IDE hangs on startup: "YAMLException: end of the stream or a document separator is expected"

Development Tools IDE 2.x
1

@ptillisch I have the same problem running Arduino IDE 2.3.7. Here is the powershell output.

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PS C:\Program Files\Arduino IDE> & ".\Arduino IDE"
PS C:\Program Files\Arduino IDE>
Arduino IDE 2.3.7
Checking for frontend application configuration customizations. Module path: C:\Program Files\Arduino IDE\resources\app\lib\backend\electron-main.js, destination 'package.json': C:\Program Files\Arduino IDE\resources\app\package.json
Setting 'theia.frontend.config.appVersion' application configuration value to: "2.3.7" (type of string)
Setting 'theia.frontend.config.cliVersion' application configuration value to: "1.3.1" (type of string)
Setting 'theia.frontend.config.buildDate' application configuration value to: "2025-12-17T16:05:25.389Z" (type of string)
Frontend application configuration after modifications: {"applicationName":"Arduino IDE","defaultTheme":{"light":"arduino-theme","dark":"arduino-theme-dark"},"defaultIconTheme":"none","electron":{"windowOptions":{},"showWindowEarly":true,"splashScreenOptions":{},"uriScheme":"arduino-ide"},"defaultLocale":"","validatePreferencesSchema":false,"reloadOnReconnect":true,"uriScheme":"theia","preferences":{"window.title":"${rootName}${activeEditorShort}${appName}","files.autoSave":"afterDelay","editor.minimap.enabled":false,"editor.tabSize":2,"editor.scrollBeyondLastLine":false,"editor.quickSuggestions":{"other":false,"comments":false,"strings":false},"editor.maxTokenizationLineLength":500,"editor.bracketPairColorization.enabled":false,"breadcrumbs.enabled":false,"workbench.tree.renderIndentGuides":"none","explorer.compactFolders":false},"appVersion":"2.3.7","cliVersion":"1.3.1","buildDate":"2025-12-17T16:05:25.389Z"}
Starting backend process. PID: 12564
Showing main window early
Using browser-only version of superagent in non-browser environment
Configuration directory URI: 'file:///c%3A/Users/ethanho/.arduinoIDE'
Configuring to accept webviews on '^.+\.webview\..+$' hostname.
2026-01-04T15:05:17.466Z root WARN Backend u.initialize took longer than the expected maximum 50 milliseconds: 59.0 ms [Finished 1.241 s after backend start]
2026-01-04T15:05:17.469Z root WARN Backend Object.initialize took longer than the expected maximum 50 milliseconds: 55.8 ms [Finished 1.241 s after backend start]
2026-01-04T15:05:17.469Z root INFO Backend a.initialize: 3.2 ms [Finished 1.241 s after backend start]
2026-01-04T15:05:17.469Z root INFO Backend a.initialize: 2.7 ms [Finished 1.241 s after backend start]
2026-01-04T15:05:17.469Z root INFO Backend u.initialize: 3.3 ms [Finished 1.242 s after backend start]
2026-01-04T15:05:17.475Z root WARN Backend l.initialize took longer than the expected maximum 50 milliseconds: 58.2 ms [Finished 1.244 s after backend start]
2026-01-04T15:05:17.491Z root INFO configured all backend app contributions
2026-01-04T15:05:17.492Z root INFO Backend l.onStart: 3.6 ms [Finished 1.269 s after backend start]
2026-01-04T15:05:17.494Z root INFO Backend d.onStart: 0.4 ms [Finished 1.270 s after backend start]
2026-01-04T15:05:17.496Z root INFO Backend a.onStart: 1.4 ms [Finished 1.271 s after backend start]
2026-01-04T15:05:17.496Z config INFO >>> Initializing CLI configuration...
2026-01-04T15:05:17.496Z root INFO Backend x.onStart: 1.0 ms [Finished 1.272 s after backend start]
2026-01-04T15:05:17.497Z config INFO Loading CLI configuration from c:\Users\ethanho\.arduinoIDE\arduino-cli.yaml...
2026-01-04T15:05:17.497Z daemon INFO Starting daemon from C:\Program Files\Arduino IDE\resources\app\lib\backend\resources\arduino-cli.exe...
2026-01-04T15:05:17.497Z root INFO Backend w.onStart: 0.9 ms [Finished 1.274 s after backend start]
2026-01-04T15:05:17.497Z discovery-log INFO start
2026-01-04T15:05:17.497Z discovery-log INFO start new deferred
2026-01-04T15:05:17.497Z root INFO Backend v.onStart: 0.6 ms [Finished 1.275 s after backend start]
2026-01-04T15:05:17.497Z root INFO Backend a.onStart: 0.2 ms [Finished 1.275 s after backend start]
2026-01-04T15:05:17.508Z root INFO Theia app listening on http://127.0.0.1:52831.
2026-01-04T15:05:17.508Z root INFO Finished starting backend application: 11.0 ms [Finished 1.286 s after backend start]
2026-01-04T15:05:17.511Z root WARN The local plugin referenced by local-dir:/c%3A/Users/ethanho/.arduinoIDE/plugins does not exist.
2026-01-04T15:05:17.511Z root WARN The local plugin referenced by local-dir:/c%3A/Users/ethanho/.arduinoIDE/deployedPlugins does not exist.
2026-01-04T15:05:17.511Z root WARN The local plugin referenced by local-dir:C:\Users\ethanho\.arduinoIDE\plugins does not exist.
2026-01-04T15:05:17.563Z config ERROR Failed to initialize the CLI configuration. YAMLException: end of the stream or a document separator is expected at line 4, column 1:
    network:
    ^
    at T (C:\Program Files\Arduino IDE\resources\app\lib\backend\main.js:2:2584560)
    at D (C:\Program Files\Arduino IDE\resources\app\lib\backend\main.js:2:2584658)
    at U (C:\Program Files\Arduino IDE\resources\app\lib\backend\main.js:2:2598478)
    at W (C:\Program Files\Arduino IDE\resources\app\lib\backend\main.js:2:2598915)
    at V (C:\Program Files\Arduino IDE\resources\app\lib\backend\main.js:2:2599120)
    at e.exports.safeLoad (C:\Program Files\Arduino IDE\resources\app\lib\backend\main.js:2:2599452)
    at x.loadCliConfig (C:\Program Files\Arduino IDE\resources\app\lib\backend\main.js:2:5544800)
    at async x.initConfig (C:\Program Files\Arduino IDE\resources\app\lib\backend\main.js:2:5543665)
2026-01-04T15:05:17.665Z root INFO Resolve plugins list: 164.6 ms [Finished 1.443 s after backend start]
2026-01-04T15:05:17.843Z daemon INFO time="2026-01-04T23:05:17+08:00" level=info msg="arduino-cli version 1.3.1"
time="2026-01-04T23:05:17+08:00" level=info msg="Using config file: c:\\Users\\ethanho\\.arduinoIDE\\arduino-cli.yaml"
time="2026-01-04T23:05:17+08:00" level=info msg="Executing `arduino-cli daemon`"
2026-01-04T15:05:17.864Z daemon INFO Daemon is now listening on 127.0.0.1:52833
{"IP":"127.0.0.1","Port":"52833"}
2026-01-04T15:05:17.864Z daemon INFO Daemon is running.
2026-01-04T15:05:17.944Z daemon INFO time="2026-01-04T23:05:17+08:00" level=info msg="Checking signature" index="C:\\Users\\ethanho\\AppData\\Local\\Arduino15\\package_index.json" signatureFile="C:\\Users\\ethanho\\AppData\\Local\\Arduino15\\package_index.json.sig" trusted=true
2026-01-04T15:05:17.952Z daemon INFO time="2026-01-04T23:05:17+08:00" level=info msg="Missing signature file" index="C:\\Users\\ethanho\\AppData\\Local\\Arduino15\\package_esp32_index.json"
time="2026-01-04T23:05:17+08:00" level=info msg="Loading hardware from: C:\\Users\\ethanho\\AppData\\Local\\Arduino15\\packages"
2026-01-04T15:05:17.966Z daemon INFO time="2026-01-04T23:05:17+08:00" level=info msg="Loading package arduino from: C:\\Users\\ethanho\\AppData\\Local\\Arduino15\\packages\\arduino\\hardware"
time="2026-01-04T23:05:17+08:00" level=info msg="Missing signature file" index="C:\\Users\\ethanho\\AppData\\Local\\Arduino15\\packages\\arduino\\hardware\\avr\\1.8.6\\installed.json"
time="2026-01-04T23:05:17+08:00" level=info msg="Adding monitor tool" protocol=serial tool="builtin:serial-monitor"
time="2026-01-04T23:05:17+08:00" level=info msg="Loaded platform" platform="arduino:avr@1.8.6"
time="2026-01-04T23:05:17+08:00" level=info msg="Checking existence of 'tools' path: C:\\Users\\ethanho\\AppData\\Local\\Arduino15\\packages\\arduino\\tools"
time="2026-01-04T23:05:17+08:00" level=info msg="Loading tools from dir: C:\\Users\\ethanho\\AppData\\Local\\Arduino15\\packages\\arduino\\tools"
time="2026-01-04T23:05:17+08:00" level=info msg="Loaded tool" tool="arduino:arduinoOTA@1.3.0"
time="2026-01-04T23:05:17+08:00" level=info msg="Loaded tool" tool="arduino:avr-gcc@7.3.0-atmel3.6.1-arduino7"
time="2026-01-04T23:05:17+08:00" level=info msg="Loaded tool" tool="arduino:avrdude@6.3.0-arduino17"
time="2026-01-04T23:05:17+08:00" level=info msg="Loading package builtin from: C:\\Users\\ethanho\\AppData\\Local\\Arduino15\\packages\\builtin"
2026-01-04T15:05:17.967Z daemon INFO time="2026-01-04T23:05:17+08:00" level=info msg="Checking existence of 'tools' path: C:\\Users\\ethanho\\AppData\\Local\\Arduino15\\packages\\builtin\\tools"
time="2026-01-04T23:05:17+08:00" level=info msg="Loading tools from dir: C:\\Users\\ethanho\\AppData\\Local\\Arduino15\\packages\\builtin\\tools"
2026-01-04T15:05:17.971Z daemon INFO time="2026-01-04T23:05:17+08:00" level=info msg="Loaded tool" tool="builtin:ctags@5.8-arduino11"
time="2026-01-04T23:05:17+08:00" level=info msg="Loaded tool" tool="builtin:dfu-discovery@0.1.2"
time="2026-01-04T23:05:17+08:00" level=info msg="Loaded tool" tool="builtin:mdns-discovery@1.0.12"
time="2026-01-04T23:05:17+08:00" level=info msg="Loaded tool" tool="builtin:serial-discovery@1.4.3"
2026-01-04T15:05:17.978Z daemon INFO time="2026-01-04T23:05:17+08:00" level=info msg="Loaded tool" tool="builtin:serial-monitor@0.15.0"
2026-01-04T15:05:18.026Z daemon INFO time="2026-01-04T23:05:18+08:00" level=info msg="Adding libraries dir" dir="C:\\Users\\ethanho\\AppData\\Local\\Arduino15\\packages\\arduino\\hardware\\avr\\1.8.6\\libraries" isSingleLibrary=false location=platform
time="2026-01-04T23:05:18+08:00" level=info msg="Loading libraries index file" index="C:\\Users\\ethanho\\AppData\\Local\\Arduino15\\library_index.json"
2026-01-04T15:05:18.546Z root INFO Deploy plugins list: 1046.1 ms [Finished 2.324 s after backend start]
2026-01-04T15:05:18.600Z daemon INFO time="2026-01-04T23:05:18+08:00" level=info msg="Adding libraries dir" dir="C:\\Users\\ethanho\\AppData\\Local\\Arduino15\\libraries" isSingleLibrary=false location=ide
2026-01-04T15:05:18.600Z daemon INFO time="2026-01-04T23:05:18+08:00" level=info msg="Adding libraries dir" dir="C:\\Users\\ethanho\\Documents\\Arduino\\libraries" isSingleLibrary=false location=user
2026-01-04T15:05:19.285Z root INFO creating connection for 1
2026-01-04T15:05:20.860Z root WARN Frontend a.configure took longer than the expected maximum 100 milliseconds: 232.8 ms [Finished 3.329 s after frontend start]
2026-01-04T15:05:20.916Z root WARN A command editor.action.toggleStickyScroll is already registered.
Opening channel for service path '/services/electron-window'.
Opening channel for service path '/services/ide-updater'.
2026-01-04T15:05:21.120Z root INFO Start frontend contributions: 592.1 ms [Finished 3.564 s after frontend start]
2026-01-04T15:05:21.120Z root INFO Changed application state from 'init' to 'started_contributions'.
2026-01-04T15:05:21.135Z root INFO Changed application state from 'started_contributions' to 'attached_shell'.
2026-01-04T15:05:21.137Z root INFO >>> Restoring the layout state...
2026-01-04T15:05:21.220Z root INFO [29a4284c-0c8a-45fd-8156-00db6194897a] Waiting for backend deployment: 127.7 ms [Finished 3.682 s after frontend start]
2026-01-04T15:05:22.317Z root INFO [hosted-plugin: 960] PLUGIN_HOST(960) starting instance
[Object: null prototype] {}
2026-01-04T15:05:23.388Z root INFO [29a4284c-0c8a-45fd-8156-00db6194897a] Sync of 23 plugins: 2176.2 ms [Finished 5.859 s after frontend start]
Closing channel on service path '/services/electron-window'.
Closing channel on service path '/services/ide-updater'.
Stored workspaces roots:
No sketches were scheduled for deletion.
My Arduino IDE 2.3.6 is stuck on the loading screen (it keep showing the piping Arduino logo)
2

Can you highlight the line that shows that error?

3

Hi @wcho. The problem is caused by corruption in the configuration file located at the following path on your hard drive:

c:\Users\ethanho\.arduinoIDE\arduino-cli.yaml

I would guess you manually edited this file in an attempt to increase the download timeout, as described here:

Is that correct?

1 Like
4

Hi @ptillisch. Thank. You for your response. I did not edit any files throughout the process of installation.

5

OK, then I think the easiest solution will be to simply delete the configuration file. Arduino IDE will automatically generate a new file on startup. Hopefully that newly generated file will not be corrupted like the current one is.

I'll provide instructions you can follow to do that:

  1. Select File > Quit from the Arduino IDE menus if it is running.
    All Arduino IDE windows will close.
  2. Delete the file at the following path:
    c:\Users\ethanho\.arduinoIDE\arduino-cli.yaml
    :warning: Please be careful when deleting things from your computer. When in doubt, back up!
  3. Start Arduino IDE.

Arduino IDE should now start up fully and be functional as usual.

Note that the file you deleted is used to store some of your IDE preferences. These will all have been reset to the default values. If you had customized either the "Sketchbook location" or "Additional Boards Manager URLs" preferences before, you'll need to select File > Preferences... from the Arduino IDE menus and then set those preferences once again after performing the instructions I provided above.

Please let us know if you have any questions or problems while following those instructions.

6

It works like a charm! Thanks for your help.

7

You are welcome. I'm glad it is working now.

Regards, Per