Google Consent Parser

A Google Tag Manager Server-side variable template that decodes Google Consent Mode v2 strings from the gcd parameter into a readable format.

Overview

This template automatically parses Google's encoded consent strings and returns the consent states for various categories (ad_storage, analytics_storage, ad_user_data, ad_personalization) in either object or string format.

Features

• ✅ Parses Google Consent Mode v2 gcd parameter
• ✅ Supports all 4 consent categories
• ✅ Returns data in object or string format
• ✅ Works with server-side Google Tag Manager
• ✅ Handles missing consent parameters gracefully

Installation

1. Download the template file
2. In your GTM Server container, go to Templates
3. Click New > Variable Template
4. Import the template file
5. Save and publish

Usage

Create a Variable

1. Go to Variables in your GTM Server container
2. Click New > User-Defined Variables
3. Select Google Consent Parser
4. Configure the template parameters

Template Parameters

Parameter	Description	Options
Consent Category	Which consent category to return	ad_storage, analytics_storage, ad_user_data, ad_personalization, all
Output Format	How to format the response	object, string

Example Configurations

Get All Consent States (Object Format)

Consent Category: All
Output Format: Object

Returns:

{
  ad_storage: { default: "denied", update: "granted" },
  analytics_storage: { default: "denied", update: "granted" },
  ad_user_data: { default: "denied", update: "granted" },
  ad_personalization: { default: "denied", update: "granted" }
}

Get Single Category (String Format)

Consent Category: ad_storage
Output Format: String

Returns:

"default:denied|update:granted"

How It Works

The template:

1. Reads the gcd parameter from incoming requests
2. Parses the base64url-encoded consent string
3. Extracts consent states using bitwise operations
4. Returns formatted consent information

Google Consent String Structure - Corrected

Actual Format Pattern

Based on your example: 13r3r3r3r5l1

┌───┬─────────┬─────────┬─────────┬─────────┬─────────────┬─────────────────────┐
│ 1 │( 3 | r )│( 3 | r )│( 3 | r )│( 3 | r )│     (5)     │       ( 1 | l )     │
└───┴─────────┴─────────┴─────────┴─────────┴─────────────┴─────────────────────┘
 Ver  ad_storage analytics ad_user_data ad_personal Global Privacy Container Scoped
      ↑       ↑   ↑       ↑   ↑        ↑   ↑        ↑    Controls      Defaults
   impl   expl impl   expl impl    expl impl    expl

Position Breakdown

Position:  1   2  3   4  5   6  7   8  9  10       11 12
String:    1   3  r   3  r   3  r   3  r   5        1  l
          ┌─┐ ┌───┐ ┌───┐ ┌───┐ ┌───┐ ┌─────┐ ┌───────┐
          │V│ │I│E│ │I│E│ │I│E│ │I│E│ │ GPC │ │ CSD │
          └─┘ └───┘ └───┘ └───┘ └───┘ └─────┘ └───────┘
           │   │ │   │ │   │ │   │ │      │       │ │
        Version│ │   │ │   │ │   │ │   Global   │ │
               │ └─→ │ └─→ │ └─→ │ └─→ Privacy  │ │
               │  r  │  r  │  r  │  r  Controls │ l
               │     │     │     │              │
            ad_storage  analytics  ad_user   Container
                                  ad_personal Scoped
                                              Defaults

Parsing Logic for Your Template

consentString.substring(1, 3) → "3r" → pair[1] = 'r'  // ad_storage
consentString.substring(3, 5) → "3r" → pair[1] = 'r'  // analytics_storage  
consentString.substring(5, 7) → "3r" → pair[1] = 'r'  // ad_user_data
consentString.substring(7, 9) → "3r" → pair[1] = 'r'  // ad_personalization

String Components Legend

┌─────────────────┬─────────────────────────────────────────────┐
│ Component       │ Description                                 │
├─────────────────┼─────────────────────────────────────────────┤
│ 1               │ Version marker                              │
│ (3│r)           │ Consent pair: Implicit│Explicit             │
│ 5               │ Global Privacy Controls marker              │
│ (1│l)           │ Container Scoped Defaults (optional)        │
└─────────────────┴─────────────────────────────────────────────┘

Consent Values

Value	Meaning
granted	User has granted consent
denied	User has denied consent
-	Consent state not defined

Use Cases

Tag Firing Logic

// Fire tag only if analytics consent granted
if ({{Consent Parser}}.analytics_storage.update === "granted") {
  // Fire analytics tag
}

Server-Side Processing

// Process data based on consent
const consent = {{Consent Parser}};
if (consent.ad_personalization.update === "granted") {
  // Enable personalized features
}

Debugging & Monitoring

// Log consent states for debugging
console.log("Current consent:", {{Consent Parser}});

API Reference

Return Values

When consentCategory: "all"

Returns object with all consent categories:

{
  ad_storage: { default: string, update: string },
  analytics_storage: { default: string, update: string },
  ad_user_data: { default: string, update: string },
  ad_personalization: { default: string, update: string }
}

When specific category selected

Returns single category data in chosen format:

Object format:

{ default: string, update: string }

String format:

"default:value|update:value"

Error Cases

• Returns false when no gcd parameter exists
• Returns null for invalid output format

Testing

The template includes comprehensive tests covering:

• ✅ Valid consent strings with all categories
• ✅ Single category extraction
• ✅ Missing parameter handling
• ✅ Object vs string format consistency

Run tests in GTM's template editor before deploying.

Troubleshooting

Common Issues

Returns false

• Check that gcd parameter exists in request
• Verify Consent Mode v2 is properly implemented

Returns null values

• Check outputFormat parameter is valid (object or string)
• Verify template configuration

Unexpected consent values

• Ensure Consent Mode v2 is correctly configured on client-side
• Check consent string format in browser network tab

Debugging

1. Check browser network requests for gcd parameter
2. Use GTM Preview mode to inspect variable values
3. Enable debug logging in server container

Technical Details

Base64url Decoding

The template uses a custom base64url (Safe) alphabet for decoding:

0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-_

Bitwise Operations

For each consent category, the template:

1. Extracts the second character from each 2-character pair
2. Gets the base64url index value
3. Uses bitwise operations to extract default and update values:
   • defaultVal = (value >> 2) & 3 (bits 2-3)
   • updateVal = value & 3 (bits 0-1)

Contributing

1. Fork the repository
2. Create a feature branch (git checkout -b feature/amazing-feature)
3. Add tests for new features
4. Commit your changes (git commit -m 'Add some amazing feature')
5. Push to the branch (git push origin feature/amazing-feature)
6. Open a Pull Request

Support

For issues and questions:

• Open an issue on GitHub
• Contact Analytics Debugger S.L.U.

Credits

Created by Analytics Debugger S.L.U. - David Vallejo, 2025