After all the attempts to fix OpenAPI document, I noticed that Orval intepreted one of the property as unknown type which is supposed to be integer, thus number type. I checked the OpenAPI document and it has the following type and format:

type: "^-?(?:0|[1-9]\d*)$",
format: "int32"

For Orval to intepret it properly as number, I need it to be:

type: "integer",
format: "int32"

So, I check the backend and found out that it is due to JsonSerializeOptions.NumberHandling is set to AllowReadingFromString by default. https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/configure-options#web-defaults-for-jsonserializeroptions

I want to set NumberHandling to Strict globally. Since I use minimal api, this article helps https://learn.microsoft.com/en-us/aspnet/core/fundamentals/minimal-apis/responses?view=aspnetcore-10.0#configure-json-serialization-options-globally. I just need to add the following on my backend code:

builder.Services.ConfigureHttpJsonOptions(options =>
{
    options.SerializerOptions.TypeInfoResolverChain.Insert(0, HttpApiJsonSerializerContext.Default);
});