CFDI XML Invoice Validation

CFDI (Comprobante Fiscal Digital por Internet) XML invoices are the foundation of fiscal expense verification in SMAF. This guide explains the XML structure, validation process, and SAT integration requirements.

What is CFDI XML?

CFDI XML is the digital format required by SAT (Servicio de Administración Tributaria) for all fiscal transactions in Mexico. Each valid invoice must include:

Digital stamp (Timbre Fiscal Digital)
UUID (Folio Fiscal) - Unique universal identifier
Structured tax information (IVA, IEPS, ISR, etc.)
Issuer and recipient details
Concept details with amounts

While XML upload is optional in SMAF’s simplified workflow, the UUID and amount are required for validation against SAT records.

XML File Structure

SMAF’s XML parser reads the following key elements:

Core Elements

<?xml version="1.0" encoding="UTF-8"?>
<cfdi:Comprobante 
    xmlns:cfdi="http://www.sat.gob.mx/cfd/4" 
    Total="1500.00"
    ...>
    
    <!-- Issuer Information -->
    <cfdi:Emisor 
        Rfc="ABC123456XYZ" 
        Nombre="Proveedor SA de CV"
        .../>
    
    <!-- Concepts/Items -->
    <cfdi:Conceptos>
        <cfdi:Concepto 
            Descripcion="Hospedaje"
            Importe="1293.10"
            .../>
    </cfdi:Conceptos>
    
    <!-- Tax Information -->
    <cfdi:Impuestos 
        TotalImpuestosTrasladados="206.90"
        TotalImpuestosRetenidos="0.00">
        <cfdi:Traslados>
            <cfdi:Traslado 
                Impuesto="IVA" 
                Importe="206.90"/>
        </cfdi:Traslados>
    </cfdi:Impuestos>
    
    <!-- Digital Stamp -->
    <cfdi:Complemento>
        <tfd:TimbreFiscalDigital 
            xmlns:tfd="http://www.sat.gob.mx/TimbreFiscalDigital"
            UUID="12345678-1234-1234-1234-123456789012"
            FechaTimbrado="2024-03-15T14:30:00"
            .../>
    </cfdi:Complemento>
    
</cfdi:Comprobante>

XML Parsing Logic

SMAF uses an XmlTextReader to extract invoice data:

ComprobacionSInXml.aspx.cs:248-457

public void Lee_XMl(string psArchivo, List<Entidad> plEntidad, Xml poXml)
{
    XmlTextReader reader = new XmlTextReader(psArchivo);
    
    while (reader.Read())
    {
        switch (reader.Name)
        {
            case "cfdi:Comprobante":
                if ((poXml.TOTAL == "") & (reader["total"] != null))
                {
                    poXml.TOTAL = reader["total"];
                }
                break;
            
            case "cfdi:Emisor":
            case "Emisor":
                if ((poXml.RFC_EMISOR == "") & (reader["rfc"] != null))
                {
                    poXml.RFC_EMISOR = reader["rfc"];
                }
                break;
            
            case "cfdi:Concepto":
            case "Concepto":
                if (reader["descripcion"] != null)
                {
                    Entidad obj = new Entidad();
                    obj.Codigo = reader["importe"];
                    obj.Descripcion = reader["descripcion"];
                    plEntidad.Add(obj);
                }
                break;
            
            case "cfdi:Impuestos":
            case "Impuestos":
                if (reader["totalImpuestosTrasladados"] != null)
                {
                    poXml.TOTAL_IMPUESTOS_TRASLADADOS = 
                        reader["totalImpuestosTrasladados"];
                }
                if (reader["totalImpuestosRetenidos"] != null)
                {
                    poXml.TOTAL_IMPUESTOS_RETENIDOS = 
                        reader["totalImpuestosRetenidos"];
                }
                break;
            
            // ... (continued)
        }
    }
}

Extracting Tax Details

The system reads transferred taxes (Traslados) and withheld taxes (Retenciones):

ComprobacionSInXml.aspx.cs:315-375

case "cfdi:Traslado":
case "Traslado":
    if (reader["impuesto"] != null)
    {
        // IVA (Value Added Tax)
        if (reader["impuesto"] == "IVA")
        {
            poXml.IVA = reader["importe"];
        }
        
        // IEPS (Special Tax on Production and Services)
        if (reader["impuesto"] == "IEPS")
        {
            poXml.IEPS = reader["importe"];
        }
        
        // TUA (Airport Use Tax)
        if (reader["impuesto"] == "TUA")
        {
            poXml.TUA = reader["importe"];
        }
        
        // ISR (Income Tax)
        if (reader["impuesto"] == "ISR")
        {
            poXml.ISR = reader["importe"];
        }
        
        // SFP (not commonly used)
        if (reader["impuesto"] == "SFP")
        {
            poXml.SFP = reader["importe"];
        }
    }
    break;

case "cfdi:Retencion":
case "Retencion":
    // Similar logic for withheld taxes
    if (reader["impuesto"] == "IVA")
    {
        poXml.IVA_RETENIDO = reader["importe"];
    }
    // ... etc
    break;

Extracting Digital Stamp (Timbre Fiscal)

ComprobacionSInXml.aspx.cs:441-444

case "tfd:TimbreFiscalDigital":
    if ((poXml.TIMBRE_FISCAL == "") & (reader["UUID"] != null))
    {
        poXml.TIMBRE_FISCAL = reader["UUID"];
    }
    if ((poXml.FECHA_TIMBRADO == "") & (reader["FechaTimbrado"] != null))
    {
        poXml.FECHA_TIMBRADO = FormatFecha(reader["FechaTimbrado"]);
    }
    break;

Local Taxes (State Level)

Some states impose additional local taxes:

ComprobacionSInXml.aspx.cs:446-453

case "implocal:TrasladosLocales":
case "TrasladosLocales":
    if ((poXml.ISH == "0") & (reader["Importe"] != null))
    {
        poXml.ISH = reader["Importe"]; // State lodging tax
    }
    break;

Simplified Workflow (Without XML)

SMAF allows expense verification without uploading XML files by manually entering key data:

Upload PDF only

Upload just the PDF version of the invoice

Enter UUID manually

Type the Folio Fiscal (UUID) from your invoice

ComprobacionSInXml.aspx.cs:476-480

if ((txtUUID.Text == "") | (txtUUID.Text == null))
{
    ClientScript.RegisterStartupScript(
        this.GetType(), "Inapesca", 
        "alert('Folio Fiscal es necesario.');", true);
    return;
}

Enter amount

Type the total invoice amount

ComprobacionSInXml.aspx.cs:482-491

if ((txtImporteFac.Text == "") | (txtImporteFac.Text == null))
{
    alert('Importe necesario');
    return;
}
else if (!IsNumeric(txtImporteFac.Text))
{
    alert('Importe debe ser numérico');
    return;
}

Enter date

Select the invoice date using the calendar control

ComprobacionSInXml.aspx:311-313

<asp:TextBox ID="TextBox1" runat="server" Width="60%"></asp:TextBox>
<cc1:CalendarExtender ID="TextBox1_CalendarExtender" 
    TargetControlID="TextBox1" Format="yyyy-MM-dd"/>

This simplified approach trades automation for flexibility, making the system accessible even when XML files are unavailable or corrupted.

UUID Validation and Uniqueness

The system enforces UUID uniqueness to prevent invoice reuse:

ComprobacionSInXml.aspx.cs:558-586

try
{
    // Check if UUID already exists in the system
    string existe = MngNegocioComprobacion.Exist_UUUID(txtUUID.Text);
    
    if ((existe == "") | (existe == null))
    {
        // UUID is new - proceed with saving
        Valida_Carpeta(detalleComision.Ruta, false, true);
        
        double totalImporteXml = Convert_Double(txtImporteFac.Text);
        
        // Save files
        if (fuplXML.HasFile)
        {
            fuplXML.PostedFile.SaveAs(
                Crip_Ruta + "/" + fuplXML.FileName);
        }
        fuplPDF.PostedFile.SaveAs(
            Crip_Ruta + "/" + fuplPDF.FileName);
        
        // Insert verification record
        sube = Inserta_Comprobacion_Comision(
            Oficio, Archivo, Usuario, Ubicacion,
            TextBox1.Text, // Invoice date
            Proyecto, Dep_Proy,
            dplFiscales.SelectedValue.ToString(), // "2" for fiscal
            ClaveConcepto, Concepto,
            fuplPDF.FileName, 
            ConvertString(totalImporteXml),
            fuplXML.FileName,
            "01", "01", // Payment method codes
            Observaciones,
            NombreArchivo, 
            "", // Ticket (for fuel)
            txtUUID.Text, // UUID
            Periodo
        );
        
        // Reload display
        Crear_Tabla();
        Carga_Detalle(detalleComision);
    }
    else
    {
        // UUID already exists - reject
        alert('La factura que intenta subir ya fue usada para ' +
              'otra comprobación, favor de ingresar una válida');
        return;
    }
}
catch (Exception x)
{
    Console.Write(x.Message);
}

Tax Validation Rules

When XML is provided, the system can validate:

IVA (Value Added Tax)

Standard rate in Mexico: 16%

// Expected calculation:
if (poXml.IVA != null)
{
    double subtotal = Convert_Double(poXml.TOTAL) - 
                      Convert_Double(poXml.TOTAL_IMPUESTOS_TRASLADADOS);
    double expectedIVA = subtotal * 0.16;
    
    // Validate within tolerance
    if (Math.Abs(Convert_Double(poXml.IVA) - expectedIVA) > 0.50)
    {
        // Flag for review
    }
}

IEPS (Special Production Tax)

Applies to specific goods:

Gasoline: Variable rate
Alcohol: 26.5% - 53%
Tobacco: 160%

if (poXml.IEPS != null && poXml.IEPS != "0")
{
    // IEPS present - flag for special review
    // Rate varies by product category
}

ISR (Income Tax Withholding)

For professional services:

if (poXml.ISR_RETENIDO != null)
{
    // Typically 10% for professional fees
    // Validate withholding is properly documented
}

File Upload Validation

The system validates file types before processing:

ComprobacionSInXml.aspx.cs:504-546

bool xmlOK = false;
bool fileOk = false;

// Validate PDF
if (!fuplPDF.HasFile)
{
    alert('Archivo pdf es necesario.');
    return;
}
else
{
    String fileExtension = 
        System.IO.Path.GetExtension(fuplPDF.FileName).ToLower();
    String[] allowedExtensions = { ".pdf", ".PDF" };
    
    for (int i = 0; i < allowedExtensions.Length; i++)
    {
        if (fileExtension == allowedExtensions[i])
        {
            fileOk = true;
        }
    }
}

// Validate XML (optional)
if (fuplXML.HasFile)
{
    String fileExtension1 = 
        System.IO.Path.GetExtension(fuplXML.FileName).ToLower();
    String[] allowedExtensions1 = { ".pdf", ".PDF", ".xml", ".XML" };
    
    for (int i = 0; i < allowedExtensions1.Length; i++)
    {
        if (fileExtension1 == allowedExtensions1[i])
        {
            xmlOK = true;
        }
    }
}
else {
    xmlOK = true; // XML is optional
}

if ((fileOk) & (xmlOK))
{
    // Proceed with upload
}
else
{
    alert('Tipo de Archivo no válido.');
    return;
}

File Storage Structure

Uploaded files are organized by commission:

/Comprobaciones/
    /Usuario_Ubicacion/
        /Folio_Comision/
            factura_001.pdf
            factura_001.xml
            factura_002.pdf
            factura_002.xml
            reintegro_baucher.pdf
            peaje_atentanota.pdf

XML Data Model

The Xml entity stores parsed data:

public class Xml
{
    public string TOTAL { get; set; }
    public string RFC_EMISOR { get; set; }
    public string CONCEPTO { get; set; }
    
    // Taxes
    public string TOTAL_IMPUESTOS_TRASLADADOS { get; set; }
    public string TOTAL_IMPUESTOS_RETENIDOS { get; set; }
    public string IVA { get; set; }
    public string IEPS { get; set; }
    public string ISR { get; set; }
    public string TUA { get; set; }
    public string SFP { get; set; }
    public string ISH { get; set; } // State tax
    
    // Withholdings
    public string IVA_RETENIDO { get; set; }
    public string IEPS_RETENIDO { get; set; }
    public string ISR_RETENIDO { get; set; }
    public string TUA_RETENIDO { get; set; }
    public string SFP_RETENIDO { get; set; }
    
    // Stamp
    public string TIMBRE_FISCAL { get; set; }
    public string FECHA_TIMBRADO { get; set; }
}

SAT Integration (Future Enhancement)

Current implementation does not perform real-time SAT validation. UUID uniqueness is checked only within the SMAF database.

For enhanced validation, consider integrating with SAT web services:

// Pseudocode for future SAT validation
public bool ValidateUUID_WithSAT(string uuid, string rfcEmisor, string total)
{
    // Call SAT SOAP service
    var satService = new SAT.ValidacionService();
    var response = satService.Consulta(
        uuid: uuid,
        rfcEmisor: rfcEmisor,
        rfcReceptor: "INSTITUCION_RFC",
        total: total
    );
    
    return response.Estado == "Vigente";
}

Common XML Issues

Missing Timbre Fiscal

Problem: XML file doesn’t contain <tfd:TimbreFiscalDigital>Solution: Request properly stamped CFDI from vendor. Unstamped invoices are not valid.

Invalid RFC Format

Problem: RFC (Tax ID) doesn’t match expected formatSolution: Verify RFC is 12 characters (moral) or 13 characters (física) with proper check digit

Mismatched Totals

Problem: Subtotal + IVA ≠ TotalSolution: Check for rounding errors. SAT allows ±$0.50 tolerance

Encoding Issues

Problem: Special characters (ñ, á, é) appear corruptedSolution: Ensure XML is UTF-8 encoded:

<?xml version="1.0" encoding="UTF-8"?>

Namespace Variations

Problem: CFDI version 3.3 vs 4.0 have different namespacesSolution: Parser handles both cfdi: prefixed and unprefixed elements

Best Practices

Always Verify UUID

Manually confirm the UUID from your paper/PDF invoice matches what you type

Keep XML Copies

Store XML files even if upload is optional - needed for audits

Check Dates

Invoice date must fall within travel period (Fecha_Inicio to Fecha_Final)

Validate Vendor RFC

Ensure vendor RFC is registered with SAT and active

Troubleshooting

UUID Already Exists

if (existe != "" && existe != null)
{
    alert('La factura ya fue usada para otra comprobación');
}

Causes:

Vendor issued duplicate invoice
Invoice was used in another employee’s verification
Database corruption

Resolution:

Contact vendor for corrected invoice with new UUID
If legitimate duplicate, contact SMAF administrator

XML Parse Errors

Causes:

Malformed XML structure
Missing required namespaces
Invalid characters

Resolution:

Validate XML against SAT schema: http://www.sat.gob.mx/sitio_internet/cfd/4/cfdv40.xsd
Use XML validator tools before upload

Get Started

User Guide

Solicitudes

Comprobaciones

Autorizaciones

Pagos y Ministraciones

Reportes

Catalogos

CFDI XML Invoice Validation

What is CFDI XML?

XML File Structure

Core Elements

XML Parsing Logic

Extracting Tax Details

Extracting Digital Stamp (Timbre Fiscal)

Local Taxes (State Level)

Simplified Workflow (Without XML)

UUID Validation and Uniqueness

Tax Validation Rules

IVA (Value Added Tax)

IEPS (Special Production Tax)

ISR (Income Tax Withholding)

File Upload Validation

File Storage Structure

XML Data Model

SAT Integration (Future Enhancement)

Common XML Issues

Best Practices

Always Verify UUID

Keep XML Copies

Check Dates

Validate Vendor RFC

Troubleshooting

UUID Already Exists

XML Parse Errors

Next Steps

Expense Verification

International Travel

Build docs developers (and LLMs) love

Get Started

User Guide

Solicitudes

Comprobaciones

Autorizaciones

Pagos y Ministraciones

Reportes

Catalogos

​What is CFDI XML?

​XML File Structure

​Core Elements

​XML Parsing Logic

​Extracting Tax Details

​Extracting Digital Stamp (Timbre Fiscal)

​Local Taxes (State Level)

​Simplified Workflow (Without XML)

​UUID Validation and Uniqueness

​Tax Validation Rules

​IVA (Value Added Tax)

​IEPS (Special Production Tax)

​ISR (Income Tax Withholding)

​File Upload Validation

​File Storage Structure

​XML Data Model

​SAT Integration (Future Enhancement)

​Common XML Issues

​Best Practices

Always Verify UUID

Keep XML Copies

Check Dates

Validate Vendor RFC

​Troubleshooting

​UUID Already Exists

​XML Parse Errors

​Next Steps

Expense Verification

International Travel

Build docs developers (and LLMs) love

What is CFDI XML?

XML File Structure

Core Elements

XML Parsing Logic

Extracting Tax Details

Extracting Digital Stamp (Timbre Fiscal)

Local Taxes (State Level)

Simplified Workflow (Without XML)

UUID Validation and Uniqueness

Tax Validation Rules

IVA (Value Added Tax)

IEPS (Special Production Tax)

ISR (Income Tax Withholding)

File Upload Validation

File Storage Structure

XML Data Model

SAT Integration (Future Enhancement)

Common XML Issues

Best Practices

Troubleshooting

UUID Already Exists

XML Parse Errors

Next Steps