diff --git a/src/bin/mathml2text.rs b/src/bin/mathml2text.rs
index ff05b521..3b55225c 100644
--- a/src/bin/mathml2text.rs
+++ b/src/bin/mathml2text.rs
@@ -18,6 +18,10 @@ fn get_rules_dir() -> String {
     return rules_path.as_os_str().to_str().unwrap().to_string();
 }
 
+fn resolve_rules_dir(rules_dir: Option<PathBuf>) -> PathBuf {
+    return rules_dir.unwrap_or_else(|| PathBuf::from(get_rules_dir()));
+}
+
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, ValueEnum)]
 enum OutputType {
     Text,
@@ -60,14 +64,15 @@ fn main() -> Result<()> {
 		"#.to_string()
     };
 
-    if let Err(e) = set_rules_dir(get_rules_dir()) {
+    let rules_dir = resolve_rules_dir(cli.rules_dir);
+    if let Err(e) = set_rules_dir(rules_dir.to_string_lossy()) {
 	panic!("Error: exiting -- {}", errors_to_string(&e));
     }
     debug!("Languages: {}", libmathcat::interface::get_supported_languages()?.join(", "));
 
     #[cfg(feature = "include-zip")]
     info!("***********include-zip is present**********");
-    info!("Version = '{}' using Rules dir {}", get_version(), get_rules_dir());
+    info!("Version = '{}' using Rules dir {}", get_version(), rules_dir.display());
     set_preference("Language", cli.language)?;
 
     set_preference("DecimalSeparator", "Auto").unwrap();
diff --git a/tests/mathml2text.rs b/tests/mathml2text.rs
new file mode 100644
index 00000000..80193a8b
--- /dev/null
+++ b/tests/mathml2text.rs
@@ -0,0 +1,97 @@
+mod common;
+
+use std::path::PathBuf;
+use std::process::Command;
+use std::time::{SystemTime, UNIX_EPOCH};
+
+fn mathml2text_command() -> Command { Command::new(env!("CARGO_BIN_EXE_mathml2text")) }
+
+fn rules_dir() -> String { common::abs_rules_dir_path() }
+
+fn unique_temp_file(name: &str) -> PathBuf {
+    let timestamp = SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .expect("system time should be after UNIX_EPOCH")
+        .as_nanos();
+    std::env::temp_dir().join(format!("mathml2text-{name}-{timestamp}.mml"))
+}
+
+/// Verifies that `mathml2text` uses an explicitly provided `--rules-dir`.
+/// This protects the CLI override path instead of silently falling back to the default rules location.
+#[test]
+fn accepts_explicit_rules_dir() {
+    let input_file = unique_temp_file("explicit-rules");
+    std::fs::write(
+        &input_file,
+        "<math><mn>4</mn></math>",
+    )
+    .expect("should write temp input file");
+
+    let output = mathml2text_command()
+        .args([
+            "--rules-dir",
+            rules_dir().as_str(),
+        ])
+        .arg(&input_file)
+        .output()
+        .expect("mathml2text should run");
+
+    let _ = std::fs::remove_file(&input_file);
+
+    assert!(output.status.success(), "stderr: {}", String::from_utf8_lossy(&output.stderr));
+    assert_eq!("4\n", String::from_utf8_lossy(&output.stdout));
+}
+
+/// Verifies that the positional input-file argument still works on its own.
+/// This keeps the default CLI file-input path covered while testing the rules-dir changes separately.
+#[test]
+fn still_accepts_input_file() {
+    let input_file = unique_temp_file("input-file");
+    std::fs::write(
+        &input_file,
+        "<math><mn>2</mn></math>",
+    )
+    .expect("should write temp input file");
+
+    let output = mathml2text_command()
+        .arg(&input_file)
+        .output()
+        .expect("mathml2text should run");
+
+    let _ = std::fs::remove_file(&input_file);
+
+    assert!(output.status.success(), "stderr: {}", String::from_utf8_lossy(&output.stderr));
+    assert_eq!("2\n", String::from_utf8_lossy(&output.stdout));
+}
+
+/// Verifies that an invalid explicit `--rules-dir` causes the CLI to fail.
+/// This protects against regressions where the flag is parsed but then ignored at runtime.
+#[test]
+fn rejects_invalid_explicit_rules_dir() {
+    let missing_rules_dir = std::env::temp_dir().join(format!(
+        "mathml2text-missing-rules-{}",
+        SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .expect("system time should be after UNIX_EPOCH")
+            .as_nanos()
+    ));
+    let input_file = unique_temp_file("invalid-rules");
+    std::fs::write(
+        &input_file,
+        "<math><mn>5</mn></math>",
+    )
+    .expect("should write temp input file");
+
+    let output = mathml2text_command()
+        .args([
+            "--rules-dir",
+            missing_rules_dir.to_str().expect("temp path should be valid UTF-8"),
+        ])
+        .arg(&input_file)
+        .output()
+        .expect("mathml2text should run");
+
+    let _ = std::fs::remove_file(&input_file);
+
+    assert!(!output.status.success(), "stdout: {}", String::from_utf8_lossy(&output.stdout));
+}