diff --git a/core/src/main/kotlin/org/evomaster/core/problem/httpws/HttpWsCallResult.kt b/core/src/main/kotlin/org/evomaster/core/problem/httpws/HttpWsCallResult.kt
index 4f2ffe7a2e..eb79d8b41f 100644
--- a/core/src/main/kotlin/org/evomaster/core/problem/httpws/HttpWsCallResult.kt
+++ b/core/src/main/kotlin/org/evomaster/core/problem/httpws/HttpWsCallResult.kt
@@ -487,6 +487,12 @@ abstract class HttpWsCallResult : EnterpriseActionResult {
                 return observed.deepCopy<JsonNode>()
             }
 
+            // Preserve a size difference recorded by an earlier observation.
+            // Its elements can no longer be merged using the original indices.
+            if (merged.size() != original.size()) {
+                return merged
+            }
+
             for (i in 0 until observed.size()) {
                 merged.set(i, mergeJsonDiffFromOriginal(original[i], observed[i], merged[i]))
             }
diff --git a/core/src/test/kotlin/org/evomaster/core/problem/rest/RestCallResultTest.kt b/core/src/test/kotlin/org/evomaster/core/problem/rest/RestCallResultTest.kt
index 8876f415b8..3f1bed4061 100644
--- a/core/src/test/kotlin/org/evomaster/core/problem/rest/RestCallResultTest.kt
+++ b/core/src/test/kotlin/org/evomaster/core/problem/rest/RestCallResultTest.kt
@@ -269,4 +269,24 @@ internal class RestCallResultTest {
 
         assertEquals("first", original.getMergedFlakyBody())
     }
+
+    @Test
+    fun testMergedFlakyBodyPreservesArraySizeChangesAcrossObservations(){
+        for (changedArray in listOf("[]", "[1]", "[1,2,3]")) {
+            val original = createCallResult("""{"items":[1,2]}""")
+            original.recordFlakyObservation(createCallResult("""{"items":$changedArray}"""), 1)
+            original.recordFlakyObservation(createCallResult("""{"items":[1,3]}"""), 2)
+
+            assertEquals("""{"items":$changedArray}""", original.getMergedFlakyBody())
+        }
+    }
+
+    @Test
+    fun testMergedFlakyBodyPreservesNestedArraySizeChangeAndOtherDeltas(){
+        val original = createCallResult("""{"items":[{"values":[1,2]}],"first":"a","second":"b"}""")
+        original.recordFlakyObservation(createCallResult("""{"items":[{"values":[]}],"first":"x","second":"b"}"""), 1)
+        original.recordFlakyObservation(createCallResult("""{"items":[{"values":[1,3]}],"first":"a","second":"y"}"""), 2)
+
+        assertEquals("""{"items":[{"values":[]}],"first":"x","second":"y"}""", original.getMergedFlakyBody())
+    }
 }