espflash flash --monitor stops after bootloader on ESP32-C6 – defmt logs never appear

[kenggua211]

Description

When flashing an ESP32-C6 binary built with esp-generate (default Embassy template) using espflash flash --monitor --chip esp32c6 --log-format defmt, the flashing succeeds, but serial monitoring stops immediately after the ESP-IDF bootloader messages. No defmt (or any user) logs are printed.

Expected behavior
After flashing, espflash --monitor should:
Complete flashing.
Display defmt logs (e.g., [INFO ] Hello world!).

Actual behavior

ESP-ROM:esp32c6-20220919
Build:Sep 19 2022
rst:0x1 (POWERON),boot:0xc (SPI_FAST_FLASH_BOOT)
I (191) boot: Disabling RNG early entropy source...

Monitor output stops here. No further logs, no defmt decoding.

Reproduction steps
Using esp-generate to generate project with necessary configs. And use cargo run to flash into board.

Contrast: The same binary works perfectly with probe-rs

Env:
espflash 4.2.0
rustc 1.91.0 (f8297e351 2025-10-28)
OS Windows11
rust toolchain
channel = "stable"
components = ["rust-src"]
targets = ["riscv32imac-unknown-none-elf"]

[bugadani]

Contrast: The same binary works perfectly with probe-rs

This means you output logs using RTT. That will not show up in espflash, they use a different transport method. probe-rs literally reads the logs out of memory, while espflash relies on the chip dumping the logs over UART or USB.

[kenggua211]

This means you output logs using RTT. That will not show up in espflash, they use a different transport method. probe-rs literally reads the logs out of memory, while espflash relies on the chip dumping the logs over UART or USB.

Thank you for the quick and helpful response.
I tried the monitor subcommand in espflash as a way to open the serial monitor without flashing, which seemed like the closest match to what you described. Unfortunately, it still doesn't capture the RTT logs.
Interestingly, I came across a Bilibili tutorial on Rust + ESP32 development where the workflow is nearly identical to what I've been following, except I'm targeting the ESP32-C6. In the comments, another user with an ESP32-C6 reported the exact same issue with logs not appearing in espflash.

[bugadani]

espflash will never be able to display RTT logs.

[kenggua211]

espflash will never be able to display RTT logs.

Thanks for the reply. I tested command espflash flash --monitor on ESP32-S3 with defmt and it's able to print logs. Here's the code:

use defmt::info;
#[esp_rtos::main] async fn main(_: Spawner) -> ! {
    info!("Embassy initialized!");
    loop { info!("Hello world!"); Timer::after_secs(1).await; }
}

Output shows defmt logs on S3. But same code and same config with ESP32-C6 can't produce the expected behavior.
Why the difference?

[bugadani]

Output shows defmt logs on S3. [...] Why the difference?

Try using the same binary with probe-rs, and you'll see that this time your S3 does not print logs in a way that probe-rs can read. The transport methods between probe-rs and espflash are different, either one works, or the other. They work in different, incompatible ways.

• espflash can read logs that you write to a serial port. (usually with esp-println)
• probe-rs can read logs that you write to an RTT buffer that it then reads out of memory. (usually you use defmt-rtt, or rtt-target in this case)

[kenggua211]

Output shows defmt logs on S3. [...] Why the difference?

Try using the same binary with probe-rs, and you'll see that this time your S3 does not print logs in a way that probe-rs can read. The transport methods between probe-rs and espflash are different, either one works, or the other. They work in different, incompatible ways.

* espflash can read logs that you write to a serial port. (usually with esp-println)

* probe-rs can read logs that you write to an RTT buffer that it then reads out of memory. (usually you use defmt-rtt, or rtt-target in this case)

That's very clear, thank you.