"Additional properties false" added to every object in schema in UI when switching to OpenAPI 3.1

Describe the bug
After switching to OpenAPI 3.1 I am seeing "Additional properties false" added to every root/nested object in schema

OpenApi File To Reproduce

{
  "openapi": "3.1.1",
  "info": {
    "title": "API",
    "version": "1"
  },
  "paths": {
    "/": {
      "get": {
        "parameters": [],
        "responses": {
          "200": {
            "description": "OK",
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/BooleanModelResponse"
                }
              }
            }
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "BooleanModel": {
        "type": "object",
        "properties": {
          "showBanner": {
            "type": "boolean"
          }
        },
        "additionalProperties": false
      },
      "BooleanModelResponse": {
        "type": "object",
        "properties": {
          "data": {
            "$ref": "#/components/schemas/BooleanModel"
          }
        },
        "additionalProperties": false
      }
    },
    "securitySchemes": {}
  },
  "security": [],
  "tags": []
}

Expected behavior
Additional properties false property not part of schema in UI

Actual behavior
Additional properties false property part of schema in UI

Additional context
Pasting provided sample swagger.json file into (I believe) official generator yields exact same behavior. ( Changing version to "openapi": "3.0.6" hides Additional proerties forbidden field).

Is there any way to prevent "additionalProperties": false from being added to generated swagger.json?
Or is that the correct and expected behavior?

Linked Issue

I looked through related code and it seems change (if any) should be made here:

OpenAPI.NET/src/Microsoft.OpenApi/Models/OpenApiSchema.cs

Lines 474 to 484 in f9a6796

    
           if (AdditionalPropertiesAllowed) 
        
           { 
        
               writer.WriteOptionalObject( 
        
                   OpenApiConstants.AdditionalProperties, 
        
                   AdditionalProperties, 
        
                   callback); 
        
           } 
        
           else 
        
           { 
        
               writer.WriteProperty(OpenApiConstants.AdditionalProperties, AdditionalPropertiesAllowed); 
        
           }

commenting line 483 fixes my issue.

I believe there should be ability of passing / configuring a parameter so that writing of OpenApiConstants.AdditionalProperties can be skipped.

It seems doing that removes additionalProperties property from generated document:

        builder.Services.AddSwaggerGen(c =>
        {
            c.SchemaFilter<RemoveAdditionalPropertiesSchemaFilter>();
        });

    private class RemoveAdditionalPropertiesSchemaFilter : ISchemaFilter
    {
        public void Apply(IOpenApiSchema schema, SchemaFilterContext context)
        {
            if (schema is OpenApiSchema s)
            {
                s.AdditionalPropertiesAllowed = true;
            }
        }
    }

Produces:

Can devs please comment on that solution?

ntark

Hi ntark
Thank you for using the SDK and for reaching out.

Thank you for your patience.

I have authored the following PRs to address the issue:

fix: additional properties serialization should not emit a schema in v2 fix: additional properties serialization should not emit booleans in v3.1+ #2630
fix/additional properties to v3 #2631

Let us know if you have any additional comments or questions.

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"Additional properties false" added to every object in schema in UI when switching to OpenAPI 3.1 #2619

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	if (AdditionalPropertiesAllowed)
	{
	writer.WriteOptionalObject(
	OpenApiConstants.AdditionalProperties,
	AdditionalProperties,
	callback);
	}
	else
	{
	writer.WriteProperty(OpenApiConstants.AdditionalProperties, AdditionalPropertiesAllowed);
	}

"Additional properties false" added to every object in schema in UI when switching to OpenAPI 3.1 #2619

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ntark commented on Nov 19, 2025

ntark commented on Nov 22, 2025

baywet commented on Dec 8, 2025

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