diff --git a/tests/Issues/Issue/Issue133Test.php b/tests/Issues/Issue/Issue133Test.php
new file mode 100644
index 0000000..02960f7
--- /dev/null
+++ b/tests/Issues/Issue/Issue133Test.php
@@ -0,0 +1,67 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPModelGenerator\Tests\Issues\Issue;
+
+use PHPModelGenerator\ModelGenerator;
+use PHPModelGenerator\SchemaProcessor\PostProcessor\EnumPostProcessor;
+use PHPModelGenerator\Tests\Issues\AbstractIssueTestCase;
+
+class Issue133Test extends AbstractIssueTestCase
+{
+    /**
+     * Demonstrates the bug: oneOf branches define the same property name (metric_id)
+     * with conflicting constraints (enum in branch 1, minLength/pattern in branch 2).
+     *
+     * ERROR:
+     *   Invalid value for property metric_id denied by filter php_model_generator_enum:
+     *   "custom_metric" is not a valid backing value for enum
+     *   Enum\Issue133Test_..._ItemOfArrayCommitted_Metrics..._Metric_Id
+     *
+     * WHERE:
+     *   Thrown at /tmp/PHPModelGeneratorTest/Models/Issue133Test_*.php:239
+     *   Inside the enum filter validator applied to the "metric_id" property
+     *
+     * WHY (speculation):
+     *   Both oneOf branches define "metric_id" — branch 1 with an enum validator
+     *   (["impressions", "spend", "clicks", "ctr"]) and branch 2 with
+     *   minLength/maxLength/pattern validators. During composition merging, the enum
+     *   validator from branch 1 is incorrectly applied to the merged property regardless
+     *   of which branch is active. Since "custom_metric" does not satisfy the enum constraint,
+     *   validation fails even though the input matches branch 2 perfectly.
+     *
+     *   The root cause is that shared property names across oneOf branches lose their
+     *   branch-level scoping — validators from all branches are merged/unconditionally
+     *   applied instead of being kept separate per-branch.
+     */
+    public function testOneOfWithConflictingPropertyConstraintsCausesSerializationError(): void
+    {
+        $this->modifyModelGenerator = static function (ModelGenerator $modelGenerator): void {
+            $modelGenerator->addPostProcessor(new EnumPostProcessor(
+                implode(DIRECTORY_SEPARATOR, [sys_get_temp_dir(), 'PHPModelGeneratorTest', 'Enum']),
+                'Enum',
+                true,
+            ));
+        };
+
+        $className = $this->generateClassFromFile('oneOfWithMetricIdConflict.json');
+
+        // Create a valid instance matching the second oneOf branch (vendor scope)
+        // metric_id "custom_metric" satisfies minLength/pattern but NOT the enum in branch 1
+        $object = new $className([
+            'committed_metrics' => [
+                ['scope' => 'standard', 'metric_id' => 'custom_metric'],
+            ],
+        ]);
+
+        // Serialize back to array - this should work but currently fails
+        // because the merged metric_id property loses its validators
+        $serialized = $object->toArray();
+
+        $this->assertSame(
+            ['committed_metrics' => [['scope' => 'standard', 'metric_id' => 'custom_metric']]],
+            $serialized,
+        );
+    }
+}
diff --git a/tests/Schema/Issues/133/oneOfWithMetricIdConflict.json b/tests/Schema/Issues/133/oneOfWithMetricIdConflict.json
new file mode 100644
index 0000000..ed6402e
--- /dev/null
+++ b/tests/Schema/Issues/133/oneOfWithMetricIdConflict.json
@@ -0,0 +1,58 @@
+{
+  "type": "object",
+  "properties": {
+    "committed_metrics": {
+      "type": "array",
+      "items": {
+        "type": "object",
+        "discriminator": {
+          "propertyName": "scope"
+        },
+        "oneOf": [
+          {
+            "properties": {
+              "scope": {
+                "type": "string",
+                "const": "vendor"
+              },
+              "metric_id": {
+                "enum": [
+                  "impressions",
+                  "spend",
+                  "clicks",
+                  "ctr"
+                ]
+              }
+            },
+            "required": [
+              "scope",
+              "metric_id"
+            ]
+          },
+          {
+            "properties": {
+              "scope": {
+                "type": "string",
+                "const": "standard"
+              },
+              "metric_id": {
+                "type": "string",
+                "minLength": 1,
+                "maxLength": 64,
+                "pattern": "^[a-z][a-z0-9_]*$"
+              }
+            },
+            "required": [
+              "scope",
+              "metric_id"
+            ]
+          }
+        ]
+      },
+      "minItems": 1
+    }
+  },
+  "required": [
+    "committed_metrics"
+  ]
+}