> ## Documentation Index
> Fetch the complete documentation index at: https://docs.hypersender.com/llms.txt
> Use this file to discover all available pages before exploring further.

# What is chatId?

> Understand the identifier formats used by Hypersender WhatsApp endpoints, including @c.us and @lid.

`chatId` is the destination identifier used by Hypersender WhatsApp endpoints. The two formats you will work with most often are `@c.us` and `@lid`.

## `@c.us`

* Format: `[phoneNumber]@c.us`
* Use this when you know the contact's phone number.
* This is the standard phone-number-based identifier for direct messaging.
* Do not include `+`, spaces, or punctuation in the numeric part.
* Example: `12132132131@c.us`

## `@lid`

* Format: `[numericIdentifier]@lid`
* Use this when the contact is represented by a linked identifier instead of a visible phone number.
* This is a privacy-preserving identifier. The same person can have both an `@lid` value and an `@c.us` value.
* Example: `1111111@lid`

## Difference Between `@c.us` And `@lid`

* `@c.us` is based on the phone number.
* `@lid` is based on a linked identifier.
* Both can refer to the same contact.
* The contact conversion endpoints let you map one format to the other when the mapping is already known by your instance.

## Formatting Rules

* Keep the numeric part clean. Do not send `+`, spaces, or punctuation in phone-number-based values.
* When an endpoint accepts the identifier inside the URL path, encode `@` as `%40`.
* For example, use `1111111%40lid` or `201553730661%40c.us` in path-based endpoints.
* Some conversion endpoints also accept only the numeric part, such as `1111111` or `201553730661`.

## When To Convert

Use the contact conversion endpoints when you need to:

* Resolve a linked identifier to a phone-number chat ID.
* Resolve a phone-number chat ID to a linked identifier.
* List or count the mappings your instance already knows.