diff --git a/nixos/modules/services/web-servers/garage.nix b/nixos/modules/services/web-servers/garage.nix
index 39ea8f21b126..d2a5109e266a 100644
--- a/nixos/modules/services/web-servers/garage.nix
+++ b/nixos/modules/services/web-servers/garage.nix
@@ -52,13 +52,6 @@ in
             type = types.path;
             description = "The main data storage, put this on your large storage (e.g. high capacity HDD)";
           };
-
-          replication_mode = mkOption {
-            default = "none";
-            type = types.enum ([ "none" "1" "2" "3" "2-dangerous" "3-dangerous" "3-degraded" 1 2 3 ]);
-            apply = v: toString v;
-            description = "Garage replication mode, defaults to none, see: <https://garagehq.deuxfleurs.fr/documentation/reference-manual/configuration/#replication-mode> for reference.";
-          };
         };
       };
       description = "Garage configuration, see <https://garagehq.deuxfleurs.fr/documentation/reference-manual/configuration/> for reference.";
@@ -71,6 +64,44 @@ in
   };
 
   config = mkIf cfg.enable {
+
+    assertions = [
+      # We removed our module-level default for replication_mode. If a user upgraded
+      # to garage 1.0.0 while relying on the module-level default, they would be left
+      # with a config which evaluates and builds, but then garage refuses to start
+      # because either replication_factor or replication_mode is required.
+      # The replication_factor option also was `toString`'ed before, which is
+      # now not possible anymore, so we prompt the user to change it to a string
+      # if present.
+      # These assertions can be removed in NixOS 24.11, when all users have been
+      # warned once.
+      {
+        assertion = (cfg.settings ? replication_factor || cfg.settings ? replication_mode) || lib.versionOlder cfg.package "1.0.0";
+        message = ''
+          Garage 1.0.0 requires an explicit replication factor to be set.
+          Please set replication_factor to 1 explicitly to preserve the previous behavior.
+          https://git.deuxfleurs.fr/Deuxfleurs/garage/src/tag/v1.0.0/doc/book/reference-manual/configuration.md#replication_factor
+
+        '';
+      }
+      {
+        assertion = lib.isString (cfg.settings.replication_mode or "");
+        message = ''
+          The explicit `replication_mode` option in `services.garage.settings`
+          has been removed and is now handled by the freeform settings in order
+          to allow it being completely absent (for Garage 1.x).
+          That module option previously `toString`'ed the value it's configured
+          with, which is now no longer possible.
+
+          You're still using a non-string here, please manually set it to
+          a string, or migrate to the separate setting keys introduced in 1.x.
+
+          Refer to https://garagehq.deuxfleurs.fr/documentation/working-documents/migration-1/
+          for the migration guide.
+        '';
+      }
+    ];
+
     environment.etc."garage.toml" = {
       source = configFile;
     };
diff --git a/nixos/tests/garage/with-3node-replication.nix b/nixos/tests/garage/with-3node-replication.nix
index d4387b198d97..266a1082893f 100644
--- a/nixos/tests/garage/with-3node-replication.nix
+++ b/nixos/tests/garage/with-3node-replication.nix
@@ -7,10 +7,10 @@ args@{ mkNode, ver, ... }:
   };
 
   nodes = {
-    node1 = mkNode { replicationMode = 3; publicV6Address = "fc00:1::1"; };
-    node2 = mkNode { replicationMode = 3; publicV6Address = "fc00:1::2"; };
-    node3 = mkNode { replicationMode = 3; publicV6Address = "fc00:1::3"; };
-    node4 = mkNode { replicationMode = 3; publicV6Address = "fc00:1::4"; };
+    node1 = mkNode { replicationMode = "3"; publicV6Address = "fc00:1::1"; };
+    node2 = mkNode { replicationMode = "3"; publicV6Address = "fc00:1::2"; };
+    node3 = mkNode { replicationMode = "3"; publicV6Address = "fc00:1::3"; };
+    node4 = mkNode { replicationMode = "3"; publicV6Address = "fc00:1::4"; };
   };
 
   testScript = ''