diff --git a/src/dolibarr_mcp/dolibarr_client.py b/src/dolibarr_mcp/dolibarr_client.py
index 09fac6d..d5ee24a 100644
--- a/src/dolibarr_mcp/dolibarr_client.py
+++ b/src/dolibarr_mcp/dolibarr_client.py
@@ -405,6 +405,15 @@ class DolibarrClient:
         if "customer_id" in payload and "socid" not in payload:
             payload["socid"] = payload.pop("customer_id")
 
+        # Fix: Map product_id to fk_product in lines
+        if "lines" in payload and isinstance(payload["lines"], list):
+            for line in payload["lines"]:
+                if "product_id" in line:
+                    line["fk_product"] = line.pop("product_id")
+                # Ensure product_type is passed if present (0=Product, 1=Service)
+                if "product_type" in line:
+                    line["product_type"] = line["product_type"]
+
         result = await self.request("POST", "invoices", data=payload)
         return self._extract_identifier(result)
 
diff --git a/src/dolibarr_mcp/dolibarr_mcp_server.py b/src/dolibarr_mcp/dolibarr_mcp_server.py
index e992d11..a1989b4 100644
--- a/src/dolibarr_mcp/dolibarr_mcp_server.py
+++ b/src/dolibarr_mcp/dolibarr_mcp_server.py
@@ -364,7 +364,9 @@ async def handle_list_tools():
                                 "subprice": {"type": "number", "description": "Unit price"},
                                 "total_ht": {"type": "number", "description": "Total excluding tax"},
                                 "total_ttc": {"type": "number", "description": "Total including tax"},
-                                "vat": {"type": "number", "description": "VAT rate"}
+                                "vat": {"type": "number", "description": "VAT rate"},
+                                "product_id": {"type": "integer", "description": "Product ID to link (optional)"},
+                                "product_type": {"type": "integer", "description": "Type of line (0=Product, 1=Service)"}
                             },
                             "required": ["desc", "qty", "subprice"]
                         }
diff --git a/tests/test_crud_operations.py b/tests/test_crud_operations.py
index fde7a5a..7724087 100644
--- a/tests/test_crud_operations.py
+++ b/tests/test_crud_operations.py
@@ -140,6 +140,52 @@ class TestCRUDOperations:
             mock_request.return_value = {"success": True}
             result = await client.delete_invoice(100)
             assert result["success"] is True
+
+    @pytest.mark.asyncio
+    async def test_invoice_creation_with_product_lines(self, client):
+        """Test invoice creation with linked product lines."""
+        with patch.object(client, 'request') as mock_request:
+            mock_request.return_value = {"id": 101}
+            
+            # Create invoice with product link and service type
+            await client.create_invoice({
+                "socid": 1,
+                "lines": [
+                    {
+                        "desc": "Linked Product", 
+                        "qty": 1, 
+                        "subprice": 50, 
+                        "product_id": 10,
+                        "product_type": 0
+                    },
+                    {
+                        "desc": "Service Line", 
+                        "qty": 2, 
+                        "subprice": 100,
+                        "product_type": 1
+                    }
+                ]
+            })
+            
+            # Verify payload transformation
+            call_args = mock_request.call_args
+            assert call_args is not None
+            method, endpoint = call_args[0]
+            kwargs = call_args[1]
+            
+            assert method == "POST"
+            assert endpoint == "invoices"
+            
+            payload = kwargs["data"]
+            lines = payload["lines"]
+            
+            # Check first line (Product)
+            assert lines[0]["fk_product"] == 10
+            assert "product_id" not in lines[0]
+            assert lines[0]["product_type"] == 0
+            
+            # Check second line (Service)
+            assert lines[1]["product_type"] == 1
     
     # Order CRUD Tests