There are several options for managing node/JS/TS projects, and not all of them are enabled within the Cloudflare Pages build process. While there is a Community Feedback post for an official solution, there are a couple of quick steps we can take to publish with pnpm.
Firstly, we will need to tell the Cloudflare Pages build agent to use a more up-to-date version of node. At time of writing the default is 12.x, but we can instruct the agent to use a version that will allow us to install and run pnpm. This is achieved by adding an environment variable in the Cloudflare Pages settings with the desired version.
NODE_VERSION=16.7.0
Lastly, in the build command configuration for the page, we need to add a one-liner that will install pnpm and then use pnpm to build our project.
npm install -g pnpm && pnpm i && pnpm build
Double check the build output and root directories are correct for your project and there we go, Cloudflare Pages with pnpm!