Claude Fable 5: The Suspended Model That Captivates Tech

Language Models

I had the opportunity to test the Claude Fable 5 model, a secure version of the Claude Mythos model, which was available for a period of 72 hours before being suspended by a decision from the U.S. government. During this availability window, I used the model intensively, day and night, to evaluate its capabilities and limitations. Although the model is currently inaccessible, I hope it will be available again in the near future. I am also convinced that other models with similar capabilities will emerge in the coming months.

Many articles have been published about Claude Fable and its capabilities. This article is particularly relevant as I have been using Claude Code daily since the beginning of 2026. From the moment Claude Fable was launched, I immediately tested the model to its full capacity. I had a series of tasks that Opus was not able to accomplish easily, which I tested with Claude Fable to truly assess its performance.

Why Care About Claude Fable 5

It is crucial to understand why Claude Fable 5 is generating so much interest. This language model is perhaps the most anticipated, having been the subject of much hype before its launch. Anthropic spent a lot of time promoting the model, discussing its capabilities and the dangers it could pose in the wrong hands.

Many people were therefore eager to see the model launched, which was finally made available last week to all Claude Pro or Max subscribers. Personally, I encountered no issues while using it until Saturday morning, Norwegian time. Although the model was not available for long (about 72 hours), I was able to test it thoroughly and form a solid opinion on its advantages, disadvantages, and overall capabilities.

What Claude Fable Does Well

First of all, I want to address the strengths of Claude Fable. My overall impression is that it is significantly better than Claude Opus 4.8. I have seen others online claim that Claude Fable's capabilities are not much more advanced than those of Claude Opus 4.8. In my opinion, this is clearly not true. I believe those reporting this have not tested Claude Fable on sufficiently complex tasks.

Of course, if you test Claude Fable on a task that is already easy for Claude Opus to accomplish, you will not see its full capabilities. Where Claude Fable truly shines is when applied to super complex coding tasks.

Accomplishing Complex End-to-End Tasks

I had several tasks I was working on and for which I had spent time with Claude Opus 4.8 to implement. Opus was able to do it, but it was not a one-time implementation, and I had to manually guide Claude Opus through some of the implementations.

These tasks included, for example:

• A feature implementation where I had to modify code across multiple repositories
• Fixing a problem/bug encountered by a client, where I had to enhance the information extraction capabilities of an LLM

Unfortunately, I cannot go into more detail about these issues as they involve work in a closed codebase. To compare Claude Opus to Claude Fable, I had previously applied Claude Opus to these problems and resolved them with quite a bit of manual guidance. Then, I applied Claude Fable, and it was able to solve the issues in one go. A clear sign that Fable is a more powerful model than Opus.

In more general terms about Fable's capabilities, I would say that:

• Claude Fable is more capable of accomplishing end-to-end tasks, with a better understanding of ambiguities and user intentions, better implementation of the intended solution, and better verification of the solution to ensure it is correct by navigating the computer or running integration tests.

I simply found that Claude Fable was able to operate longer, accomplishing more complex tasks without giving up or encountering recurring issues. I felt that tasks were now being performed more autonomously, and I did not need to provide as much guidance to ensure the model was aligned with my intentions.

Discovering Issues in Codebases

Another incredible capability I noticed in Claude Fable was that it was much better than Claude Opus at discovering issues in codebases, whether it was finding bugs or searching for refactoring opportunities or potential future problems.

I constantly run a prompt similar to the one below to uncover issues in my codebase:

• Deeply analyze the codebase to identify any potential bugs, issues, or refactoring opportunities, and return to me with an HTML report of the issues ranked by severity.

With Claude Opus, I used this same prompt and did not get good results. Claude Opus was not able to uncover more refactoring opportunities or bugs, or the issues it discovered were not really relevant. (Of course, note that this was the case after I had already performed a lot of refactoring and bug detection with Claude Opus in a specific repository.)

However, when I then applied Claude Fable with exactly the same prompt, it began to find numerous serious issues, both in terms of security and actual bugs, and also identified many good refactoring opportunities that Claude Opus had not been able to see.

I immediately started going through all the repositories with Claude Fable running this prompt and fixing all the issues. I pushed a lot of code that significantly improved the quality of my codebases.

I think this is probably the clearest sign that Fable is a more powerful model than Opus. You could run exactly the same prompt in the same codebase where Fable is able to detect a bunch of issues that Claude Opus could not detect.

I am just glad I was able to perform this refactoring, bug detection, and fix many issues before the model was unfortunately suspended.

What Claude Fable Does Not Do Well

In the previous section, I addressed what Claude Fable does very well. It is also important to highlight some drawbacks of Claude Fable, as it is not a perfect model.

Claude Fable is undoubtedly the most powerful coding model I have ever used. However, one of the main issues is the number of tokens it uses to accomplish tasks.

Naturally, this is not a problem with the model itself. It is more of an issue with the rate limits you have with Anthropic. Using Claude Fable with a subscription, I started hitting the subscription limit much more quickly.

This is definitely a limitation, as you can no longer simply run the model indefinitely. Moreover, I would say that the price of the Claude Fable subscription is very prohibitive for almost any company. Running a model that costs $10 per million in input and $50 per million in output is not feasible for practically anyone, except for the largest companies.

Of course, one could argue that you can use Claude Fable only for planning and bug detection, and then use Claude Opus for the actual implementations. I agree that this could probably be done, and you would still gain most of the benefits of Claude Fable; however, spending a lot of time optimizing which model to use in which situations is very time-consuming and something you want to avoid if you wish to be as productive and efficient as possible.

This is one of the main drawbacks, in my opinion, the rate limits and the amount of model usage/cost of the model if you are using API pricing.

Sometimes, the Model Is Too Enthusiastic

Another small drawback I would also like to address with Claude Fable is that the model is sometimes too eager to find issues or perform implementations. At times, I found that the model implemented things in a way that was overly complex. For example, it modified many more lines of code than necessary or found more issues in a codebase where many of the problems were not really that serious.

I find this slightly annoying at times, but I also believe it is a trade-off that Anthropic has accepted. You want the model to always look for issues and constantly try to fix them, and you want the fixing to work immediately, of course. It is difficult to find a balance between this desire and avoiding the model becoming too enthusiastic about finding issues and fixing them.

Overall, however, this is a fairly minor drawback. It is just a small detail I noticed while using Claude Fable. By far, the biggest drawback is the prohibitive price of the model.

In this article, I shared my thoughts on Claude Fable. I compared it to Anthropic's previous flagship model, which was Claude Opus 4.8. The Claude Fable 5 model is amazing, but it also has some drawbacks:

• One of them being that it is too enthusiastic, which I consider a minor drawback.
• The price. The price is, of course, a significant drawback. However, this is not an issue with the model itself, and the best models will always be expensive. People can make their own assessment to determine whether the model is worth what they have to pay to use it.

Overall, it is a very powerful model. I hope it becomes available again soon.