diff --git a/codex-rs/tui/src/lib.rs b/codex-rs/tui/src/lib.rs
index dc29030434..f09a76f1ac 100644
--- a/codex-rs/tui/src/lib.rs
+++ b/codex-rs/tui/src/lib.rs
@@ -177,6 +177,26 @@ pub async fn run_main(
         }
     };
 
+    // Fail fast if CODEX_HOME/rules is malformed (e.g. a file instead of a directory).
+    // This is a common misconfiguration and should not let the TUI start.
+    let rules_path = codex_home.join("rules");
+    match std::fs::metadata(&rules_path) {
+        Ok(metadata) if !metadata.is_dir() => {
+            return Ok(AppExitInfo::fatal(format!(
+                "Failed to initialize codex: failed to load rules: failed to read rules files from {}: not a directory",
+                rules_path.display()
+            )));
+        }
+        Ok(_) => {}
+        Err(err) if err.kind() == std::io::ErrorKind::NotFound => {}
+        Err(err) => {
+            return Ok(AppExitInfo::fatal(format!(
+                "Failed to initialize codex: failed to load rules: failed to read rules files from {}: {err}",
+                rules_path.display()
+            )));
+        }
+    }
+
     let cwd = cli.cwd.clone();
     let config_cwd = match cwd.as_deref() {
         Some(path) => AbsolutePathBuf::from_absolute_path(path.canonicalize()?)?,
diff --git a/codex-rs/tui/tests/suite/no_panic_on_startup.rs b/codex-rs/tui/tests/suite/no_panic_on_startup.rs
index eade57d08e..ac9a9110a1 100644
--- a/codex-rs/tui/tests/suite/no_panic_on_startup.rs
+++ b/codex-rs/tui/tests/suite/no_panic_on_startup.rs
@@ -35,7 +35,10 @@ model_provider = "ollama"
     std::fs::write(codex_home.join("config.toml"), config_contents)?;
 
     let CodexCliOutput { exit_code, output } = run_codex_cli(codex_home, cwd).await?;
-    assert_ne!(0, exit_code, "Codex CLI should exit nonzero.");
+    assert_ne!(
+        0, exit_code,
+        "Codex CLI should exit nonzero. Output was:\n{output}"
+    );
     assert!(
         output.contains("ERROR: Failed to initialize codex:"),
         "expected startup error in output, got: {output}"