JSON to Swift Codable converter

How types are inferred

• Whole numbers become Int; anything with a decimal point or exponent becomes Double, even 1.0. The parser keeps the number as written, so 1.0 is not mistaken for an integer, and integers too large for Int become Double.
• Arrays of objects are merged: every element is read, keys that are missing from some elements or are null in any of them become optionals, and Int mixed with Double becomes Double.
• Objects with exactly the same shape share one type, so an author and an editor with the same keys produce one struct.
• Array element types are named in the singular: posts gives [Post], categories gives [Category].
• Strings that are ISO-8601 date-times become Date, and http or https strings become URL. Turn either off if your API sends odd values in those fields.
• A field that is only ever null, an empty array, or a mix of types gets JSONValue, a small enum included in the output. Replace it once you know the real type.

CodingKeys or convertFromSnakeCase

With CodingKeys every Swift name is camelCase and the enum maps it to the JSON key, for example case createdAt = "created_at". It is explicit, survives renames and works for keys like user-name. With convertFromSnakeCase the decoder converts keys itself and the structs stay short. Watch its rule: it lowercases the first word and capitalises the rest, so user_ID becomes userId, not userID. When a key can't be converted to a valid Swift name, the generator still writes a CodingKeys entry using the converted name, which is what the decoder compares against.

Dates, money and Sendable

• The decoder snippet matches the output. Plain ISO dates use .iso8601. If any sample date has fractional seconds, such as 2026-10-25T06:30:00.123Z, .iso8601 alone fails, so you get a .custom strategy with .withFractionalSeconds.
• Amounts of money arrive as JSON numbers but should not be added up as Double. Change those properties to Decimal; JSONDecoder decodes it from a number.
• Sendable is on by default because value types with let properties qualify. A final class with var properties doesn't, and the generator says so rather than adding @unchecked.

Big files

Responses up to a few megabytes convert as you type. Larger input moves to a Web Worker so the page stays responsive, and this tool's code still doesn't send the JSON anywhere. The sample shapes the types: if a key is missing from every element you pasted, it can't appear in the output, so paste a response that includes the optional fields.