JetBrains、Claude Fable 5 の評価と導入プロセスを公開

JetBrains builds the tools developers use worldwide, from IntelliJ IDEA and PyCharm to the Kotlin programming language, serving more than 12.5 million active users and 88 of the Fortune Global 100. Vladislav Tankov, CTO at JetBrains, spoke with Anthropic about how his team evaluates new models, decides when to use Claude Fable 5, and thinks about data retention and safeguards when working with frontier models.

How has frontier AI changed for JetBrains in 2026?

I've been with JetBrains for 10 years, and we were among the very first customers of LLM providers. Over the last year, we moved from having AI skeptics among our customers and inside the company to seeing that AI is here to stay. It's a big and foundational change in the technology industry. Literally every skeptic in the company has changed.

How do you evaluate new models and decide when to use them?

We're a coding company, so we have a big evaluation pipeline: large eval sets on private repositories, including our monorepo. We take a close look at whether a model lives up to its benchmark scores on real work—some models are tuned to score well on public benchmarks but fall down on actual tasks. With a private repository, that's a lot easier to check. We also keep leaderboards for best quality, best cost per task, and fastest model. While Claude Fable 5 is more expensive per token, its cost per task is lower in some cases, particularly for more complicated, long-running work.

How did Claude Fable 5 score on your evals relative to previous models?

Claude Fable 5 is both more accurate and more efficient than prior models. It posted the best Python pass rate in our suite at 44.3%, against 28.2% for Opus 4.8, a 16-point jump. In a head-to-head comparison, Claude Fable 5 solved 18 Python tasks that Opus 4.8 missed and lost only 2. Its answers are also more trustworthy: when its code ran, it passed our tests far more often than either Opus model. That matters because code that runs but produces wrong answers is the most expensive kind of failure to catch.

The efficiency story is just as interesting. Claude Fable 5 needed about 22% fewer steps than Opus 4.8 to reach a solution, so it gets to working code with less trial and error. It also spends its effort in the right places. On Java tasks, Opus 4.8 repeatedly tried to pull in outside resources that almost never help in our environment, while Claude Fable 5 skipped that entirely and worked with the code in front of it. It shows better engineering habits more generally.

When do you use Claude Fable 5 over other models?

Opus is seen as a workhorse: you can be very sure it will do the work. You go to Claude Fable 5 when you really need good reasoning, when you almost need a partner, and you're not sure yourself how to do the thing. For example, one of our tech leads decided to implement a rich text editor component we had attempted a few times over the years, and Claude Fable 5 almost one-shotted it.

Another popular Claude Fable 5 use case is long-running agentic-coding experimentation. We provide an agent running Claude Fable 5 with specifications (in the form of text and images) and make it implement sophisticated IDE-like apps. The interesting thing here is that specifications can also be generated by the agent, based on the existing app. Joining these two components allows us to rewrite the app from one runtime, framework, or language to another in a nearly black-box setup.

How are you thinking about safety and data retention with today's frontier models?

We're not a company trying to create the safest model ourselves. We expect that the red teaming and everything else done on Anthropic's side is enough to believe the model is safe. Then we take a systematic approach to deployment, where we can guarantee safety: creating the infrastructure and the safety net around the model and the harness, rather than tweaking the model itself.

Security is also one of our biggest Claude Fable 5 uses. We run white-box testing against our own products to find vulnerabilities, and our security team is preparing for the fact that not only are we running the model—people outside the company will be running Claude Fable 5, or similar-class models, to probe for vulnerabilities across all of our products. Since we serve large enterprises in regulated industries, it's important for us to be prepared. Claude Fable 5 supports our work rather than blocking.

So it's a tight balance: the less aggressive the classifier is on your side, the more vulnerabilities someone will find in our products—including ones nobody knew about.

And it's no secret: we'd prefer zero data retention. But I don't see any other way for you to understand what was asked and where a classifier may have worked incorrectly. As long as reviews are only to investigate the most serious cases flagged , I'm okay with it. I think it's a fair tradeoff for access to frontier intelligence that allows my team to do their best work.

What's next on JetBrains's AI roadmap?

We expect the underlying models built by the LLM providers to keep getting more capable. What matters now is a kind of cockpit for software development: a space in which agents and people collaborate, and where people can manage the development process.

For JetBrains, it’s a big transformation. We see an opportunity to build the next generation of products across the agentic software development lifecycle that powers that cockpit. Developers will get more and better code shipped with agents, non-technical roles will have a larger role in software creation, and organisations will get the governance and clarity on the return on investment they need.

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