Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• 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

• When using the scala-sttp generator with jsonLibrary=circe, the generated code relies on circe's AutoDerivation to derive Encoder/Decoder instances. This derives codecs from Scala field names (camelCase), not the original JSON property names from the OpenAPI spec.
• Properties like first-name (kebab-case), phone_number (snake_case), or ZipCode (PascalCase) are converted to camelCase Scala fields (firstName, phoneNumber, zipCode). Circe's AutoDerivation then serializes/deserializes using these Scala field names instead of the original JSON names, breaking the wire contract.
• Additionally, AutoDerivation cannot derive codecs for File (from format: binary) or Any (from type: object),
  causing compilation failures when those types appear in model properties.

openapi-generator version

7.22.0-SNAPSHOT (latest master as of 2026-04-07), also affects 7.21.0 and all prior versions with circe support.

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: Mixed Case Test
  version: 1.0.0
paths:
  /test:
    get:
      operationId: getTest
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/MixedCaseModel'
components:
  schemas:
    MixedCaseModel:
      type: object
      properties:
        first-name:
          type: string
        phone_number:
          type: string
        lastName:
          type: string
        ZipCode:
          type: string
        address:
          type: string
    BinaryPayload:
      type: object
      properties:
        data:
          type: string
          format: binary
        metadata:
          type: object

Note: format: binary maps to java.io.File and type: object (without properties) maps to Any in the scala-sttp generator. Circe's AutoDerivation cannot derive codecs for either type, causing compilation failures.

Generation Details

$ openapi-generator-cli version
7.22.0-SNAPSHOT

$ openapi-generator-cli generate \
    -i mixed-case-fields.yaml \
    -g scala-sttp \
    -o /tmp/scala-sttp-circe-test \
    --additional-properties=jsonLibrary=circe
[main] INFO  o.o.codegen.DefaultGenerator - Generating with dryRun=false
[main] INFO  o.o.codegen.DefaultGenerator - OpenAPI Generator: scala-sttp (client)
[main] INFO  o.o.codegen.DefaultGenerator - Generator 'scala-sttp' is considered stable.
[main] INFO  o.o.c.languages.AbstractScalaCodegen - Environment variable SCALA_POST_PROCESS_FILE not defined so the Scala code may not be properly formatted. To define it, try 'export SCALA_POST_PROCESS_FILE=/usr/local/bin/scalafmt' (Linux/Mac)
[main] INFO  o.o.c.languages.AbstractScalaCodegen - NOTE: To enable file post-processing, 'enablePostProcessFile' must be set to `true` (--enable-post-process-file for CLI).
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/scala-sttp-circe-test/src/main/scala/org/openapitools/client/model/MixedCaseModel.scala
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/scala-sttp-circe-test/src/main/scala/org/openapitools/client/api/DefaultApi.scala
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/scala-sttp-circe-test/src/main/scala/org/openapitools/client/core/JsonSupport.scala
[main] INFO  o.o.codegen.TemplateManager - writing file /tmp/scala-sttp-circe-test/src/main/scala/org/openapitools/client/core/AdditionalTypeSerializers.scala

Steps to reproduce

1. Save the spec above as mixed-case-fields.yaml
2. Generate:

   openapi-generator-cli generate \
     -i mixed-case-fields.yaml \
     -g scala-sttp \
     -o /tmp/scala-sttp-circe-test \
     --additional-properties=jsonLibrary=circe

3. Inspect the generated MixedCaseModel.scala — there is no companion object with explicit encoder/decoder. The code
   relies on AutoDerivation which uses Scala field names on the wire.
4. Inspect JsonSupport.scala:

   object JsonSupport extends SttpCirceApi with AutoDerivation with DateSerializers with AdditionalTypeSerializers {

   AutoDerivation will ser/deserialize phoneNumber instead of the original phone_number, firstName instead of
   first-name, and zipCode instead of ZipCode.
5. Attempting to compile the generated project fails:

   [error] could not find implicit value for evidence parameter of type io.circe.Encoder[java.io.File]
   [error] could not find implicit value for evidence parameter of type io.circe.Encoder[Any]
   

Actual wire behavior:

{
  "firstName": "John",
  "phoneNumber": "555-1234",
  "zipCode": "90210"
}

Expected wire behavior (matching the spec):

{
  "first-name": "John",
  "phone_number": "555-1234",
  "ZipCode": "90210"
}

Related issues/PRs

• The scala-http4s generator already handles this correctly by generating explicit Encoder/Decoder instances using
  baseName in model.mustache
• The scala-sttp4-jsoniter generator handles this correctly using @named("{{baseName}}") annotations
• The type: object -> Any mapping also differs: scala-http4s maps it to Json (which has built-in circe codecs),
  while scala-sttp maps it to Any (which does not)
• Related issue for inline enum codecs not being generated: [scala-sttp]: fix for missing EnumNameSerializer for inner enum definitions #17697

Suggest a fix

1. model.mustache: For circe ({{#circe}}), generate a companion object with explicit Encoder.instance/
   Decoder.instance per model using {{baseName}} for JSON field names, following the scala-http4s pattern.
2. jsonSupport.mustache: Remove AutoDerivation mixin since explicit instances are now generated per model.
3. additionalTypeSerializers.mustache: Add circe Encoder/Decoder for File (delegating to Array[Byte]
   codec) and Any (delegating to Json codec).
4. ScalaSttpClientCodegen.java: Map type: object to io.circe.Json for circe (and org.json4s.JValue for
   json4s) instead of Any, matching scala-http4s.

I'm planning to submit a pull request with this fix.