diff --git a/.changes/unreleased/fixed-20260702-235813.yaml b/.changes/unreleased/fixed-20260702-235813.yaml
new file mode 100644
index 0000000..739fbb6
--- /dev/null
+++ b/.changes/unreleased/fixed-20260702-235813.yaml
@@ -0,0 +1,3 @@
+kind: Fixed
+body: '64-bit integer JSON helpers now reject unquoted decimal/exponent numbers at or above 2^52 in magnitude. serde_json''s default float parsing is not correctly rounded, and from 2^52 a one-ulp parse error is a whole integer — an unquoted integer token there could silently decode to the adjacent `i64`/`u64`. Below 2^52 the same error breaks integrality and is rejected loudly by the existing exactness check. Quote large integer values to parse them exactly. (#274)'
+time: 2026-07-02T23:58:13.000000000Z
diff --git a/buffa/src/json_helpers.rs b/buffa/src/json_helpers.rs
index e868e8f..bb0a6e8 100644
--- a/buffa/src/json_helpers.rs
+++ b/buffa/src/json_helpers.rs
@@ -908,6 +908,21 @@ fn is_exact_integer(f: f64) -> bool {
     f.is_finite() && f == (f as i128 as f64)
 }
 
+/// Largest integer-valued `f64` this visitor accepts from an unquoted JSON
+/// decimal/exponent token: 2^52 - 1.
+///
+/// The hazard is not only f64 representability (integers up to 2^53 are
+/// exact) but `serde_json`'s *default* float parsing, which is not
+/// correctly rounded (that is its opt-in `float_roundtrip` feature) and can
+/// be off by an ulp. In `[2^52, 2^53)` an ulp is exactly 1, so a one-ulp
+/// parse error silently turns an integer token into the *adjacent* integer
+/// — observed: `"9007199254740991.0"` parsing as `…990.0`. Below 2^52 an
+/// ulp is at most 0.5, so the same parse error breaks integrality instead
+/// and the exactness check below rejects the token loudly rather than
+/// mis-decoding it. Quote larger integer values to parse them exactly —
+/// the quoted-string path parses the token text digit-by-digit.
+const MAX_SAFE_JSON_INTEGER_F64: f64 = 4_503_599_627_370_495.0;
+
 /// Try to parse a string as an integer, handling float notation like `"1.0"`,
 /// `"1e5"`, `"1.0e2"`.  Returns `None` if the value is not an exact integer.
 fn parse_int_from_str<I: TryFrom<i128>>(v: &str) -> Option<I> {
@@ -981,8 +996,13 @@ fn parse_exact_decimal_int(v: &str) -> Option<i128> {
     }
 }
 
-/// Try to interpret an f64 as an exact integer.
+/// Try to interpret an f64 as an exact integer, rejecting magnitudes above
+/// [`MAX_SAFE_JSON_INTEGER_F64`] where the value no longer uniquely
+/// identifies the JSON token it was parsed from.
 fn f64_to_int<I: TryFrom<i128>>(v: f64) -> Option<I> {
+    if !(-MAX_SAFE_JSON_INTEGER_F64..=MAX_SAFE_JSON_INTEGER_F64).contains(&v) {
+        return None;
+    }
     if !is_exact_integer(v) {
         return None;
     }
diff --git a/buffa/src/json_helpers/tests.rs b/buffa/src/json_helpers/tests.rs
index d9fc4e3..6588baa 100644
--- a/buffa/src/json_helpers/tests.rs
+++ b/buffa/src/json_helpers/tests.rs
@@ -266,6 +266,45 @@ fn int64_deserializes_exact_negative_float_notation_string() {
     assert_eq!(val.0, -9007199254740993i64);
 }
 
+#[test]
+fn int64_deserializes_safe_unquoted_float_notation() {
+    let cases: &[(&str, i64)] = &[
+        ("4503599627370495.0", 4503599627370495),
+        ("4.503599627370495e15", 4503599627370495),
+        ("-4503599627370495.0", -4503599627370495),
+        ("-4.503599627370495e15", -4503599627370495),
+    ];
+    for &(json, expected) in cases {
+        let val: SerdeInt64 = serde_json::from_str(json).unwrap();
+        assert_eq!(val.0, expected, "input: {json}");
+    }
+}
+
+#[test]
+fn int64_rejects_unsafe_unquoted_float_notation() {
+    // From 2^52 an ulp is a whole integer, so serde_json's default
+    // (not-correctly-rounded) float parse can silently produce the adjacent
+    // integer — "9007199254740991.0" is observed to parse as …990.0.
+    let cases: &[&str] = &[
+        "4503599627370496.0",
+        "9007199254740991.0",
+        "9007199254740992.0",
+        "9007199254740993.0",
+        "9.007199254740993e15",
+        "-4503599627370496.0",
+        "-9007199254740991.0",
+        "-9007199254740992.0",
+        "-9007199254740993.0",
+        "-9.007199254740993e15",
+    ];
+    for &json in cases {
+        assert!(
+            serde_json::from_str::<SerdeInt64>(json).is_err(),
+            "input should reject: {json}"
+        );
+    }
+}
+
 // ── uint64 ──────────────────────────────────────────────────────────────
 
 #[test]
@@ -286,6 +325,29 @@ fn uint64_deserializes_exact_float_notation_string() {
     assert_eq!(val.0, u64::MAX);
 }
 
+#[test]
+fn uint64_deserializes_safe_unquoted_float_notation() {
+    let val: SerdeUint64 = serde_json::from_str("4503599627370495.0").unwrap();
+    assert_eq!(val.0, 4503599627370495);
+}
+
+#[test]
+fn uint64_rejects_unsafe_unquoted_float_notation() {
+    let cases: &[&str] = &[
+        "4503599627370496.0",
+        "9007199254740991.0",
+        "9007199254740992.0",
+        "9007199254740993.0",
+        "9.007199254740993e15",
+    ];
+    for &json in cases {
+        assert!(
+            serde_json::from_str::<SerdeUint64>(json).is_err(),
+            "input should reject: {json}"
+        );
+    }
+}
+
 // ── float ───────────────────────────────────────────────────────────────
 
 #[test]