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+# Umbraco CMS Development Guide
+
+Always reference these instructions first and fallback to search or bash commands only when you encounter unexpected information that does not match the info here.
+
+## Working Effectively
+
+Bootstrap, build, and test the repository:
+
+- Install .NET SDK (version specified in global.json):
+  - `curl -sSL https://dot.net/v1/dotnet-install.sh | bash /dev/stdin --version $(jq -r '.sdk.version' global.json)`
+  - `export PATH="/home/runner/.dotnet:$PATH"`
+- Install Node.js (version specified in src/Umbraco.Web.UI.Client/.nvmrc):
+  - `curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.40.0/install.sh | bash`
+  - `export NVM_DIR="$HOME/.nvm" && [ -s "$NVM_DIR/nvm.sh" ] && \. "$NVM_DIR/nvm.sh"`
+  - `nvm install $(cat src/Umbraco.Web.UI.Client/.nvmrc) && nvm use $(cat src/Umbraco.Web.UI.Client/.nvmrc)`
+- Fix shallow clone issue (required for GitVersioning):
+  - `git fetch --unshallow`
+- Restore packages:
+  - `dotnet restore` -- takes 50 seconds. NEVER CANCEL. Set timeout to 90+ seconds.
+- Build the solution:
+  - `dotnet build` -- takes 4.5 minutes. NEVER CANCEL. Set timeout to 10+ minutes.
+- Install and build frontend:
+  - `cd src/Umbraco.Web.UI.Client`
+  - `npm ci --no-fund --no-audit --prefer-offline` -- takes 11 seconds.
+  - `npm run build:for:cms` -- takes 1.25 minutes. NEVER CANCEL. Set timeout to 5+ minutes.
+- Install and build Login
+  - `cd src/Umbraco.Web.UI.Login`
+  - `npm ci --no-fund --no-audit --prefer-offline`
+  - `npm run build`
+- Run the application:
+  - `cd src/Umbraco.Web.UI`
+  - `dotnet run --no-build` -- Application runs on https://localhost:44339 and http://localhost:11000
+
+## Validation
+
+- ALWAYS run through at least one complete end-to-end scenario after making changes.
+- Build and unit tests must pass before committing changes.
+- Frontend build produces output in src/Umbraco.Web.UI.Client/dist-cms/ which gets copied to src/Umbraco.Web.UI/wwwroot/umbraco/backoffice/
+- Always run `dotnet build` and `npm run build:for:cms` before running the application to see your changes.
+- For login-only changes, you can run `npm run build` from src/Umbraco.Web.UI.Login and then `dotnet run --no-build` from src/Umbraco.Web.UI.
+- For frontend-only changes, you can run `npm run dev:server` from src/Umbraco.Web.UI.Client for hot reloading.
+- Frontend changes should be linted using `npm run lint:fix` which uses Eslint.
+
+## Testing
+
+### Unit Tests (.NET)
+- Location: tests/Umbraco.Tests.UnitTests/
+- Run: `dotnet test tests/Umbraco.Tests.UnitTests/Umbraco.Tests.UnitTests.csproj --configuration Release --verbosity minimal`
+- Duration: ~1 minute with 3,343 tests
+- NEVER CANCEL: Set timeout to 5+ minutes
+
+### Integration Tests (.NET)
+- Location: tests/Umbraco.Tests.Integration/
+- Run: `dotnet test tests/Umbraco.Tests.Integration/Umbraco.Tests.Integration.csproj --configuration Release --verbosity minimal`
+- NEVER CANCEL: Set timeout to 10+ minutes
+
+### Frontend Tests
+- Location: src/Umbraco.Web.UI.Client/
+- Run: `npm test` (requires `npx playwright install` first)
+- Frontend tests use Web Test Runner with Playwright
+
+### Acceptance Tests (E2E)
+- Location: tests/Umbraco.Tests.AcceptanceTest/
+- Requires running Umbraco application and configuration
+- See tests/Umbraco.Tests.AcceptanceTest/README.md for detailed setup (requires `npx playwright install` first)
+
+## Project Structure
+
+The solution contains 30 C# projects organized as follows:
+
+### Main Application Projects
+- **Umbraco.Web.UI**: Main web application project (startup project)
+- **Umbraco.Web.UI.Client**: TypeScript frontend (backoffice)
+- **Umbraco.Web.UI.Login**: Separate login screen frontend
+- **Umbraco.Core**: Core domain models and interfaces
+- **Umbraco.Infrastructure**: Data access and infrastructure
+- **Umbraco.Cms**: Main CMS package
+
+### API Projects
+- **Umbraco.Cms.Api.Management**: Management API
+- **Umbraco.Cms.Api.Delivery**: Content Delivery API
+- **Umbraco.Cms.Api.Common**: Shared API components
+
+### Persistence Projects
+- **Umbraco.Cms.Persistence.SqlServer**: SQL Server support
+- **Umbraco.Cms.Persistence.Sqlite**: SQLite support
+- **Umbraco.Cms.Persistence.EFCore**: Entity Framework Core abstractions
+
+### Test Projects
+- **Umbraco.Tests.UnitTests**: Unit tests
+- **Umbraco.Tests.Integration**: Integration tests
+- **Umbraco.Tests.AcceptanceTest**: End-to-end tests with Playwright
+- **Umbraco.Tests.Common**: Shared test utilities
+
+## Common Tasks
+
+### Frontend Development
+For frontend-only changes:
+1. Configure backend for frontend development:
+   ```json
+   <!-- Add to src/Umbraco.Web.UI/appsettings.json under Umbraco:Cms:Security: -->
+   ```json
+   "BackOfficeHost": "http://localhost:5173",
+   "AuthorizeCallbackPathName": "/oauth_complete",
+   "AuthorizeCallbackLogoutPathName": "/logout",
+   "AuthorizeCallbackErrorPathName": "/error"
+   ```
+2. Run backend: `cd src/Umbraco.Web.UI && dotnet run --no-build`
+3. Run frontend dev server: `cd src/Umbraco.Web.UI.Client && npm run dev:server`
+
+### Backend-Only Development
+For backend-only changes, disable frontend builds:
+- Comment out the target named "BuildStaticAssetsPreconditions" in src/Umbraco.Cms.StaticAssets.csproj:
+  ```
+  <!--<Target Name="BuildStaticAssetsPreconditions" BeforeTargets="AssignTargetPaths">
+    [...]
+  </Target>-->
+  ```
+- Remember to uncomment before committing
+
+### Building NuGet Packages
+To build custom NuGet packages for testing:
+```bash
+dotnet pack -c Release -o Build.Out
+dotnet nuget add source [Path to Build.Out folder] -n MyLocalFeed
+```
+
+### Regenerating Frontend API Types
+When changing Management API:
+```bash
+cd src/Umbraco.Web.UI.Client
+npm run generate:server-api-dev
+```
+Also update OpenApi.json from /umbraco/swagger/management/swagger.json
+
+## Database Setup
+
+Default configuration supports SQLite for development. For production-like testing:
+- Use SQL Server/LocalDb for better performance
+- Configure connection string in src/Umbraco.Web.UI/appsettings.json
+
+## Clean Up / Reset
+
+To reset development environment:
+```bash
+# Remove configuration and database
+rm src/Umbraco.Web.UI/appsettings.json
+rm -rf src/Umbraco.Web.UI/umbraco/Data
+
+# Full clean (removes all untracked files)
+git clean -xdf .
+```
+
+## Version Information
+
+- Target Framework: .NET (version specified in global.json)
+- Current Version: (specified in version.json)
+- Node.js Requirement: (specified in src/Umbraco.Web.UI.Client/.nvmrc)
+- npm Requirement: Latest compatible version
+
+## Known Issues
+
+- Build requires full git history (not shallow clone) due to GitVersioning
+- Some NuGet package security warnings are expected (SixLabors.ImageSharp vulnerabilities)
+- Frontend tests require Playwright browser installation: `npx playwright install`
+- Older Node.js versions may show engine compatibility warnings (check .nvmrc for current requirement)
+
+## Timing Expectations
+
+**NEVER CANCEL** these operations - they are expected to take time:
+
+| Operation | Expected Time | Timeout Setting |
+|-----------|--------------|-----------------|
+| `dotnet restore` | 50 seconds | 90+ seconds |
+| `dotnet build` | 4.5 minutes | 10+ minutes |
+| `npm ci` | 11 seconds | 30+ seconds |
+| `npm run build:for:cms` | 1.25 minutes | 5+ minutes |
+| `npm test` | 2 minutes | 5+ minutes |
+| `npm run lint` | 1 minute | 5+ minutes |
+| Unit tests | 1 minute | 5+ minutes |
+| Integration tests | Variable | 10+ minutes |
+
+Always wait for commands to complete rather than canceling and retrying.
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