Summary

Between v2.5.1 and v2.6.0, pkg/codegen/schema.go dropped a !p.Nullable guard from the omitempty calculation for generated struct fields. As a result, any property with nullable: true in the OpenAPI schema now gets ,omitempty added to its Go JSON tag, where prior versions did not add it. This is a wire-format ("shape") change with no corresponding change to the input spec — the same spec generates code that serializes differently across this version boundary.

There are two distinct manifestations of this:

Case 1: required + nullable fields

A property that is both required: true and nullable: true in a JSON Schema / OpenAPI schema is meant to always appear in the serialized object — its value may be null, but the key is expected to always be present.

Minimal repro:

components:
  schemas:
    Widget:
      type: object
      required:
        - token
      properties:
        token:
          type: string
          nullable: true
          readOnly: true

oapi-codegen v2.5.1:

Token *string `json:"token"`

oapi-codegen v2.8.0 (current):

Token *string `json:"token,omitempty"`

Case 2: optional + nullable fields

For a property that's nullable: true but not required, prior versions did not add omitempty unless the nullable-type output option was enabled — this preserved the distinction between "explicitly cleared" (null) and "field absent" (undefined/missing key), which is a meaningful and common REST/JSON convention (e.g. PATCH semantics).

Minimal repro:

components:
  schemas:
    Widget:
      type: object
      properties:
        note:
          type: string
          nullable: true

v2.5.1:

Note *string `json:"note"`

Always present in output, null when unset.

v2.8.0:

Note *string `json:"note,omitempty"`

Omitted when unset.

Root cause

Directly diff on pkg/codegen/schema.go across installed module versions shows:

v2.5.1 (last unaffected):  omitEmpty := !p.Nullable && shouldOmitEmpty
v2.6.0 (first changed):    omitEmpty := shouldOmitEmpty

(confirmed unchanged through v2.7.0, v2.7.1, v2.7.2, v2.8.0 — the latest release as of this report)

Versions

• github.com/oapi-codegen/oapi-codegen/v2: behavior changed starting at v2.6.0, present through v2.8.0 (latest at time of writing); confirmed as v2.5.1's behavior in v2.4.1, v2.5.0, v2.5.1.
• Reproduced with github.com/getkin/kin-openapi v0.145.0, but the change is entirely internal to oapi-codegen's own codegen logic — it reproduces identically regardless of kin-openapi version, using the exact same input spec across the version boundary above.
• Go 1.26.0 (shouldn't matter, but noting for completeness).
• Codegen config: generate: {models: true} (also reproduces with strict-server/chi-server enabled).

Workaround found

Adding x-omitempty: false in the spec seem to mitigate, but it is undesirable to change the specs to preserve the older behavior