Decode GS1 AI

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When to use

• A scanner / OCR / EDI feed produced a raw element string and you need structured fields (GTIN, lot, expiry, serial, SSCC, GRAI, GIAI).
• The agent received a GS1 Digital Link (https://id.gs1.org/01/...) and needs to extract the AIs the path encodes.
• A user pasted a GS1-128 barcode payload and asked “what does this mean?”.

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The two encodings

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Step 1 — Detect the variant

def detect_variant(s: str) -> str:
    if s.startswith("("):
        return "bracketed"
    if s.startswith("https://") or s.startswith("http://"):
        return "digital_link"
    return "unbracketed"

1. (01)09506000134352(17)270101(10)ABC123 → bracketed
2. 010950600013435217270101\x1d10ABC123 → unbracketed
3. https://id.gs1.org/01/09506000134352/10/ABC123 → Digital Link path

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Step 2 — Know the fixed-length AIs

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Step 3 — Know the variable-length AIs

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Step 4 — Parse, bracketed → structured

import re

BRACKETED_RE = re.compile(r"\((\d{2,4})\)([^(]*)")

def parse_bracketed(s: str) -> dict[str, str]:
    return dict(BRACKETED_RE.findall(s))

parse_bracketed("(01)09506000134352(17)270101(10)ABC123")
# {'01': '09506000134352', '17': '270101', '10': 'ABC123'}

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Step 5 — Parse, unbracketed → structured

1. Read 2-4 leading digits and try them against the AI table. Try the longest prefix that matches a known AI (so 8003 is preferred over 80 + 03).
2. If the AI is fixed-length, slice length characters as the value.
3. If it’s variable-length, slice up to the next \x1d (or end of string, if there’s no separator).
4. Repeat from the position after the value.

FIXED = {"00":18, "01":14, "02":14, "11":6, "12":6, "13":6,
         "15":6, "16":6, "17":6, "20":2, "8005":6, "8006":18}
VAR_MAX = {"10":20, "21":20, "22":20, "240":30, "241":30,
           "242":6, "8003":30, "8004":30}
FNC1 = "\x1d"

def parse_unbracketed(s: str) -> dict[str, str]:
    out, i = {}, 0
    while i < len(s):
        # try longest AI prefix first (4 → 2 chars)
        for n in (4, 3, 2):
            ai = s[i:i+n]
            if ai in FIXED or ai in VAR_MAX:
                i += n
                break
        else:
            raise ValueError(f"Unknown AI at offset {i}: {s[i:i+4]!r}")

        if ai in FIXED:
            length = FIXED[ai]
            out[ai] = s[i:i+length]
            i += length
        else:
            end = s.find(FNC1, i)
            end = len(s) if end == -1 else end
            out[ai] = s[i:end]
            i = end + (1 if end < len(s) else 0)
    return out

• Normalize bracketed input by stripping brackets and re-emitting an unbracketed string with FNC1 separators inserted, then run the unbracketed algorithm. One code path is easier to maintain than two.
• Validate the Mod-10 check digit on AI 01 (GTIN). The Closient apps.core.types.GTIN.from_raw() does this — reuse if you’re in Python.
• Accept the literal characters ^ or ~ as FNC1 sentinels (some scanners emit those instead of \x1d).

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Step 6 — Parse, GS1 Digital Link → structured

import re
from urllib.parse import urlparse

PATH_SEG_RE = re.compile(r"/(\d{2,4})/([^/]+)")

def parse_digital_link(url: str) -> dict[str, str]:
    parts = PATH_SEG_RE.findall(urlparse(url).path)
    return dict(parts)

parse_digital_link("https://id.gs1.org/01/09506000134352/10/ABC123/21/SN9999")
# {'01': '09506000134352', '10': 'ABC123', '21': 'SN9999'}

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Step 7 — Validate

• GTIN check digit (AI 01). Mod-10; the last digit is computed from the first 13. Reject silently-malformed scans rather than continuing with a corrupt GTIN.
• Date plausibility (AI 17 in particular). YYMMDD with MM in 01-12, DD in 00-31 (where 00 is sentinel for “last day of month”). The Closient scanner emits structured freshness_meta for AI 17 — see apps.scanner.services.gs1_dates.

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Example end-to-end

0109506000134352173009011045454GH\x1d21SN0001

1. 01 → fixed 14 → 09506000134352 (GTIN).
2. 17 → fixed 6 → 300901 (expiry 2030-09-01).
3. 10 → variable, up to FNC1 → 45454GH (lot).
4. 21 → variable, no separator left → SN0001 (serial).

{
  "01": "09506000134352",
  "17": "300901",
  "10": "45454GH",
  "21": "SN0001"
}

GET /products/api/v1/products?gtin=09506000134352
GET /resolver/api/v1/verify/01/09506000134352/21/SN0001

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Common mistakes

• Forgetting FNC1 after a variable-length AI before another AI starts — produces a corrupted parse where 10ABC12321SN0001 is read as a 20-character lot ending in 21SN0001.
• Not zero-padding GTIN-8 or GTIN-12 to GTIN-14 before lookups — Closient stores GTINs as GTIN-14 internally; pad with leading zeros.
• Treating (01) from a Digital Link query string as the SSCC AI 00 because the parser was eager about 2-digit prefixes; always try the longest AI prefix first.
• Trusting YY as 4-digit year. GS1 says YY in 00-49 → 20YY, 50-99 → 19YY (sliding window). Don’t hardcode the century.

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Related skills

• build-gs1-digital-link — inverse: given GTIN + AIs, emit a valid Digital Link URI.
• resolve-gtin — fetch product detail once you have the GTIN.
• check-product-availability — find local stock for a GTIN.
• capture-epcis-event — once you have GTIN + lot + serial, record a supply-chain event.
• closient-api-quickstart — authenticate before calling any of the above.

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Authoritative references

• GS1 General Specifications (v25 or later) — section 3 (AIs) and section 7 (data carriers).
• GS1 Digital Link Standard 1.4.
• ISO/IEC 15418 — GS1 Application Identifiers.
• Closient internal: backend/apps/products/gs1_digital_link.py (parser) and backend/apps/core/types/_gs1.py (typed value objects).