# How to define custom Mutipart codec?

**URL:** <https://softwaremill.community/t/how-to-define-custom-mutipart-codec/349>\
**Category:** tapir\
**Created:** [January 4, 2024, 11:39am UTC](https://softwaremill.community/t/how-to-define-custom-mutipart-codec/349 "2024-01-04T11:39:25Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![counter2015](https://dub1.discourse-cdn.com/flex005/user_avatar/softwaremill.community/counter2015/32/228_2.png) [@counter2015](https://softwaremill.community/u/counter2015)\
**Post date:** [January 4, 2024, 11:39am UTC](https://softwaremill.community/t/how-to-define-custom-mutipart-codec/349/1 "2024-01-04T11:39:25Z")

</div>

suppose we have such endpoint for uploading file.

```auto

// post /file/<id>
// form format:
// uploader: name text
// file: binary
case class FileForm(uploader: String, file: Part[File])

val uploadEndpoint =
    endpoint
      .post
      .in("file")
      .in(path[String]("id"))
      .in(multipartBody[FileForm])
      .out(stringBody)
      .tag("File")

```

How can I use custom codec `filenameEncoded` like this one ?

```auto
import sttp.model.Part
import sttp.model.Part.FileNameDispositionParam
import sttp.tapir.CodecFormat.OctetStream
import sttp.tapir.macros.MultipartCodecMacros
import sttp.tapir.{Codec, MultipartCodec, PartCodec, RawBodyType}

import java.net.URLEncoder
import java.nio.charset.StandardCharsets
import scala.collection.immutable.ListMap

object CustomMultipartCodec extends MultipartCodecMacros {
  private val arrayBytePartListCodec: Codec[List[Part[Array[Byte]]], List[Part[Array[Byte]]], OctetStream] =
    implicitly[Codec[List[Part[Array[Byte]]], List[Part[Array[Byte]]], OctetStream]]

  def encode(s: String) = URLEncoder.encode(s, StandardCharsets.UTF_8)
  
  val filenameEncoded: MultipartCodec[Seq[Part[Array[Byte]]]] =
    Codec
      .multipart(Map(), Some(PartCodec(RawBodyType.ByteArrayBody, arrayBytePartListCodec)))
      // we know that all parts will end up as byte arrays; also, removing/restoring the by-name grouping of parts
      .map(_.values.toSeq.flatMap(_.asInstanceOf[List[Part[Array[Byte]]]]))(l =>
        ListMap(l.groupBy(_.name).toList.map {
          case (name, parts) =>
            if (name == FileNameDispositionParam) name -> parts.toList.map(_.fileName.map(encode)) else name -> parts.toList
        }: _*)
      )
}

```

I want to encode `filename` part to UTF8 to avoid [the problem](https://github.com/http4s/http4s/issues/5809) which will be trigger when upload file with non-ascii name.

This seems not the right way to use

```auto
      .in(multipartBody[FileForm](filenameEncoded))

```

Is there any way to let the auto generated schema use my custom codec, or is there any way to provide codec explictly ?

---

<div class="post-metadata">

**Author:** ![kciesielski](https://dub1.discourse-cdn.com/flex005/user_avatar/softwaremill.community/kciesielski/32/133_2.png) [@kciesielski](https://softwaremill.community/u/kciesielski)\
**Post date:** [January 5, 2024, 9:49am UTC](https://softwaremill.community/t/how-to-define-custom-mutipart-codec/349/2 "2024-01-05T09:49:38Z")

</div>

It’s impossible on the level of `MultipartCodec`, at this stage it’s already too late.

What Tapir does is (simplified):

1. Tapir first decodes `org.http4s.Media[F]` into `org.http4s.multipart.Multipart[F]` using implicit `org.http4s.EntityDecoder[F, Multipart[F]]`
2. It then converts the `Multipart[F]` into a set of `sttp.model.Part`
3. The codec transforms this set of `Part` into a your case classes.

Plugging a custom codec happens at point 3. I guess the problem happens in step 1, where the `EntityDecoder` reads the `Content-Disposition` header. I don’t see how this could be intercepted. Even if you decided to write your custom `EntityDecoder[F, Multipart[F]]`, there’s no way to make Tapir use it instead of the default one, summoned in Tapir internals from imports (namely in `sttp.tapir.server.http4s.Http4sRequestBody`).