diff --git a/api/routes.py b/api/routes.py
index ac0d143f..7252fed7 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -9407,6 +9407,7 @@ from api.workspace import (
     EscapeAuthorizationExpiredError,
     list_dir,
     list_authorized_escape_dir,
+    serialize_workspace_entries_for_browser,
     dir_signature,
     list_workspace_suggestions,
     read_file_content,
@@ -16636,7 +16637,7 @@ def _handle_list_dir(handler, parsed):
         return j(
             handler,
             {
-                "entries": entries,
+                "entries": serialize_workspace_entries_for_browser(entries),
                 "signature": dir_signature(Path(workspace), rel_path, entries),
                 "path": rel_path,
             },
@@ -16706,6 +16707,7 @@ def _handle_escape_list_dir(handler, parsed):
     rel_path = qs.get("path", ["."])[0]
     try:
         payload = list_authorized_escape_dir(Path(s.workspace), sid, token, rel_path)
+        payload["entries"] = serialize_workspace_entries_for_browser(payload.get("entries"))
         return j(handler, payload)
     except FileNotFoundError as exc:
         return bad(handler, _sanitize_error(exc), 404)
diff --git a/api/workspace.py b/api/workspace.py
index 8f03b490..5c0ba580 100644
--- a/api/workspace.py
+++ b/api/workspace.py
@@ -16,6 +16,7 @@ import secrets
 import shutil
 import stat
 import subprocess
+import sys
 import concurrent.futures
 import threading
 import time
@@ -1231,6 +1232,40 @@ def rename_anchored(root: Path, source: Path, dest: Path) -> None:
         os.close(src_parent_fd)
 
 
+def _birthtime_ns(lst) -> int | None:
+    """Return creation time in ns, or None when the platform lacks birthtime."""
+    value = getattr(lst, 'st_birthtime_ns', None)
+    if value is not None:
+        return value
+    value = getattr(lst, 'st_birthtime', None)
+    if value is not None:
+        return int(value * 1_000_000_000)
+    if sys.platform == 'win32':
+        return getattr(lst, 'st_ctime_ns', None)
+    return None
+
+
+def _browser_timestamp_ns(value) -> str | None:
+    if value is None:
+        return None
+    try:
+        return str(int(value))
+    except (TypeError, ValueError):
+        return None
+
+
+def serialize_workspace_entries_for_browser(entries: list[dict] | None) -> list[dict]:
+    payload = []
+    for entry in entries or []:
+        item = dict(entry or {})
+        if 'mtime_ns' in item:
+            item['mtime_ns'] = _browser_timestamp_ns(item.get('mtime_ns'))
+        if 'birthtime_ns' in item:
+            item['birthtime_ns'] = _browser_timestamp_ns(item.get('birthtime_ns'))
+        payload.append(item)
+    return payload
+
+
 def list_dir(workspace: Path, rel: str='.'):
     target = safe_resolve_ws(workspace, rel)
     if not target.is_dir():
@@ -1295,6 +1330,7 @@ def list_dir(workspace: Path, rel: str='.'):
                     'is_dir': False,
                     'target_outside_workspace': True,
                     'mtime_ns': mtime_ns,
+                    'birthtime_ns': _birthtime_ns(lstat_result) if lstat_result is not None else None,
                 }
                 entries.append(entry)
             else:
@@ -1307,6 +1343,7 @@ def list_dir(workspace: Path, rel: str='.'):
                     'is_dir': is_dir,
                     'target_outside_workspace': False,
                     'mtime_ns': mtime_ns,
+                    'birthtime_ns': _birthtime_ns(lstat_result) if lstat_result is not None else None,
                 }
                 if not is_dir:
                     try:
@@ -1333,6 +1370,7 @@ def list_dir(workspace: Path, rel: str='.'):
                 'type': 'dir' if is_dir_entry else 'file',
                 'size': size,
                 'mtime_ns': mtime_ns,
+                'birthtime_ns': _birthtime_ns(lstat_result) if lstat_result is not None else None,
             })
 
     if _DIR_FD_OK:
diff --git a/tests/test_issue6066_workspace_sort_api_payload.py b/tests/test_issue6066_workspace_sort_api_payload.py
new file mode 100644
index 00000000..e7e06894
--- /dev/null
+++ b/tests/test_issue6066_workspace_sort_api_payload.py
@@ -0,0 +1,31 @@
+"""Route-facing regression coverage for exact workspace timestamp transport."""
+
+from api import workspace as workspace_api
+
+
+def test_browser_workspace_entries_stringify_nanosecond_fields():
+    entries = [
+        {
+            "name": "newer.txt",
+            "path": "newer.txt",
+            "type": "file",
+            "mtime_ns": 1_752_598_800_000_000_001,
+            "birthtime_ns": 1_752_598_800_000_000_000,
+        },
+        {
+            "name": "missing.txt",
+            "path": "missing.txt",
+            "type": "file",
+            "mtime_ns": None,
+            "birthtime_ns": None,
+        },
+    ]
+
+    payload = workspace_api.serialize_workspace_entries_for_browser(entries)
+
+    assert payload[0]["mtime_ns"] == "1752598800000000001"
+    assert payload[0]["birthtime_ns"] == "1752598800000000000"
+    assert payload[1]["mtime_ns"] is None
+    assert payload[1]["birthtime_ns"] is None
+    assert entries[0]["mtime_ns"] == 1_752_598_800_000_000_001
+    assert entries[0]["birthtime_ns"] == 1_752_598_800_000_000_000