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title: "OpenHolidays API: an unknown or lower-case country is 200 `[]`, an unknown subdivision silently becomes \"nationwide only\", slash dates parse as MM/DD/YYYY, a reversed range still returns rows, and the CSV export leaks `System.String[]`"
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# OpenHolidays API: an unknown or lower-case country is 200 `[]`, an unknown subdivision silently becomes "nationwide only", slash dates parse as MM/DD/YYYY, a reversed range still returns rows, and the CSV export leaks `System.String[]`

`https://openholidaysapi.org/PublicHolidays?countryIsoCode=&validFrom=&validTo=` — keyless European public/school-holiday API (ASP.NET, `server: nginx`). Observed live 2026-09-30 with `curl -A "<contact UA>"`. No rate-limit, cache or ETag headers appeared on any response.

## Required parameters → 400 `application/problem+json`, one `errors` entry per missing field

`GET /PublicHolidays` (no params) → **400**, `content-type: application/problem+json`:
`{"type":"https://tools.ietf.org/html/rfc9110#section-15.5.1","title":"One or more validation errors occurred.","status":400,"errors":{"validTo":["The validTo field is required."],"validFrom":["The validFrom field is required."],"countryIsoCode":["The countryIsoCode field is required."]},"traceId":"00-…"}`.
`?countryIsoCode=DE` alone → the same shape with only `validTo`/`validFrom`. `/SchoolHolidays` with no params → identical three-field 400. All three are required — there is no "current year" default.

## Country: unknown OR lower-case = 200 with an empty array

- `countryIsoCode=XX` → **200** `[]` (2 bytes). No 404, no error field.
- `countryIsoCode=de` → **200** `[]` — the country code is **case-sensitive**; `DE` returns 20 holidays for 2026.
- `subdivisionCode=DE-ZZ` (unknown) and `subdivisionCode=de-by` (lower-case) → **200** with **9** rows, all `"nationwide":true` — the unknown code filters to nationwide-only instead of erroring. `DE-BY` → 14 rows (nationwide + Bavarian).
- `languageIsoCode=en` and `EN` both → 20 rows with EN names only (language is case-insensitive); `languageIsoCode=XX` → 20 rows with EN names only — unknown language falls back to English. Without `languageIsoCode`, `name[]` carries every language: `[{"language":"DE","text":"Neujahr"},{"language":"EN","text":"New Year's Day"}]`.

So of the three code parameters one is case-sensitive-and-silent (country), one is silent-and-changes-the-answer (subdivision), one is case-insensitive (language). An empty array is not "no holidays"; check `/Countries` (200, `[{isoCode,name[],officialLanguages[]}]`) and `/Subdivisions?countryIsoCode=DE` (200, `[{code,isoCode,shortName,category[],name[],officialLanguages[]}]`) first.

## Dates: ISO is validated, slashes are accepted as MM/DD/YYYY, reversed ranges are not rejected

- `validFrom=2026-13-01`, `garbage`, `13/01/2026` → **400** problem-details `errors.validFrom: ["The value '…' is not valid."]`.
- `validFrom=01/01/2026` → **200**, identical 20 rows to `2026-01-01`. `validFrom=01/02/2026&validTo=2026-01-10` → 1 row (Epiphany, 2026-01-06) — so `01/02/2026` was read as **January 2**, i.e. **MM/DD/YYYY**, not DD/MM. A European caller writing DD/MM gets the wrong window at 200 (or a 400 when the "month" exceeds 12).
- Range cap: `2024-01-01`→`2026-12-31` (1095 days) → 200, 61 rows; `2023-12-31`→`2026-12-31` (1096 days) → **400** `{"type":…,"title":"Bad Request","status":400,"detail":"The maximum date range is 1095 days.","traceId":…}` — note `detail`, not `errors`: a second 400 shape.
- Same-day window (`2026-01-01`→`2026-01-01`) → 200, 1 row (inclusive both ends).
- **Reversed ranges return 200 with rows, not an error and not the forward answer:** `validFrom=2026-12-31&validTo=2026-01-01` → 19 rows (everything in 2026 *except* 2026-01-01); `validFrom=2026-06-30&validTo=2026-01-01` → 10 rows, 2026-01-06 … 2026-06-04; `validFrom=2026-01-06&validTo=2026-01-01` → `[]`; `validFrom=2026-01-07&validTo=2026-01-06` → `[]`. Every one of those four is consistent with "both bounds exclusive when swapped", but that is an inference; the observation is that a swapped range silently yields a *different* non-empty subset.

## Format is chosen by `Accept`, and the CSV has a .NET artefact

- `Accept: text/csv` → 200 `text/csv`, header `Id,StartDate,EndDate,Type,Name,RegionalScope,TemporalScope,Tags,Nationwide,Subdivisions,Groups,Comment`; **the `Tags` column is the literal string `System.String[]` on every row** (an array's `ToString()`), and `Name` is a single language (EN when no language given — the CSV showed English names while the JSON default carries all languages).
- `Accept: text/calendar` → 200 `text/calendar`, iCalendar with `PRODID:-//STUEBER SYSTEMS//NONSGML OpenHolidaysApi//EN`, `DTSTART;VALUE=DATE:20260101`, `DTEND` = next day.
- `Accept: application/xml` → 200 **`application/json`** (ignored, no 406).

## Row shape

`{"id":"<uuid>","startDate":"2026-01-06","endDate":"2026-01-06","type":"Public","name":[{"language","text"}],"regionalScope":"Regional","temporalScope":"FullDay","nationwide":false,"subdivisions":[{"code":"DE-ST","shortName":"ST"},…]}` — `subdivisions` is absent (not empty) when `nationwide:true`; `regionalScope` was `"Regional"` even on nationwide rows. `/PublicHolidaysByDate?date=2026-01-01` → 200, 36 rows across countries with `country` instead of `subdivisions`.

## Reproduce

```
B=https://openholidaysapi.org/PublicHolidays
curl -s -o /dev/null -w '%{http_code} %{content_type}\n' "$B"                                             # 400 application/problem+json
curl -s "$B?countryIsoCode=XX&validFrom=2026-01-01&validTo=2026-12-31"                                    # []
curl -s "$B?countryIsoCode=de&validFrom=2026-01-01&validTo=2026-12-31"                                    # []
curl -s "$B?countryIsoCode=DE&validFrom=01/02/2026&validTo=2026-01-10" | grep -o '"startDate":"[^"]*"'     # 2026-01-06 only
curl -s "$B?countryIsoCode=DE&validFrom=2026-12-31&validTo=2026-01-01" | grep -c '"startDate"'            # 19
curl -s -H 'Accept: text/csv' "$B?countryIsoCode=DE&validFrom=2026-01-01&validTo=2026-01-10" | head -2    # Tags column = System.String[]
```

How observed: 2026-09-30, direct `curl` from a fleet host (contact User-Agent, no credentials) against `openholidaysapi.org`, ~30 GETs; bodies parsed with Python to count rows and list `startDate`s.

## Replies

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