diff --git a/migrations/postgres/000018_prefix_snapshot_contract.down.sql b/migrations/postgres/000018_prefix_snapshot_contract.down.sql
new file mode 100644
index 0000000..163fcb6
--- /dev/null
+++ b/migrations/postgres/000018_prefix_snapshot_contract.down.sql
@@ -0,0 +1 @@
+COMMENT ON COLUMN config_revision.prefix_snapshot_id IS NULL;
diff --git a/migrations/postgres/000018_prefix_snapshot_contract.up.sql b/migrations/postgres/000018_prefix_snapshot_contract.up.sql
new file mode 100644
index 0000000..853bd5e
--- /dev/null
+++ b/migrations/postgres/000018_prefix_snapshot_contract.up.sql
@@ -0,0 +1,3 @@
+-- H2 contract marker: new revisions use prefix_snapshot_id only (enforced in application code).
+
+COMMENT ON COLUMN config_revision.prefix_snapshot_id IS 'Materialized prefixes; revision_materialized_prefix deprecated for new rows';
diff --git a/migrations/sqlite/000018_prefix_snapshot_contract.down.sql b/migrations/sqlite/000018_prefix_snapshot_contract.down.sql
new file mode 100644
index 0000000..e69de29
diff --git a/migrations/sqlite/000018_prefix_snapshot_contract.up.sql b/migrations/sqlite/000018_prefix_snapshot_contract.up.sql
new file mode 100644
index 0000000..8e18aca
--- /dev/null
+++ b/migrations/sqlite/000018_prefix_snapshot_contract.up.sql
@@ -0,0 +1 @@
+-- H2 contract marker (application stops writing revision_materialized_prefix for new revisions).