Kubernetes API Compatibility Model

Kubernetes API Compatibility Model is the set of explicit guarantees and constraints governing how the API is permitted to change over time, defining what kinds of changes are considered backward-compatible and therefore safe to make within a stable version, versus what kinds of changes require a new version entirely, giving both cluster operators and application authors confidence about how much their existing manifests, clients, and automation will continue working as the platform evolves. Without this model, every Kubernetes upgrade would carry unpredictable risk of silently breaking existing workloads and tooling.

The Core Compatibility Guarantee

What Stable Means in Practice

For a stable (GA) API version, Kubernetes guarantees that valid objects created against that version will continue to be readable and functionally equivalent in future releases for a long time horizon, and that a client written against that version's schema will continue to function without modification, barring the class of allowed additive changes described below.

Scope of the Guarantee

This guarantee applies specifically to the API surface and its documented semantics; it does not promise that internal implementation details, undocumented behavior, or explicitly alpha/beta-gated features carry the same stability, which is precisely why the maturity levels in the API version model exist as a separate, narrower-guarantee tier.

Backward-Compatible Changes

Additive Changes Are Safe

Adding a new optional field, adding a new resource type, or adding a new optional API group is considered backward compatible, since existing clients that do not know about the addition simply continue operating as before, ignoring fields or types they were never written to expect.

Relaxing Constraints Is Generally Safe

Loosening a validation constraint — accepting a wider range of values for a field that was previously more restrictive — is typically compatible, since any request that was previously valid remains valid, though care is still taken because client-side assumptions built around the old, tighter constraint could theoretically be affected in edge cases.

Breaking Changes

What Counts as Breaking

Removing a field, renaming a field, changing a field's type, making a previously optional field required, or tightening a validation constraint so that previously valid requests become invalid are all considered breaking changes, since each of these can cause an existing, unmodified client to fail where it previously succeeded.

Breaking Changes Require a New Version

Because breaking changes cannot be made safely within an already-stable version, any such change must be introduced as part of a new API version, coexisting alongside the old one for a transition period governed by the deprecation policy, giving users an explicit, bounded migration window rather than an unannounced behavioral shift.

The Deprecation Policy as the Enforcement Mechanism

Minimum Support Windows

The Kubernetes project's formal deprecation policy specifies minimum durations a deprecated API version must continue to be served after a replacement becomes available, varying by maturity level, which operationalizes the compatibility model by ensuring "we're changing this" always comes with a concrete, predictable runway before the old behavior actually disappears.

Deprecation Communication

Deprecated API usage is surfaced through both release notes and, at runtime, through API response warning headers, giving operators multiple independent signals to catch deprecated usage in their manifests and automation before a version is actually removed rather than discovering it only when an upgrade breaks something.

Compatibility Boundaries for Custom Resources

The Same Principles Apply by Convention

While the formal deprecation policy is a project-level commitment specific to Kubernetes' own built-in APIs, the same compatibility principles — additive changes are safe, breaking changes require a new version, deprecated versions get a support window — are the recommended convention for CustomResourceDefinition authors to follow, since consumers of a custom API reasonably expect the same category of stability guarantees from any API they build automation against, regardless of who defines it.