Toolpaper

Send multipart form-data with cURL in Kotlin

Paste a curl command that sends multipart form-data and get idiomatic Kotlin code. We pick the right serialization for OkHttp and set the correct Content-Type.

Quick answer

To convert a cURL command to Kotlin, paste it into the Toolpaper workbench below. It parses the command in your browser and emits idiomatic Kotlin code — headers, query string, request body, authentication (Bearer, Basic, API key), and cookies are all wired up automatically. No data is uploaded; conversion is fully client-side.

Prefer a different target? See the full cURL to Kotlin reference, or jump to the same guide in JavaScript, Node.js, or Python.

InputcURL command
MethodPOST·Headers0·Body—·Form2 fields· Chars 100
OutputJVM · OkHttp / Kotlin(idiomatic)
import okhttp3.*
import okhttp3.MediaType.Companion.toMediaTypeOrNull
import okhttp3.RequestBody.Companion.toRequestBody
import java.io.File
import okhttp3.RequestBody.Companion.asRequestBody

val client = OkHttpClient.Builder().followRedirects(false).followSslRedirects(false).build()
val body = MultipartBody.Builder().setType(MultipartBody.FORM)
  .addFormDataPart("file", "/path/to/photo.png", File("/path/to/photo.png").asRequestBody("application/octet-stream".toMediaTypeOrNull()))
  .addFormDataPart("caption", "Hello")
  .build()
val request = Request.Builder()
  .url("https://api.example.com/upload")
  .method("POST", body)
  .build()

client.newCall(request).execute().use { response ->
  println(response.body?.string())
}
Inspect

Example

A representative curl command for this scenario. Paste your own above to convert an exact match.

curl input
curl -X POST https://api.example.com/upload \
  -F "file=@/path/to/photo.png" \
  -F "caption=Hello"

Implementation notes

  • Content-Type is detected from -H headers first, then inferred from the body shape when the header is missing.
  • Each -F flag becomes a distinct part; file@ paths are surfaced as OkHttp file handles so the multipart boundary stays valid.
  • Charset directives on Content-Type (e.g. application/json; charset=utf-8) survive the round trip.

Common mistakes

  • Setting Content-Type manually and then also using OkHttp's helper — you end up with duplicate headers or the wrong one wins.
  • Forgetting @ in -F "file=@upload.png" — without it curl sends the literal string, not the file. The generator flags this.
  • Trusting the browser DevTools "Copy as cURL" to preserve charset — it often drops it. Clean panel restores common defaults.

Common questions

How is multipart form-data encoded in the generated Kotlin?
-F flags translate to multipart form-data with proper boundary handling — files={...} in Python, MultipartFormDataContent in C#, MultipartBody.Part in Java/Kotlin.
Does it preserve Content-Type?
Yes. The Content-Type header is honored and matched against the body so serialization stays consistent.
Is anything uploaded?
No. Parsing and generation happen entirely in your browser.

More payload types in Kotlin

Kotlin hub →

JSON in Kotlin

/how-to/send-json-with-curl-in-kotlin

form-urlencoded in Kotlin

/how-to/send-form-with-curl-in-kotlin

XML in Kotlin

/how-to/send-xml-with-curl-in-kotlin

GraphQL in Kotlin

/how-to/send-graphql-with-curl-in-kotlin

The same guide in other languages

All guides →

In JavaScript

/how-to/send-multipart-with-curl-in-javascript

In Node.js

/how-to/send-multipart-with-curl-in-node

In Python

/how-to/send-multipart-with-curl-in-python

In Go

/how-to/send-multipart-with-curl-in-go

In PHP

/how-to/send-multipart-with-curl-in-php

In Ruby

/how-to/send-multipart-with-curl-in-ruby

In Rust

/how-to/send-multipart-with-curl-in-rust

In Java

/how-to/send-multipart-with-curl-in-java

Full Kotlin reference on the cURL to Kotlin hub, or open the main converter for all 18 targets.