Verifica Codice Fiscale

Validate and decode an Italian codice fiscale (16-character alphanumeric fiscal code).

When to activate

Activate when a user:

• Asks "Il mio codice fiscale è corretto?"
• Provides a CF and wants to know what it encodes
• Gets a mismatch error when submitting their 730
• Needs to check a dependent family member's CF

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Structure of the codice fiscale

S S S N N N A A D D D C C C C X
│ │ │ │ │ │ │ │ │ │ │ │ │ │ │ └─ Check character (1)
│ │ │ │ │ │ │ │ │ │ └─────────── Codice comune nascita (4)
│ │ │ │ │ │ │ │ └─────────────── Giorno nascita + sesso (2)
│ │ │ │ │ │ └───────────────── Mese nascita (1 letter)
│ │ │ │ │ └─────────────────── Anno nascita (2 digits)
│ │ │ └───────────────────── Nome consonants (3)
└─────────────────────────── Cognome consonants (3)

Total: 16 characters - uppercase letters and digits.

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Step-by-step validation

1 - Use Dichiarino MCP (if available)

valida_codice_fiscale(codice_fiscale="RSSMRA85M01H501Z")
→ Returns: valid (bool), sesso, data_nascita, codice_comune, errors

2 - Manual validation (fallback)

Step 2a - Format check

• Must be exactly 16 characters
• Regex: ^[A-Z]{6}[0-9LMNPQRSTUV]{2}[ABCDEHLMPRST]{1}[0-9LMNPQRSTUV]{2}[A-Z]{1}[0-9LMNPQRSTUV]{3}[A-Z]{1}$

Step 2b - Decode each field

Pos	Field	How to read
0–2	Cognome	3 consonants; if < 3 consonants, fill with vowels then X
3–5	Nome	If ≥ 4 consonants: take 1st, 3rd, 4th. Else: consonants then vowels then X
6–7	Anno nascita	Last 2 digits of birth year
8	Mese nascita	Letter code (see table below)
9–10	Giorno + sesso	Day 01–31 = male; day 41–71 = female (day − 40)
11–14	Codice comune	Starts with letter for Italian comuni; Z+3 digits for foreign countries
15	Carattere controllo	Computed from positions 0–14

Month letter codes: A=Gen, B=Feb, C=Mar, D=Apr, E=Mag, H=Giu, L=Lug, M=Ago, P=Set, R=Ott, S=Nov, T=Dic

Step 2c - Verify the check digit (position 15)

Odd positions (1,3,5,…): use odd-position lookup table. Even positions (0,2,4,…): face value (0→0, A→0, B→1 … Z→25). Sum all 16 partial values → check = chr(ord('A') + (total % 26))

See references/CHECK_DIGIT.md for the full lookup tables.

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Presenting results

Always show:

1. ✅ / ❌ Validity (with reason if invalid)
2. Decoded fields: surname hint, name hint, birth date, gender (M/F), comune/nazione
3. Any detected omocodia substitution (digits replaced with letters)
4. Suggested fix if invalid

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Common errors

Error	Cause	Fix
Wrong check digit	Typo anywhere in the CF	Re-derive from personal data
Day > 40 reported as male	Encoding is correct - female day = day+40	Inform user it's normal
Omocodia variant	Comune has duplicate CFs; Agenzia Entrate substitutes digits with letters	Both variants are valid
Foreign-born	Codice comune = Z + 3 digits (Belcalis table)	Not an error