diff --git a/src/auth/src/supabase_auth/_async/gotrue_base_api.py b/src/auth/src/supabase_auth/_async/gotrue_base_api.py
index ae0bcaa9..76e73b3b 100644
--- a/src/auth/src/supabase_auth/_async/gotrue_base_api.py
+++ b/src/auth/src/supabase_auth/_async/gotrue_base_api.py
@@ -21,7 +21,11 @@ def __init__(
         proxy: Optional[str] = None,
     ) -> None:
         self._url = url
-        self._headers = headers
+        # Copy: callers pass a dict they keep mutating, and GoTrueClient hands
+        # its own `_headers` to the admin API. Sharing the object means a later
+        # `_headers["Authorization"] = <user token>` silently downgrades every
+        # other holder of that dict.
+        self._headers = dict(headers)
         self._http_client = http_client or AsyncClient(
             verify=bool(verify),
             proxy=proxy,
diff --git a/src/auth/src/supabase_auth/_sync/gotrue_base_api.py b/src/auth/src/supabase_auth/_sync/gotrue_base_api.py
index 727478eb..627d13f0 100644
--- a/src/auth/src/supabase_auth/_sync/gotrue_base_api.py
+++ b/src/auth/src/supabase_auth/_sync/gotrue_base_api.py
@@ -21,7 +21,11 @@ def __init__(
         proxy: Optional[str] = None,
     ) -> None:
         self._url = url
-        self._headers = headers
+        # Copy: callers pass a dict they keep mutating, and GoTrueClient hands
+        # its own `_headers` to the admin API. Sharing the object means a later
+        # `_headers["Authorization"] = <user token>` silently downgrades every
+        # other holder of that dict.
+        self._headers = dict(headers)
         self._http_client = http_client or Client(
             verify=bool(verify),
             proxy=proxy,
diff --git a/src/auth/tests/_async/test_admin_headers.py b/src/auth/tests/_async/test_admin_headers.py
new file mode 100644
index 00000000..47192f22
--- /dev/null
+++ b/src/auth/tests/_async/test_admin_headers.py
@@ -0,0 +1,38 @@
+from supabase_auth import AsyncGoTrueAdminAPI, AsyncGoTrueClient
+
+
+async def test_admin_api_does_not_share_the_client_header_dict() -> None:
+    """The admin API must hold its own headers.
+
+    `GoTrueClient` passes its own `_headers` to the admin API. If that dict is
+    shared, anything that later rewrites the client's `Authorization` header
+    also rewrites the admin one, and admin calls silently run with the user's
+    token instead of the service key.
+    """
+    client = AsyncGoTrueClient(
+        url="http://localhost:9998",
+        headers={"Authorization": "Bearer service-key"},
+        auto_refresh_token=False,
+        persist_session=False,
+    )
+
+    assert client.admin._headers is not client._headers
+
+    client._headers["Authorization"] = "Bearer user-token"
+
+    assert client.admin._headers["Authorization"] == "Bearer service-key"
+
+
+async def test_caller_headers_are_copied() -> None:
+    """A caller that keeps mutating the dict it passed in must not reach us."""
+    caller_headers = {"Authorization": "Bearer service-key"}
+
+    admin = AsyncGoTrueAdminAPI(
+        url="http://localhost:9998",
+        headers=caller_headers,
+        http_client=None,
+    )
+
+    caller_headers["Authorization"] = "Bearer user-token"
+
+    assert admin._headers["Authorization"] == "Bearer service-key"
diff --git a/src/auth/tests/_sync/test_admin_headers.py b/src/auth/tests/_sync/test_admin_headers.py
new file mode 100644
index 00000000..05997b83
--- /dev/null
+++ b/src/auth/tests/_sync/test_admin_headers.py
@@ -0,0 +1,38 @@
+from supabase_auth import SyncGoTrueAdminAPI, SyncGoTrueClient
+
+
+def test_admin_api_does_not_share_the_client_header_dict() -> None:
+    """The admin API must hold its own headers.
+
+    `GoTrueClient` passes its own `_headers` to the admin API. If that dict is
+    shared, anything that later rewrites the client's `Authorization` header
+    also rewrites the admin one, and admin calls silently run with the user's
+    token instead of the service key.
+    """
+    client = SyncGoTrueClient(
+        url="http://localhost:9998",
+        headers={"Authorization": "Bearer service-key"},
+        auto_refresh_token=False,
+        persist_session=False,
+    )
+
+    assert client.admin._headers is not client._headers
+
+    client._headers["Authorization"] = "Bearer user-token"
+
+    assert client.admin._headers["Authorization"] == "Bearer service-key"
+
+
+def test_caller_headers_are_copied() -> None:
+    """A caller that keeps mutating the dict it passed in must not reach us."""
+    caller_headers = {"Authorization": "Bearer service-key"}
+
+    admin = SyncGoTrueAdminAPI(
+        url="http://localhost:9998",
+        headers=caller_headers,
+        http_client=None,
+    )
+
+    caller_headers["Authorization"] = "Bearer user-token"
+
+    assert admin._headers["Authorization"] == "Bearer service-key"