Claude Opus 5.5 vs Fable 5.1: Is Fable Still Worth 2.5x the Price?

The short answer

Default to Opus 5.5. Set the effort level on purpose, measure it on your own tasks, and move a task up to Fable 5.1 only when Opus 5.5 at high effort demonstrably fails. For most coding and knowledge work, that means you will rarely need Fable at all.

This matches how Anthropic positions the two models. It built Opus 5.5 for agentic coding and knowledge work, and says it performs at Fable 5.1’s level on most tasks. Fable 5.1 remains Anthropic’s most capable widely released model for long, demanding reasoning, but the benchmark lead it held over Opus 5 did not carry over to Opus 5.5.

Benchmarks side by side

The gap is widest where the work is long and agentic: 10.6 points on Terminal-Bench 4.0, 8.6 on AutomationBench and 111 Elo on the GDPval-AA knowledge-work arena. It is narrowest on tasks that are mostly about perception, such as desktop control on OSWorld 2.0 (1.1 points) and chart reading on Chartography (0.6 points). There, the two models are effectively tied.

Anthropic’s customer examples show the same pattern. Translating HAProxy from C to Rust, Opus 5.5 passed nearly all regression tests in 9.5 hours against 12 hours for Fable 5.1, at 51% lower cost. On a document-research task, 16 of its 18 reports cleared a quality bar with no invented figures or quotes. Fable 5.1 and Opus 5 cleared none.

Price comparison

Output is where agent costs pile up, because thinking tokens bill as output. An agent that produces 5 million output tokens a day costs $250 a day on Fable 5.1 and $100on Opus 5.5 before input costs. Anthropic’s customers also report Opus 5.5 finishing tasks in fewer steps, so the real saving per finished task is usually larger than the rate card shows. The Opus 5.5 pricing guide works through a full monthly estimate, including rupee figures.

Where Fable 5.1 still earns its place

Benchmarks average over many tasks, and the ones you care about may be in the tail. Move a task up to Fable 5.1 when Opus 5.5, even at high or xhigh effort:

• keeps missing a condition buried in a long specification,
• overlooks a conflict between two source documents,
• cannot resolve a bug after several full attempts, or
• loses the thread on a very long research task that needs sustained reasoning.

The two models also differ on data handling. Fable 5.1 requires 30-day data retention unless Anthropic expressly authorizes zero data retention. Anthropic lists zero data retention as an option for Opus 5.5. If your contracts rule out retention, that alone may decide it.

The API behaves almost the same on both. Thinking is always on, forced tool_choice returns a 400, and thinking blocks are tied to the conversation. Code written for one runs on the other with a model string change. Coming from Opus 5 is a bigger job, covered in the Opus 5.5 migration guide.

How to route between them

The simplest setup is to run every task on Opus 5.5 and escalate the ones that fail a check, such as a test suite, a schema validator or a reviewer model. Keep the same conversation when you escalate.

import anthropic

client = anthropic.Anthropic()

def run(model: str, messages: list):
    with client.messages.stream(
        model=model,
        max_tokens=64000,
        output_config={"effort": "high"},
        messages=messages,
    ) as stream:
        return stream.get_final_message()

response = run("claude-opus-5-5", messages)

if not passes_checks(response):
    # Append the full content, thinking blocks included, unchanged.
    messages.append({"role": "assistant", "content": response.content})
    messages.append({"role": "user", "content": "The tests still fail:\n" + test_output})
    response = run("claude-fable-5-1", messages)

This works well in one direction only. On the Claude API, Fable 5.1 reads Opus 5.5’s thinking blocks, so it picks up the reasoning behind the failed attempt. Opus 5.5 cannot read Fable’s blocks, so if you drop back down afterwards, the API removes Fable’s thinking and Opus 5.5 continues from the visible text only. Whether Fable keeps Opus 5.5’s blocks on Bedrock and Google Cloud was not confirmed at launch.

Which model for which job

Still weighing an upgrade from the previous Opus? See Claude Opus 5.5 vs Opus 5.