[BUG] protobuf-schema generator wraps standalone enums in message types but does not update field references

[NielsDoucet]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Since 7.20.0, in the protobuf-schema generator, standalone enum schemas (top-level $ref enums in OpenAPI) are unconditionally wrapped in a message type with a nested Enum:

v7.19.0:

enum ResourceType {
  RESOURCE_TYPE_UNSPECIFIED = 0;
  RESOURCE_TYPE_SYNC = 1;
  RESOURCE_TYPE_ASYNC = 2;
}

v7.20.0+:

message ResourceType {
  enum Enum {
    RESOURCE_TYPE_UNSPECIFIED = 0;
    RESOURCE_TYPE_SYNC = 1;
    RESOURCE_TYPE_ASYNC = 2;
  }
}

However, field references to that enum in other generated messages are not updated. Instead they still use the bare type name:

message BulkOperationMessage {
  ResourceType resource_type = 5;  // ResourceType is now a message, not an enum
}

The field should reference the nested enum type:

message BulkOperationMessage {
  ResourceType.Enum resource_type = 5;  // correct
}

The generated .proto files are syntactically valid but semantically broken. ResourceType is now an empty message, so the resource_type field carries no information. In Java:

• message.getResourceType() returns an empty ResourceType message object instead of an enum value
• ResourceType.RESOURCE_TYPE_SYNC no longer compiles (the constants are now on ResourceType.Enum)
• message.getResourceType().name() does not compile (no .name() method on a proto message)

Affected versions: 7.20.0, 7.21.0 (confirmed working: 7.19.0)

openapi-generator version

7.21.0
protobuf-schema generator

OpenAPI declaration file content or url

Minimal reproducer spec:

components:                                                                                                                                                                                 
  schemas:                                                
    ResourceType:
      type: string
      enum:
        - SYNC
        - ASYNC                                                                                                                                                                             
 
    BulkOperationMessage:                                                                                                                                                                   
      type: object                                        
      properties:
        resourceType:
          $ref: '#/components/schemas/ResourceType'

Generation Details

Generation through the gradle plugin, with the following additional options set:

generatorName = "protobuf-schema"
library = ""
configOptions.put("startEnumsWithUnspecified", "true")
configOptions.put("numberedFieldNumberList", "true")

Steps to reproduce

Related issues/PRs

Introduced by: #22740 (merged 2026-01-30)

Suggest a fix

enum.mustache was changed to always generate the message wrapper, but the field-reference was not updated to match. The two halves of the generated output are inconsistent with each other.
This should be fixed in ProtobufSchemaCodegen.java: when setting x-protobuf-data-type for a property whose $ref target is a wrapped enum (i.e. isEnum is true on the referenced model), it should set the value to ResourceType.Enum instead of ResourceType.

Workaround

There is a workaround possible, by reverting the enum.mustache file to its v7.19.0 state. This reverts the generated enums to a flat structure.
The gradle plugin provides a templateDir option to allow for that.

[angus-lord]

I also encountered this. I have a fix/workaround which I made in model.mustache using the isEnumRef flag -> see below

{{>partial_header}}
syntax = "proto3";

package {{#lambda.lowercase}}{{{packageName}}};{{/lambda.lowercase}}

{{#customOptionsModel}}
{{{.}}}
{{/customOptionsModel}}
{{#imports}}
{{#import}}
import public "{{{.}}}.proto";
{{/import}}
{{/imports}}

{{#models}}
{{#model}}
{{#isEnum}}{{>enum}}{{/isEnum}}{{^isEnum}}message {{classname}} {

{{#oneOf}}
{{#-first}}
    oneof {{classVarName}} {
        {{#vars}}
        {{#description}}
        // {{{.}}}
        {{/description}}
        {{#vendorExtensions.x-protobuf-type}}{{{.}}} {{/vendorExtensions.x-protobuf-type}}{{{vendorExtensions.x-protobuf-data-type}}} {{{name}}} = {{vendorExtensions.x-protobuf-index}}{{#vendorExtensions.x-protobuf-packed}} [packed=true]{{/vendorExtensions.x-protobuf-packed}};
        {{/vars}}
    }
{{/-first}}
{{/oneOf}}
{{^oneOf}}
  {{#vars}}
  {{#description}}
  // {{{.}}}
  {{/description}}
  {{^isEnum}}
  {{#isEnumRef}}
  {{#vendorExtensions.x-protobuf-type}}{{{.}}} {{/vendorExtensions.x-protobuf-type}}{{{vendorExtensions.x-protobuf-data-type}}}.Enum {{{name}}} = {{vendorExtensions.x-protobuf-index}}{{#vendorExtensions.x-protobuf-packed}} [packed=true]{{/vendorExtensions.x-protobuf-packed}}{{#vendorExtensions.x-protobuf-json-name}} [json_name="{{vendorExtensions.x-protobuf-json-name}}"]{{/vendorExtensions.x-protobuf-json-name}};
  {{/isEnumRef}}
  {{^isEnumRef}}
  {{#vendorExtensions.x-protobuf-type}}{{{.}}} {{/vendorExtensions.x-protobuf-type}}{{{vendorExtensions.x-protobuf-data-type}}} {{{name}}} = {{vendorExtensions.x-protobuf-index}}{{#vendorExtensions.x-protobuf-packed}} [packed=true]{{/vendorExtensions.x-protobuf-packed}}{{#vendorExtensions.x-protobuf-json-name}} [json_name="{{vendorExtensions.x-protobuf-json-name}}"]{{/vendorExtensions.x-protobuf-json-name}};
  {{/isEnumRef}}
  {{/isEnum}}
  {{#isEnum}}
  {{^vendorExtensions.x-protobuf-enum-reference-import}}
  enum {{enumName}} {
    {{#allowableValues}}
    {{#enumVars}}
    {{{name}}} = {{{protobuf-enum-index}}};
    {{/enumVars}}
    {{/allowableValues}}
  }

  {{/vendorExtensions.x-protobuf-enum-reference-import}}
  {{#vendorExtensions.x-protobuf-type}}{{{.}}} {{/vendorExtensions.x-protobuf-type}}{{{vendorExtensions.x-protobuf-data-type}}} {{name}} = {{vendorExtensions.x-protobuf-index}};
  {{/isEnum}}

  {{/vars}}
{{/oneOf}}
}
{{/isEnum}}
{{/model}}

{{/models}}

[swapneswarsundarray]

I can take a look at this and work on a fix. Planning to investigate the enum reference handling and align the generated field type with the wrapped enum structure.