I’m integrating ERPNext (Frappe Framework) with the Pakistan FBR DI API.
I’m facing a strange issue:

When I send the request using cURL, the invoice is accepted successfully.

When I send the exact same payload through ERPNext using Python requests, the API returns:

500 Internal Server Error

1. Working cURL Command

This request succeeds and FBR accepts the invoice:

curl -X POST "https://gw.fbr.gov.pk/di_data/v1/di/postinvoicedata_sb" \
  -H "Authorization: Bearer <TOKEN>" \
  -H "Content-Type: application/json" \
  -H "Accept: */*" \
  -H "Accept-Encoding: gzip, deflate, br" \
  -H "User-Agent: Mozilla/5.0" \
  -d '{
    "invoiceType": "",
    "invoiceDate": "",
    "sellerNTNCNIC": "",
    "sellerBusinessName": "",
    "sellerAddress": "",
    "sellerProvince": "",
    "buyerNTNCNIC": "",
    "buyerBusinessName": "",
    "buyerAddress": "",
    "buyerProvince": "",
    "invoiceRefNo": "",
    "scenarioId": "SN001",
    "buyerRegistrationType": "",
    "items": [{
        "hsCode": "4819.1000",
        "productDescription": "2Sum 2g Injection TP New 2D R#06",
        "rate": "18%",
        "uoM": "Numbers,pieces,units",
        "quantity": 2500.0,
        "totalValues": 2767.1,
        "valueSalesExcludingST": 2345.0,
        "fixedNotifiedValueOrRetailPrice": 0,
        "salesTaxApplicable": 422.1,
        "salesTaxWithheldAtSource": 0,
        "extraTax": "",
        "furtherTax": 0.0,
        "sroScheduleNo": "",
        "fedPayable": 0,
        "discount": 0.0,
        "saleType": "Goods at standard rate (default)",
        "sroItemSerialNo": ""
    }]
  }'

This works perfectly.

2. Failing ERPNext / Python requests code

Inside ERPNext custom app (fbr_api.py), this block is used:

headers = {
    "Authorization": f"Bearer {token}",
    "Content-Type": "application/json",
    "Accept": "*/*",
    "Accept-Encoding": "gzip, deflate, br",
    "User-Agent": "Mozilla/5.0"
}

payload = {... same JSON structure as curl ...}

response = requests.post(
    api_url,
    headers=headers,
    json=payload,  
    verify=False
)

response.raise_for_status()

This results in:

500 Server Error: Internal Server Error

4. The payload logged inside ERPNext is IDENTICAL to the working cURL payload

The headers also match.
Yet ERPNext → FBR always gives a 500 error, while cURL → FBR works.

Question:

Why does the API accept the exact same request via cURL but produces 500 error when sent using Python requests inside ERPNext (Frappe)?

Is there something different about:

• encoding?

• gzip handling?

• SSL?

• how Python constructs the raw body?

• any hidden headers FBR expects?

Any insight would be greatly appreciated.

✔ Additional notes:

• ERPNext is running Python 3.10

• API endpoint is the official DI API sandbox

• Token is valid

• Same token + same payload always work in curl