Serial read, collate, write AT commands - c++

Question

This is most probably a software question, not sure where to post this (Arduino sub? electronics sub?).
I have an Arduino Mega, that is connected to :
an RFID card read via SPI,
a wifi chip (esp8266-12f) via Serial1.
The wifi chip currently acts as a web Server as I intend to send commands to it via the internet. I also want that wifi chip to act as a Client, that is to say : when the RFID reads a card, I want the Arduino Mega to forward the RFID's card id to the wifi chip via Serial1 so that the wifi chip can send an http request to a dedicated server that would save it into sql database.
This is how I coded it on the wifi chip so far :
// esp as a Server
// [...]
// esp as a Client
timeout = millis() + (10 * 1000UL); // 10s
String input;
while (Serial.available() >= 0 && millis() < timeout) {
input += Serial.readStringUntil('\r');
int pos = input.indexOf("AT+TEST");
if (pos >= 0) {
Serial.println(F("AT+TEST"));
// collate, aka confirm to Arduino Mega we understood the command and we are processing it
// todo: process the command, aka parse the command, send an http request, etc
Serial.println(F("\r\nOK"));
} else {
Serial.println(F("\r\nNOPE"));
}
}
As you might have guessed, the chip is printing "AT+TEST" over and over again (until it times out, then loops infinite NOPE), as it is trying to process the command over and over, but I just want it to say "ok Arduino Mega, I understood you want me to execute AT+TEST"
What would be an elegant way to make it so that the wifi chip "ignores" the stuff it itself prints to it's own serial? It should only execute commands coming from the Arduino Mega.
This is something the factory default firmware does very well : when I send "AT", it echoes back "AT\r\n\r\nOK" only once.
I found this but I cannot understand how they implemented the idea (irrelevant code everywhere) : https://github.com/JiriBilek/ESP_ATMod/blob/d6ad63f71ef3189518ef4740f16a9d697c3e843a/ESP_ATMod/ESP_ATMod.ino#L581
Looking for guidance on a solid way to achieve this as there are lots of datas coming in and out to that serial of the wifi chip.

Answer 1

It looped because of the Line Feed. Not only that, it also needed a 1ms delay somehow.
Sorry for the poor explanation of my issue.
The following works as I expected :
// esp as a client
String input;
timeout = millis() + (5 * 1000UL); // 5s
while (Serial.available() && millis() < timeout) {
input = Serial.readStringUntil('\r'); // up to CR
delay(1); Serial.read(); // discard LF
// collate
Serial.println(input);
// AT+TEST
if (input.indexOf("AT+TEST") >= 0) {
Serial.println(F("\r\nOK"));
// todo : verbose for debug
}
// AT+GET=https://arduino.example.tld/api/arduino.php?token=123&key1=val1
else if (input.indexOf("AT+GET") >= 0) {
Serial.println(F("\r\nOK"));
// todo : regex + http get req
} else {
Serial.println(F("\r\nERROR"));
}
}