diff --git a/rdagent/components/coder/factor_coder/auto_fixer.py b/rdagent/components/coder/factor_coder/auto_fixer.py
index 6c7b1b78..b9a277d5 100644
--- a/rdagent/components/coder/factor_coder/auto_fixer.py
+++ b/rdagent/components/coder/factor_coder/auto_fixer.py
@@ -654,6 +654,25 @@ class FactorAutoFixer:
                 f"groupby: {df_var}.groupby(level={level})['{col}'].apply() → transform()"
             )
 
+        # === FIX: .transform(...).reset_index(level=N, drop=True) ===
+        # transform() already returns the same index as the input — adding reset_index()
+        # after it drops an index level and causes ValueError on assignment back to df['col'].
+        # Detected line-by-line: if a line contains both .transform( and .reset_index(level=
+        reset_suffix = re.compile(r'\s*\.reset_index\s*\(\s*level\s*=[^,)]+,\s*drop\s*=\s*True\s*\)\s*$')
+        new_lines = []
+        changed = False
+        for line in fixed_code.splitlines():
+            if '.transform(' in line and '.reset_index(' in line:
+                cleaned = reset_suffix.sub('', line)
+                if cleaned != line:
+                    new_lines.append(cleaned)
+                    changed = True
+                    continue
+            new_lines.append(line)
+        if changed:
+            fixed_code = '\n'.join(new_lines)
+            self.fixes_applied.append("groupby: removed spurious .reset_index() after .transform()")
+
         # Pattern: Simple groupby().apply() with rolling().method()
         # df.groupby(level=N).apply(lambda x: x['col'].rolling(...).method())
         apply_pattern = r"df\.groupby\(level=(\d+)\)\.apply\(\s*lambda\s+x:\s+x\['([^']+)'\]\.rolling\([^)]+\)\.(\w+)\([^)]*\)\s*\)"
diff --git a/test/qlib/test_auto_fixer.py b/test/qlib/test_auto_fixer.py
index e56c27d6..09698ef6 100644
--- a/test/qlib/test_auto_fixer.py
+++ b/test/qlib/test_auto_fixer.py
@@ -197,6 +197,13 @@ class TestGroupbyApplyToTransform:
         assert "lambda x: x.cumsum()" in result
         assert ".transform(" in result
 
+    def test_transform_reset_index_stripped(self, fixer):
+        # .transform() already preserves index — .reset_index() after it is wrong
+        code = "df['v'] = df.groupby(level=1)['x'].transform(lambda x: x.rolling(20).mean()).reset_index(level=0, drop=True)"
+        result = fixer.fix(code)
+        assert ".reset_index(level=0, drop=True)" not in result
+        assert ".transform(" in result
+
 
 class TestRollingDdof:
     def test_removes_ddof_from_rolling_args(self, fixer):