diff --git a/crates/goose/src/config/base.rs b/crates/goose/src/config/base.rs
index 0cd9a22ad..9bfabbba5 100644
--- a/crates/goose/src/config/base.rs
+++ b/crates/goose/src/config/base.rs
@@ -19,14 +19,16 @@ use thiserror::Error;
 fn write_secrets_file(path: &Path, content: &str) -> std::io::Result<()> {
     #[cfg(unix)]
     {
-        use std::os::unix::fs::OpenOptionsExt;
+        use std::os::unix::fs::{OpenOptionsExt, PermissionsExt};
         let mut file = OpenOptions::new()
             .write(true)
             .create(true)
-            .truncate(true)
+            .truncate(false)
             .mode(0o600)
             .open(path)?;
 
+        file.set_permissions(std::fs::Permissions::from_mode(0o600))?;
+        file.set_len(0)?;
         file.write_all(content.as_bytes())
     }
 
@@ -2068,6 +2070,28 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    #[cfg(unix)]
+    fn test_existing_secrets_file_permissions_tightened_on_write() -> Result<(), ConfigError> {
+        use std::os::unix::fs::PermissionsExt;
+
+        let dir = TempDir::new().unwrap();
+        let config_file = NamedTempFile::new().unwrap();
+        let secrets_path = dir.path().join("secrets.yaml");
+        std::fs::write(&secrets_path, "existing: old\n")?;
+        std::fs::set_permissions(&secrets_path, std::fs::Permissions::from_mode(0o644))?;
+
+        let config = Config::new_with_file_secrets(config_file.path(), &secrets_path)?;
+        config.set_secret("key", &"value")?;
+
+        let value: String = config.get_secret("key")?;
+        assert_eq!(value, "value");
+        let mode = std::fs::metadata(&secrets_path)?.permissions().mode() & 0o777;
+        assert_eq!(mode, 0o600);
+
+        Ok(())
+    }
+
     #[test]
     fn test_merge_config_values_basic_override() {
         let mut base = Mapping::new();