While attempting to create a load test for a client, a post request to a critical endpoint failed due to JMeter converting "\\\" to "\\" in the post's body data. After the conversion, JMeter sends an invalid JSON to the server and it responds with 400 Bad Request.

The client has a really weird (bad) endpoint where it accepts json in json in json which seems to throw off JMeter parser.

Body Data in HTTP Request (POST):

{"order":"{\"id\":0,\"order_date\":\"3/8/2019\",\"order_data\":{\"items\":[{\"id\":1876318,\"quantity\":1,\"name\":\"Air Filter\",\"price\":8.54,\"retail_price\":8.54,\"msrp\":0.0,\"price_level\":{\"price_level_id\":1,\"price_level\":\"RETAIL\",\"retail_multipler\":1.00,\"description\":\"Regular retail\",\"price_type\":{\"price_type_id\":1,\"price_type\":\"Regular\",\"label\":\"\"}},\"is_hazmat\":false,\"is_accessory\":false,\"oversize_category_id\":null,\"oem_number\":\"W10311524\",\"image_url\":\"https://www.redacted.com/dbImages/i/00004039/Air-Filter-W10311524-01215703.jpg\",\"quantity_by_dc\":[{\"id\":1,\"quantity\":0},{\"id\":2,\"quantity\":0}],\"repair_time\":null,\"repair_savings\":null,\"skill_level\":1,\"labor_hours\":null,\"prodcut_categories\":[{\"id\":1,\"name\":\"Appliances\"}]}],\"email\":\"${email}\",\"billing_address\":{\"first_name\":\"${first_name}\",\"last_name\":\"${last_name}\",\"street1\":\"2600 S River Rd\",\"street2\":\"\",\"city\":\"Des Plaines\",\"state\":\"IL\",\"country\":\"US\",\"zipcode\":\"60018\",\"phone\":\"1231231234\",\"ext\":\"\",\"business_name\":\"\",\"is_business_address\":false,\"is_po_box\":false},\"shipping_address\":{\"first_name\":\"${first_name}\",\"last_name\":\"${last_name}\",\"street1\":\"2600 S River Rd\",\"street2\":\"\",\"city\":\"Des Plaines\",\"state\":\"IL\",\"country\":\"US\",\"zipcode\":\"60018\",\"phone\":\"1231231234\",\"ext\":\"\",\"business_name\":\"\",\"is_business_address\":false,\"is_po_box\":false},\"shipping_choice\":{\"shipping_option_id\":1,\"ship_to_po_box\":false,\"is_saturday_delivery\":false,\"ship_complete\":true,\"is_home_business\":false},\"total\":15.71,\"discount\":0.0,\"subtotal\":8.54,\"shipping_charge\":6.25,\"offshore_surcharge\":0.0,\"saturday_shipping_surcharge\":0.0,\"saturday_shipping_surcharge_for_ship_method\":15.00,\"oversize_surcharge\":0.0,\"weight_surcharge\":0.0,\"international_processing_fee\":0.0,\"sales_tax\":0.92,\"sales_tax_label\":\"Estimated Sales Tax\",\"purchase_order_number\":null,\"promotion_code\":\"\",\"device_data\":\"{\\\"device_session_id\\\":\\\"${sessionId}\\\",\\\"fraud_merchant_id\\\":\\\"600000\\\"}\",\"is_promo_code_valid\":false},\"sop_number\":\"\",\"security_code\":null,\"rc_order_form\":\"\",\"o_checksum\":\"${o_checksum}\",\"rc_checksum\":\"${rc_checksum}\"}","payment":{"payment_type_id":0,"credit_card":{"nonce":"${creditCardToken}","expiration_month":"03","expiration_year":2019},"paypal":null}}

Request Body in View Results Tree for the above request:

{"order":"{\"id\":0,\"order_date\":\"3/8/2019\",\"order_data\":{\"items\":[{\"id\":1876318,\"quantity\":1,\"name\":\"Air Filter\",\"price\":8.54,\"retail_price\":8.54,\"msrp\":0.0,\"price_level\":{\"price_level_id\":1,\"price_level\":\"RETAIL\",\"retail_multipler\":1.00,\"description\":\"Regular retail\",\"price_type\":{\"price_type_id\":1,\"price_type\":\"Regular\",\"label\":\"\"}},\"is_hazmat\":false,\"is_accessory\":false,\"oversize_category_id\":null,\"oem_number\":\"W10311524\",\"image_url\":\"https://www.redacted.com/dbImages/i/00004039/Air-Filter-W10311524-01215703.jpg\",\"quantity_by_dc\":[{\"id\":1,\"quantity\":0},{\"id\":2,\"quantity\":0}],\"repair_time\":null,\"repair_savings\":null,\"skill_level\":1,\"labor_hours\":null,\"prodcut_categories\":[{\"id\":1,\"name\":\"Appliances\"}]}],\"email\":\"test12@example.com\",\"billing_address\":{\"first_name\":\"John\",\"last_name\":\"Doe\",\"street1\":\"2600 S River Rd\",\"street2\":\"\",\"city\":\"Des Plaines\",\"state\":\"IL\",\"country\":\"US\",\"zipcode\":\"60018\",\"phone\":\"1231231234\",\"ext\":\"\",\"business_name\":\"\",\"is_business_address\":false,\"is_po_box\":false},\"shipping_address\":{\"first_name\":\"John\",\"last_name\":\"Doe\",\"street1\":\"2600 S River Rd\",\"street2\":\"\",\"city\":\"Des Plaines\",\"state\":\"IL\",\"country\":\"US\",\"zipcode\":\"60018\",\"phone\":\"1231231234\",\"ext\":\"\",\"business_name\":\"\",\"is_business_address\":false,\"is_po_box\":false},\"shipping_choice\":{\"shipping_option_id\":1,\"ship_to_po_box\":false,\"is_saturday_delivery\":false,\"ship_complete\":true,\"is_home_business\":false},\"total\":15.71,\"discount\":0.0,\"subtotal\":8.54,\"shipping_charge\":6.25,\"offshore_surcharge\":0.0,\"saturday_shipping_surcharge\":0.0,\"saturday_shipping_surcharge_for_ship_method\":15.00,\"oversize_surcharge\":0.0,\"weight_surcharge\":0.0,\"international_processing_fee\":0.0,\"sales_tax\":0.92,\"sales_tax_label\":\"Estimated Sales Tax\",\"purchase_order_number\":null,\"promotion_code\":\"\",\"device_data\":\"{\\"device_session_id\\":\\"6E0E291D4A295930E103D3F89D9DA58F\\",\\"fraud_merchant_id\\":\\"600000\\"}\",\"is_promo_code_valid\":false},\"sop_number\":\"\",\"security_code\":null,\"rc_order_form\":\"\",\"o_checksum\":\"7AB4149F2F0E47E9A3F7546FB0E21ABD\",\"rc_checksum\":\"703BE7B71AA016C82132378323E98B9E\"}","payment":{"payment_type_id":0,"credit_card":{"nonce":"tokencc_bd_v3q2pq_nr8xwy_w8k5dt_qcvdvw_yxy","expiration_month":"03","expiration_year":2019},"paypal":null}}

Seems like JMeter converts \\\"device_session_id\\\" to \\"device_session_id\\" (with the latter being invalid JSON).

Please let me know if you need more information. Sorry about the big wall of text.

Thanks,
uTosTan