What is the Semantic Versioning (SemVer) and Go Modules compatibility rule?

Go Modules strictly enforce SemVer. The import compatibility rule dictates that if an old and new package share the same import path, they must be backward compatible.

When you release a major breaking change (e.g., v2.0.0), you must change the module path (usually appending /v2). This allows a consumer to import v1 and v2 of the same module simultaneously without naming conflicts, a feature critical for large monorepos upgrading dependencies incrementally.