diff --git a/docs/changelog/100886.yaml b/docs/changelog/100886.yaml
new file mode 100644
index 0000000000000..b926f924c7a7c
--- /dev/null
+++ b/docs/changelog/100886.yaml
@@ -0,0 +1,5 @@
+pr: 100886
+summary: Use the correct writable name for model assignment metadata in mixed version clusters. Prevents a node failure due to IllegalArgumentException Unknown NamedWriteable [trained_model_assignment]
+area: Machine Learning
+type: bug
+issues: []
diff --git a/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/inference/assignment/TrainedModelAssignmentMetadata.java b/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/inference/assignment/TrainedModelAssignmentMetadata.java
index 8391d287a6847..aabedfc4351b5 100644
--- a/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/inference/assignment/TrainedModelAssignmentMetadata.java
+++ b/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/inference/assignment/TrainedModelAssignmentMetadata.java
@@ -309,7 +309,8 @@ private TrainedModeAssignmentDiff(final StreamInput in, String writeableName) th
         @Override
         public Metadata.Custom apply(Metadata.Custom part) {
             return new TrainedModelAssignmentMetadata(
-                new TreeMap<>(modelRoutingEntries.apply(((TrainedModelAssignmentMetadata) part).deploymentRoutingEntries))
+                new TreeMap<>(modelRoutingEntries.apply(((TrainedModelAssignmentMetadata) part).deploymentRoutingEntries)),
+                writeableName
             );
         }