[BUG][TypeScript] JSON-content header parameter is not serialized as JSON

[AndreyVMarkelov]

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

TypeScript clients do not correctly serialize a header parameter declared with Parameter.content: application/json. The required representation is one header containing JSON, using the model’s OpenAPI wire property names:

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

typescript-fetch: the generated header assignment uses the ordinary string path:

headerParameters['X-Json-Arg'] = String(requestParameters['xJsonArg']);

For an object this produces [object Object]. Even direct JSON.stringify of a generated model with filePath: "/x" would produce {"filePath":"/x"}; the generator’s existing model-to-wire conversion is needed to produce file_path.

typescript-node: the generated assignment passes the serialized model object as the header value without converting it to a JSON string:

localVarHeaderParams['X-Json-Arg'] = ObjectSerializer.serialize(xJsonArg, "HeaderArg");

The same problem applies to a present optional JSON-content header and to application/vnd.example+json. An ordinary schema-based string header should retain its current behavior.

typescript-axios was also checked. Its generated model uses the wire member name file_path, and its header path already uses JSON.stringify; this reproduction did not show the bug there.

openapi-generator version

7.26.0-SNAPSHOT, reproduced from master commit 271e4dd6e6d03ee456d8acef6542617371ef827c.

OpenAPI declaration file content or url

openapi: 3.1.0
info:
  title: JSON header serialization
  version: 1.0.0
paths:
  /json:
    get:
      operationId: getJson
      parameters:
        - name: X-Json-Arg
          in: header
          required: true
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/HeaderArg'
        - name: X-Optional-Json-Arg
          in: header
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/HeaderArg'
        - name: X-Plain-Arg
          in: header
          required: true
          schema:
            type: string
      responses:
        '204':
          description: No content
  /vendor:
    get:
      operationId: getVendor
      parameters:
        - name: X-Vendor-Arg
          in: header
          required: true
          content:
            application/vnd.example+json:
              schema:
                $ref: '#/components/schemas/HeaderArg'
      responses:
        '204':
          description: No content
components:
  schemas:
    HeaderArg:
      type: object
      properties:
        file_path:
          type: string

Generation Details

Generate clients from the YAML above with the default options for typescript-fetch and typescript-node, for example:

openapi-generator-cli generate -i json-header.yaml -g typescript-fetch -o /tmp/json-header-fetch
openapi-generator-cli generate -i json-header.yaml -g typescript-node -o /tmp/json-header-node

Steps to reproduce

1. Generate either client from the specification above.
2. Call getJson with X-Json-Arg represented by a generated model value equivalent to { filePath: "/x" } and X-Plain-Arg set to "plain". Optionally provide the same model value for X-Optional-Json-Arg.
3. Inspect the outgoing headers or the generated assignments shown above. Also call getVendor with the same model value.

Expected: exactly one X-Json-Arg header with the value {"file_path":"/x"}. The present optional and vendor JSON headers should have the same JSON value under their declared names. X-Plain-Arg should remain plain.

Actual: typescript-fetch stringifies the object as [object Object]; typescript-node assigns a serialized object as the header value rather than a JSON string.

Related issues/PRs

The same OpenAPI parameter serialization problem has been reported for other client generators:

• Java: #25055 / #25056
• Python: #25067 / #25068
• Go: #25084 / #25085

Suggest a fix

Use the common CodegenParameter.headerIsJsonMimeType metadata populated by DefaultCodegen to identify JSON-content headers, including application/json and JSON structured-suffix media types such as application/vnd.example+json.

For affected TypeScript clients, serialize using their existing model-to-wire paths before forming the JSON header string: typescript-fetch should apply its model ToJSON conversion before JSON.stringify; typescript-node should apply ObjectSerializer.serialize before JSON.stringify. Preserve ordinary schema-based header handling.