Why might Dependabot updates break a build even when following semantic versioning correctly?

Semantic versioning is a convention, not an enforced guarantee — it depends entirely on the dependency's own maintainers correctly classifying their changes, and even well-intentioned maintainers occasionally misclassify a breaking change as a minor or patch bump, whether through oversight or differing interpretation of what counts as "breaking."

flowchart TD
    A[Dependency author labels a change as 'patch'] --> B{Change actually alters observable behavior?}
    B -->|Yes, unintentionally breaking| C[Consuming code breaks despite correct semver category]

Beyond maintainer error, other real causes include: a transitive dependency shifting versions as a side effect of the direct update (even if the direct dependency itself followed semver correctly), behavior that was never part of the dependency's actual documented/tested public contract but your code relied on anyway (technically not a semver violation, since undocumented behavior isn't covered by the guarantee), or a subtle interaction with another dependency in your specific combination that the updated package's own test suite never covered. This is precisely why CI validation remains essential even for "safe" patch/minor updates, rather than treating semver compliance as a substitute for actually testing the change.