DLT Variable Tagging: Tags, Limits & Fixes

1. What TRAI directed

TRAI's direction F.No. D-2711/(2)/2024-QoS of 18 November 2025 (announced in press release 133/2025) requires each variable field in every SMS content template to be "pre-tagged with descriptive labels specifying the content type, purpose, and validation criteria". The reason, in TRAI's words: the absence of tagging "has been routinely exploited for fraudulent and phishing activities, allowing unregistered or malicious URLs, app links, and callback numbers to be inserted into approved templates".

Directions from February and May 2023 had already asked for this; TRAI notes they "have not been implemented till date". COAI proposed the tag set on 5 September 2025 and all access providers agreed to it on 17 September 2025.

2. Enforcement dates

3. Tag names by operator

TRAI's Annexure-I names the tags with bare hashes. The portals wrap them in braces, and Vi and BSNL shorten them:

Airtel's tag list also contains {#purpose#}, which no public Airtel material explains. Its pre-tagging notice asks for three things per variable: the "Variable Type", the "Purpose of the variable" and a "Sample value matching validation rules". For URL, OTT, callback and email tags, the value dropdown is filled from your whitelisted CTAs. If you have none of that type, Airtel shows "Values not found for type… kindly register for more in Manage CTA".

We could not confirm the tag spelling for Jio TrueConnect or SmartPing from their public portal code, so use what their template screens offer. Operator-by-operator portal notes are in our Jio, Airtel, Vi and BSNL guides.

4. Length and validation rules

Sources: the Vi and BSNL content template manuals, and TRAI Annexure-I, which sets "not more than 40 characters" for alphanumeric values. Airtel's equivalents follow the same Annexure. Link formats for whitelisting are covered in our template registration guide.

5. 30 or 40 characters? Why sources disagree

Search for the variable limit and you will find both numbers. The conflict exists inside Airtel's own portal. Its older template page says "You can pass up to 30 characters excluding exempted special characters in {#var#}". Its newer template component says "up to 40 characters (excluding exempted special characters)". Infobip's help article, updated in July 2026, says 30. MSG91 says 40.

Our reading: 30 applies to legacy untagged {#var#} on Airtel, and 40 applies to tagged number, text and email variables everywhere. Airtel does not list the "exempted special characters", so do not count on them. Until every template you use is tagged, keep free-text values to 30 characters.

6. Which tag for which value

7. Migrating old {#var#} templates

Vi and BSNL provide a bulk CSV tool, described in the same words in both manuals:

1. Go to Content Templates → Templates Tagging and open the migration upload page.
2. Click Download eligible templates. Only templates that are Active and Approved, contain at least one variable, and have not been migrated already are included.
3. Fill only two columns: Modified Template and Sample Message. Template ID, name and message are pre-filled and must not be edited.
4. Replace each {#var#} with a tag. Do not change one character of the fixed text. "Even a single extra space… will cause the row to fail." Keep the same number of variables.
5. The file arrives with auto-suggested values. The manuals warn they "may not always be accurate", so check every row.
6. Save as CSV, not XLSX, under 2 MB, with no duplicate template IDs. Upload it, accept the TRAI compliance declaration and submit.
7. Check Migration History, download the error details, and re-upload only the rows that failed.

Before:   Dear {#var#}, your order {#var#} of Rs {#var#} is confirmed. -Acme Stores
Vi/BSNL:  Dear {#alp#}, your order {#alp#} of Rs {#num#} is confirmed. -Acme Stores
Airtel:   Dear {#alphanumeric#}, your order {#alphanumeric#} of Rs {#numeric#} is confirmed. -Acme Stores
Sample:   Dear Priya, your order AZ10234 of Rs 1499 is confirmed. -Acme Stores

Airtel's public material describes no bulk migration tool; its template screen swaps each placeholder for the tag you pick from a type dropdown. For existing Airtel templates, ask dlt.care@airtel.com or your telemarketer. One trap: an old {#var#} that carried a link now needs a URL tag and a whitelisted CTA.

8. Fixing error 635: variable exceeded max length

Airtel's scrubbing code 635 means "Template variable exceeded max length". The 600-series table used for Vi and BSNL has the same number, TEMPLATE_VARIABLE_EXCEEDED_MAX_LENGTH. Aggregators re-label it; Infobip, for example, reports it as error 4131. The usual causes:

• A name or address over 40 characters, or over 30 in an untagged Airtel template
• A long tracking URL over 120 characters
• Two values joined into one variable, such as a name and an order number

Fixes, in order of preference:

• Truncate display values such as names in code.
• Move constant parts into the fixed text.
• Register a whitelisted short URL for long links.
• Split one long value across two variables, if your variable count allows.

Never truncate an OTP, an amount or a link. A cut-off value is worse than a rejected one, so validate these and stop the send instead. Other codes are covered in our DLT error codes guide.

9. Validate values in PHP before sending

Scrubbing rejects a message only after you have sent it. This function applies the published limits first and accepts both Airtel and Vi/BSNL tag names. It is a pre-check; the operator's scrubber has the final word.

<?php
// dlt-check.php: catch DLT variable errors before the operator does (PHP 8.0+).
// Limits: TRAI Annexure-I (18 Nov 2025) and the Vi/BSNL template manuals.

// Airtel's long tag names and Vi/BSNL's short ones map to the same rule.
const DLT_TAG_TYPES = [
    'numeric' => 'num', 'number' => 'num', 'num' => 'num',
    'alphanumeric' => 'alp', 'alp' => 'alp',
    'url' => 'url', 'urg' => 'url',
    'urlott' => 'ott', 'uro' => 'ott',
    'cbn' => 'cbn',
    'email' => 'eml', 'eml' => 'eml',
    'var' => 'var',   // legacy, untagged
];

/**
 * Returns a list of problems; an empty array means every value passes.
 * $ctas holds your whitelisted values: ['url' => [...], 'ott' => [...], 'cbn' => [...]].
 * A whitelisted link ending in "?" or "/" is treated as a prefix (dynamic or
 * short URL); anything else must match exactly.
 */
function dlt_check(string $template, array $values, array $ctas = []): array
{
    preg_match_all('/\{#(\w+)#\}/', $template, $m);
    if (count($m[1]) !== count($values)) {
        return [sprintf('Template has %d variables but %d values were given', count($m[1]), count($values))];
    }

    $errors = [];
    foreach (array_values($values) as $i => $value) {
        $value = (string) $value;
        $tag   = strtolower($m[1][$i]);
        $type  = DLT_TAG_TYPES[$tag] ?? null;
        $len   = mb_strlen($value);
        $at    = sprintf('Variable %d {#%s#}', $i + 1, $tag);

        $error = match ($type) {
            'num' => preg_match('/^[0-9]{1,40}$/', $value) ? null : 'digits only, 1-40 characters',
            'alp' => match (true) {
                $len > 40 => "$len characters, limit 40",
                (bool) preg_match('~https?://|www\.~i', $value) => 'contains a URL; use a URL tag',
                default => null,
            },
            'url', 'ott' => match (true) {
                $len > 120 => "$len characters, limit 120",
                !dlt_whitelisted($value, $ctas[$type] ?? []) => 'link is not whitelisted',
                default => null,
            },
            'cbn' => match (true) {
                !preg_match('/^[0-9]{3,14}$/', $value) => '3-14 digits, no + or spaces',
                !in_array($value, $ctas['cbn'] ?? [], true) => 'number is not whitelisted',
                default => null,
            },
            'eml' => ($len <= 40 && filter_var($value, FILTER_VALIDATE_EMAIL)) ? null : 'not a valid email of 40 characters or fewer',
            'var' => $len > 30 ? "$len characters; keep untagged {#var#} to 30" : null,
            default => 'unknown tag',
        };
        if ($error !== null) {
            $errors[] = "$at: $error";
        }
    }
    return $errors;
}

function dlt_whitelisted(string $link, array $whitelist): bool
{
    foreach ($whitelist as $allowed) {
        $prefix = str_ends_with($allowed, '?') || str_ends_with($allowed, '/');
        if ($link === $allowed || ($prefix && str_starts_with($link, $allowed))) {
            return true;
        }
    }
    return false;
}

// Try it
$ctas = [
    'url' => ['https://acme.example/track?', 'https://shrt.example/ACMEST/'],
    'cbn' => ['18001234567'],
];
$tpl = 'Your Acme Stores order {#alphanumeric#} has been shipped. Track it here: {#url#} -Acme Stores';
$tests = [
    [$tpl, ['AZ10234', 'https://acme.example/track?id=AZ10234']],
    [$tpl, ['AZ10234 for Priya Ramasamy, Door 12, Kalangal Road', 'https://bit.ly/3xYz']],
    ['Your OTP is {#num#}. -Acme', ['48 29 13']],
    ['Call {#cbn#} for help. -Acme', ['+91 1800 123 4567']],
];
foreach ($tests as [$template, $values]) {
    $errors = dlt_check($template, $values, $ctas);
    echo $errors ? implode("\n", $errors) : 'OK', "\n";
}
// OK
// Variable 1 {#alphanumeric#}: 50 characters, limit 40
// Variable 2 {#url#}: link is not whitelisted
// Variable 1 {#num#}: digits only, 1-40 characters
// Variable 1 {#cbn#}: 3-14 digits, no + or spaces

Run it where you build the message, then send with the matching dlt_template_id as shown in our PHP SMS guide. Operators do not publish how they count Tamil or other Indic text against variable limits, so leave a margin.

Related DLT guides

• DLT registration for bulk SMS, the end-to-end process
• Entity ID (PE ID) and header (sender ID) rules
• Transactional vs promotional SMS, to choose the category before tagging