diff --git a/loopx/state_migration.py b/loopx/state_migration.py
index cbd9fc2a18..d11c0a44f9 100644
--- a/loopx/state_migration.py
+++ b/loopx/state_migration.py
@@ -135,6 +135,37 @@ def copy_rewritten_text_file(source: Path, target: Path, *, goal_id_map: dict[st
     atomic_write_state_text(target, rewrite_text(text, goal_id_map=goal_id_map, path_map=path_map))
 
 
+def _planned_state_bytes(source: Path, *, goal_id_map: dict[str, str], path_map: dict[str, str]) -> bytes:
+    """The exact bytes ``copy_rewritten_text_file`` publishes for ``source``."""
+    try:
+        text = source.read_text(encoding="utf-8")
+    except UnicodeDecodeError:
+        return source.read_bytes()
+    return rewrite_text(text, goal_id_map=goal_id_map, path_map=path_map).encode("utf-8")
+
+
+def _require_no_active_state_clobber(
+    source_path: Path,
+    target_path: Path,
+    *,
+    goal_id_map: dict[str, str],
+    path_map: dict[str, str],
+) -> None:
+    """Refuse to replace a different live target state with the legacy copy.
+
+    A repeated migration (for example ``--all-goals`` after more legacy Goals
+    appear) must not roll back Todos completed or added at the target since the
+    first copy. Identical content and in-place rewrites remain idempotent.
+    """
+    if not target_path.exists() or source_path.resolve() == target_path.resolve():
+        return
+    if target_path.read_bytes() != _planned_state_bytes(source_path, goal_id_map=goal_id_map, path_map=path_map):
+        raise ValueError(
+            f"migrated active state already exists with different content: {target_path}; "
+            "refusing to overwrite it with the legacy copy"
+        )
+
+
 def copy_active_state_files(
     pairs: list[tuple[dict[str, Any], dict[str, Any]]],
     *,
@@ -142,10 +173,23 @@ def copy_active_state_files(
     path_map: dict[str, str],
     execute: bool,
 ) -> list[dict[str, Any]]:
+    resolved_pairs = [
+        (
+            source_goal,
+            target_goal,
+            resolve_goal_state(source_goal.get("repo"), source_goal.get("state_file")),
+            resolve_goal_state(target_goal.get("repo"), target_goal.get("state_file")),
+        )
+        for source_goal, target_goal in pairs
+    ]
+    # Check every target before the first copy so a conflict leaves no partial write.
+    for _source_goal, _target_goal, source_path, target_path in resolved_pairs:
+        if source_path and target_path and source_path.exists():
+            _require_no_active_state_clobber(
+                source_path, target_path, goal_id_map=goal_id_map, path_map=path_map
+            )
     results: list[dict[str, Any]] = []
-    for source_goal, target_goal in pairs:
-        source_path = resolve_goal_state(source_goal.get("repo"), source_goal.get("state_file"))
-        target_path = resolve_goal_state(target_goal.get("repo"), target_goal.get("state_file"))
+    for _source_goal, target_goal, source_path, target_path in resolved_pairs:
         row: dict[str, Any] = {
             "goal_id": target_goal.get("id"),
             "source": str(source_path) if source_path else None,
diff --git a/tests/test_state_migration.py b/tests/test_state_migration.py
index bf46235a7d..4661e08a23 100644
--- a/tests/test_state_migration.py
+++ b/tests/test_state_migration.py
@@ -76,3 +76,70 @@ def test_migration_rejects_target_goal_id_collisions_before_writing(
         for target_repo in target_repos.values()
     )
     assert not (target_runtime / "goals" / "goal-b").exists()
+
+
+def _active_state_migration_fixture(tmp_path: Path) -> dict[str, Path]:
+    repo = tmp_path / "repo"
+    source_state = repo / ".codex" / "goals" / "goal-a" / "ACTIVE_GOAL_STATE.md"
+    source_state.parent.mkdir(parents=True)
+    source_state.write_text("## Agent Todo\n\n- [ ] legacy todo\n", encoding="utf-8")
+    legacy_registry = tmp_path / "legacy-registry.json"
+    legacy_registry.write_text(
+        json.dumps(
+            {
+                "schema_version": "0.1",
+                "goals": [
+                    {
+                        "id": "goal-a",
+                        "status": "active",
+                        "repo": str(repo),
+                        "state_file": ".codex/goals/goal-a/ACTIVE_GOAL_STATE.md",
+                    }
+                ],
+            }
+        ),
+        encoding="utf-8",
+    )
+    return {
+        "repo": repo,
+        "legacy_registry": legacy_registry,
+        "target_registry": repo / ".loopx" / "registry.json",
+        "target_state": repo / ".loopx" / "goals" / "goal-a" / "ACTIVE_GOAL_STATE.md",
+        "target_runtime": tmp_path / "target-runtime",
+    }
+
+
+def _migrate_active_state(paths: dict[str, Path], tmp_path: Path, *, execute: bool) -> dict:
+    return migrate_legacy_state(
+        legacy_registry_path=paths["legacy_registry"],
+        target_registry_path=paths["target_registry"],
+        legacy_runtime_root=tmp_path / "legacy-runtime",
+        target_runtime_root=paths["target_runtime"],
+        goal_ids=["goal-a"],
+        goal_id_map={},
+        path_map={".codex/goals": ".loopx/goals"},
+        copy_active_state=True,
+        copy_runtime=False,
+        execute=execute,
+    )
+
+
+def test_repeated_migration_does_not_overwrite_diverged_target_active_state(
+    tmp_path: Path,
+) -> None:
+    paths = _active_state_migration_fixture(tmp_path)
+    first = _migrate_active_state(paths, tmp_path, execute=True)
+    assert first["active_state"][0]["copied"] is True
+    # An identical rerun is idempotent.
+    assert _migrate_active_state(paths, tmp_path, execute=True)["ok"] is True
+
+    progressed = "## Agent Todo\n\n- [x] legacy todo\n- [ ] work added after migration\n"
+    paths["target_state"].write_text(progressed, encoding="utf-8")
+    registry_before = paths["target_registry"].read_bytes()
+
+    for execute in (False, True):
+        with pytest.raises(ValueError, match="already exists with different content"):
+            _migrate_active_state(paths, tmp_path, execute=execute)
+
+    assert paths["target_state"].read_text(encoding="utf-8") == progressed
+    assert paths["target_registry"].read_bytes() == registry_before