The caret symbol (^) indicates that Poetry is free to install any version matching the leftmost non-zero digit of the version string. For example, if the Requests library releases a new version 2.99.99, then Poetry will consider it an acceptable candidate for your project. However, version 3.0 wouldn’t be allowed.

The idea behind this follows the semantic versioning scheme (major.minor.patch), where minor and patch updates shouldn’t introduce backward-incompatible changes. Still, this is just an assumption that could break your builds, and some notable Python developers disagree with such a choice of defaults on behalf of Poetry.

Ref: https://realpython.com/dependency-management-python-poetry/