Understanding UART Overrun Error When Sending AT Commands from STM32 to ESP8266

I’m interfacing an ESP8266 with an NUCLEO-L073RZ using UART1 PA9, PA10 for communication with the ESP and UART2 for communication with a PC for debugging. I also used a CP2102 USB to TTL converter to tap the Tx signal from UART1 and monitor AT commands sent from the STM32 The STM32 sends the first AT command correctly, but when I send the second command AT+RST, only part of it is received by the ESP8266. After debugging, I found that HAL_UART_Transmit returns HAL_ERROR, and the cause seems to be an overrun error. See how I set things up: _Baud rate : 115200 for both STM32 and ESP8266 _Buffer size: 256 bytes _Communication method: Polling for UART communication _Power supply: 5V regulated power source for ESP8266 Using HAL_MAX_DELAY as the timeout fixed the issue, but I’m unsure why. Increasing the delay in HAL_Delay() didn’t resolve the issue, so I’m still confused about what’s causing the overrun error. Here’s the code I’m using to send AT commands;