diff --git a/crates/cli_bin/fixtures/match_filename/.grit/patterns/rewrite_filename.md b/crates/cli_bin/fixtures/match_filename/.grit/patterns/rewrite_filename.md
new file mode 100644
index 000000000..a5f7ae222
--- /dev/null
+++ b/crates/cli_bin/fixtures/match_filename/.grit/patterns/rewrite_filename.md
@@ -0,0 +1,11 @@
+---
+name: rewrite_filename
+language: js
+engine: marzano(0.1)
+---
+
+```grit
+file($name, $body) where {
+  $name <: `index.js`
+}
+```
diff --git a/crates/cli_bin/fixtures/windows_path_match/.grit/grit.yaml b/crates/cli_bin/fixtures/windows_path_match/.grit/grit.yaml
new file mode 100644
index 000000000..3a8b4b6f8
--- /dev/null
+++ b/crates/cli_bin/fixtures/windows_path_match/.grit/grit.yaml
@@ -0,0 +1,35 @@
+version: 0.0.1
+patterns:
+  - name: windows_path_match
+    body: |
+      engine marzano(0.1)
+      language js
+
+      file($name, $body) where {
+        $name <: `src/index.js`
+      }
+    samples:
+      - input: |
+          // @filename: src/index.js
+
+          console.log('hello from unix path');
+        output: |
+          // @filename: src/index.js
+
+          console.log('hello from unix path');
+      - input: |
+          // @filename: src\index.js
+
+          console.log('hello from windows path');
+        output: |
+          // @filename: src\index.js
+
+          console.log('hello from windows path');
+      - input: |
+          // @filename: src\nested\index.js
+
+          // This should not match
+      - input: |
+          // @filename: other/index.js
+
+          // This should not match
diff --git a/crates/cli_bin/fixtures/windows_path_match/.grit/patterns/windows_path_match.md b/crates/cli_bin/fixtures/windows_path_match/.grit/patterns/windows_path_match.md
new file mode 100644
index 000000000..2639f25ab
--- /dev/null
+++ b/crates/cli_bin/fixtures/windows_path_match/.grit/patterns/windows_path_match.md
@@ -0,0 +1,9 @@
+---
+name: windows_path_match
+language: js
+engine: marzano(0.1)
+---
+
+file($name, $body) where {
+  $name <: `src/index.js`
+}
diff --git a/crates/cli_bin/tests/patterns_test.rs b/crates/cli_bin/tests/patterns_test.rs
index 3a524d253..a2eaf4a90 100644
--- a/crates/cli_bin/tests/patterns_test.rs
+++ b/crates/cli_bin/tests/patterns_test.rs
@@ -326,6 +326,22 @@ fn tests_match_only_with_file_name() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn test_windows_path_match() -> Result<()> {
+    // Verifies the fix for GitHub Issue #639:
+    // Filename checks should work regardless of Windows (`\`) vs Unix (`/`) separators.
+    let (_temp_dir, dir) = get_fixture("windows_path_match", false)?;
+
+    let mut test = get_test_cmd()?;
+    test.arg("patterns").arg("test").current_dir(dir);
+
+    let output = test.output()?;
+
+    assert!(output.status.success());
+
+    Ok(())
+}
+
 #[test]
 fn tests_python_pattern_with_file_name() -> Result<()> {
     let (_temp_dir, dir) = get_fixture("python_filename", false)?;
diff --git a/crates/core/src/api.rs b/crates/core/src/api.rs
index 1558e1fa7..b8d8213a0 100644
--- a/crates/core/src/api.rs
+++ b/crates/core/src/api.rs
@@ -61,16 +61,20 @@ impl MatchResult {
 
 /// Make a path look the way provolone expects it to
 /// Removes leading "./", or the root path if it's provided
-fn normalize_path_in_project<'a>(path: &'a str, root_path: Option<&'a PathBuf>) -> &'a str {
-    if let Some(root_path) = root_path {
+fn normalize_path_in_project(path: &str, root_path: Option<&PathBuf>) -> String {
+    // First, trim project prefix / leading "./"
+    let trimmed = if let Some(root_path) = root_path {
         let basic = path
             .strip_prefix(root_path.to_string_lossy().as_ref())
             .unwrap_or(path);
-        // Stip the leading / if it's there
+        // Strip the leading '/' if it's there
         basic.strip_prefix('/').unwrap_or(basic)
     } else {
         path.strip_prefix("./").unwrap_or(path)
-    }
+    };
+
+    // Finally, ensure we always use forward slashes for path separators
+    trimmed.replace('\\', "/")
 }
 
 impl MatchResult {
@@ -861,4 +865,24 @@ mod tests {
         let path = "src/main.rs";
         assert_eq!(normalize_path_in_project(path, None), "src/main.rs");
     }
+
+    #[test]
+    fn test_normalize_path_in_project_windows_separators() {
+        // Simulate a Windows-style relative path with back-slashes
+        let path = ".\\src\\module\\main.rs";
+        assert_eq!(
+            normalize_path_in_project(path, None),
+            "src/module/main.rs"
+        );
+    }
+
+    #[test]
+    fn test_normalize_path_in_project_mixed_separators() {
+        // A path that has mixed separators should still normalize correctly
+        let path = "./src\\nested/main.rs";
+        assert_eq!(
+            normalize_path_in_project(path, None),
+            "src/nested/main.rs"
+        );
+    }
 }