[BUG] [cpp-httplib-server] Primitive and binary response types incorrectly generated as models

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 cpp-httplib-server generator incorrectly generates or references model headers for OpenAPI primitive/built-in types.

In particular, responses using primitive string or binary/file schemas can result in generated code containing invalid model includes such as:

#include "model/String.h"

or:

#include "model/File.h"

These models are not defined in the OpenAPI specification and should not be generated.

For example, a response defined as:

content:
  text/plain:
    schema:
      type: string

should be mapped to the corresponding C++ primitive type, such as std::string, rather than a generated String model.

Similarly, a binary response such as:

content:
  application/octet-stream:
    schema:
      type: string
      format: binary

should be mapped to the generator's appropriate binary/file representation rather than resulting in a reference to a non-existent model/File.h.

This makes the generated server code fail to compile because the referenced model headers do not exist.

openapi-generator version

OpenAPI Generator: 7.25.0

Generator:

cpp-httplib-server

OpenAPI version:

3.1.0

OpenAPI declaration file content or url

Minimal reproducible specification:

openapi: 3.1.0

info:
  title: Primitive Response Test
  version: 1.0.0

paths:

  /text:
    get:
      operationId: getText
      responses:
        '200':
          description: Successful response
          content:
            text/plain:
              schema:
                type: string

  /file:
    get:
      operationId: getFile
      responses:
        '200':
          description: Binary response
          content:
            application/octet-stream:
              schema:
                type: string
                format: binary

Generation Details

Generator:

cpp-httplib-server

Example CLI command:

docker run --rm \
    --user "$(id -u):$(id -g)" \
    -v "$SCRIPT_DIR:/local" \
    openapitools/openapi-generator-cli:v7.25.0 \
    generate \
    -i /local/v1.0/vsat_endpoint_v1.json \
    -g cpp-httplib-server \
    -o /local/generated/generated_api_server_cpp_httplib 

Steps to reproduce

1. Create the openapi.yaml shown above.
2. Generate the server using:

    --user "$(id -u):$(id -g)" \
    -v "$SCRIPT_DIR:/local" \
    openapitools/openapi-generator-cli:v7.25.0 \
    generate \
    -i /local/v1.0/vsat_endpoint_v1.json \
    -g cpp-httplib-server \
    -o /local/generated/generated_api_server_cpp_httplib 

3. Inspect the generated C++ source code.
4. The generated code contains references to model headers for primitive/built-in types, for example:

#include "model/String.h"

and:

#include "model/File.h"

5. These headers are not present in the generated project.
6. Compilation therefore fails because the generated code references non-existent model classes.

Actual output vs expected output

Actual:

The generator generates/references model classes for primitive or built-in OpenAPI types.

Examples include:

#include "model/String.h"

and:

#include "model/File.h"

However, the OpenAPI specification does not define String or File as schemas.

As a result, the generated C++ server code contains references to non-existent headers/classes and does not compile.

Expected:

Primitive and built-in OpenAPI types should be mapped to their corresponding C++ representations without generating model classes.

For example:

schema:
  type: string

should resolve to something equivalent to:

std::string

and:

schema:
  type: string
  format: binary

should resolve to the generator's appropriate binary/file representation without creating or referencing:

model/File.h

No String.h or File.h model should be generated or referenced unless the user explicitly defines corresponding schemas in the OpenAPI document.

Related issues/PRs

I have not yet identified a related issue or PR specifically covering primitive string and binary/file response schemas in the cpp-httplib-server generator.

Suggest a fix

The generator should correctly distinguish between:

1. Primitive OpenAPI types (string, integer, number, boolean, etc.)
2. OpenAPI formatted primitive types such as string/binary
3. User-defined schemas/models

Primitive types should be resolved directly to their corresponding C++ types rather than being passed through the model generation mechanism.

In particular:

type: string

should resolve to the generator's C++ string type, and:

type: string
format: binary

should resolve to the appropriate binary representation.

The response type resolution and model/include generation logic should avoid producing model references such as:

#include "model/String.h"
#include "model/File.h"

when the corresponding types originate from primitive OpenAPI schemas rather than user-defined models.

Start by reproducing the issue with the minimal OpenAPI 3.1.0 specification and the cpp-httplib-server generator command described in the report. Trace the response type resolution and model/include generation logic for primitive string and binary schemas. Done means generated server code uses appropriate C++ representations and no longer references nonexistent String.h or File.h models.