diff --git a/crates/oxc_diagnostics/src/named_source.rs b/crates/oxc_diagnostics/src/named_source.rs
index e46e59d0fd..05b85b88cd 100644
--- a/crates/oxc_diagnostics/src/named_source.rs
+++ b/crates/oxc_diagnostics/src/named_source.rs
@@ -1,18 +1,33 @@
-use std::{fmt, sync::OnceLock};
+use std::{
+    fmt,
+    sync::{Mutex, OnceLock},
+};
 
 use crate::{SourceCode, source_impls};
 
 /// Utility struct for when you have a regular [`SourceCode`] type that doesn't
 /// implement `name`. For example [`String`]. Or if you want to override the
 /// `name` returned by the `SourceCode`.
-#[derive(Clone)]
 pub struct NamedSource<S: SourceCode> {
     source: S,
     name: String,
-    /// Line starts of `source`, computed on first use. Reporters resolve every
-    /// diagnostic's line and column through this, so a file's many diagnostics
-    /// share one scan instead of each rescanning the file.
+    /// Complete index, built only when explicitly requested.
     line_starts: OnceLock<Option<Box<[u32]>>>,
+    /// Position queries retain only the line starts scanned so far.
+    line_index: Mutex<source_impls::SourceLineIndex>,
+}
+
+impl<S: SourceCode + Clone> Clone for NamedSource<S> {
+    fn clone(&self) -> Self {
+        Self {
+            source: self.source.clone(),
+            name: self.name.clone(),
+            line_starts: self.line_starts.clone(),
+            line_index: Mutex::new(
+                self.line_index.lock().unwrap_or_else(std::sync::PoisonError::into_inner).clone(),
+            ),
+        }
+    }
 }
 
 impl<S: SourceCode> fmt::Debug for NamedSource<S> {
@@ -29,7 +44,12 @@ impl<S: SourceCode> NamedSource<S> {
     /// a name.
     #[must_use]
     pub fn new(name: impl AsRef<str>, source: S) -> Self {
-        Self { source, name: name.as_ref().to_string(), line_starts: OnceLock::new() }
+        Self {
+            source,
+            name: name.as_ref().to_string(),
+            line_starts: OnceLock::new(),
+            line_index: Mutex::default(),
+        }
     }
 }
 
@@ -45,4 +65,63 @@ impl<S: SourceCode> SourceCode for NamedSource<S> {
     fn line_starts(&self) -> Option<&[u32]> {
         self.line_starts.get_or_init(|| source_impls::line_starts(self.source.data())).as_deref()
     }
+
+    fn line_column(&self, offset: usize) -> Option<(usize, usize)> {
+        let input = self.source.data();
+        if let Some(line_starts) = self.line_starts.get().and_then(Option::as_deref) {
+            return source_impls::line_column(input, Some(line_starts), offset);
+        }
+        if u32::try_from(input.len()).is_err() {
+            return source_impls::line_column(input, None, offset);
+        }
+        self.line_index
+            .lock()
+            .unwrap_or_else(std::sync::PoisonError::into_inner)
+            .line_column(input, offset)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::sync::Arc;
+
+    use oxc_span::Span;
+
+    use crate::{OxcDiagnostic, reporter::Info};
+
+    use super::*;
+
+    #[test]
+    fn early_diagnostic_does_not_initialize_complete_index() {
+        let source = Arc::new(NamedSource::new("file.js", "line\n".repeat(200_000)));
+        let diagnostic = OxcDiagnostic::error("test")
+            .with_label(Span::new(0, 1))
+            .with_source_code(Arc::clone(&source));
+        let info = Info::new(&diagnostic);
+        assert_eq!((info.start.line, info.start.column), (1, 1));
+        assert_eq!((info.end.line, info.end.column), (1, 2));
+        assert!(source.line_starts.get().is_none());
+    }
+
+    #[test]
+    fn cloned_and_shared_indexes_match_direct_scan() {
+        let text = "first\r\nsecond\n\u{00e9}\r\u{1f980}\n";
+        let source = NamedSource::new("file.js", text);
+        source.line_column(8).unwrap();
+        let cloned = Arc::new(source.clone());
+        std::thread::scope(|scope| {
+            for _ in 0..4 {
+                scope.spawn(|| {
+                    for offset in (0..=text.len() + 1).rev() {
+                        assert_eq!(cloned.line_column(offset), text.line_column(offset));
+                    }
+                });
+            }
+        });
+        assert!(source.line_starts.get().is_none());
+        cloned.line_starts().unwrap();
+        for offset in 0..=text.len() + 1 {
+            assert_eq!(source.line_column(offset), cloned.line_column(offset));
+        }
+    }
 }
diff --git a/crates/oxc_diagnostics/src/protocol.rs b/crates/oxc_diagnostics/src/protocol.rs
index ebbb1844bc..2827419e06 100644
--- a/crates/oxc_diagnostics/src/protocol.rs
+++ b/crates/oxc_diagnostics/src/protocol.rs
@@ -84,4 +84,12 @@ pub trait SourceCode: Send + Sync {
     fn line_starts(&self) -> Option<&[u32]> {
         None
     }
+
+    /// Returns the one-based line and byte column at `offset`.
+    ///
+    /// Returns `None` if the offset is past the end of the source. An offset
+    /// inside a CRLF pair refers to the position of its CR.
+    fn line_column(&self, offset: usize) -> Option<(usize, usize)> {
+        crate::source_impls::line_column(self.data(), self.line_starts(), offset)
+    }
 }
diff --git a/crates/oxc_diagnostics/src/reporter.rs b/crates/oxc_diagnostics/src/reporter.rs
index bf8722da06..dbe0ff2108 100644
--- a/crates/oxc_diagnostics/src/reporter.rs
+++ b/crates/oxc_diagnostics/src/reporter.rs
@@ -140,36 +140,8 @@ pub struct InfoPosition {
     pub column: usize,
 }
 
-fn line_column(source: &dyn SourceCode, mut offset: usize) -> Option<InfoPosition> {
-    let data = source.data();
-    if offset > data.len() {
-        return None;
-    }
-    if offset > 0 && offset < data.len() && data[offset - 1] == b'\r' && data[offset] == b'\n' {
-        offset -= 1;
-    }
-
-    if let Some(line_starts) = source.line_starts() {
-        // `line_starts[0] == 0`, so at least one line starts at or before `offset`.
-        let line = line_starts.partition_point(|&start| start as usize <= offset);
-        let line_start = line_starts[line - 1] as usize;
-        return Some(InfoPosition { line, column: offset - line_start + 1 });
-    }
-
-    let mut line = 1;
-    let mut line_start = 0;
-    for index in memchr::memchr2_iter(b'\r', b'\n', &data[..offset]) {
-        if data[index] == b'\n' && index > 0 && data[index - 1] == b'\r' {
-            continue;
-        }
-        line += 1;
-        line_start = if data[index] == b'\r' && index + 1 < offset && data[index + 1] == b'\n' {
-            index + 2
-        } else {
-            index + 1
-        };
-    }
-    Some(InfoPosition { line, column: offset - line_start + 1 })
+fn line_column(source: &dyn SourceCode, offset: usize) -> Option<InfoPosition> {
+    source.line_column(offset).map(|(line, column)| InfoPosition { line, column })
 }
 
 impl Info {
@@ -212,7 +184,9 @@ impl Info {
 
 #[cfg(test)]
 mod test {
-    use crate::{Error, NamedSource, OxcDiagnostic};
+    use std::sync::Arc;
+
+    use crate::{Error, NamedSource, OxcDiagnostic, SourceCode};
 
     use super::{Info, line_column};
 
@@ -228,7 +202,8 @@ mod test {
                     text.push(ALPHABET[code % 3] as char);
                     code /= 3;
                 }
-                let indexed = NamedSource::new("file.js", text.clone());
+                let indexed = Arc::new(NamedSource::new("file.js", text.clone()));
+                let cloned = indexed.as_ref().clone();
                 for offset in 0..=text.len() + 1 {
                     assert_eq!(
                         line_column(&indexed, offset),
@@ -236,6 +211,14 @@ mod test {
                         "{text:?} at {offset}"
                     );
                 }
+                for offset in (0..=text.len() + 1).rev() {
+                    assert_eq!(line_column(&indexed, offset), line_column(&text, offset));
+                    assert_eq!(line_column(&cloned, offset), line_column(&text, offset));
+                }
+                indexed.line_starts().unwrap();
+                for offset in 0..=text.len() + 1 {
+                    assert_eq!(line_column(&indexed, offset), line_column(&text, offset));
+                }
             }
         }
     }
diff --git a/crates/oxc_diagnostics/src/source_impls.rs b/crates/oxc_diagnostics/src/source_impls.rs
index c957f7355d..b4b6c721a3 100644
--- a/crates/oxc_diagnostics/src/source_impls.rs
+++ b/crates/oxc_diagnostics/src/source_impls.rs
@@ -803,6 +803,74 @@ impl<T: ?Sized + SourceCode> SourceCode for Arc<T> {
     fn line_starts(&self) -> Option<&[u32]> {
         self.as_ref().line_starts()
     }
+
+    fn line_column(&self, offset: usize) -> Option<(usize, usize)> {
+        self.as_ref().line_column(offset)
+    }
+}
+
+/// Resolve a position using a complete index or a direct prefix scan.
+pub fn line_column(
+    input: &[u8],
+    line_starts: Option<&[u32]>,
+    offset: usize,
+) -> Option<(usize, usize)> {
+    if offset > input.len() {
+        return None;
+    }
+    let offset = normalize_offset(input, offset);
+    if let Some(line_starts) = line_starts {
+        let line = line_starts.partition_point(|&start| start as usize <= offset);
+        return Some((line, offset - line_starts[line - 1] as usize + 1));
+    }
+    let mut line = 1;
+    let mut line_start = 0;
+    for index in memchr::memchr2_iter(b'\r', b'\n', &input[..offset]) {
+        if input[index] == b'\n' && index > 0 && input[index - 1] == b'\r' {
+            continue;
+        }
+        line += 1;
+        line_start = if input[index] == b'\r' && index + 1 < offset && input[index + 1] == b'\n' {
+            index + 2
+        } else {
+            index + 1
+        };
+    }
+    Some((line, offset - line_start + 1))
+}
+
+fn normalize_offset(input: &[u8], offset: usize) -> usize {
+    if offset > 0 && offset < input.len() && input[offset - 1] == b'\r' && input[offset] == b'\n' {
+        offset - 1
+    } else {
+        offset
+    }
+}
+
+/// Shared reporters extend this index only through their requested positions.
+#[derive(Clone, Default)]
+pub struct SourceLineIndex {
+    line_starts: Vec<u32>,
+    frontier: usize,
+}
+
+impl SourceLineIndex {
+    pub fn line_column(&mut self, input: &[u8], offset: usize) -> Option<(usize, usize)> {
+        if offset > input.len() {
+            return None;
+        }
+        let offset = normalize_offset(input, offset);
+        if offset > self.frontier {
+            for line_break in LineBreaks::new(&input[self.frontier..offset]) {
+                self.line_starts
+                    .push(u32::try_from(self.frontier + line_break.next_line_start()).ok()?);
+            }
+            self.frontier = offset;
+        }
+        let line = self.line_starts.partition_point(|&start| start as usize <= offset);
+        let line_start = line.checked_sub(1).map_or(0, |index| self.line_starts[index] as usize);
+        Some((line + 1, offset - line_start + 1))
+    }
 }
 
 /// Byte offsets at which each line of `input` starts, with CRLF counted as one line break.
@@ -820,6 +888,30 @@ pub fn line_starts(input: &[u8]) -> Option<Box<[u32]>> {
 mod tests {
     use super::*;
 
+    #[test]
+    fn source_line_index_only_scans_requested_prefixes() {
+        let input = b"first\r\nsecond\nthird\rfourth";
+        let mut index = SourceLineIndex::default();
+        assert_eq!(index.line_column(input, 1), Some((1, 2)));
+        assert_eq!(index.frontier, 1);
+        assert_eq!(index.line_starts, [] as [u32; 0]);
+
+        // An offset on the LF keeps the whole CRLF beyond the frontier.
+        assert_eq!(index.line_column(input, 6), Some((1, 6)));
+        assert_eq!(index.frontier, 5);
+        assert_eq!(index.line_starts, [] as [u32; 0]);
+        assert_eq!(index.line_column(input, 8), Some((2, 2)));
+        assert_eq!(index.frontier, 8);
+        assert_eq!(index.line_starts, [7]);
+
+        assert_eq!(index.line_column(input, 0), Some((1, 1)));
+        assert_eq!(index.frontier, 8);
+        assert_eq!(index.line_column(input, input.len() + 1), None);
+        assert_eq!(index.frontier, 8);
+        assert_eq!(index.line_column(input, input.len()), Some((4, 7)));
+        assert_eq!(index.line_starts, [7, 14, 20]);
+    }
+
     #[test]
     fn zero_context_prefix_matches_generic_path() {
         for input in [