IDE hangs on startup: "... package_index.json=: The system cannot find the file specified"

Arduino IDE was fine before a restart of my PC. Now, the IDE refuses to launch and is stuck in a booting loop with the logo pulsing. I have the snippet of my powershell output here:

Windows PowerShell Copyright (C) Microsoft Corporation. All rights reserved. Install the latest PowerShell for new features and improvements! https://aka.ms/PSWindows PS C:\Users\calvi\AppData\Local\Programs\Arduino IDE> & ".\Arduino IDE" PS C:\Users\calvi\AppData\Local\Programs\Arduino IDE> Arduino IDE 2.3.4 Checking for frontend application configuration customizations. Module path: C:\Users\calvi\AppData\Local\Programs\Arduino IDE\resources\app\lib\backend\electron-main.js, destination 'package.json': C:\Users\calvi\AppData\Local\Programs\Arduino IDE\resources\app\package.json Setting 'theia.frontend.config.appVersion' application configuration value to: "2.3.4" (type of string) Setting 'theia.frontend.config.cliVersion' application configuration value to: "1.1.1" (type of string) Setting 'theia.frontend.config.buildDate' application configuration value to: "2024-12-03T10:03:03.490Z" (type of string) Frontend application configuration after modifications: {"applicationName":"Arduino IDE","defaultTheme":{"light":"arduino-theme","dark":"arduino-theme-dark"},"defaultIconTheme":"none","electron":{"windowOptions":{}},"defaultLocale":"","validatePreferencesSchema":false,"preferences":{"window.title":"${rootName}${activeEditorShort}${appName}","files.autoSave":"afterDelay","editor.minimap.enabled":false,"editor.tabSize":2,"editor.scrollBeyondLastLine":false,"editor.quickSuggestions":{"other":false,"comments":false,"strings":false},"editor.maxTokenizationLineLength":500,"editor.bracketPairColorization.enabled":false,"breadcrumbs.enabled":false,"workbench.tree.renderIndentGuides":"none","explorer.compactFolders":false},"appVersion":"2.3.4","cliVersion":"1.1.1","buildDate":"2024-12-03T10:03:03.490Z"} Starting backend process. PID: 15344 Using browser-only version of superagent in non-browser environment Configuration directory URI: 'file:///c%3A/Users/calvi/.arduinoIDE' Configuring to accept webviews on '^.+\.webview\..+$' hostname. 2025-03-30T03:36:26.699Z root INFO Backend Object.initialize: 4.9 ms [Finished 0.618 s after backend start] 2025-03-30T03:36:26.700Z root INFO Backend Object.configure: 3.9 ms [Finished 0.618 s after backend start] 2025-03-30T03:36:26.700Z root INFO Backend s.onStart: 2.5 ms [Finished 0.618 s after backend start] 2025-03-30T03:36:26.701Z root INFO Backend s.configure: 0.4 ms [Finished 0.642 s after backend start] 2025-03-30T03:36:26.701Z root INFO Backend d.onStart: 0.3 ms [Finished 0.642 s after backend start] 2025-03-30T03:36:26.702Z root INFO Backend d.configure: 0.1 ms [Finished 0.642 s after backend start] 2025-03-30T03:36:26.703Z root INFO Backend d.initialize: 24.2 ms [Finished 0.642 s after backend start] 2025-03-30T03:36:26.704Z root INFO Backend c.onStart: 0.3 ms [Finished 0.642 s after backend start] 2025-03-30T03:36:26.704Z root INFO Backend a.configure: 1.1 ms [Finished 0.643 s after backend start] 2025-03-30T03:36:26.706Z config INFO >>> Initializing CLI configuration... 2025-03-30T03:36:26.706Z root INFO Backend c.initialize: 0.5 ms [Finished 0.644 s after backend start] 2025-03-30T03:36:26.706Z root INFO Backend x.onStart: 0.7 ms [Finished 0.644 s after backend start] 2025-03-30T03:36:26.707Z config INFO Loading CLI configuration from c:\Users\calvi\.arduinoIDE\arduino-cli.yaml... 2025-03-30T03:36:26.708Z daemon INFO Starting daemon from C:\Users\calvi\AppData\Local\Programs\Arduino IDE\resources\app\lib\backend\resources\arduino-cli.exe... 2025-03-30T03:36:26.708Z root INFO Backend a.initialize: 0.5 ms [Finished 0.645 s after backend start] 2025-03-30T03:36:26.708Z root INFO Backend w.onStart: 1.3 ms [Finished 0.646 s after backend start] 2025-03-30T03:36:26.708Z root INFO Backend r.configure: 2.2 ms [Finished 0.646 s after backend start] 2025-03-30T03:36:26.708Z discovery-log INFO start 2025-03-30T03:36:26.708Z discovery-log INFO start new deferred 2025-03-30T03:36:26.708Z root INFO Backend d.initialize: 0.6 ms [Finished 0.647 s after backend start] 2025-03-30T03:36:26.708Z root INFO Backend y.onStart: 2.5 ms [Finished 0.649 s after backend start] 2025-03-30T03:36:26.708Z root INFO Backend r.configure: 2.1 ms [Finished 0.649 s after backend start] 2025-03-30T03:36:26.708Z root INFO Backend a.onStart: 0.5 ms [Finished 0.650 s after backend start] 2025-03-30T03:36:26.708Z root INFO Backend h.configure: 0.4 ms [Finished 0.651 s after backend start] 2025-03-30T03:36:26.708Z root INFO Backend s.configure: 0.2 ms [Finished 0.651 s after backend start] 2025-03-30T03:36:26.708Z root INFO Backend l.configure: 0.1 ms [Finished 0.651 s after backend start] 2025-03-30T03:36:26.711Z root INFO Theia app listening on http://127.0.0.1:50253. 2025-03-30T03:36:26.711Z root INFO Finished starting backend application: 3.6 ms [Finished 0.654 s after backend start] 2025-03-30T03:36:26.712Z root WARN The local plugin referenced by local-dir:/c%3A/Users/calvi/.arduinoIDE/plugins does not exist. 2025-03-30T03:36:26.712Z root WARN The local plugin referenced by local-dir:/c%3A/Users/calvi/.arduinoIDE/extensions does not exist. 2025-03-30T03:36:26.712Z root WARN The local plugin referenced by local-dir:C:\Users\calvi\.arduinoIDE\plugins does not exist. 2025-03-30T03:36:26.722Z config INFO Loaded CLI configuration: {"board_manager":{"additional_urls":["https://downloads.arduino.cc/packages/package_index.json=","https://github.com/stm32duino/BoardManagerFiles/raw/main/package_stmicroelectronics_index.json","https://github.com/stm32duino/BoardManagerFiles/raw/master/package_stmicroelectronics_index.json"]},"directories":{"builtin":{"libraries":"C:\\Users\\calvi\\AppData\\Local\\Arduino15\\libraries"},"data":"c:\\Users\\calvi\\AppData\\Local\\Arduino15","user":"c:\\Users\\calvi\\OneDrive\\Documents\\Arduino"},"locale":"en"} 2025-03-30T03:36:26.722Z config INFO 'directories.data' and 'directories.user' are set in the CLI configuration model. 2025-03-30T03:36:26.722Z config INFO Loaded the CLI configuration. Restoring workspace roots: c:\Users\calvi\Downloads\SDP_Final_Code_3_24_2025\SDP_Final_Code_3_24_2025\SDP_Final_Code_3_24_2025.ino isTempSketch: false. Input was c:\Users\calvi\Downloads\SDP_Final_Code_3_24_2025\SDP_Final_Code_3_24_2025\SDP_Final_Code_3_24_2025.ino 2025-03-30T03:36:26.796Z config INFO Mapped the CLI configuration: {"dataDirUri":"file:///c%3A/Users/calvi/AppData/Local/Arduino15","sketchDirUri":"file:///c%3A/Users/calvi/OneDrive/Documents/Arduino","additionalUrls":["https://downloads.arduino.cc/packages/package_index.json=","https://github.com/stm32duino/BoardManagerFiles/raw/main/package_stmicroelectronics_index.json","https://github.com/stm32duino/BoardManagerFiles/raw/master/package_stmicroelectronics_index.json"],"network":"none","locale":"en"} 2025-03-30T03:36:26.796Z config INFO Validating the CLI configuration... 2025-03-30T03:36:26.797Z config INFO The CLI config is valid. 2025-03-30T03:36:26.797Z config INFO <<< Initialized the CLI configuration. 2025-03-30T03:36:26.817Z root INFO Resolve plugins list: 110.0 ms [Finished 0.759 s after backend start] 2025-03-30T03:36:26.883Z daemon INFO time="2025-03-29T20:36:26-07:00" level=info msg="arduino-cli version 1.1.1" time="2025-03-29T20:36:26-07:00" level=info msg="Using config file: c:\\Users\\calvi\\.arduinoIDE\\arduino-cli.yaml" time="2025-03-29T20:36:26-07:00" level=info msg="Executing `arduino-cli daemon`" Daemon is now listening on 127.0.0.1:50254 {"IP":"127.0.0.1","Port":"50254"} 2025-03-30T03:36:26.883Z daemon INFO Daemon is running. 2025-03-30T03:36:26.910Z daemon INFO time="2025-03-29T20:36:26-07:00" level=info msg="Updating index" url="https://downloads.arduino.cc/packages/package_index.tar.bz2" time="2025-03-29T20:36:26-07:00" level=info msg="Starting download" url="https://downloads.arduino.cc/packages/package_index.tar.bz2" 2025-03-30T03:36:27.157Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Updating index" url="https://downloads.arduino.cc/packages/package_index.json=" time="2025-03-29T20:36:27-07:00" level=info msg="Starting download" url="https://downloads.arduino.cc/packages/package_index.json=" 2025-03-30T03:36:27.301Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Updating index" url="https://github.com/stm32duino/BoardManagerFiles/raw/main/package_stmicroelectronics_index.json" time="2025-03-29T20:36:27-07:00" level=info msg="Starting download" url="https://github.com/stm32duino/BoardManagerFiles/raw/main/package_stmicroelectronics_index.json" 2025-03-30T03:36:27.540Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Updating index" url="https://github.com/stm32duino/BoardManagerFiles/raw/master/package_stmicroelectronics_index.json" 2025-03-30T03:36:27.541Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Starting download" url="https://github.com/stm32duino/BoardManagerFiles/raw/master/package_stmicroelectronics_index.json" 2025-03-30T03:36:27.577Z root INFO Deploy plugins list: 110.0 ms [Finished 1.519 s after backend start] 2025-03-30T03:36:27.749Z root ERROR Detected an error response during the gRPC core client initialization: code: 3, message: Some indexes could not be updated. 2025-03-30T03:36:27.759Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Checking signature" index="c:\\Users\\calvi\\AppData\\Local\\Arduino15\\package_index.json" signatureFile="c:\\Users\\calvi\\AppData\\Local\\Arduino15\\package_index.json.sig" trusted=true 2025-03-30T03:36:27.761Z root ERROR Detected an error response during the gRPC core client initialization: code: 9, message: Loading index file: loading json index file c:\Users\calvi\AppData\Local\Arduino15\package_index.json=: open c:\Users\calvi\AppData\Local\Arduino15\package_index.json=: The system cannot find the file specified. 2025-03-30T03:36:27.763Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Missing signature file" index="c:\\Users\\calvi\\AppData\\Local\\Arduino15\\package_stmicroelectronics_index.json" 2025-03-30T03:36:27.763Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Missing signature file" index="c:\\Users\\calvi\\AppData\\Local\\Arduino15\\package_stmicroelectronics_index.json" 2025-03-30T03:36:27.763Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Loading hardware from: c:\\Users\\calvi\\AppData\\Local\\Arduino15\\packages" 2025-03-30T03:36:27.764Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Loading package builtin from: C:\\Users\\calvi\\AppData\\Local\\Arduino15\\packages\\builtin" 2025-03-30T03:36:27.765Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Checking existence of 'tools' path: C:\\Users\\calvi\\AppData\\Local\\Arduino15\\packages\\builtin\\tools" 2025-03-30T03:36:27.765Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Loading tools from dir: C:\\Users\\calvi\\AppData\\Local\\Arduino15\\packages\\builtin\\tools" 2025-03-30T03:36:27.765Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Loaded tool" tool="builtin:ctags@5.8-arduino11" 2025-03-30T03:36:27.765Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Loaded tool" tool="builtin:dfu-discovery@0.1.2" 2025-03-30T03:36:27.766Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Loaded tool" tool="builtin:mdns-discovery@1.0.9" 2025-03-30T03:36:27.766Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Loaded tool" tool="builtin:serial-discovery@1.4.1" 2025-03-30T03:36:27.766Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Loaded tool" tool="builtin:serial-monitor@0.15.0" 2025-03-30T03:36:27.778Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Loading libraries index file" index="c:\\Users\\calvi\\AppData\\Local\\Arduino15\\library_index.json" 2025-03-30T03:36:27.973Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Adding libraries dir" dir="C:\\Users\\calvi\\AppData\\Local\\Arduino15\\libraries" isSingleLibrary=false location=ide 2025-03-30T03:36:27.973Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Adding libraries dir" dir="c:\\Users\\calvi\\OneDrive\\Documents\\Arduino\\libraries" isSingleLibrary=false location=user 2025-03-30T03:36:27.984Z root ERROR The primary packages indexes are missing. Running indexes update before initializing the core gRPC client The index of the cores and libraries must be updated before initializing the core gRPC client. The following problems were detected during the gRPC client initialization: [platform-index] - code: 9, message: Loading index file: loading json index file c:\Users\calvi\AppData\Local\Arduino15\package_index.json=: open c:\Users\calvi\AppData\Local\Arduino15\package_index.json=: The system cannot find the file specified. 2025-03-30T03:36:27.985Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Updating index" url="https://downloads.arduino.cc/packages/package_index.tar.bz2" 2025-03-30T03:36:27.986Z daemon INFO time="2025-03-29T20:36:27-07:00" level=info msg="Starting download" url="https://downloads.arduino.cc/packages/package_index.tar.bz2" 2025-03-30T03:36:27.987Z root INFO core-client-provider [platform-index] Downloading index: package_index.tar.bz2 Opening channel for service path '/services/electron-window'. 2025-03-30T03:36:28.186Z daemon INFO time="2025-03-29T20:36:28-07:00" level=info msg="Updating index" url="https://downloads.arduino.cc/packages/package_index.json=" 2025-03-30T03:36:28.186Z daemon INFO time="2025-03-29T20:36:28-07:00" level=info msg="Starting download" url="https://downloads.arduino.cc/packages/package_index.json=" 2025-03-30T03:36:28.187Z root INFO core-client-provider [platform-index] Downloading index: package_index.json= Opening channel for service path '/services/ide-updater'. 2025-03-30T03:36:28.376Z root INFO core-client-provider [platform-index] Downloading index: package_stmicroelectronics_index.json 2025-03-30T03:36:28.376Z daemon INFO time="2025-03-29T20:36:28-07:00" level=info msg="Updating index" url="https://github.com/stm32duino/BoardManagerFiles/raw/main/package_stmicroelectronics_index.json" time="2025-03-29T20:36:28-07:00" level=info msg="Starting download" url="https://github.com/stm32duino/BoardManagerFiles/raw/main/package_stmicroelectronics_index.json" 2025-03-30T03:36:28.435Z root WARN Frontend a.configure took longer than the expected maximum 100 milliseconds: 138.5 ms [Finished 1.652 s after frontend start] 2025-03-30T03:36:28.435Z root WARN A command editor.action.toggleStickyScroll is already registered. 2025-03-30T03:36:28.585Z daemon INFO time="2025-03-29T20:36:28-07:00" level=info msg="Updating index" url="https://github.com/stm32duino/BoardManagerFiles/raw/master/package_stmicroelectronics_index.json" 2025-03-30T03:36:28.586Z daemon INFO time="2025-03-29T20:36:28-07:00" level=info msg="Starting download" url="https://github.com/stm32duino/BoardManagerFiles/raw/master/package_stmicroelectronics_index.json" 2025-03-30T03:36:28.587Z root INFO core-client-provider [platform-index] Downloading index: package_stmicroelectronics_index.json 2025-03-30T03:36:28.789Z daemon INFO time="2025-03-29T20:36:28-07:00" level=info msg="Updating index" url="https://downloads.arduino.cc/packages/package_index.tar.bz2" 2025-03-30T03:36:28.790Z daemon INFO time="2025-03-29T20:36:28-07:00" level=info msg="Starting download" url="https://downloads.arduino.cc/packages/package_index.tar.bz2" 2025-03-30T03:36:28.982Z daemon INFO time="2025-03-29T20:36:28-07:00" level=info msg="Updating index" url="https://downloads.arduino.cc/packages/package_index.json=" 2025-03-30T03:36:28.983Z daemon INFO time="2025-03-29T20:36:28-07:00" level=info msg="Starting download" url="https://downloads.arduino.cc/packages/package_index.json=" 2025-03-30T03:36:29.100Z daemon INFO time="2025-03-29T20:36:29-07:00" level=info msg="Updating index" url="https://github.com/stm32duino/BoardManagerFiles/raw/main/package_stmicroelectronics_index.json" 2025-03-30T03:36:29.101Z daemon INFO time="2025-03-29T20:36:29-07:00" level=info msg="Starting download" url="https://github.com/stm32duino/BoardManagerFiles/raw/main/package_stmicroelectronics_index.json" 2025-03-30T03:36:29.324Z daemon INFO time="2025-03-29T20:36:29-07:00" level=info msg="Updating index" url="https://github.com/stm32duino/BoardManagerFiles/raw/master/package_stmicroelectronics_index.json" 2025-03-30T03:36:29.325Z daemon INFO time="2025-03-29T20:36:29-07:00" level=info msg="Starting download" url="https://github.com/stm32duino/BoardManagerFiles/raw/master/package_stmicroelectronics_index.json" 2025-03-30T03:36:29.541Z root ERROR Detected an error response during the gRPC core client initialization: code: 3, message: Some indexes could not be updated. 2025-03-30T03:36:29.557Z daemon INFO time="2025-03-29T20:36:29-07:00" level=info msg="Checking signature" index="c:\\Users\\calvi\\AppData\\Local\\Arduino15\\package_index.json" signatureFile="c:\\Users\\calvi\\AppData\\Local\\Arduino15\\package_index.json.sig" trusted=true 2025-03-30T03:36:29.558Z root ERROR Detected an error response during the gRPC core client initialization: code: 9, message: Loading index file: loading json index file c:\Users\calvi\AppData\Local\Arduino15\package_index.json=: open c:\Users\calvi\AppData\Local\Arduino15\package_index.json=: The system cannot find the file specified. 2025-03-30T03:36:29.558Z daemon INFO time="2025-03-29T20:36:29-07:00" level=info msg="Missing signature file" index="c:\\Users\\calvi\\AppData\\Local\\Arduino15\\package_stmicroelectronics_index.json" time="2025-03-29T20:36:29-07:00" level=info msg="Missing signature file" index="c:\\Users\\calvi\\AppData\\Local\\Arduino15\\package_stmicroelectronics_index.json" time="2025-03-29T20:36:29-07:00" level=info msg="Loading hardware from: c:\\Users\\calvi\\AppData\\Local\\Arduino15\\packages" 2025-03-30T03:36:29.559Z daemon INFO time="2025-03-29T20:36:29-07:00" level=info msg="Loading package builtin from: C:\\Users\\calvi\\AppData\\Local\\Arduino15\\packages\\builtin" 2025-03-30T03:36:29.559Z daemon INFO time="2025-03-29T20:36:29-07:00" level=info msg="Checking existence of 'tools' path: C:\\Users\\calvi\\AppData\\Local\\Arduino15\\packages\\builtin\\tools" time="2025-03-29T20:36:29-07:00" level=info msg="Loading tools from dir: C:\\Users\\calvi\\AppData\\Local\\Arduino15\\packages\\builtin\\tools" 2025-03-30T03:36:29.560Z daemon INFO time="2025-03-29T20:36:29-07:00" level=info msg="Loaded tool" tool="builtin:ctags@5.8-arduino11" 2025-03-30T03:36:29.560Z daemon INFO time="2025-03-29T20:36:29-07:00" level=info msg="Loaded tool" tool="builtin:dfu-discovery@0.1.2" 2025-03-30T03:36:29.560Z daemon INFO time="2025-03-29T20:36:29-07:00" level=info msg="Loaded tool" tool="builtin:mdns-discovery@1.0.9" 2025-03-30T03:36:29.560Z daemon INFO time="2025-03-29T20:36:29-07:00" level=info msg="Loaded tool" tool="builtin:serial-discovery@1.4.1" 2025-03-30T03:36:29.560Z daemon INFO time="2025-03-29T20:36:29-07:00" level=info msg="Loaded tool" tool="builtin:serial-monitor@0.15.0" 2025-03-30T03:36:29.573Z daemon INFO time="2025-03-29T20:36:29-07:00" level=info msg="Loading libraries index file" index="c:\\Users\\calvi\\AppData\\Local\\Arduino15\\library_index.json" 2025-03-30T03:36:29.752Z daemon INFO time="2025-03-29T20:36:29-07:00" level=info msg="Adding libraries dir" dir="C:\\Users\\calvi\\AppData\\Local\\Arduino15\\libraries" isS> & ".\Arduino IDE"location=ide 2025-03-30T03:36:29.752Z daemon INFO time="2025-03-29T20:36:29-07:00" level=info msg="Adding libraries dir" dir="c:\\Users\\calvi\\OneDrive\\Documents\\Arduino\\libraries" isSingleLibrary=false location=user 2025-03-30T03:36:29.761Z root ERROR Uncaught Exception: Error: The index of the cores and libraries must be updated before initializing the core gRPC client. The following problems were detected during the gRPC client initialization: [platform-index] - code: 9, message: Loading index file: loading json index file c:\Users\calvi\AppData\Local\Arduino15\package_index.json=: open c:\Users\calvi\AppData\Local\Arduino15\package_index.json=: The system cannot find the file specified. 2025-03-30T03:36:29.761Z root ERROR Error: The index of the cores and libraries must be updated before initializing the core gRPC client. The following problems were detected during the gRPC client initialization: [platform-index] - code: 9, message: Loading index file: loading json index file c:\Users\calvi\AppData\Local\Arduino15\package_index.json=: open c:\Users\calvi\AppData\Local\Arduino15\package_index.json=: The system cannot find the file specified. at C:\Users\calvi\AppData\Local\Programs\Arduino IDE\resources\app\lib\backend\main.js:2:777546 at v.evaluateErrorStatus (C:\Users\calvi\AppData\Local\Programs\Arduino IDE\resources\app\lib\backend\main.js:2:777562) at process.processTicksAndRejections (node:internal/process/task_queues:95:5) at async a.<anonymous> (C:\Users\calvi\AppData\Local\Programs\Arduino IDE\resources\app\lib\backend\main.js:2:777232) 

Hi @calvinbao1337.

The cause of the problem is that you put an invalid URL in Arduino IDE's "Additional Boards Manager URLs" preference.

I'll provide instructions you can follow to recover the IDE from this broken state:

  1. Close the Arduino IDE window if it is running.
  2. Delete the file at the following path on your hard drive:
    c:\Users\calvi\.arduinoIDE\arduino-cli.yaml 
    • :warning: Please be careful when deleting things from your computer. When in doubt, make a backup!
  3. Start Arduino IDE.
    Arduino IDE should now start up and work as expected.
  4. Select File > Preferences... from the Arduino IDE menus.
    The "Preferences" dialog will open.
  5. Enter the following text into the "Additional Boards Manager URLs" field in the "Preferences" dialog:
    https://github.com/stm32duino/BoardManagerFiles/raw/main/package_stmicroelectronics_index.json 
  6. Click the "OK" button.
    The "Preferences" dialog will close.

Please note that there is no need to add the Arduino primary package index URL https://downloads.arduino.cc/packages/package_index.json to the "Additional Boards Manager URLs" preference, as you attempted to do when you broke the IDE by fumbling the URL. Arduino IDE uses this primary package index URL by default. You only need to add URLs to the "Additional Boards Manager URLs" preference for 3rd party boards platforms.

If you are having some problem with the primary package index, then you can provide a detailed description of the problem here on the forum and the forum helpers will provide assistance with that. Adding that URL to your "Additional Boards Manager URLs" preference is not going to solve any problems, and as you saw it even has the chance of causing its own problems!


FYI, the Arduino developers have already made a fix for the problem of the IDE not handling invalid URLs gracefully. The fix was made after the time of the last Arduino IDE release (which is why you are still encountering the problem when using the current version of the IDE), but at least we can be glad that users will not encounter this problem going forward.

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