The "Request contains an invalid argument" error in the FCM (Firebase Cloud Messaging) API typically occurs when there is an issue with the request payload or some missing required fields. Firebase Cloud Messaging is quite strict with the structure of the request, so any inconsistency or missing data can result in this error.

Here are some common reasons and troubleshooting steps to resolve the "Request contains an invalid argument" error:

  1. Check Authorization: Ensure that you are sending the request with the correct FCM Server Key or Legacy Server Key. The server key must be valid and properly set in the Authorization header of the request.

  2. Verify Endpoint: Make sure that you are sending the request to the correct FCM API endpoint:

  1. Check the Request Format: The FCM API expects the request payload to be in a specific format. Typically, the payload should be a JSON object with specific fields like to, data, or notification.

Example of a valid FCM request payload:

{ "to": "device_token_or_topic", "data": { "title": "Notification Title", "body": "Notification Body" } }
  1. Check for Special Characters: Ensure that there are no special characters, invalid characters, or whitespace issues in the payload. The JSON payload must be properly encoded.

  2. Verify the Device Token or Topic: Ensure that the to field in the request contains a valid device token or topic to which the notification should be sent. If it is a device token, it should be a valid FCM registration token associated with the target device.

  3. Validate Payload Size: Make sure the payload size does not exceed the maximum limit set by FCM, which is 4KB for a single message.

  4. Verify Field Types: Check the data types of the fields you are sending. For example, if you are sending custom data, ensure that it is properly formatted, especially if you are sending nested objects.

  5. Use Valid Parameters: Review the FCM documentation to ensure that you are using valid parameters for the request.

If you have checked all the above points and still face the error, consider reviewing the specific error response from FCM. It usually provides more details about the exact cause of the issue.

Additionally, you can consult the official Firebase Cloud Messaging documentation and error reference for more detailed information and examples:

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