diff --git a/inventory.schema.json b/inventory.schema.json
index 222cb4d..6d525a0 100644
--- a/inventory.schema.json
+++ b/inventory.schema.json
@@ -2983,5 +2983,16 @@
       "type": "string"
     }
   },
+  "if": {
+    "properties": {
+      "action": { "const": "netdiscovery" }
+    },
+    "required": ["action"]
+  },
+  "then": {
+    "properties": {
+      "tag": false
+    }
+  },
   "additionalProperties": false
 }
diff --git a/lib/php/Schema.php b/lib/php/Schema.php
index b7719e8..0a1900c 100644
--- a/lib/php/Schema.php
+++ b/lib/php/Schema.php
@@ -210,13 +210,22 @@ public function getPatterns(): array
      */
     public function validate($json): bool
     {
+        $rawSchema = null;
         try {
-            $schema = \Swaggest\JsonSchema\Schema::import($this->build());
+            $rawSchema = $this->build();
+            $schema = \Swaggest\JsonSchema\Schema::import($rawSchema);
 
             $context = new Context();
             $context->tolerateStrings = (!defined('TU_USER'));
             $schema->in($json, $context);
             return true;
+        } catch (\Swaggest\JsonSchema\InvalidValue $e) {
+            throw new RuntimeException(
+                sprintf(
+                    "JSON does not validate. Violations:\n%1\$s\n",
+                    $this->improveErrorMessage($e, $rawSchema)
+                )
+            );
         } catch (Exception $e) {
             throw new RuntimeException(
                 sprintf(
@@ -227,6 +236,32 @@ public function validate($json): bool
         }
     }
 
+    /**
+     * Translate a swaggest InvalidValue exception into a human-readable message.
+     * Handles the case where a property is forbidden by an if/then conditional rule.
+     *
+     * @param \Swaggest\JsonSchema\InvalidValue $e
+     * @param object $rawSchema The raw (decoded) schema object
+     * @return string
+     */
+    private function improveErrorMessage(\Swaggest\JsonSchema\InvalidValue $e, object $rawSchema): string
+    {
+        $path = $e->path ?? '';
+        // Pattern: "->then->properties:PROP->not" means PROP is forbidden by a conditional rule
+        if (preg_match('/->then->properties:(\w+)->not$/', $path, $matches)) {
+            $property = $matches[1];
+            if (isset($rawSchema->{'if'}->properties->action->const)) {
+                return sprintf(
+                    'Property "%s" is not allowed when action is "%s"',
+                    $property,
+                    $rawSchema->{'if'}->properties->action->const
+                );
+            }
+            return sprintf('Property "%s" is not allowed in this context', $property);
+        }
+        return $e->getMessage();
+    }
+
     /**
      * Get current schema version
      *
diff --git a/tests/Glpi/Inventory/tests/units/Schema.php b/tests/Glpi/Inventory/tests/units/Schema.php
index bda21a5..718a77a 100644
--- a/tests/Glpi/Inventory/tests/units/Schema.php
+++ b/tests/Glpi/Inventory/tests/units/Schema.php
@@ -29,7 +29,7 @@ public function testSchemaPath(): void
 
     public function testValidateOK(): void
     {
-        $json = json_decode(json_encode(['deviceid' => 'myid', 'content' => ['versionclient' => 'GLPI-Agent_v1.0', 'hardware' => ['name' => 'my inventory']]]));
+        $json = json_decode(json_encode(['deviceid' => 'myid', 'tag' => 'mytag', 'content' => ['versionclient' => 'GLPI-Agent_v1.0', 'hardware' => ['name' => 'my inventory']]]));
         $instance = new \Glpi\Inventory\Schema();
         $this->assertTrue($instance->validate($json));
     }
@@ -155,4 +155,31 @@ public function testVersion(): void
         $instance = new \Glpi\Inventory\Schema();
         $this->assertIsFloat($instance->getVersion());
     }
+
+    public function testNoTagOnNetdiscoveryRequests(): void
+    {
+        $json = json_decode(json_encode([
+            'content' => [
+                'hardware' => [
+                    'workgroup' => 'WORKGROUP',
+                ],
+                'versionclient' => '5.1',
+                'network_device' => [
+                    'type' => 'Unmanaged',
+                    'mac' => '4c:cc:6a:02:13:a9',
+                    'name' => 'DESKTOP-A3J16LF',
+                    'ips' => ['192.168.1.20'],
+                ],
+            ],
+            'deviceid' => 'asus-desktop-2022-09-20-16-43-09',
+            'action' => 'netdiscovery',
+            'jobid' => 189,
+            'itemtype' => 'Unmanaged',
+            'tag' => 'tag_is_not_allowed_in_netdiscovery',
+        ]));
+        $instance = new \Glpi\Inventory\Schema();
+        $this->expectException(\RuntimeException::class);
+        $this->expectExceptionMessage('Property "tag" is not allowed when action is "netdiscovery"');
+        $instance->validate($json);
+    }
 }
diff --git a/tests/data/1.xml b/tests/data/1.xml
index ecd4683..4957116 100644
--- a/tests/data/1.xml
+++ b/tests/data/1.xml
@@ -1734,4 +1734,5 @@
   </CONTENT>
   <DEVICEID>LF014-2016-06-13-14-03-53</DEVICEID>
   <QUERY>INVENTORY</QUERY>
+  <TAG>Mytag</TAG>
 </REQUEST>
diff --git a/tests/data/5.xml b/tests/data/5.xml
index 4cb1958..26f6227 100644
--- a/tests/data/5.xml
+++ b/tests/data/5.xml
@@ -311,4 +311,5 @@ ARM Bios Ver 7.59u v46 454MHz B987-M995-F80-O0,0 MAC:00042d076b88</COMMENTS>
   </CONTENT>
   <DEVICEID>foo</DEVICEID>
   <QUERY>SNMPQUERY</QUERY>
+  <TAG>Mytag</TAG>
 </REQUEST>
diff --git a/tests/data/8.xml b/tests/data/8.xml
index 9fa63b6..5df79c8 100644
--- a/tests/data/8.xml
+++ b/tests/data/8.xml
@@ -1,5 +1,6 @@
 <?xml version="1.0"?>
-<REQUEST><CONTENT><DEVICE>
+<REQUEST>
+  <CONTENT><DEVICE>
       <CARTRIDGES>
         <DRUMBLACK>0</DRUMBLACK>
         <TONERBLACK>0g</TONERBLACK>