fix: write to real file if config.yaml is symlink

crates/goose/src/config/base.rs

@@ -459,20 +459,57 @@ impl Config {
         load_init_config_from_workspace()
     }
 
+    fn config_write_target_path(&self) -> Result<PathBuf, ConfigError> {
+        let mut path = self.config_path.clone();
+
+        // Follow symlinks so we update the target file without replacing the link itself.
+        const MAX_SYMLINK_HOPS: usize = 1;
+        let mut hops = 0usize;
+        loop {
+            match std::fs::symlink_metadata(&path) {
+                Ok(meta) if meta.file_type().is_symlink() => {
+                    if hops >= MAX_SYMLINK_HOPS {
+                        return Err(std::io::Error::new(
+                            std::io::ErrorKind::InvalidInput,
+                            format!(
+                                "Too many symlink levels (or a cycle) while resolving config path: {:?}",
+                                self.config_path
+                            ),
+                        )
+                        .into());
+                    }
+                    hops += 1;
+
+                    let link = std::fs::read_link(&path)?;
+                    path = if link.is_absolute() {
+                        link
+                    } else {
+                        path.parent().unwrap_or_else(|| Path::new(".")).join(link)
+                    };
+                }
+                Ok(_) => return Ok(path),
+                Err(e) if e.kind() == std::io::ErrorKind::NotFound => return Ok(path),
+                Err(e) => return Err(e.into()),
+            }
+        }
+    }
+
     fn save_values(&self, values: &Mapping) -> Result<(), ConfigError> {
         // Create backup before writing new config
         self.create_backup_if_needed()?;
 
+        let target_path = self.config_write_target_path()?;
+
         // Convert to YAML for storage
         let yaml_value = serde_yaml::to_string(values)?;
 
-        if let Some(parent) = self.config_path.parent() {
+        if let Some(parent) = target_path.parent() {
             std::fs::create_dir_all(parent)
                 .map_err(|e| ConfigError::DirectoryError(e.to_string()))?;
         }
 
         // Write to a temporary file first for atomic operation
-        let temp_path = self.config_path.with_extension("tmp");
+        let temp_path = target_path.with_extension("tmp");
 
         {
             let mut file = OpenOptions::new()
@@ -493,7 +530,7 @@ impl Config {
         }
 
         // Atomically replace the original file
-        std::fs::rename(&temp_path, &self.config_path)?;
+        std::fs::rename(&temp_path, &target_path)?;
 
         Ok(())
     }
@@ -1029,7 +1066,7 @@ pub fn load_init_config_from_workspace() -> Result<Mapping, ConfigError> {
 mod tests {
     use super::*;
     use serial_test::serial;
-    use tempfile::NamedTempFile;
+    use tempfile::{NamedTempFile, TempDir};
     #[test]
     fn test_basic_config() -> Result<(), ConfigError> {
         let config = new_test_config();
@@ -1250,6 +1287,78 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    #[cfg(unix)]
+    fn test_write_follows_symlink() -> Result<(), ConfigError> {
+        use std::os::unix::fs as unix_fs;
+
+        let dir = TempDir::new().unwrap();
+        let target_path = dir.path().join("real_config.yaml");
+        let symlink_path = dir.path().join("config.yaml");
+
+        std::fs::write(&target_path, "{}\n")?;
+        unix_fs::symlink(&target_path, &symlink_path)?;
+
+        let secrets_file = NamedTempFile::new().unwrap();
+        let config = Config::new_with_file_secrets(&symlink_path, secrets_file.path())?;
+
+        config.set_param("key1", "value1")?;
+
+        let meta = std::fs::symlink_metadata(&symlink_path)?;
+        assert!(
+            meta.file_type().is_symlink(),
+            "config path should remain a symlink"
+        );
+
+        let content = std::fs::read_to_string(&symlink_path)?;
+        assert!(content.contains("key1: value1"));
+
+        let content = std::fs::read_to_string(&target_path)?;
+        assert!(content.contains("key1: value1"));
+
+        Ok(())
+    }
+
+    #[test]
+    #[cfg(unix)]
+    fn test_write_fails_on_long_symlink_chain() -> Result<(), ConfigError> {
+        use std::os::unix::fs as unix_fs;
+
+        let dir = TempDir::new().unwrap();
+        let target_path = dir.path().join("real_config.yaml");
+        std::fs::write(&target_path, "{}\n")?;
+
+        // config.yaml -> link1.yaml -> real_config.yaml
+        // We only allow following one symlink hop. If there's another symlink, we should fail
+        // rather than overwrite the intermediate symlink.
+        let config_symlink = dir.path().join("config.yaml");
+        let link1 = dir.path().join("link1.yaml");
+        unix_fs::symlink(&target_path, &link1)?;
+        unix_fs::symlink(&link1, &config_symlink)?;
+
+        let secrets_file = NamedTempFile::new().unwrap();
+        let config = Config::new_with_file_secrets(&config_symlink, secrets_file.path())?;
+
+        let err = config.set_param("key1", "value1").unwrap_err();
+        assert!(
+            err.to_string().contains("Too many symlink levels"),
+            "unexpected error: {err}"
+        );
+
+        let meta = std::fs::symlink_metadata(&config_symlink)?;
+        assert!(
+            meta.file_type().is_symlink(),
+            "config path should remain a symlink"
+        );
+        let meta = std::fs::symlink_metadata(&link1)?;
+        assert!(
+            meta.file_type().is_symlink(),
+            "intermediate link should remain a symlink"
+        );
+
+        Ok(())
+    }
+
     #[test]
     fn test_config_recovery_from_backup() -> Result<(), ConfigError> {
         let config_file = NamedTempFile::new().unwrap();