CUSIP Validation in Python

1. What is a CUSIP?

A CUSIP (Committee on Uniform Securities Identification Procedures) is a 9-character alphanumeric code that uniquely identifies a financial instrument in North America — stocks, bonds, mutual funds, and other registered securities. CUSIPs are assigned by CUSIP Global Services, operated by FactSet under license from the American Bankers Association (ABA).

CUSIPs are the backbone of trade settlement, clearing, and custody in the United States and Canada. They are also embedded in ISINs: a US ISIN is simply US + the 9-character CUSIP + a Luhn check digit. If you work with North American securities, you will encounter CUSIPs everywhere — from DTCC settlement instructions to Bloomberg terminal lookups.

2. CUSIP anatomy

Every CUSIP is exactly 9 characters long and consists of three parts:

Putting it together: 037833 + 10 + 0 = 037833100 (Apple Inc. common stock). The issuer number 037833 is assigned to Apple, and 10 refers to the common stock issue. Other issue numbers for the same issuer would represent different securities — preferred stock, bonds, warrants, etc.

3. The check digit algorithm

The CUSIP check digit uses a Luhn-variant algorithm that operates on the first 8 characters. Letters are converted to numeric values (A=10, B=11, ..., Z=35), and special characters like *, @, and # map to 36, 37, and 38 respectively. Characters at even positions (0-indexed) are doubled, then digit sums are computed.

Here is what a naive Python implementation looks like:

# cusip_check.py — naive implementation (format + checksum only)

import re

CUSIP_RE = re.compile(r'^[A-Z0-9*@#]{8}[0-9]$')

SPECIAL_CHARS = {'*': 36, '@': 37, '#': 38}


def char_value(ch: str) -> int:
    """Convert a CUSIP character to its numeric value."""
    if ch.isdigit():
        return int(ch)
    if ch.isalpha():
        return ord(ch) - 55  # A=10, B=11, ..., Z=35
    if ch in SPECIAL_CHARS:
        return SPECIAL_CHARS[ch]
    raise ValueError(f"Invalid CUSIP character: {ch}")


def cusip_check_digit(cusip8: str) -> int:
    """Compute the CUSIP check digit for the first 8 characters."""
    total = 0
    for i, ch in enumerate(cusip8):
        value = char_value(ch)
        if i % 2 == 1:  # odd position (0-indexed) → double
            value *= 2
        # Sum the individual digits of the (possibly doubled) value
        total += value // 10 + value % 10
    return (10 - (total % 10)) % 10


def validate_cusip_format(cusip: str) -> dict:
    cusip = cusip.replace(" ", "").replace("-", "").upper()

    if len(cusip) != 9:
        return {"valid": False, "reason": "length"}

    if not CUSIP_RE.match(cusip):
        return {"valid": False, "reason": "format"}

    expected = cusip_check_digit(cusip[:8])
    actual = int(cusip[8])

    if expected != actual:
        return {"valid": False, "reason": "check_digit"}

    return {
        "valid": True,
        "issuer_number": cusip[:6],
        "issue_number": cusip[6:8],
        "check_digit": cusip[8],
    }


# ── Example ───────────────────────────────────────────────────────────────────
print(validate_cusip_format("037833100"))   # valid — Apple Inc.
print(validate_cusip_format("037833105"))   # invalid check digit
print(validate_cusip_format("03783310"))    # invalid length

4. Why manual validation is not enough

5. The right solution: one API call

Instead of implementing the check digit algorithm yourself and maintaining a CUSIP database, use the IsValid CUSIP API. A single GET request validates the format, verifies the check digit, and breaks down the CUSIP into its component parts — issuer number, issue number, and check digit.

Get your free API key at isvalid.dev. The free tier includes 100 calls per day — enough for most development and low-volume production use.

Full parameter reference and response schema: CUSIP Validation API docs →

6. Python code example

Using the isvalid-sdk package or the requests library. Install with pip install isvalid-sdk or pip install requests.

# cusip_validator.py
import os
from isvalid_sdk import IsValidConfig, create_client

iv = create_client(IsValidConfig(api_key=os.environ["ISVALID_API_KEY"]))

result = iv.cusip("037833100")  # Apple Inc.

if not result["valid"]:
    print("Invalid CUSIP: failed format or check digit")
else:
    print(f"Valid        : {result['valid']}")
    print(f"Issuer number: {result['issuerNumber']}")
    print(f"Issue number : {result['issueNumber']}")
    print(f"Check digit  : {result['checkDigit']}") 

Expected output for 037833100:

Valid        : True
Issuer number: 037833
Issue number : 10
Check digit  : 0

7. cURL example

Validate a CUSIP and get the component breakdown:

curl -H "Authorization: Bearer YOUR_API_KEY" \
  "https://api.isvalid.dev/v0/cusip?value=037833100"

Invalid CUSIP — wrong check digit:

curl -H "Authorization: Bearer YOUR_API_KEY" \
  "https://api.isvalid.dev/v0/cusip?value=037833105"

8. Understanding the response

Response for a valid CUSIP:

{
  "valid": true,
  "issuerNumber": "037833",
  "issueNumber": "10",
  "checkDigit": "0"
}

Response for an invalid CUSIP:

{
  "valid": false
}

9. Edge cases to handle

result = iv.cusip("037833105")  # Wrong check digit

if not result["valid"]:
    print("CUSIP is invalid — check digit mismatch")
    # Prompt the user to re-enter or verify the source

# CUSIPs with letters are perfectly valid
result = iv.cusip("17275R102")  # Cisco Systems
print(result["valid"])          # True
print(result["issuerNumber"])   # 17275R

# CINS code — letter in first position indicates country
result = iv.cusip("D18190898")  # International security

if result["valid"]:
    print(f"Issuer: {result['issuerNumber']}")  # D18190

import requests


def validate_cusip_safe(cusip: str) -> dict | None:
    """Validate a CUSIP with graceful error handling."""
    try:
        return validate_cusip(cusip)
    except requests.Timeout:
        logger.warning("CUSIP API timed out for %s", cusip)
        return None
    except requests.HTTPError as exc:
        logger.error("CUSIP API error %s for %s", exc.response.status_code, cusip)
        return None

from concurrent.futures import ThreadPoolExecutor

cusips = ["037833100", "17275R102", "594918104", "88160R101"]

with ThreadPoolExecutor(max_workers=4) as pool:
    results = list(pool.map(iv.cusip, cusips))

for cusip, result in zip(cusips, results):
    status = "valid" if result["valid"] else "INVALID"
    print(f"{cusip}: {status}")