[BUG][scala-sttp][circe] `format: byte` (Base64) properties fail to deserialize - `Decoder[Array[Byte]]` expects JSON array

[nikhilsu]

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?

Description

When a schema property uses type: string, format: byte (Base64-encoded binary data in JSON), the scala-sttp generator with jsonLibrary: circe produces Option[Array[Byte]] for the field type. At runtime, circe's derived Decoder[Array[Byte]] expects a JSON array of numbers ([10, -20, 30, ...]), but the actual wire format for format: byte is a Base64-encoded JSON string (e.g. "rO0ABX..."). Decoding fails with:

DecodingFailure at .fieldName: Array[Byte]

The same issue affects format: binary properties used in JSON object schemas (not multipart bodies): they generate Option[File], whose FileDecoder also relies on Decoder[Array[Byte]] and fails identically.

No Decoder[Array[Byte]] is provided in AdditionalTypeSerializers that handles Base64 strings, leaving the field permanently undecodable regardless of input.

openapi-generator version

Master (6be478dd7d8)

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: Byte Test
  version: 1.0.0
paths: {}
components:
  schemas:
    Payload:
      type: object
      properties:
        checksum:
          type: string
          format: byte

Generation Details

generatorName: scala-sttp
additionalProperties:
  jsonLibrary: circe

Steps to reproduce

1. Generate a client from the spec above with jsonLibrary: circe
2. Call an endpoint that returns {"checksum": "rO0ABXNy..."}
3. Observe DecodingFailure at .checksum: Array[Byte]

Related issues/PRs

• [Fix][scala-sttp][circe] Add NaN-tolerant Decoder[Double] for non-finite values #23611 — added NanTolerantDoubleDecoder for non-finite Double values (same pattern)

Suggest a fix

Add a FlexibleByteArrayDecoder: Decoder[Array[Byte]] to additionalTypeSerializers.mustache that tries a JSON number array first (strict OAS compliance), then falls back to Base64 string decoding (Jackson's actual wire format):

implicit final lazy val FlexibleByteArrayDecoder: Decoder[Array[Byte]] =
  Decoder.decodeArray[Byte].or(Decoder.decodeString.emap { s =>
    try Right(Base64.getDecoder.decode(s))
    catch { case _: IllegalArgumentException => Left(s"Cannot decode '$s' as Base64 Array[Byte]") }
  })

This also fixes FileDecoder for format: binary JSON properties, since it internally calls Decoder[Array[Byte]].