diff --git a/doc/release-notes/rl-2511.section.md b/doc/release-notes/rl-2511.section.md
index a62e6bc0c001d..6267735ddb2a7 100644
--- a/doc/release-notes/rl-2511.section.md
+++ b/doc/release-notes/rl-2511.section.md
@@ -177,7 +177,13 @@
 ### Deprecations {#sec-nixpkgs-release-25.11-lib-deprecations}
 
 - Create the first release note entry in this section!
+- `types.either` silently accepted mismatching types when used in `freeformType`. Module maintainers should fix the used type
+  In most cases wrapping `either` with `attrsOf` should be sufficient.
 
+  Since types.either was used to bootstrap other types. This also affects the following types:
+  - `oneOf`
+  - `number`
+  - `numbers.*`
 
 ### Additions and Improvements {#sec-nixpkgs-release-25.11-lib-additions-improvements}
 
diff --git a/lib/tests/modules.sh b/lib/tests/modules.sh
index fbbccd8172f67..5c715bc7f8c90 100755
--- a/lib/tests/modules.sh
+++ b/lib/tests/modules.sh
@@ -257,6 +257,10 @@ checkConfigError 'A definition for option .* is not of type.*between.*-21 and 43
 # types.either
 checkConfigOutput '^42$' config.value ./declare-either.nix ./define-value-int-positive.nix
 checkConfigOutput '^"24"$' config.value ./declare-either.nix ./define-value-string.nix
+
+# Check regression detected in https://github.com/NixOS/nixpkgs/issues/438459
+# Can be removed after 26.05 release.
+checkConfigOutput '^"bar"$' config.foo ./freeform-either.nix
 # types.oneOf
 checkConfigOutput '^42$' config.value ./declare-oneOf.nix ./define-value-int-positive.nix
 checkConfigOutput '^\[\]$' config.value ./declare-oneOf.nix ./define-value-list.nix
diff --git a/lib/tests/modules/freeform-either.nix b/lib/tests/modules/freeform-either.nix
new file mode 100644
index 0000000000000..edbd359315434
--- /dev/null
+++ b/lib/tests/modules/freeform-either.nix
@@ -0,0 +1,7 @@
+{ lib, ... }:
+{
+  # Proper type would be attrsOf (either int str)
+  # This checks backwards compatibility for the incorrect usage of either
+  freeformType = lib.types.either lib.types.int lib.types.str;
+  foo = "bar";
+}
diff --git a/lib/types.nix b/lib/types.nix
index 6b51f9254a00c..33acd09f1304f 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -106,6 +106,14 @@ let
     in
     if invalidDefs != [ ] then { message = "Definition values: ${showDefs invalidDefs}"; } else null;
 
+  # Compat helper for merge to safely access the .value of merge.v2
+  checkHeadError =
+    loc: descr: checkedAndMerged:
+    if checkedAndMerged.headError or null != null then
+      throw "A definition for option `${showOption loc}' is not of type `${descr}'. TypeError: ${checkedAndMerged.headError.message}"
+    else
+      checkedAndMerged.value;
+
   outer_types = rec {
     isType = type: x: (x._type or "") == type;
 
@@ -715,7 +723,7 @@ let
           merge = {
             __functor =
               self: loc: defs:
-              (self.v2 { inherit loc defs; }).value;
+              checkHeadError loc description (self.v2 { inherit loc defs; });
             v2 =
               { loc, defs }:
               let
@@ -828,7 +836,7 @@ let
           merge = {
             __functor =
               self: loc: defs:
-              (self.v2 { inherit loc defs; }).value;
+              checkHeadError loc description (self.v2 { inherit loc defs; });
             v2 =
               { loc, defs }:
               let
@@ -1252,7 +1260,7 @@ let
           merge = {
             __functor =
               self: loc: defs:
-              (self.v2 { inherit loc defs; }).value;
+              checkHeadError loc description (self.v2 { inherit loc defs; });
             v2 =
               { loc, defs }:
               let
@@ -1417,7 +1425,16 @@ let
           merge = {
             __functor =
               self: loc: defs:
-              (self.v2 { inherit loc defs; }).value;
+              let
+                cm = (self.v2 { inherit loc defs; });
+              in
+              if cm.headError ? fallback then
+                lib.warn (
+                  cm.headError.message
+                  + "\nIf you are the module maintainer, please fix the type. This will be turned into an error after 26.05"
+                ) cm.headError.fallback
+              else
+                checkHeadError loc description cm;
             v2 =
               { loc, defs }:
               let
@@ -1452,6 +1469,9 @@ let
                       };
                       headError = {
                         message = "The option `${showOption loc}` is neither a value of type `${t1.description}` nor `${t2.description}`, Definition values: ${showDefs defs}";
+                        # Specific for either, to ease migrations
+                        # until 26.05 this is a warning, afterwards it will be removed
+                        fallback = mergeOneOption loc defs;
                       };
                       value = abort "(t.merge.v2 defs).value must only be accessed when `.headError == null`. This is a bug in code that consumes a module system type.";
                     };
@@ -1501,7 +1521,7 @@ let
           merge = {
             __functor =
               self: loc: defs:
-              (self.v2 { inherit loc defs; }).value;
+              checkHeadError loc description (self.v2 { inherit loc defs; });
             v2 =
               { loc, defs }:
               let
@@ -1568,7 +1588,7 @@ let
             merge = {
               __functor =
                 self: loc: defs:
-                (self.v2 { inherit loc defs; }).value;
+                checkHeadError loc elemType.description (self.v2 { inherit loc defs; });
               v2 =
                 { loc, defs }:
                 let