Slack Incoming webhook not sending complete data

I am using incoming webhooks in SLACK to send data using my python code.

slack_data = {
        "username": "Test",
        "icon_emoji": ":robot_face:",
        "attachments": [
            {
                "pretext": f"Test",
                "fields": [
                    {
                        "title": self.__title,
                        "value": message
                    }
                ]
            }
        ]
    }

    headers = {'Content-Type': "application/json"}
    response = requests.post(SLACK_CHANNEL_URL, json=slack_data, headers=headers)
    

But when I run the code, I find if the 'value' in 'fields' is very long, it gets truncated in the slack channel.

for example:

long message long messagelong message long messagelong message long messagelong message long messagelong message long messagelong message long messagelong message long messagelong message long messagelong message long messagelong message long messagelong message long m...

I tried to search a lot but couldn't find anything which helps in preventing the data from getting truncated.