The Execute Code, Execute Function and Execute Tool endpoints are all backed by a set of runtime engines. Every time you run code, it runs using exactly one of these engines. Unless you’ve created a Custom Runtime, your code runs using the default WASI engine. When you create a custom runtime, the flags on your account determine which engine the custom runtime uses.

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WASI

The default runtime engine is WASI, which is a server-side interface for WebAssembly. This engine is extremely performant but has some limitations, notably limited support for third-party dependencies.

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V8

When you run code in a Custom Runtime for Python or JavaScript, by default that code runs using our V8 runtime engine. This engine is more flexible than WASI but isn’t quite as fast for Python.

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microVM (deprecated)

The microVM runtime engine is deprecated. We recommend migrating to the V8 engine for improved performance and continued support. For more information about this deprecation and migration guidance, see our blog post.
For Python and JavaScript, we previously had a runtime engine based on microVMs. This engine offered full JavaScript and Python third-party package support but was slower than the other engines and didn’t work with our self-hosted solution, so we deprecated it.