Debugging into devcontainer

[Ninitage]

Hi,

Context: I'm failing to start a debugging session into a Zed editor instance attached to a running devcontainer.

Host: Arch Linux system
Zed version: 0.219.4+stable (installed through pacman)
Zed PHP extension version: 0.4.6 (installed from zed extension menu)
Docker version: 29.1.4, build 0e6fee6c52
Docker Compose version: 5.0.1
devcontainer-cli version: 0.80.3 (installed from AUR)

Dev container is composed of a docker compose file. The micro service used for php is based on FrankenPHP, PHP v8.3 onto a Debian Bookworm system. Xdebug is installed and configured on it.

devcontainer.json

{
    "name": "my app",
    "dockerComposeFile": [
        "../docker-compose.yaml",
        "../docker-compose.override.yaml"
    ],
    "service": "app",
    "workspaceFolder": "/app",
    "remoteEnv": {
        "XDEBUG_MODE": "debug",
        "XDEBUG_CONFIG": "client_host=host.docker.internal client_port=9003"
    },
    "customizations": {
        "zed": {
            "extensions": ["php"]
        }
    }
}

docker-compose.yaml

services:
  app:
    build:
      context: ./
      dockerfile: ./Dockerfile
    # restart: always
    ports:
      - "80:80" # HTTP
      - "443:443" # HTTPS
    volumes:
      - ./:/app
      - caddy_data:/data
      - caddy_config:/config
    depends_on:
      - db
      - messenger
    networks:
      - db
      - messenger

  db:
    image: amd64/mariadb:11.8.3
    ports:
      - "3306:3306"
    environment:
      MARIADB_USER: "user"
      MARIADB_PASSWORD: "password"
      MARIADB_ROOT_PASSWORD: "root-password"
      MARIADB_DATABASE: "my_database"
    networks:
      - db

  messenger:
    image: amd64/redis:8.2-bookworm
    networks:
      - messenger

volumes:
  caddy_data:
  caddy_config:

networks:
  db:
    driver: bridge
  messenger:
    driver: bridge

docker-compose.override.yaml

services:
    app:
        tty: true
        extra_hosts:
            - "host.docker.internal:host-gateway"

Dockerfile

FROM dunglas/frankenphp:1.9.1-builder-php8.3.25-bookworm
WORKDIR /app
RUN set -eux \
    && install-php-extensions \
    @composer \
    apcu \
    intl \
    gd \
    opcache \
    openssl \
    pdo_mysql \
    redis \
    xdebug \
    zip \
    && cp /usr/local/etc/php/php.ini-development /usr/local/etc/php/php.ini \
    && sed -i "s/memory_limit = 128M/memory_limit = 256M/g" /usr/local/etc/php/php.ini \
    && echo "xdebug.mode=debug,develop" >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini \
    && echo "xdebug.start_with_request=yes" >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini \
    && echo 'xdebug.client_host="host.docker.internal"' >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini \
    && echo "xdebug.client_port=9003" >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini \
    && echo "xdebug.discover_client_host=0" >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini \
    && echo 'xdebug.log="/app/var/log/xdebug.log"' >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini

.zed/debug.json

[
    {
        "label": "PHP: Listen to Xdebug",
        "adapter": "Xdebug",
        "request": "launch",
        "port": 9003,
        "hostname": "127.0.0.1",
        "pathMappings": {
            "/app": "$ZED_WORKTREE_ROOT"
        },
        "log": true
    },
    {
        "label": "PHP: Debug this test",
        "adapter": "Xdebug",
        "request": "launch",
        "port": 9003,
        "hostname": "127.0.0.1",
        "program": "vendor/bin/phpunit",
        "args": ["--filter", "$ZED_SYMBOL"],
        "pathMappings": {
            "/app": "$ZED_WORKTREE_ROOT"
        },
        "log": true
    }
]

remote php xdebug extension is installed into /data/zed/remote_extensions/work/php/Xdebug/Xdebug_v1.39.1/extension/ into the dev container.
LSP is working.

Process

1. I start the project running the command devcontainer up --workspace-folder /home/user/my_app.
2. I launch Zed editor.
3. I click on yes into this popup:

4. Once inside, I try to start a new debug session, the first one listed into the configuration above for example.

Result

• failing almost instantly
• debug console is showing:

error: 
error: process exited before debugger attached.

• no logs are written where it should be defined or into the related button "Open Debug Adapter Logs"
• host does not seem to listen on port 9003 using the command ss -lntp | grep 9003 to check it (firewall rules have been added to make sure, even disabled, it does not change anything, not sure it's related and not sure host should be listening here, correct me if I'm wrong)

I've watched most related issues onto Zed repository and this one, tried different configurations, like replacing the ip address into .zed/debug.json by 0.0.0.0 or "host.docker.internal", tried another port even if most people seems to discourage it, make sure the log file have full permission access to get more info about what is going on without any success. Let Xdebug discover host does not help either.

So, I would like to know if anyone has noticed an error into my configuration please? (the same one on another computer seems to work well with another editor, but I really would like to switch to Zed, seems promising)
Or did I miss something? Maybe debugging into a devcontainer is not implemented yet?

Edit

I've succeeded to get some logs:

[140475505965760] Log opened at 2026-01-19 23:02:02.736359
[140475505965760] [Config] INFO: Control socket set up successfully: '@xdebug-ctrl.140475505965760'
[140475505965760] [Step Debug] INFO: Connecting to configured address/port: host.docker.internal:9003.
[140475505965760] [Step Debug] WARN: Creating socket for 'host.docker.internal:9003', poll success, but error: Operation now in progress (29).
[140475505965760] [Step Debug] ERR: Could not connect to debugging client. Tried: host.docker.internal:9003 (through xdebug.client_host/xdebug.client_port).
[140475505965760] Log closed at 2026-01-19 23:02:02.737016

[Ninitage]

Here the result of this command that i run into the container php -r 'xdebug_info();':

__   __   _      _
\ \ / /  | |    | |
 \ V / __| | ___| |__  _   _  __ _
  > < / _` |/ _ \ '_ \| | | |/ _` |
 / . \ (_| |  __/ |_) | |_| | (_| |
/_/ \_\__,_|\___|_.__/ \__,_|\__, |
                              __/ |
                             |___/

Version => 3.5.0
Support Xdebug on Patreon, GitHub, or as a business: https://xdebug.org/support

             Enabled Features (through 'xdebug.mode' setting)
Feature => Enabled/Disabled
Development Helpers => ✘ disabled
Coverage => ✘ disabled
GC Stats => ✘ disabled
Profiler => ✘ disabled
Step Debugger => ✔ enabled
Tracing => ✘ disabled

                            Optional Features
Compressed File Support => yes (gzip)
Clock Source => clock_gettime
TSC Clock Source => available
'xdebug://gateway' pseudo-host support => yes
'xdebug://nameserver' pseudo-host support => yes
Systemd Private Temp Directory => not enabled

                              Diagnostic Log
[Step Debug] WARN: Creating socket for 'host.docker.internal:9003', poll success, but error: Operation now in progress (29).
[Step Debug] ERR: Could not connect to debugging client. Tried: host.docker.internal:9003 (through xdebug.client_host/xdebug.client_port).

                              Step Debugging
Debugger is not active

                                   PHP
                           Build Configuration
Version (Run Time) => 8.3.25
Version (Compile Time) => 8.3.25
Debug Build => no
Thread Safety => enabled
Thread API => POSIX Threads
                                 Settings
Configuration File (php.ini) Path => /usr/local/etc/php
Loaded Configuration File => /usr/local/etc/php/php.ini
Scan this dir for additional .ini files => /usr/local/etc/php/conf.d
Additional .ini files parsed => /usr/local/etc/php/conf.d/docker-php-ext-apcu.ini,
/usr/local/etc/php/conf.d/docker-php-ext-gd.ini,
/usr/local/etc/php/conf.d/docker-php-ext-intl.ini,
/usr/local/etc/php/conf.d/docker-php-ext-opcache.ini,
/usr/local/etc/php/conf.d/docker-php-ext-pdo_mysql.ini,
/usr/local/etc/php/conf.d/docker-php-ext-redis.ini,
/usr/local/etc/php/conf.d/docker-php-ext-sodium.ini,
/usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini,
/usr/local/etc/php/conf.d/docker-php-ext-zip.ini


Directive => Local Value => Master Value
xdebug.mode => debug => debug
xdebug.start_with_request => yes => yes
xdebug.start_upon_error => default => default
xdebug.output_dir => /tmp => /tmp
xdebug.use_compression => 1 => 1
xdebug.trigger_value => no value => no value
xdebug.file_link_format => no value => no value
xdebug.filename_format => no value => no value
xdebug.control_socket => time: 25ms => time: 25ms
xdebug.path_mapping => 0 => 0
xdebug.log => /app/var/log/xdebug.log => /app/var/log/xdebug.log
xdebug.log_level => 7 => 7
xdebug.var_display_max_children => 128 => 128
xdebug.var_display_max_data => 512 => 512
xdebug.var_display_max_depth => 3 => 3
xdebug.max_nesting_level => 512 => 512
xdebug.cli_color => 0 => 0
xdebug.force_display_errors => Off => Off
xdebug.force_error_reporting => 0 => 0
xdebug.halt_level => 0 => 0
xdebug.max_stack_frames => -1 => -1
xdebug.show_error_trace => Off => Off
xdebug.show_exception_trace => Off => Off
xdebug.show_local_vars => Off => Off
xdebug.dump.COOKIE => no value => no value
xdebug.dump.ENV => no value => no value
xdebug.dump.FILES => no value => no value
xdebug.dump.GET => no value => no value
xdebug.dump.POST => no value => no value
xdebug.dump.REQUEST => no value => no value
xdebug.dump.SERVER => no value => no value
xdebug.dump.SESSION => no value => no value
xdebug.dump_globals => On => On
xdebug.dump_once => On => On
xdebug.dump_undefined => Off => Off
xdebug.profiler_output_name => cachegrind.out.%p => cachegrind.out.%p
xdebug.profiler_append => Off => Off
xdebug.cloud_id => no value => no value
xdebug.client_host => host.docker.internal => host.docker.internal
xdebug.client_port => 9003 => 9003
xdebug.discover_client_host => Off => Off
xdebug.client_discovery_header => HTTP_X_FORWARDED_FOR,REMOTE_ADDR => HTTP_X_FORWARDED_FOR,REMOTE_ADDR
xdebug.idekey => no value => no value
xdebug.connect_timeout_ms => 200 => 200
xdebug.scream => Off => Off
xdebug.gc_stats_output_name => gcstats.%p => gcstats.%p
xdebug.trace_output_name => trace.%c => trace.%c
xdebug.trace_format => 0 => 0
xdebug.trace_options => 0 => 0
xdebug.collect_assignments => Off => Off
xdebug.collect_params => On => On
xdebug.collect_return => Off => Off

I've rebuilt the container using pecl install xdebug && docker-php-ext-enable xdebug into the Dockerfile instead of using install-php-extensions xdebug. It seems to more verbose about logs:

[140359837805248] Log opened at 2026-01-21 19:28:28.506504
[140359837805248] [Config] INFO: Control socket set up successfully: '@xdebug-ctrl.140359837805248'
[140359837805248] [Step Debug] INFO: Connecting to configured address/port: host.docker.internal:9003.
[140359837805248] [Step Debug] WARN: Creating socket for 'host.docker.internal:9003', poll success, but error: Operation now in progress (29).
[140359837805248] [Step Debug] ERR: Could not connect to debugging client. Tried: host.docker.internal:9003 (through xdebug.client_host/xdebug.client_port).
[140248638278336] Log opened at 2026-01-21 19:28:29.904722
[140248638278336] [Config] INFO: Control socket set up successfully: '@xdebug-ctrl.140248638278336'
[140248638278336] [Step Debug] INFO: Connecting to configured address/port: host.docker.internal:9003.
[140248638278336] [Step Debug] WARN: Creating socket for 'host.docker.internal:9003', poll success, but error: Operation now in progress (29).
[140248638278336] [Step Debug] ERR: Could not connect to debugging client. Tried: host.docker.internal:9003 (through xdebug.client_host/xdebug.client_port).
[140248638278336] Log closed at 2026-01-21 19:28:30.051773

[140098431011520] Log opened at 2026-01-21 19:28:30.404350
[140134335093440] Log opened at 2026-01-21 19:28:30.404349
[140098431011520] [Config] INFO: Control socket set up successfully: '@xdebug-ctrl.140098431011520'
[140134335093440] [Config] INFO: Control socket set up successfully: '@xdebug-ctrl.140134335093440'
[140098431011520] [Step Debug] INFO: Connecting to configured address/port: host.docker.internal:9003.
[140098431011520] [Step Debug] WARN: Creating socket for 'host.docker.internal:9003', poll success, but error: Operation now in progress (29).
[140098431011520] [Step Debug] ERR: Could not connect to debugging client. Tried: host.docker.internal:9003 (through xdebug.client_host/xdebug.client_port).
[140704281234112] Log opened at 2026-01-21 19:28:30.405043
[140704281234112] [Config] INFO: Control socket set up successfully: '@xdebug-ctrl.140704281234112'
[140134335093440] [Step Debug] INFO: Connecting to configured address/port: host.docker.internal:9003.
[140134335093440] [Step Debug] WARN: Creating socket for 'host.docker.internal:9003', poll success, but error: Operation now in progress (29).
[140134335093440] [Step Debug] ERR: Could not connect to debugging client. Tried: host.docker.internal:9003 (through xdebug.client_host/xdebug.client_port).
[140704281234112] Log closed at 2026-01-21 19:28:30.406148

[140098431011520] Log closed at 2026-01-21 19:28:30.754240

[139638290074304] Log opened at 2026-01-21 19:28:30.781751
[139638290074304] [Config] INFO: Control socket set up successfully: '@xdebug-ctrl.139638290074304'
[139638290074304] [Step Debug] INFO: Connecting to configured address/port: host.docker.internal:9003.
[139638290074304] [Step Debug] WARN: Creating socket for 'host.docker.internal:9003', poll success, but error: Operation now in progress (29).
[139638290074304] [Step Debug] ERR: Could not connect to debugging client. Tried: host.docker.internal:9003 (through xdebug.client_host/xdebug.client_port).
[140134335093440] Log closed at 2026-01-21 19:28:31.102469

[139638290074304] Log closed at 2026-01-21 19:28:33.429373

This log occurred when I'm starting zed editor into the devcontainer, this is strange it's like it would spam it without I've asked to start a debug session.