fix(indexes)!: keep IndexConstituent.founded verbatim as a string

crates/fmp-py/python/fmp/_native/indexes/__init__.pyi

@@ -93,7 +93,7 @@ class IndexConstituent:
     @property
     def cik(self) -> builtins.str: ...
     @property
-    def founded(self) -> datetime.date: ...
+    def founded(self) -> builtins.str: ...
     def __eq__(self, other: builtins.object, /) -> builtins.bool: ...
     def __new__(
         cls,
@@ -105,7 +105,7 @@ class IndexConstituent:
         head_quarter: builtins.str,
         date_first_added: typing.Optional[datetime.date],
         cik: builtins.str,
-        founded: datetime.date,
+        founded: builtins.str,
     ) -> IndexConstituent: ...
     def __getnewargs_ex__(self) -> tuple[tuple, dict]: ...
     def __repr__(self) -> builtins.str: ...

crates/fmp-py/src/models/indexes.rs

@@ -130,7 +130,7 @@ pub(crate) struct IndexConstituent {
     #[pyo3(get)]
     pub cik: String,
     #[pyo3(get)]
-    pub founded: ::chrono::NaiveDate,
+    pub founded: String,
 }
 
 #[gen_stub_pymethods]
@@ -148,7 +148,7 @@ impl IndexConstituent {
         head_quarter: String,
         date_first_added: Option<::chrono::NaiveDate>,
         cik: String,
-        founded: ::chrono::NaiveDate,
+        founded: String,
     ) -> Self {
         Self {
             symbol,
@@ -279,7 +279,7 @@ impl From<libfmp::responses::indexes::IndexConstituent> for IndexConstituent {
             head_quarter: value.head_quarter,
             date_first_added: value.date_first_added.map(|value| value.into_inner()),
             cik: value.cik.into_inner(),
-            founded: value.founded.into_inner(),
+            founded: value.founded,
         }
     }
 }

crates/fmp-py/tests/test_indexes_b.py

@@ -23,7 +23,7 @@
             "/sp500-constituent",
             "indexes_sp500_constituents.json",
             "HONA",
-            datetime.date(1913, 12, 31),
+            "1902/1985",
             datetime.date(2026, 6, 29),
             id="sp500",
         ),
@@ -32,7 +32,7 @@
             "/nasdaq-constituent",
             "indexes_nasdaq_constituents.json",
             "ADBE",
-            datetime.date(1982, 12, 1),
+            "1982-12-01",
             None,
             id="nasdaq",
         ),
@@ -41,7 +41,7 @@
             "/dowjones-constituent",
             "indexes_dow_jones_constituents.json",
             "GOOGL",
-            datetime.date(1998, 9, 4),
+            "1998-09-04",
             datetime.date(2026, 6, 29),
             id="dow-jones",
         ),
@@ -54,7 +54,7 @@ def test_current_constituents_take_no_arguments(
     path: str,
     fixture: str,
     symbol: str,
-    founded: datetime.date,
+    founded: str,
     date_first_added: datetime.date | None,
 ) -> None:
     """Each current-constituent method hits its own path with no query and decodes the date fields."""
@@ -72,6 +72,18 @@ def test_current_constituents_take_no_arguments(
     assert row.head_quarter
 
 
+@pytest.mark.parametrize("founded", ["1994", "1902/1985", "1902/1985/2001"])
+def test_constituent_founded_keeps_the_year_forms_verbatim(
+    client: Any, fixture_server: FixtureServer, founded: str
+) -> None:
+    """``founded`` is the provider's founding year or years, returned as the exact string."""
+    body = load_fixture("indexes_sp500_constituents.json")
+    body[0]["founded"] = founded
+    fixture_server.route("/sp500-constituent", body)
+
+    assert client.indexes.sp500_constituents()[0].founded == founded
+
+
 @pytest.mark.parametrize(
     ("method", "path", "fixture", "symbol", "removed_ticker", "date"),
     [

crates/fmp-py/tests/typecheck_contract.py

@@ -238,7 +238,7 @@ def check_indexes_contract(client: FmpClient) -> None:
     bar_time: datetime.datetime = intraday[0].date
     members: list[IndexConstituent] = client.indexes.sp500_constituents()
     first_added: datetime.date | None = members[0].date_first_added
-    founded: datetime.date = members[0].founded
+    founded: str = members[0].founded
     _ = (currency, bar_date, vwap, bar_time, first_added, founded)
 
 

crates/libfmp/src/responses/indexes.rs

@@ -42,7 +42,10 @@ pub struct IndexConstituent {
     #[serde(deserialize_with = "required_option")]
     pub date_first_added: Option<Date>,
     pub cik: Cik,
-    pub founded: Date,
+    /// Founding year or years as the provider sends them, kept verbatim: a
+    /// year (`"1994"`), several years joined by `/` (`"1902/1985"`), or a full
+    /// date (`"1982-12-01"`) on some indexes. It is not a date.
+    pub founded: String,
 }
 
 /// One documented historical addition to and removal from a major US index.

crates/libfmp/tests/fixtures/indexes_sp500_constituents.json

@@ -2,7 +2,7 @@
   {
     "cik": "0002089271",
     "dateFirstAdded": "2026-06-29",
-    "founded": "1913-12-31",
+    "founded": "1902/1985",
     "headQuarter": "Charlotte, North Carolina",
     "name": "Honeywell Aerospace Inc",
     "sector": "Technology",

crates/libfmp/tests/indexes_constituent_responses.rs

@@ -29,9 +29,11 @@ fn all_six_documented_constituent_fixtures_match_the_typed_wire_contract_exactly
         current[0].date_first_added.unwrap().to_string(),
         "2026-06-29"
     );
+    assert_eq!(current[0].founded, "1902/1985");
 
     let nullable: Vec<IndexConstituent> = serde_json::from_slice(NASDAQ).unwrap();
     assert_eq!(nullable[0].cik.as_str(), "0000796343");
+    assert_eq!(nullable[0].founded, "1982-12-01");
     assert_eq!(nullable[0].date_first_added, None);
 
     let history: Vec<HistoricalIndexConstituent> =
@@ -98,6 +100,18 @@ fn current_constituent_fields_are_required_and_nullable_date_stays_required() {
     assert_eq!(decoded.symbol.as_str(), "ADBE");
 }
 
+#[test]
+fn founded_keeps_year_multi_year_and_date_forms_verbatim() {
+    let source: Value = serde_json::from_slice(NASDAQ).unwrap();
+    for founded in ["1994", "1902/1985", "1902/1985/2001", "1982-12-01", ""] {
+        let mut row = source[0].clone();
+        row["founded"] = json!(founded);
+        let decoded: IndexConstituent = serde_json::from_value(row.clone()).unwrap();
+        assert_eq!(decoded.founded, founded);
+        assert_eq!(serde_json::to_value(decoded).unwrap(), row);
+    }
+}
+
 #[test]
 fn historical_fields_are_required_while_omittable_members_accept_null_or_empty() {
     let source: Value = serde_json::from_slice(HISTORICAL_NASDAQ).unwrap();

sdk/go/indexes_parity_test.go

@@ -108,7 +108,7 @@ func TestDocumentedIndexConstituentFixturesDecodeExactValues(t *testing.T) {
 	t.Parallel()
 	sp500 := assertFixtureParity[IndexConstituent](t, "indexes_sp500_constituents.json")
 	if len(sp500) != 1 || sp500[0].Symbol != "HONA" || sp500[0].CIK != "0002089271" ||
-		sp500[0].SubSector != "Aerospace & Defense" || sp500[0].Founded != mustParseDate(t, "1913-12-31") {
+		sp500[0].SubSector != "Aerospace & Defense" || sp500[0].Founded != "1902/1985" {
 		t.Fatalf("indexes_sp500_constituents = %+v", sp500)
 	}
 	if got := sp500[0].DateFirstAdded; got == nil || got.String() != "2026-06-29" {
@@ -117,7 +117,7 @@ func TestDocumentedIndexConstituentFixturesDecodeExactValues(t *testing.T) {
 
 	nasdaq := assertFixtureParity[IndexConstituent](t, "indexes_nasdaq_constituents.json")
 	if len(nasdaq) != 1 || nasdaq[0].Symbol != "ADBE" || nasdaq[0].CIK != "0000796343" ||
-		nasdaq[0].HeadQuarter != "San Jose, CA" {
+		nasdaq[0].HeadQuarter != "San Jose, CA" || nasdaq[0].Founded != "1982-12-01" {
 		t.Fatalf("indexes_nasdaq_constituents = %+v", nasdaq)
 	}
 	if nasdaq[0].DateFirstAdded != nil {
@@ -172,6 +172,12 @@ func TestIndexesRequiredMembersAreEnforcedLikeSerde(t *testing.T) {
 		if err := json.Unmarshal([]byte(wire), &rows); err != nil || len(rows) != 1 || rows[0].DateFirstAdded != nil {
 			t.Fatalf("null dateFirstAdded with unknown member: %+v, %v", rows, err)
 		}
+		for _, founded := range []string{"1994", "1902/1985", "1902/1985/2001"} {
+			wire = `[{"dateFirstAdded":null,` + strings.Replace(constituent, "1982-12-01", founded, 1) + `}]`
+			if err := json.Unmarshal([]byte(wire), &rows); err != nil || len(rows) != 1 || rows[0].Founded != founded {
+				t.Fatalf("founded %q: %+v, %v", founded, rows, err)
+			}
+		}
 		var changes []HistoricalIndexConstituent
 		wire = `[{"removedSecurity":null,"removedTicker":null,` + history + `,"futureField":[1,2,3]}]`
 		if err := json.Unmarshal([]byte(wire), &changes); err != nil || len(changes) != 1 ||