diff --git a/errors/runner-environment/ubuntu-azure-artifacts-credprovider-missing-june2026.yml b/errors/runner-environment/ubuntu-azure-artifacts-credprovider-missing-june2026.yml
new file mode 100644
index 0000000..c3d9262
--- /dev/null
+++ b/errors/runner-environment/ubuntu-azure-artifacts-credprovider-missing-june2026.yml
@@ -0,0 +1,115 @@
+id: runner-environment-519
+title: 'Azure Artifacts Credential Provider Removed from Ubuntu Runners — NuGet Private Feed Auth Fails'
+category: runner-environment
+severity: error
+tags:
+  - azure-artifacts
+  - nuget
+  - credential-provider
+  - ubuntu
+  - dotnet
+  - private-feed
+  - regression
+  - june-2026
+patterns:
+  - regex: 'Path to the Azure Artifacts Credential Provider is null or empty'
+    flags: 'i'
+  - regex: 'CredentialProvider\.Microsoft.*not found|artifacts-credprovider.*missing'
+    flags: 'i'
+  - regex: 'Unable to obtain credentials for feed.*Azure Artifacts'
+    flags: 'i'
+  - regex: 'Response status code does not indicate success.*401.*nuget|nuget.*401.*azure\.artifacts'
+    flags: 'i'
+error_messages:
+  - "Value cannot be null. (Parameter 'Path to the Azure Artifacts Credential Provider is null or empty. See https://github.com/NuGet/Home/wiki/NuGet-cross-plat-authentication-plugin#plugin-installation-and-discovery to set up the Credential Provider.')"
+  - "Unable to obtain credentials for feed 'https://pkgs.dev.azure.com/...'"
+  - "CredentialProvider.Microsoft.exe: plugin not found"
+root_cause: |
+  As of runner image build `20260611.554` (deployed June 17, 2026), the Azure Artifacts
+  Credential Provider (`Microsoft.Net.Http.Headers.CredentialProvider.Microsoft.exe`) was
+  silently removed from the pre-installed tool set on all GitHub-hosted Ubuntu runner images.
+  Previously it was bundled at `~/.nuget/plugins/netcore/CredentialProvider.Microsoft/`.
+
+  Affected runner labels (as of June 17, 2026):
+  - `ubuntu-22.04`, `ubuntu-22.04-arm`
+  - `ubuntu-24.04`, `ubuntu-24.04-arm`
+  - `ubuntu-26.04`, `ubuntu-26.04-arm`
+  - `ubuntu-latest`, `ubuntu-slim`
+
+  macOS and Windows runners may be affected as well (see actions/runner-images#14270).
+
+  Any workflow that uses `dotnet restore`, `nuget restore`, or `msbuild /restore` against a
+  private Azure Artifacts NuGet feed relies on this provider for MSAL-based authentication.
+  The restore fails with HTTP 401 when the provider is absent, even if `NUGET_CREDENTIALPROVIDERS_PATH`
+  is set or `nugetAuthenticate@1` task is configured — that task only configures credentials, it
+  does not install the provider binary.
+fix: |
+  Add an explicit install step for the Azure Artifacts Credential Provider **before** any NuGet
+  restore or build step:
+
+  ```bash
+  sh -c "$(curl -fsSL https://aka.ms/install-artifacts-credprovider.sh)"
+  ```
+
+  This installs the provider to `~/.nuget/plugins/netcore/CredentialProvider.Microsoft/`.
+  Subsequent `dotnet restore`, `nuget restore`, and `msbuild /restore` commands will find and
+  use it automatically.
+
+  For workflows using `NuGetAuthenticate@1` or similar Azure DevOps task wrappers, the install
+  step must still be added — those tasks configure credentials but don't install the binary.
+fix_code:
+  - language: yaml
+    label: 'Install Azure Artifacts Credential Provider before dotnet restore'
+    code: |
+      jobs:
+        build:
+          runs-on: ubuntu-latest
+          steps:
+            - uses: actions/checkout@v6
+
+            - name: Install Azure Artifacts Credential Provider
+              run: sh -c "$(curl -fsSL https://aka.ms/install-artifacts-credprovider.sh)"
+
+            - name: Restore NuGet packages
+              run: dotnet restore
+              env:
+                NUGET_CREDENTIALPROVIDERS_PATH: "${{ runner.temp }}/plugins/netcore"
+                VSS_NUGET_EXTERNAL_FEED_ENDPOINTS: |
+                  {
+                    "endpointCredentials": [{
+                      "endpoint": "https://pkgs.dev.azure.com/YOUR_ORG/_packaging/YOUR_FEED/nuget/v3/index.json",
+                      "username": "any",
+                      "password": "${{ secrets.AZURE_DEVOPS_PAT }}"
+                    }]
+                  }
+
+            - name: Build
+              run: dotnet build --no-restore
+  - language: yaml
+    label: 'Reusable composite step — install credential provider once'
+    code: |
+      # .github/actions/install-azartifacts-cred/action.yml
+      name: Install Azure Artifacts Credential Provider
+      description: Installs CredentialProvider.Microsoft on the runner
+      runs:
+        using: composite
+        steps:
+          - name: Install credential provider
+            shell: bash
+            run: sh -c "$(curl -fsSL https://aka.ms/install-artifacts-credprovider.sh)"
+prevention:
+  - "Do not assume the Azure Artifacts Credential Provider is pre-installed on Ubuntu runners after June 17, 2026"
+  - "Add the `install-artifacts-credprovider.sh` install step as the first step in any job that restores NuGet packages from private Azure Artifacts feeds"
+  - "For Windows runners, use the equivalent PowerShell script: `iex \"& { $(irm https://aka.ms/install-artifacts-credprovider.ps1) }\"`"
+  - "Pin the install script URL from `aka.ms` (Microsoft short link) rather than hardcoding the GitHub release URL — Microsoft keeps the alias stable"
+docs:
+  - url: 'https://github.com/actions/runner-images/issues/14270'
+    label: 'runner-images#14270: Azure Artifacts Credential Provider missing on Ubuntu Runner'
+  - url: 'https://github.com/microsoft/artifacts-credprovider'
+    label: 'microsoft/artifacts-credprovider — official provider repo'
+  - url: 'https://aka.ms/install-artifacts-credprovider.sh'
+    label: 'Official install script (Linux/macOS)'
+  - url: 'https://github.com/NuGet/Home/wiki/NuGet-cross-plat-authentication-plugin#plugin-installation-and-discovery'
+    label: 'NuGet cross-platform authentication plugin discovery guide'
+  - url: 'https://learn.microsoft.com/en-us/azure/devops/artifacts/nuget/nuget-exe?view=azure-devops'
+    label: 'Azure Artifacts: NuGet restore authentication guide'
diff --git a/errors/runner-environment/windows-11-arm64-wsl-update-prompt-steals-focus-gui-tests.yml b/errors/runner-environment/windows-11-arm64-wsl-update-prompt-steals-focus-gui-tests.yml
new file mode 100644
index 0000000..52aa77f
--- /dev/null
+++ b/errors/runner-environment/windows-11-arm64-wsl-update-prompt-steals-focus-gui-tests.yml
@@ -0,0 +1,131 @@
+id: runner-environment-520
+title: 'Windows 11 ARM64 `wsl.exe --update` Prompt on Logon Steals Focus — GUI Automation Breaks'
+category: runner-environment
+severity: error
+tags:
+  - windows-11-arm
+  - arm64
+  - wsl
+  - gui
+  - focus
+  - regression
+  - browser-automation
+  - playwright
+  - selenium
+  - june-2026
+patterns:
+  - regex: 'wsl\.exe.*--update.*--confirm.*--prompt-before-exit'
+    flags: 'i'
+  - regex: 'windows.terminal.*stole.*focus|focus.*stolen.*wsl|wsl.*update.*window.*focus'
+    flags: 'i'
+  - regex: 'Timeout.*waiting for.*browser.*focus|element.*not.*focused.*arm64'
+    flags: 'i'
+error_messages:
+  - "Error: Timeout 30000ms exceeded. waiting for element to be focused."
+  - "browserType.launch: Target page, context or browser has been closed"
+  - "wsl.exe --update --confirm --prompt-before-exit"
+root_cause: |
+  Runner image builds shipping from late June 2026 for `windows-11-arm` introduced a Windows
+  startup logon trigger that runs `wsl.exe --update --confirm --prompt-before-exit`. This spawns
+  a visible Windows Terminal window that:
+  1. Takes foreground focus away from the test runner process.
+  2. Waits up to 60 seconds for a user interaction that never arrives in CI.
+  3. Remains topmost, preventing GUI automation frameworks from interacting with the window under test.
+
+  Effects on common frameworks:
+  - **Playwright** (headed mode): `page.click()` misses because the WSL window has focus.
+    `browserType.launch()` may fail with "target has been closed" if the runner's session
+    times out waiting for the WSL prompt to dismiss.
+  - **Selenium / WebDriver**: `driver.findElement().click()` acts on wrong window; element
+    interaction assertions fail.
+  - **WinForms / WPF UI tests**: `Form.ActiveForm` may return the WSL terminal window's handle,
+    and `SendKeys` targets the wrong window.
+  - **Screenshot-based tests**: screenshots capture the WSL terminal overlay instead of the app.
+
+  This is distinct from the known `windows-11-arm` non-interactive session limitation
+  (runner-environment-113 / ku-113). That issue is about Form.ActiveForm always being null;
+  this issue is about a pop-up window actively stealing focus from a running test session.
+
+  Tracked at actions/runner-images#14264 — open, under investigation by the runner team.
+fix: |
+  **Workaround 1 — Suppress WSL at session start (recommended while issue is open):**
+  Add a step before any GUI/browser automation to kill the WSL update window and disable
+  the startup trigger:
+
+  ```powershell
+  # Kill any WSL update window that appeared on logon
+  Get-Process -Name "WindowsTerminal" -ErrorAction SilentlyContinue | Stop-Process -Force
+  # Disable WSL update prompt for this session
+  Set-ItemProperty -Path "HKCU:\Software\Microsoft\Windows\CurrentVersion\Run" `
+    -Name "WslUpdatePrompt" -Value "" -ErrorAction SilentlyContinue
+  ```
+
+  **Workaround 2 — Avoid `windows-11-arm` for GUI automation until fixed:**
+  Route headed browser and GUI automation tests to `windows-latest` (x64 Windows Server 2025),
+  which does not exhibit this WSL update prompt regression.
+
+  **Workaround 3 — Use headless mode:**
+  Switch Playwright / Selenium to headless mode to avoid the focus-stealing issue entirely.
+  Headless browsers are not affected by the WSL window.
+
+  Monitor actions/runner-images#14264 for an official image patch.
+fix_code:
+  - language: yaml
+    label: 'Suppress WSL update window before GUI tests on windows-11-arm'
+    code: |
+      jobs:
+        gui-tests:
+          runs-on: windows-11-arm
+          steps:
+            - uses: actions/checkout@v6
+
+            - name: Suppress WSL update prompt (regression workaround — runner-images#14264)
+              shell: pwsh
+              run: |
+                # Kill any Windows Terminal window spawned by the WSL update logon trigger
+                Get-Process -Name "WindowsTerminal" -ErrorAction SilentlyContinue | Stop-Process -Force
+                Start-Sleep -Seconds 2  # allow window teardown to complete
+
+            - name: Run Playwright tests
+              run: npx playwright test
+  - language: yaml
+    label: 'Route GUI tests to windows-latest; arm-specific tests to windows-11-arm'
+    code: |
+      jobs:
+        tests:
+          strategy:
+            matrix:
+              include:
+                - os: windows-latest
+                  suite: gui        # headed browser / WinForms tests
+                - os: windows-11-arm
+                  suite: native     # ARM64 native code tests — no GUI
+          runs-on: ${{ matrix.os }}
+          steps:
+            - uses: actions/checkout@v6
+
+            - name: Run tests
+              run: dotnet test --filter Category=${{ matrix.suite }}
+  - language: yaml
+    label: 'Force headless mode on windows-11-arm to avoid focus issues'
+    code: |
+      - name: Run Playwright tests (headless on ARM64)
+        run: |
+          if [ "${{ runner.arch }}" = "ARM64" ]; then
+            npx playwright test --project=chromium-headless-shell
+          else
+            npx playwright test
+          fi
+        shell: bash
+prevention:
+  - "Avoid headed browser automation on `windows-11-arm` until actions/runner-images#14264 is resolved"
+  - "Add a 'kill WindowsTerminal' pre-step when running GUI tests on windows-11-arm"
+  - "Prefer headless mode for cross-platform browser tests to avoid session focus dependencies"
+  - "Subscribe to actions/runner-images#14264 for the official image fix"
+docs:
+  - url: 'https://github.com/actions/runner-images/issues/14264'
+    label: 'runner-images#14264: Windows 11 ARM64 image shows WSL update required prompt'
+  - url: 'https://github.com/actions/runner-images/issues/14049'
+    label: 'runner-images#14049: windows-11-arm non-interactive session (related context)'
+  - url: 'https://docs.github.com/en/actions/using-github-hosted-runners/using-github-hosted-runners/about-github-hosted-runners'
+    label: 'GitHub Docs: About GitHub-hosted runners'