The era of large model distillation is over: Fable 5.1 rewrites the API, completely cutting off all possibilities for distillation.

The booming era of large-scale distillation is coming to an end!

Back on September 2, Anthropic launched a heavy strike that directly rewrote the API rules, completely cutting off all escape routes for wrapper large models and distillation practitioners.

In the past, countless companies chose to extract the inference process of top-tier leading models through APIs to save huge computing power costs and lengthy training time, then used this data to train their own "small models".

But from today onwards, this possibility no longer exists.

Claude Fable 5.1 Blocks Tampering of "Thinking Blocks" 

The Fable 5.1 released by Anthropic this time introduces what can be called the strictest anti-distillation mechanism in history.

The core action is to lock the "thinking blocks" tightly.

What is a "thinking block"?

Simply put, it is the Chain of Thought (CoT) process that the AI goes through in its mind before giving you the final answer.

The complex, parallel paths in Claude's thinking process when performing mental calculations

During API calls, Claude will return these inference steps to users in the form of "thinking blocks".

In normal multi-turn conversations, developers will send these thinking blocks back to Claude together with system prompts, tools and historical messages, so that the model can remember the context and maintain the coherence of the conversation.

But it is precisely this mechanism that has become an opportunity for distillation practitioners.

They discovered a huge loophole: as long as they secretly modify the context before and after the thinking block in multi-turn conversations (such as tampering with early system prompts or historical messages), they can break Claude's defense mechanism, and even induce Claude to output its originally hidden underlying inference logic!

In response to this chaos, Anthropic unceremoniously launched the new rule of "context consistency verification" in Fable 5.1.

1. Return as original, not a single word changed: The API will now strictly verify whether the "thinking block" sent back by the client is completely consistent with the system prompts, tools and historical messages that originally generated it.

2. Tampering? Error returned directly! As long as the system detects that the context has been modified, even the slightest change will lead to a matching failure, and the API will directly return an error prompt and refuse service.

Moreover, to take care of legitimate developers who really need to modify the context (such as compressing the context length to save costs), Anthropic provides a "non-strict mode".

In this mode, your request can pass, but the system will delete all the "thinking blocks" directly! The model will force a response without seeing the previous inference process at all.

This move can be said to have pulled the rug from under the feet of distillation practitioners.

Examples of faithful reasoning and motivated (unfaithful) reasoning when Claude is asked a relatively simple question and a relatively difficult question

How Crazy is Industrial-grade Distillation? 

Is it necessary for Anthropic to get so angry and impose such strict restrictions?

The answer is: Yes, and it is extremely necessary. 

Because the current illegal distillation has evolved to industrial-scale.

Over the past year, Anthropic's security team has observed shocking abuse phenomena.

These professional "distillation hackers" do not use one account to ask a few questions every day, but use thousands of fake accounts, running automated scripts to frantically "leech resources" on the API end around the clock.

Their operation methods are highly concealed and aggressive.

As mentioned above, although Anthropic has long encrypted Claude's core thinking blocks, hackers have used "context injection" and "conversation rewriting" technologies.

This practice is like hypnopaedia for Claude, making it actively decrypt and print out its own encrypted inference process in a state of logical confusion.

As a result, many small-parameter models have not gone through the zero-to-one stacking of computing power and algorithm innovation, but only memorized the problem-solving ideas of top-tier AIs to achieve comparable performance.

Even more terrifying, this practice directly crosses the red line, leading to the collapse of security guarantees.

The Biggest Hidden Danger of AI Out of Control 

If the loss of commercial interests only makes Anthropic feel "painful", then the "decoupling of capability and security" makes the entire AI security community feel terrified.

This is also the core motivation for Anthropic to make up its mind to take drastic measures.

As we all know, large models like Claude and GPT-4, while having extremely high intelligence, have also undergone extremely strict "value alignment" and security training. They know they cannot teach people to make bombs, cannot write malicious ransomware, and cannot publish hate speech.

This dual binding of "capability + safety guardrails" is established by large AI companies at the cost of tens of millions of dollars through RLHF (Reinforcement Learning from Human Feedback) and red-blue team confrontation exercises.

However, distilled models perfectly bypass this safety net!

When distillation practitioners forcibly extract Claude's "thinking blocks" by modifying the context, they only extract the high-intelligence "inference capability" part, but cannot inherit the underlying security guarantees at all.

It is like an evil organization that clones Einstein's intelligence, but does not clone Einstein's sense of morality and empathy.

Systems trained with this illegal distillation method will inexplicably inherit advanced logic and code capabilities that they should not have originally.

But since they are underlying models with "low guarantees and weak guardrails", they will not hesitate to respond to hackers' requests to generate cyber attack scripts, or assist in the research and development of biological weapons.

"The decoupling of capability and security is the biggest risk of AI today."

New Rules Implemented: Who Is Affected? 

Will this crackdown affect legitimate developers?

Don't worry, Anthropic has adopted a precise "phased implementation" strategy to minimize accidental injuries.

The first to be affected are "new accounts".

According to official documents, this restriction first targets new accounts where abuse is most concentrated. For the Fable 5.1 model, this update only applies to new API accounts created after 12:00:00 AM UTC on August 31, 2026.

The following groups can rest assured completely, you are not affected at all.

1. Existing API accounts: Existing old accounts are temporarily not affected on Fable 5.1. This gives legitimate developers sufficient time to make adjustments.

2. Ordinary consumer end users: If you are just using the web version of Claude.ai, or C-end users of official products such as Claude Code and Claude Cowork, or chatting normally through third-party wrapper products, you will not be interfered in any way.

For affected API developers, what do you need to pay attention to?

If your previous application integration just used the technology of "rewriting early turns in the middle of a conversation" (such as context compression to save costs, or forcibly injecting new system reminders in the middle of a conversation), you need to start adjusting your code logic immediately.

To cope with this change, Anthropic provides a complete migration guide. Developers have two options.

Option 1: Keep the context absolutely consistent. Pass back the original thinking blocks, system prompts and tools word for word.

Option 2: Choose to enable "non-strict mode" in the API request. In this mode, even if you modify the context, the API will not report an error and abort, but will automatically and silently delete the affected thinking blocks in the request.

Although this will make the model "forget" the previous specific inference process in subsequent conversations, it can at least ensure that your conversation or task is not interrupted.

It should be reminded that although there is an exemption period for this rule at present, Anthropic has clearly stated that the "preserve thinking" mechanism will apply to all accounts in future model versions! 

The existing buffer window period is also limited.

Unexpected Benefit: Good News for Law-abiding Developers 

Moreover, after the implementation of this new rule, there is also a hidden benefit for legitimate developers — a significant reduction in costs and a leap in response speed.

How is this achieved?

The core lies in "context consistency".

In the past, because developers could modify the context at will, every request received by the model was "brand new". This not only consumed computing power, but also was extremely slow.

Now, the new rule forces everyone to keep the "thinking blocks" and their surrounding system prompts and historical messages completely consistent, without tampering.

This technically creates a perfect condition: prompt caching can achieve an extremely high hit rate!

Now, the API server can easily cache the previous conversation context directly. When the next conversation request arrives, the system directly retrieves the data from the cache and completes the matching instantly.

The result is: the API response time is greatly shortened, the latency drops sharply, and the API call cost for developers will also be significantly reduced!

This "unintentional gain" can be said to have given real money subsidies to developers who abide by the rules.

In short, this new rule is undoubtedly a watershed for the entire AI industry.

The story of small-parameter models outperforming large models will probably happen much less frequently.

When the tide recedes, who is swimming naked?

References:

https://support.claude.com/zh-CN/articles/16761192-%E4%BF%9D%E7%95%99%E6%80%9D%E8%80%83-%E6%94%B9%E5%8F%98messages-api%E5%A4%84%E7%90%86%E6%80%9D%E8%80%83%E5%9D%97%E7%9A%84%E6%96%B9%E5%BC%8F%E4%BB%A5%E9%98%B2%E6%AD%A2%E8%92%B8%E9%A6%8F 

Editor: Aeneas

This article is from WeChat Official Account "AI Era" (ID: AI_era), written by ASI Revelation, authorized for release by 36Kr.