diff --git a/src/uu/numfmt/locales/en-US.ftl b/src/uu/numfmt/locales/en-US.ftl
index bc6d6fc3dd..28b1062355 100644
--- a/src/uu/numfmt/locales/en-US.ftl
+++ b/src/uu/numfmt/locales/en-US.ftl
@@ -72,6 +72,7 @@ numfmt-error-rejecting-suffix = rejecting suffix in input: '{ $number }{ $suffix
 numfmt-error-suffix-unsupported-for-unit = This suffix is unsupported for specified unit
 numfmt-error-invalid-unit-argument = invalid argument '{$arg}' for '{$opt}'
 numfmt-error-number-too-big = Number is too big and unsupported
+numfmt-error-too-many-digits = value too large to be converted: { $input }
 numfmt-error-format-no-percent = format { $format } has no % directive
 numfmt-error-format-ends-in-percent = format { $format } ends in %
 numfmt-error-invalid-format-directive = invalid format { $format }, directive must be %[0]['][-][N][.][N]f
diff --git a/src/uu/numfmt/locales/fr-FR.ftl b/src/uu/numfmt/locales/fr-FR.ftl
index a7c55d6d98..55a6eb2d45 100644
--- a/src/uu/numfmt/locales/fr-FR.ftl
+++ b/src/uu/numfmt/locales/fr-FR.ftl
@@ -71,6 +71,7 @@ numfmt-error-rejecting-suffix = rejet du suffixe dans l'entrée : '{ $number }{
 numfmt-error-suffix-unsupported-for-unit = Ce suffixe n'est pas pris en charge pour l'unité spécifiée
 numfmt-error-unit-auto-not-supported-with-to = L'unité 'auto' n'est pas prise en charge avec les options --to
 numfmt-error-number-too-big = Le nombre est trop grand et non pris en charge
+numfmt-error-too-many-digits = valeur trop grande pour être convertie : { $input }
 numfmt-error-format-no-percent = le format { $format } n'a pas de directive %
 numfmt-error-format-ends-in-percent = le format { $format } se termine par %
 numfmt-error-invalid-format-directive = format invalide { $format }, la directive doit être %[0]['][-][N][.][N]f
diff --git a/src/uu/numfmt/src/diagnostics.rs b/src/uu/numfmt/src/diagnostics.rs
index d73dad4aa7..84f1790cfd 100644
--- a/src/uu/numfmt/src/diagnostics.rs
+++ b/src/uu/numfmt/src/diagnostics.rs
@@ -14,7 +14,7 @@ use uucore::diagnostics::{Snapshot, ValueOptions};
 use uucore::ranges::RangeError;
 use uucore::translate;
 
-use crate::format::{holds_number, invalid_span};
+use crate::format::{holds_number, invalid_span, overlong_number_len};
 use crate::options::{FormatError, FormatErrorKind, NumfmtOptions, OptionValueError};
 use crate::units::Unit;
 
@@ -97,12 +97,16 @@ pub fn render_input(
     else {
         return false;
     };
-    let span = invalid_span(input, options);
+    // A number with too many digits is refused as a whole, so it is what the
+    // caret marks, and no suffix advice would help.
+    let start = input.len() - input.trim_start().len();
+    let overlong = overlong_number_len(&input[start..]);
+    let span = overlong.map_or_else(|| invalid_span(input, options), |len| start..start + len);
     // Without --from there is no suffix to spell out, only the option to reach
     // for; an input with no number in it at all is not a suffix question, and
     // neither is one whose leading part only looks like the start of one.
     let help = match options.transform.from {
-        _ if !holds_number(input) => None,
+        _ if overlong.is_some() || !holds_number(input) => None,
         Unit::None => Some("numfmt-diag-help-input-no-from"),
         _ => Some("numfmt-diag-help-input-suffixes"),
     };
diff --git a/src/uu/numfmt/src/format.rs b/src/uu/numfmt/src/format.rs
index ee0a1dd132..cc6ba38154 100644
--- a/src/uu/numfmt/src/format.rs
+++ b/src/uu/numfmt/src/format.rs
@@ -181,6 +181,30 @@ pub fn holds_number(input: &str) -> bool {
     })
 }
 
+/// How many digits numfmt converts on either side of the decimal separator,
+/// as GNU does. Leading zeros carry no value and are not counted.
+const MAX_DIGITS: usize = 33;
+
+/// Byte length of the number `s` starts with, when its whole part or its
+/// fraction has more than [`MAX_DIGITS`] digits; `None` otherwise.
+pub fn overlong_number_len(s: &str) -> Option<usize> {
+    let dec_sep = locale_decimal_separator();
+    let digits_len = |s: &str| s.bytes().take_while(u8::is_ascii_digit).count();
+    let counted = |digits: &str| digits.trim_start_matches('0').len();
+
+    let sign_len = usize::from(s.starts_with('-'));
+    let whole_len = digits_len(&s[sign_len..]);
+    let whole = &s[sign_len..sign_len + whole_len];
+    let fraction = s[sign_len + whole_len..]
+        .strip_prefix(dec_sep)
+        .map(|rest| &rest[..digits_len(rest)]);
+
+    if counted(whole) <= MAX_DIGITS && fraction.is_none_or(|f| counted(f) <= MAX_DIGITS) {
+        return None;
+    }
+    Some(sign_len + whole_len + fraction.map_or(0, |f| dec_sep.len() + f.len()))
+}
+
 fn detailed_error_message(s: &str, unit: Unit, unit_separator: &str) -> Option<String> {
     if s.is_empty() {
         return Some(translate!("numfmt-error-invalid-number-empty"));
@@ -482,6 +506,11 @@ fn transform_from(
     opts: &TransformOptions,
     options: &NumfmtOptions,
 ) -> Result<ParsedNumber> {
+    // Checked first: whatever follows the digits, the number itself is refused.
+    if overlong_number_len(s).is_some() {
+        return Err(translate!("numfmt-error-too-many-digits", "input" => s.quote()));
+    }
+
     let (i, suffix) = parse_suffix(
         s,
         opts.from,
diff --git a/tests/by-util/test_numfmt.rs b/tests/by-util/test_numfmt.rs
index 56d127d472..121bc6e6f9 100644
--- a/tests/by-util/test_numfmt.rs
+++ b/tests/by-util/test_numfmt.rs
@@ -1854,6 +1854,134 @@ fn test_header_detached() {
         .stdout_is("1\n2\n");
 }
 
+// https://github.com/uutils/coreutils/issues/12855
+#[test]
+fn test_too_many_digits_issue_input() {
+    let input = "1091611621123456668921298.1279298262626672676226672772762626627828";
+    new_ucmd!()
+        .arg(input)
+        .fails_with_code(2)
+        .no_stdout()
+        .stderr_is(format!(
+            "numfmt: value too large to be converted: '{input}'\n"
+        ));
+}
+
+#[test]
+fn test_too_many_digits_whole_part() {
+    new_ucmd!()
+        .args(&["--to=si", &"1".repeat(33)])
+        .succeeds()
+        .stdout_only("112Q\n");
+
+    let input = "1".repeat(34);
+    new_ucmd!()
+        .args(&["--to=si", &input])
+        .fails_with_code(2)
+        .no_stdout()
+        .stderr_is(format!(
+            "numfmt: value too large to be converted: '{input}'\n"
+        ));
+}
+
+#[test]
+fn test_too_many_digits_fraction() {
+    new_ucmd!()
+        .args(&["--to=si", &format!("1.{}", "1".repeat(33))])
+        .succeeds()
+        .stdout_only("1\n");
+
+    // Trailing zeros of the fraction count.
+    for input in [
+        format!("1.{}", "1".repeat(34)),
+        format!("1.1{}", "0".repeat(33)),
+    ] {
+        new_ucmd!()
+            .args(&["--to=si", &input])
+            .fails_with_code(2)
+            .no_stdout()
+            .stderr_is(format!(
+                "numfmt: value too large to be converted: '{input}'\n"
+            ));
+    }
+}
+
+#[test]
+fn test_too_many_digits_leading_zeros_not_counted() {
+    new_ucmd!()
+        .args(&["--to=si", &format!("0000{}", "1".repeat(33))])
+        .succeeds()
+        .stdout_only("112Q\n");
+    new_ucmd!()
+        .args(&["--to=si", &format!("1.0000{}", "1".repeat(33))])
+        .succeeds()
+        .stdout_only("1\n");
+
+    let input = format!("0000{}", "1".repeat(34));
+    new_ucmd!()
+        .args(&["--to=si", &input])
+        .fails_with_code(2)
+        .no_stdout()
+        .stderr_is(format!(
+            "numfmt: value too large to be converted: '{input}'\n"
+        ));
+}
+
+#[test]
+fn test_too_many_digits_negative() {
+    new_ucmd!()
+        .args(&["--to=si", "--", &format!("-{}", "1".repeat(33))])
+        .succeeds()
+        .stdout_only("-112Q\n");
+
+    for input in [
+        format!("-{}", "1".repeat(34)),
+        format!("-1.{}", "1".repeat(34)),
+    ] {
+        new_ucmd!()
+            .args(&["--to=si", "--", &input])
+            .fails_with_code(2)
+            .no_stdout()
+            .stderr_is(format!(
+                "numfmt: value too large to be converted: '{input}'\n"
+            ));
+    }
+}
+
+#[test]
+fn test_too_many_digits_comes_before_suffix_errors() {
+    for (unit, input) in [
+        ("--from=si", format!("{}K", "1".repeat(34))),
+        ("--from=none", format!("{}x", "1".repeat(34))),
+    ] {
+        new_ucmd!()
+            .args(&[unit, &input])
+            .fails_with_code(2)
+            .no_stdout()
+            .stderr_is(format!(
+                "numfmt: value too large to be converted: '{input}'\n"
+            ));
+    }
+}
+
+#[test]
+fn test_too_many_digits_follows_invalid_mode() {
+    let input = "1".repeat(34);
+    let message = format!("numfmt: value too large to be converted: '{input}'\n");
+    new_ucmd!()
+        .args(&["--to=si", "--invalid=warn"])
+        .pipe_in(format!("1000\n{input}\n2000\n"))
+        .succeeds()
+        .stdout_is(format!("1.0k\n{input}\n2.0k\n"))
+        .stderr_is(&message);
+    new_ucmd!()
+        .args(&["--to=si", "--invalid=fail"])
+        .pipe_in(format!("1000\n{input}\n2000\n"))
+        .fails_with_code(2)
+        .stdout_is(format!("1.0k\n{input}\n2.0k\n"))
+        .stderr_is(&message);
+}
+
 #[cfg(all(feature = "feat_diagnostics", not(wasi_runner)))]
 mod diagnostics {
     use super::*;
@@ -2143,6 +2271,32 @@ numfmt: invalid number: 'abc'
         );
     }
 
+    #[cfg(unix)]
+    #[test]
+    fn test_snippet_marks_a_number_with_too_many_digits() {
+        let input = format!("{}.5K", "1".repeat(34));
+        let result = new_ucmd!()
+            .terminal_sim_stderr()
+            .args(&["--from=si", &input])
+            .fails_with_code(2);
+
+        // The number is refused whatever follows it, so the caret leaves the
+        // suffix out and no advice about suffixes is given.
+        result.no_stdout();
+        assert_eq!(
+            result.stderr_as_displayed(),
+            format!(
+                "numfmt: value too large to be converted: '{input}'
+   ╭─[ numfmt:1:18 ]
+   │
+ 1 │ numfmt --from=si {input}
+   │                  {}
+───╯",
+                "─".repeat(36)
+            )
+        );
+    }
+
     #[cfg(unix)]
     #[test]
     fn test_snippet_points_at_the_i_a_unit_does_not_take() {