Bun's value is that the runtime, the toolchain, and the batteries are one binary. Two biases decide most calls:

Reach for what already ships. A dependency earns its place by doing what Bun.*, bun:*, and the bundled toolchain do not. Adding one that duplicates a built-in is a defect.
Write node: only where Bun has no native equivalent, or where the code must also run under Node. Portability is a requirement to state, not a habit to keep.

Release Version

Bun ships one stable line and no long-lived support branches, so bun upgrade moves a machine to whatever release is newest. Pin the version in CI (oven-sh/setup-bun, bun-version) so a developer machine and a runner agree, and state the release a feature needs in the sentence that uses it. Never write "the latest version supports X".

The rules below assume 1.3.0 or later unless a version anchor says otherwise. Floors for the features these rules reference: