. Mythos 5.2 is available to verified organizations through Anthropic's programs, including its cybersecurity program. Its base prices, context window and maximum output match Fable's.

For ordinary application development, applying for Mythos is not a way to obtain a cheaper or larger-context model. Its relevance is legitimate work that needs the capabilities covered by those verification programs. Confirm the organization's access before changing a production model ID.

Check retention and conversation history before migrating

The migration guide identifies two important operational constraints. Fable and Mythos 5.2 require 30-day retention unless Anthropic expressly authorizes zero data retention. An incompatible organization or workspace configuration returns a 400 error on the Claude API. Resolve that requirement with the people responsible for your data policy before sending customer material.

Fable 5.2 also enforces its thinking-block prefix check for every account. If your application edits earlier messages, the system prompt or tools and then reuses a thinking block bound to the old history, a request can fail even if it worked on an older account with Fable 5.1. Retrying the same body will not repair the mismatch.

Downgrading a conversation from Fable 5.2 to Opus 5.5 has another cost implication: Opus cannot read Fable's thinking blocks. The API drops those blocks and the request succeeds, but the next turn runs without that reasoning. Include this transition in routing tests rather than treating a successful HTTP response as proof that nothing changed.

The sensible rollout is a narrow one: keep your current default, send a defined class of difficult work to Fable, and measure whether it reduces the cost of reaching a result you will actually accept. Fable earns a place in the workflow when it removes expensive failure, not simply because it tops a launch-day chart.

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