Send 24-Hour Text Messages via WhatChimp API

How to send text messages within the WhatsApp 24-hour session window using the WhatChimp API.

Updated 5 February 2026

WhatsApp allows businesses to send free-text replies to customers — but only within 24 hours of the customer's last message. This is called a session message or 24-hour window message. This guide shows you how to send text messages via the WhatChimp API and explains exactly how the 24-hour rule works.

Before you begin: Make sure you have your API key and phone number ID. See Getting Started with WhatChimp Developer API for setup instructions.

What is the 24-Hour Session Window?

When a customer sends you a message on WhatsApp, a 24-hour session opens. During this window, you can reply with any text message you want — no template needed, no approval required.

Once the 24-hour window closes (the customer hasn't messaged you in over 24 hours), you cannot send text messages anymore. The API will return an error.

Key points:

  • The 24-hour clock resets every time the customer messages you
  • Only the customer can open a session — you cannot open one yourself
  • Text messages within the session are part of a customer-initiated conversation (lower cost tier)
  • Outside the 24-hour window, you must use a template message instead

The API console showing the Send Message endpoint and the text vs template message explanation

When to Use Text Messages

Text messages are the right choice when:

  • A customer has messaged you in the last 24 hours and you're replying to them
  • You're building a chatbot that responds to incoming messages
  • You're handling live chat replies to customer inquiries
  • You're sending a quick follow-up while the conversation is still active

Do NOT use text messages for:

  • Sending notifications, reminders, or order updates — use a template message
  • Reaching out to customers who haven't messaged you recently
  • Broadcasting promotional messages
  • Sending OTPs or verification codes

API Endpoint

Endpoint: GET/POST /api/v1/whatsapp/send

Full URL: https://app.whatchimp.com/api/v1/whatsapp/send

Parameters:

ParameterTypeRequiredDescription
apiTokenStringYesYour WhatChimp API key
phone_number_idStringYesYour WhatsApp phone number ID
messageStringYesText message content (URL-encode for GET requests)
phone_numberStringYesRecipient phone number with country code, numeric only (e.g., 919999999999)

Sending a Text Message

GET Request:

https://app.whatchimp.com/api/v1/whatsapp/send?apiToken=API-KEY&phone_number_id=PHONE-NUMBER-ID&message=Hello%2C+thanks+for+reaching+out!&phone_number=919999999999

POST Request (curl):

curl -X POST \
  'https://app.whatchimp.com/api/v1/whatsapp/send' \
  -d 'apiToken=API-KEY' \
  -d 'phone_number_id=PHONE-NUMBER-ID' \
  -d 'message=Hello, thanks for reaching out!' \
  -d 'phone_number=919999999999'

Success Response:

{
  "status": "1",
  "wa_message_id": "wamid.HBgNODgwMTcyMzMwOTAwMxUCABEYEjlGQkY3MEFEMEVGODhCNDkxNQA=",
  "message": "Message sent successfully."
}

Error Responses

24-hour window expired:

If you try to send a text message outside the session window, the API will return an error. When this happens, switch to a template message.

Subscriber limit exceeded:

{"status": "0", "message": "Subscriber limit has been exceeded. You cannot have more subscribers."}

This means your WhatChimp plan's subscriber limit has been reached. Upgrade your plan or remove inactive subscribers.

Tips

  • URL-encode your message when using GET requests. Spaces become %20 or +, special characters need encoding.
  • Save the wa_message_id from the success response — you can use it to track delivery status later.
  • If your message fails, check whether the 24-hour window is still open. If not, use a template message instead.
  • For automated replies (chatbots), text messages work perfectly since the customer initiates the conversation.

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