[BUG][Go] JSON-content header parameter is expanded into bracketed headers instead of JSON #25084

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?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The Go client does not JSON-serialize a header parameter declared with content: application/json. A generated object model implements MappedNullable, so the generic header formatter expands its fields into bracketed header names. With path: /x, the request contains X-Json-Arg[path]: /x rather than X-Json-Arg: {"path":"/x"}.

openapi-generator version

Reproduced with a local 7.26.0-SNAPSHOT CLI. Whether this is a regression is unknown. The spec below passes openapi-generator-cli validate with no validation issues.

OpenAPI declaration file content or url

openapi: 3.1.0
info:
  title: JSON header test
  version: 1.0.0
paths:
  /test:
    post:
      operationId: testJsonHeader
      parameters:
        - name: X-Json-Arg
          in: header
          required: true
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/HeaderArg'
        - name: X-Plain-Arg
          in: header
          schema:
            type: string
      responses:
        '204':
          description: No content
components:
  schemas:
    HeaderArg:
      type: object
      properties:
        path:
          type: string

Generation Details

Generator: go, with default options.

openapi-generator-cli validate -i json-header-content.yaml
openapi-generator-cli generate -i json-header-content.yaml -g go -o /tmp/go-json-header

Steps to reproduce

1. Save the OpenAPI definition above as json-header-content.yaml.

2. Run the validation and generation commands above.

3. Inspect /tmp/go-json-header/api_default.go. It sends the JSON-content header through the generic formatter:

   parameterAddToHeaderOrQuery(localVarHeaderParams, "X-Json-Arg", r.xJsonArg, "", "")

4. Inspect /tmp/go-json-header/client.go. The formatter calls ToMap() for the generated HeaderArg and recursively expands the map into bracketed header names. In a generated Go client with HeaderArg.Path set to /x, the outgoing request has:

   X-Json-Arg[path]: /x
   

   There is no X-Json-Arg header. This was verified with a local http.RoundTripper that captured the generated request headers; no server was needed.

Expected behavior

The outgoing request should contain one header:

X-Json-Arg: {"path":"/x"}

The ordinary X-Plain-Arg header should keep its current string formatting.

Related issues/PRs

This is the Go counterpart of the Java issue/PR #25055 / #25056 and Python issue/PR #25067 / #25068.

Suggest a fix

Use CodegenParameter.contentType to identify JSON-content header parameters. Marshal those values with encoding/json before adding them to the header, return marshaling errors, and retain the existing formatter for ordinary headers. Emit the JSON header helper only when needed.