PascalCaseJSON.swift

import Foundation

let json = """
{
    "Title": "example",
    "Items": [
      "hello",
      "world"
    ]
}
""".data(using: .utf8)!

struct Model: Codable {
    let title: String
    let items: [String]
}

private extension String {
    func firstCharLowercased() -> String {
        prefix(1).lowercased() + dropFirst()
    }
}

extension JSONDecoder.KeyDecodingStrategy {
    static var convertFromPascalCase: JSONDecoder.KeyDecodingStrategy {
        return .custom { keys -> CodingKey in
            // keys array is never empty
            let key = keys.last!
            // Do not change the key for an array
            guard key.intValue == nil else {
                return key
            }

            let codingKeyType = type(of: key)
            let newStringValue = key.stringValue.firstCharLowercased()

            return codingKeyType.init(stringValue: newStringValue)!
        }
    }
}

let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromPascalCase
let model = try! decoder.decode(Model.self, from: json)

print(model)