diff --git a/crates/core/src/refs.rs b/crates/core/src/refs.rs
index 47c1705..efc89db 100644
--- a/crates/core/src/refs.rs
+++ b/crates/core/src/refs.rs
@@ -1,10 +1,15 @@
 use crate::error::AppError;
 use serde::{Deserialize, Serialize};
+use std::cell::RefCell;
 use std::collections::HashMap;
 use std::path::PathBuf;
 
 const MAX_REFMAP_BYTES: u64 = 1_048_576; // 1 MB
 
+thread_local! {
+    static HOME_OVERRIDE: RefCell<Option<PathBuf>> = const { RefCell::new(None) };
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct RefEntry {
     pub pid: i32,
@@ -144,11 +149,73 @@ fn refmap_path() -> Result<PathBuf, AppError> {
 }
 
 fn home_dir() -> Option<PathBuf> {
+    if let Some(p) = HOME_OVERRIDE.with(|cell| cell.borrow().clone()) {
+        return Some(p);
+    }
     std::env::var_os("HOME")
         .map(PathBuf::from)
         .or_else(|| std::env::var_os("USERPROFILE").map(PathBuf::from))
 }
 
+#[cfg(test)]
+pub(crate) struct HomeGuard {
+    _dir: tempdir::TempDir,
+    prev: Option<PathBuf>,
+}
+
+#[cfg(test)]
+mod tempdir {
+    use std::fs;
+    use std::path::PathBuf;
+    use std::sync::atomic::{AtomicU64, Ordering};
+    use std::time::{SystemTime, UNIX_EPOCH};
+
+    static COUNTER: AtomicU64 = AtomicU64::new(0);
+
+    pub struct TempDir(PathBuf);
+
+    impl TempDir {
+        pub fn new() -> Self {
+            let n = COUNTER.fetch_add(1, Ordering::SeqCst);
+            let nanos = SystemTime::now()
+                .duration_since(UNIX_EPOCH)
+                .map(|d| d.as_nanos())
+                .unwrap_or(0);
+            let path = std::env::temp_dir().join(format!("agent-desktop-test-{nanos}-{n}"));
+            fs::create_dir_all(&path).expect("create tempdir");
+            Self(path)
+        }
+
+        pub fn path(&self) -> &std::path::Path {
+            &self.0
+        }
+    }
+
+    impl Drop for TempDir {
+        fn drop(&mut self) {
+            let _ = fs::remove_dir_all(&self.0);
+        }
+    }
+}
+
+#[cfg(test)]
+impl HomeGuard {
+    pub fn new() -> Self {
+        let dir = tempdir::TempDir::new();
+        let prev = HOME_OVERRIDE.with(|cell| cell.borrow().clone());
+        HOME_OVERRIDE.with(|cell| *cell.borrow_mut() = Some(dir.path().to_path_buf()));
+        Self { _dir: dir, prev }
+    }
+}
+
+#[cfg(test)]
+impl Drop for HomeGuard {
+    fn drop(&mut self) {
+        let prev = self.prev.take();
+        HOME_OVERRIDE.with(|cell| *cell.borrow_mut() = prev);
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -384,6 +451,75 @@ mod tests {
         assert!(result.is_ok(), "normal-sized refmap should serialize");
     }
 
+    #[test]
+    fn test_save_load_roundtrip_with_home_override() {
+        let _guard = HomeGuard::new();
+        let mut map = RefMap::new();
+        map.allocate(RefEntry {
+            pid: 7,
+            role: "button".into(),
+            name: Some("Send".into()),
+            value: None,
+            states: vec![],
+            bounds: None,
+            bounds_hash: Some(42),
+            available_actions: vec!["Click".into()],
+            source_app: Some("TestApp".into()),
+            root_ref: None,
+        });
+        map.save().expect("save should succeed under HomeGuard");
+
+        let loaded = RefMap::load().expect("load should succeed");
+        assert_eq!(loaded.len(), 1);
+        let entry = loaded.get("@e1").unwrap();
+        assert_eq!(entry.pid, 7);
+        assert_eq!(entry.name.as_deref(), Some("Send"));
+    }
+
+    #[test]
+    fn test_save_oversize_preserves_previous_file() {
+        let _guard = HomeGuard::new();
+
+        let mut original = RefMap::new();
+        original.allocate(RefEntry {
+            pid: 1,
+            role: "button".into(),
+            name: Some("Original".into()),
+            value: None,
+            states: vec![],
+            bounds: None,
+            bounds_hash: None,
+            available_actions: vec!["Click".into()],
+            source_app: None,
+            root_ref: None,
+        });
+        original.save().expect("baseline save");
+
+        let mut oversize = RefMap::new();
+        let big = "x".repeat(2048);
+        for _ in 0..600 {
+            oversize.allocate(RefEntry {
+                pid: 1,
+                role: "button".into(),
+                name: Some(big.clone()),
+                value: None,
+                states: vec![],
+                bounds: None,
+                bounds_hash: None,
+                available_actions: vec!["Click".into()],
+                source_app: None,
+                root_ref: None,
+            });
+        }
+        let result = oversize.save();
+        assert!(result.is_err(), "oversize save must reject");
+
+        let reloaded = RefMap::load().expect("previous file must still load");
+        assert_eq!(reloaded.len(), 1);
+        let entry = reloaded.get("@e1").unwrap();
+        assert_eq!(entry.name.as_deref(), Some("Original"));
+    }
+
     #[test]
     fn test_root_ref_none_omitted() {
         let entry = RefEntry {
diff --git a/crates/core/src/snapshot_ref.rs b/crates/core/src/snapshot_ref.rs
index 43a18f3..3cda36d 100644
--- a/crates/core/src/snapshot_ref.rs
+++ b/crates/core/src/snapshot_ref.rs
@@ -112,7 +112,12 @@ fn allocate_refs_with_root(
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::action::Action;
+    use crate::adapter::{NativeHandle, PermissionStatus, PlatformAdapter};
+    use crate::error::AdapterError;
     use crate::node::AccessibilityNode;
+    use crate::refs::HomeGuard;
+    use std::cell::Cell;
 
     fn node(role: &str) -> AccessibilityNode {
         AccessibilityNode {
@@ -129,6 +134,203 @@ mod tests {
         }
     }
 
+    fn named(role: &str, name: &str) -> AccessibilityNode {
+        let mut n = node(role);
+        n.name = Some(name.into());
+        n
+    }
+
+    struct StubAdapter {
+        subtree: AccessibilityNode,
+        resolve_calls: Cell<u32>,
+    }
+
+    impl StubAdapter {
+        fn new(subtree: AccessibilityNode) -> Self {
+            Self {
+                subtree,
+                resolve_calls: Cell::new(0),
+            }
+        }
+    }
+
+    unsafe impl Send for StubAdapter {}
+    unsafe impl Sync for StubAdapter {}
+
+    impl PlatformAdapter for StubAdapter {
+        fn check_permissions(&self) -> PermissionStatus {
+            PermissionStatus::Granted
+        }
+
+        fn resolve_element(
+            &self,
+            _entry: &crate::refs::RefEntry,
+        ) -> Result<NativeHandle, AdapterError> {
+            self.resolve_calls.set(self.resolve_calls.get() + 1);
+            Ok(NativeHandle::null())
+        }
+
+        fn get_subtree(
+            &self,
+            _handle: &NativeHandle,
+            _opts: &TreeOptions,
+        ) -> Result<AccessibilityNode, AdapterError> {
+            Ok(self.subtree.clone())
+        }
+
+        fn execute_action(
+            &self,
+            _handle: &NativeHandle,
+            _action: Action,
+        ) -> Result<crate::action::ActionResult, AdapterError> {
+            Err(AdapterError::not_supported("execute_action"))
+        }
+    }
+
+    fn seed_skeleton_refmap() -> RefMap {
+        let mut map = RefMap::new();
+        let anchor = ref_entry_from_node(&named("group", "Sidebar"), 42, Some("TestApp"), None);
+        let _ = map.allocate(anchor);
+        let other = ref_entry_from_node(&named("button", "Toolbar"), 42, Some("TestApp"), None);
+        let _ = map.allocate(other);
+        map
+    }
+
+    fn drill_opts() -> TreeOptions {
+        TreeOptions {
+            interactive_only: false,
+            ..Default::default()
+        }
+    }
+
+    #[test]
+    fn test_run_from_ref_returns_subtree_and_persists_refs() {
+        let _guard = HomeGuard::new();
+        seed_skeleton_refmap().save().unwrap();
+
+        let mut child_btn = named("button", "Save");
+        child_btn.children = vec![];
+        let mut subtree_root = named("group", "Sidebar");
+        subtree_root.children = vec![child_btn];
+
+        let adapter = StubAdapter::new(subtree_root);
+        let result = run_from_ref(&adapter, &drill_opts(), "@e1").expect("drill should succeed");
+
+        let on_disk = RefMap::load().unwrap();
+        assert_eq!(on_disk.len(), result.refmap.len());
+        assert!(
+            result.refmap.len() >= 3,
+            "expected at least 2 skeleton + 1 drill ref, got {}",
+            result.refmap.len()
+        );
+
+        let drill_ref = result
+            .tree
+            .children
+            .iter()
+            .find(|c| c.role == "button")
+            .and_then(|c| c.ref_id.as_deref())
+            .expect("button child should carry a ref");
+        let drill_entry = on_disk.get(drill_ref).expect("entry persisted");
+        assert_eq!(drill_entry.root_ref.as_deref(), Some("@e1"));
+        assert_eq!(adapter.resolve_calls.get(), 1);
+    }
+
+    #[test]
+    fn test_run_from_ref_stale_root_returns_stale_ref() {
+        let _guard = HomeGuard::new();
+        RefMap::new().save().unwrap();
+
+        let adapter = StubAdapter::new(named("group", "Sidebar"));
+        let result = run_from_ref(&adapter, &drill_opts(), "@e99");
+        let err = match result {
+            Ok(_) => panic!("stale root must error"),
+            Err(e) => e,
+        };
+        match err {
+            AppError::Adapter(adapter_err) => {
+                assert_eq!(adapter_err.code, crate::error::ErrorCode::StaleRef);
+                let suggestion = adapter_err.suggestion.as_deref().unwrap_or("");
+                assert!(
+                    suggestion.contains("skeleton"),
+                    "stale-ref suggestion should mention skeleton, got: {suggestion}"
+                );
+            }
+            other => panic!("expected Adapter(StaleRef), got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn test_run_from_ref_re_drill_replaces_drill_refs_only() {
+        let _guard = HomeGuard::new();
+        seed_skeleton_refmap().save().unwrap();
+
+        let subtree = named("button", "Save");
+        let adapter = StubAdapter::new(subtree);
+
+        let first = run_from_ref(&adapter, &drill_opts(), "@e1").unwrap();
+        let first_count = first.refmap.len();
+        let first_button_ref = first.tree.ref_id.clone().expect("button should get a ref");
+
+        let second = run_from_ref(&adapter, &drill_opts(), "@e1").unwrap();
+        let second_count = second.refmap.len();
+        let second_button_ref = second.tree.ref_id.clone().expect("button should get a ref");
+
+        assert_eq!(
+            first_count, second_count,
+            "ref count stable across re-drill"
+        );
+        assert_ne!(
+            first_button_ref, second_button_ref,
+            "re-drill should issue a fresh ref id (counter continues)"
+        );
+        let on_disk = RefMap::load().unwrap();
+        assert!(on_disk.get("@e1").is_some(), "skeleton anchor preserved");
+        assert!(on_disk.get(&second_button_ref).is_some());
+        assert!(
+            on_disk.get(&first_button_ref).is_none(),
+            "first drill ref must be invalidated by remove_by_root_ref"
+        );
+    }
+
+    #[test]
+    fn test_run_from_ref_multiple_drill_downs_accumulate() {
+        let _guard = HomeGuard::new();
+        seed_skeleton_refmap().save().unwrap();
+
+        let adapter_one = StubAdapter::new(named("button", "FromE1"));
+        let first = run_from_ref(&adapter_one, &drill_opts(), "@e1").unwrap();
+        let from_e1_ref = first.tree.ref_id.clone().expect("first drill ref");
+
+        let adapter_two = StubAdapter::new(named("button", "FromE2"));
+        let second = run_from_ref(&adapter_two, &drill_opts(), "@e2").unwrap();
+        let from_e2_ref = second.tree.ref_id.clone().expect("second drill ref");
+
+        let on_disk = RefMap::load().unwrap();
+        assert!(on_disk.get("@e1").is_some(), "skeleton @e1 preserved");
+        assert!(on_disk.get("@e2").is_some(), "skeleton @e2 preserved");
+        let entry_one = on_disk.get(&from_e1_ref).expect("@e1 drill survives");
+        assert_eq!(entry_one.root_ref.as_deref(), Some("@e1"));
+        let entry_two = on_disk.get(&from_e2_ref).expect("@e2 drill survives");
+        assert_eq!(entry_two.root_ref.as_deref(), Some("@e2"));
+    }
+
+    #[test]
+    fn test_run_from_ref_empty_subtree() {
+        let _guard = HomeGuard::new();
+        seed_skeleton_refmap().save().unwrap();
+
+        let adapter = StubAdapter::new(node("group"));
+        let result = run_from_ref(&adapter, &drill_opts(), "@e1").unwrap();
+
+        assert!(result.tree.children.is_empty());
+        assert_eq!(
+            result.refmap.len(),
+            2,
+            "no new refs added for empty subtree"
+        );
+    }
+
     #[test]
     fn test_allocate_refs_with_root_tags_entries() {
         let mut btn = node("button");