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[BUG][KOTLIN] multiplatform/jvm-ktor: `DELETE` without `requestBody` sends `"Content-Type: application/json"` with an empty body

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@mohitduhan19 is already working on this.

Since Oct 13, 2026.

  • #25243 by @mohitduhan19 — open

Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
78/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
kotlin
Domain
backend

Research direction

Read modules/openapi-generator/src/main/resources/kotlin-client/libraries/multiplatform/infrastructure/ApiClient.kt.mustache and the corresponding libraries/jvm-ktor/infrastructure/ApiClient.kt.mustache; inspect the request function’s handling of EmptyContent and the methods that set content type and body. Check existing template tests for these libraries, then verify that a generated DELETE without requestBody omits the content type and body while one with a JSON body still sends them.

Written by the indexing model from the issue text.

Description

Issue: Bug
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

For operations without a requestBody, the kotlin client with library=multiplatform generates localVariableBody = io.ktor.client.utils.EmptyContent. ApiClient.request then sets a JSON content type and the body for every PUT, POST, PATCH, and DELETE, regardless of whether there is a body:

if (requestConfig.method in listOf(RequestMethod.PUT, RequestMethod.POST, RequestMethod.PATCH, RequestMethod.DELETE)) {
    val contentType = (requestConfig.headers[HttpHeaders.ContentType]?.let { ContentType.parse(it) }
        ?: ContentType.Application.Json)
    this.contentType(contentType)
    this.setBody(body)
}

As a result, a DELETE /items/{id}?row=1 without a request body goes out with Content-Type: application/json and Content-Length: 0. Servers that bind a JSON body whenever the content type says JSON reject the request. ASP.NET Core with FastEndpoints answers:

{"statusCode":400,"message":"One or more errors occurred!","errors":{"/SerializerErrors":["The input does not contain any JSON tokens. Expected the input to start with a valid JSON token, when isFinalBlock is true. LineNumber: 0 | BytePositionInLine: 0."]}}

The same DELETE succeeds when the client omits the Content-Type header. DELETE operations that do declare a JSON requestBody work as expected.

The same block exists in libraries/jvm-ktor/infrastructure/ApiClient.kt.mustache.

openapi-generator version

7.25.0 (Gradle plugin org.openapi.generator). The same code is in the 7.26.0 templates and on master (modules/openapi-generator/src/main/resources/kotlin-client/libraries/multiplatform/infrastructure/ApiClient.kt.mustache, the request function). Not a regression as far as I can tell.

OpenAPI declaration file content or url
openapi: 3.0.0
info:
  title: Delete without body
  version: 1.0.0
paths:
  /items/{id}:
    delete:
      operationId: deleteItem
      parameters:
        - name: id
          in: path
          required: true
          schema:
            type: string
        - name: rowVersion
          in: query
          required: true
          schema:
            type: integer
      responses:
        '204':
          description: Deleted
Generation Details
generatorName: kotlin
library: multiplatform
additionalProperties:
  dateLibrary: kotlinx-datetime
Steps to reproduce
  1. Generate the client from the spec above with -g kotlin --library multiplatform.
  2. Call deleteItem("1", 1) against any server, or a request logger.
  3. Observe the request headers: Content-Type: application/json, Content-Length: 0.

Actual: a JSON content type is sent although the operation has no request body.
Expected: no Content-Type header (and no body) when the operation declares no requestBody.

Related issues/PRs

None found (searched for "multiplatform DELETE content-type empty body" and "EmptyContent Content-Type").

Suggest a fix

Only set the content type and body when there is a body, for example:

if (requestConfig.method in listOf(RequestMethod.PUT, RequestMethod.POST, RequestMethod.PATCH, RequestMethod.DELETE) &&
    body != null && body !is io.ktor.client.utils.EmptyContent
) {
    val contentType = (requestConfig.headers[HttpHeaders.ContentType]?.let { ContentType.parse(it) }
        ?: ContentType.Application.Json)
    this.contentType(contentType)
    this.setBody(body)
}

The same change applies to the jvm-ktor template.

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