Pyrefly IDE extension now 18x faster

As we move closer to a stable release of Pyrefly, our efforts have moved from expanding the language server’s capabilities to tackling performance edge cases. Recently, our friends at Astral alerted us to a specific edge case that is a great example of the kind of issues we’ve been aiming to uncover. The specific example Astral highlighted showed that in some edge cases Pyrefly could take multiple seconds to update diagnostics after editing. This is much slower than expected (used across Meta’s codebases, Pyrefly usually takes less than 10 milliseconds to recheck files after saving them) and prompted us to investigate further.

Now we’d like to share the story of how that edge case led us to rethink our underlying approach to diagnostics and dependency tracking, improving the speed at which Pyrefly's type errors update by 18x in the process.

The Problem pyrefly.org/blog/2026/02/06/performance-improvements

Say you’re working in a very large codebase and have two files open in your editor, A and B.

A is a “load-bearing” file that has many reverse dependencies - that is to say, many files directly or transitively import A.

B is one of the files that imports A.

Incremental type checking should be snappy - when you edit A and save the file, any effects on the type errors in B should be reflected almost instantly. However, in a small number of cases, we found that updates could take multiple seconds.

How Incremental Updates Work in Pyrefly pyrefly.org/blog/2026/02/06/performance-improvements

To understand how this problem arises, we need to look at how incremental updates currently work. Pyrefly’s analysis operates at the module level, meaning it keeps track of the types exported from each module, as well as the dependencies between modules. Let’s illustrate with a simple example.

The above image demonstrates a file A that has 4 reverse-dependencies. B1-B4 all rely on the types exported from A.

When file A is edited & saved, we recheck A to see if any of its exported types also change. If so, we recheck every file that imports A (and if any of those files’ expor…