test: Add coinstatsindex compatibility test

diff --git a/test/functional/feature_coinstatsindex_compatibility.py b/test/functional/feature_coinstatsindex_compatibility.py
new file mode 100755
index 00000000..357700a7
--- /dev/null
+++ b/test/functional/feature_coinstatsindex_compatibility.py
@@ -0,0 +1,64 @@
+#!/usr/bin/env python3
+# Copyright (c) 2025 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test coinstatsindex across node versions.
+
+This test may be removed some time after v29 has reached end of life.
+"""
+
+import shutil
+
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import assert_equal
+
+
+class CoinStatsIndexTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 2
+        self.supports_cli = False
+        self.extra_args = [["-coinstatsindex"],["-coinstatsindex"]]
+
+    def skip_test_if_missing_module(self):
+        self.skip_if_no_previous_releases()
+
+    def setup_nodes(self):
+        self.add_nodes(
+            self.num_nodes,
+            extra_args=self.extra_args,
+            versions=[
+                None,
+                280200,
+            ],
+        )
+        self.start_nodes()
+
+    def run_test(self):
+        self._test_coin_stats_index_compatibility()
+
+    def _test_coin_stats_index_compatibility(self):
+        node = self.nodes[0]
+        legacy_node = self.nodes[1]
+        for n in self.nodes:
+            self.wait_until(lambda: n.getindexinfo()['coinstatsindex']['synced'] is True)
+
+        self.log.info("Test that gettxoutsetinfo() output is consistent between the different index versions")
+        res0 = node.gettxoutsetinfo('muhash')
+        res1 = legacy_node.gettxoutsetinfo('muhash')
+        assert_equal(res1, res0)
+
+        self.log.info("Test that gettxoutsetinfo() output is consistent for the new index running on a datadir with the old version")
+        self.stop_nodes()
+        shutil.rmtree(node.chain_path / "indexes" / "coinstatsindex")
+        shutil.copytree(legacy_node.chain_path / "indexes" / "coinstats", node.chain_path / "indexes" / "coinstats")
+        old_version_path = node.chain_path / "indexes" / "coinstats"
+        msg = f'[warning] Old version of coinstatsindex found at {old_version_path}. This folder can be safely deleted unless you plan to downgrade your node to version 29 or lower.'
+        with node.assert_debug_log(expected_msgs=[msg]):
+            self.start_node(0, ['-coinstatsindex'])
+        self.wait_until(lambda: node.getindexinfo()['coinstatsindex']['synced'] is True)
+        res2 = node.gettxoutsetinfo('muhash')
+        assert_equal(res2, res0)
+
+
+if __name__ == '__main__':
+    CoinStatsIndexTest(__file__).main()
diff --git a/test/functional/test_runner.py b/test/functional/test_runner.py
index f2e514a7..94965618 100755
--- a/test/functional/test_runner.py
+++ b/test/functional/test_runner.py
@@ -342,6 +342,7 @@ BASE_SCRIPTS = [
     'feature_anchors.py',
     'mempool_datacarrier.py',
     'feature_coinstatsindex.py',
+    'feature_coinstatsindex_compatibility.py',
     'wallet_orphanedreward.py',
     'wallet_timelock.py',
     'p2p_permissions.py',