In Magento 2, we can make use of Upsell product GraphQL query to retrieve all the upsell product items for the provided main product.

If any product has some assigned upsell products so we can fetch all the upsell items of the main product using the following GraphQL query:

Upsell Products GraphQL Query Request Payload:

{
  products(filter: { sku: { eq: "test_1" } }) {
    items {
      id
      name
      upsell_products {
        id
        sku
        stock_status
        short_description {
          html
        }
        url_key
        name
        special_price
        price_range {
          minimum_price {
            final_price {
              value
              currency
            }
          }
          maximum_price {
            final_price {
              value
              currency
            }
          }
        }
      }
    }
  }
}

Note: upsell_products{} field used to retrieve all the upsell product items of the main product.

Output:

{
  "data": {
    "products": {
      "items": [
        {
          "id": 1,
          "name": "Test Product 1",
          "upsell_products": [
            {
              "id": 1,
              "sku": "test_upsell_1",
              "stock_status": "IN_STOCK",
              "short_description": {
                "html": ""
              },
              "url_key": "test-upsell-1",
              "name": "Test Upsell 1",
              "special_price": null,
              "price_range": {
                "minimum_price": {
                  "final_price": {
                    "value": 10,
                    "currency": "USD"
                  }
                },
                "maximum_price": {
                  "final_price": {
                    "value": 10,
                    "currency": "USD"
                  }
                }
              }
            },
            {
              "id": 2,
              "sku": "test_2",
              "stock_status": "IN_STOCK",
              "short_description": {
                "html": ""
              },
              "url_key": "test-upsell-2",
              "name": "Test upsell 2",
              "special_price": null,
              "price_range": {
                "minimum_price": {
                  "final_price": {
                    "value": 20,
                    "currency": "USD"
                  }
                },
                "maximum_price": {
                  "final_price": {
                    "value": 20,
                    "currency": "USD"
                  }
                }
              }
            }
          ]
        }
      ]
    }
  }
}