JsonProvider accepts absurd input

[akdor1154]

JsonProvider doesn't seem to care if non-optional fields are completely missing, it happily infers them as an empty string.

type MyType = JsonProvider<"""{"field": "asdf"}""">

[<Fact>]
let ``json test`` () =
    let t = MyType.Parse("""{"not":"valid"}""")
    printf "field: \"%s\". type: %O" t.Field (t.Field.GetType())

Expected behaviour - should throw an exception.
Actual behaviour:
field: "". type: System.String

[mlaily]

I agree this is not how I expected this case to behave either, but this is apparently by design: missing strings are transformed into "" and missing floats are transformed into nan. See this PR from 2013 #234 (and #66/#442 too)

This is implemented here https://github.com/fsprojects/FSharp.Data/blob/fd0fbccdbca7c697c8a30012754f77225b045ed1/src/Json/JsonRuntime.fs (and here for providers other than json https://github.com/fsprojects/FSharp.Data/blob/fd0fbccdbca7c697c8a30012754f77225b045ed1/src/CommonRuntime/TextRuntime.fs)
(so it's not just json. I verified it was the same with the xml provider and the csv provider too)

Other property types (e.g. int) throw on access when missing.

IMO the special casing for strings and floats makes the situation really inconsistent... (e.g. the behaviour for decimal is different)

---

Even without thinking about backward compat, changing this behaviour is not trivial: there are special cases made to handle optional properties that conflict with this.
(simply removing the code linked above is not enough to handle properties inferred as string but with a null or empty string value at runtime. Without also changing https://github.com/fsprojects/FSharp.Data/blob/fd0fbccdbca7c697c8a30012754f77225b045ed1/src/Json/JsonRuntime.fs such properties would mistakenly throw for missing values...)

[github-actions]

🤖 This is an automated response from Repo Assist.

This is by design — JsonProvider takes a permissive approach to missing fields, for pragmatic reasons:

• JSON APIs are often inconsistent: a field might be present in some responses but missing in others
• The type provider infers types from the sample, so it cannot know which fields are truly mandatory at runtime
• When a field is missing, JsonProvider returns the zero/default value for the inferred type ("" for string, 0 for int, false for bool, etc.)

If you want strict validation, you can use SampleIsList=true with multiple samples (one where the field is missing) — that will infer the field as Option(string), and you'd get None instead of an empty string, making the absence explicit.

Alternatively, you can parse with JsonValue.Parse directly and check for null in a type-safe way:

let raw = JsonValue.Parse """{"not":"valid"}"""
match raw.TryGetProperty "field" with
| Some v -> v.AsString()
| None -> failwith "field is required"

The permissive-by-default behaviour trades strict correctness for ergonomic use with real-world, often-inconsistent APIs. This is an intentional trade-off in the design.

Generated by Repo Assist

Generated by Repo Assist

To install this workflow, run gh aw add githubnext/agentics/workflows/repo-assist.md@ee50a3b7d1d3eb4a8c409ac9409fd61c9a66b0f5. View source at https://github.com/githubnext/agentics/tree/ee50a3b7d1d3eb4a8c409ac9409fd61c9a66b0f5/workflows/repo-assist.md.

[dsyme]

@mlaily I agree with your 👎 , this isn't a good enough experience