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+# Copilot Instructions for WebmentionFs
+
+## Repository Overview
+
+**WebmentionFs** is an F# library implementing the W3C Webmention specification for sending and receiving webmentions. It's a **small, focused library** targeting .NET Standard 2.1 with modern F# idioms.
+
+- **Size**: ~8 F# source files (~35KB total code)
+- **Language**: F# (functional-first)
+- **Target**: .NET Standard 2.1 (compatible with .NET Core 3.0+, .NET 5+, .NET 6+, .NET 8+)
+- **Dependencies**: FSharp.Data 5.0.2, Microsoft.AspNetCore.Http.Abstractions 2.2.0
+- **Package**: Published to NuGet as `lqdev.WebmentionFs`
+
+## Build & Validation
+
+### Prerequisites
+- .NET SDK 6.0+ (tested with .NET 8.0 and 10.0)
+- F# compiler (included with .NET SDK)
+
+### Build Commands (VALIDATED - All Working)
+
+**ALWAYS run commands in this exact order:**
+
+```bash
+# 1. Clean build (takes ~0.7s)
+dotnet clean
+
+# 2. Restore packages (takes ~2-3s on first run, instantaneous after)
+dotnet restore
+
+# 3. Build Debug (takes ~3-4s after restore, ~10s from clean)
+dotnet build
+
+# 4. Build Release (takes ~3-4s)
+dotnet build --configuration Release
+
+# 5. Create NuGet package (only if needed)
+dotnet pack --configuration Release
+```
+
+**Build Output**: `bin/Debug/netstandard2.1/WebmentionFs.dll` or `bin/Release/netstandard2.1/WebmentionFs.dll`
+
+### Testing
+
+**Interactive test script** (`test.fsx`):
+```bash
+dotnet fsi test.fsx
+```
+
+**KNOWN ISSUE**: The test script has a version compatibility issue with FSharp.Data when running via `dotnet fsi`. The library builds successfully, but the test script may fail with `TypeLoadException` related to `FSharp.Data.HttpMethod`. This does NOT affect the compiled library - only the interactive test script. You can safely ignore test.fsx failures if the build succeeds.
+
+### Validation Checklist
+- ✓ `dotnet build` must succeed with 0 warnings, 0 errors
+- ✓ XML documentation is generated (`GenerateDocumentationFile=true`)
+- ✓ No F# specific formatter (fantomas) configured - rely on .editorconfig
+- ✓ No formal unit tests - validation done via test.fsx (interactive)
+
+### NO CI/CD Yet
+The README mentions CI, but **no .github/workflows directory exists**. There are no automated checks to replicate. When making changes:
+1. Run `dotnet build` - must succeed with 0 warnings
+2. Verify XML docs are present on public APIs
+3. Follow F# conventions in CONTRIBUTING.md
+
+## Project Structure
+
+### File Organization (CRITICAL - Order Matters)
+
+F# projects require **dependency order** in .fsproj. Files are compiled top-to-bottom:
+
+```
+WebmentionFs.fsproj (defines compilation order)
+├── Domain.fs                      # Core types (no dependencies)
+├── Constants.fs                   # String constants, CSS selectors
+├── Utils.fs                       # Pure utility functions (module)
+├── UrlDiscoveryService.fs         # Endpoint discovery service
+├── RequestValidationService.fs    # Request validation service
+├── WebmentionValidationService.fs # Content validation service
+├── WebmentionReceiverService.fs   # Complete receive pipeline
+└── WebmentionSenderService.fs     # Complete send pipeline
+```
+
+**Never reorder files in .fsproj** - it will break compilation.
+
+### Key Directories
+- `/` - Source files at root (no `src/` directory)
+- `bin/` - Build output (gitignored)
+- `obj/` - Build artifacts (gitignored)
+- `.github/` - GitHub configs (you may be creating this)
+- No tests/ or samples/ directories exist
+
+### Configuration Files
+- `.editorconfig` - F# formatting rules (4 spaces, 120 char lines, LF endings)
+- `.gitignore` - Standard Visual Studio/Rider/VS Code ignores
+- `.devcontainer.json` - VS Code devcontainer with Ionide F# extension
+- `WebmentionFs.fsproj` - Project file (XML, 2-space indent)
+
+### Documentation Files (Read These First!)
+1. **CONTRIBUTING.md** - F# conventions, naming, async patterns, error handling
+2. **ARCHITECTURE.md** - Design philosophy, module descriptions, extension points
+3. **AI_ASSISTANT_GUIDE.md** - AI-specific guidance, patterns, common tasks
+4. **README.md** - User documentation, examples, API overview
+
+## Coding Conventions (ENFORCE STRICTLY)
+
+### Must Follow
+1. **camelCase** for values/functions, **PascalCase** for types/modules
+2. **Never use magic strings** - add to Constants module
+3. **Discriminated unions for results** - never throw exceptions in normal flow
+4. **Use `task {}` for async** - never `.Result` or `.Wait()`
+5. **XML docs on ALL public APIs** - build generates documentation file
+6. **Pure functions in modules** - Utils and Constants are modules, not classes
+7. **Services are classes** - for DI compatibility with ASP.NET Core
+
+### Common Patterns
+```fsharp
+// Good: Discriminated union result
+match result with
+| ValidationSuccess data -> // handle success
+| ValidationError error -> // handle error
+
+// Good: task {} async
+let fetchAsync url =
+    task {
+        let! response = httpClient.GetAsync(url)
+        return! response.Content.ReadAsStringAsync()
+    }
+
+// Good: Constants usage
+open Constants
+let selector = EndpointSelectors.linkElement
+
+// Bad: Magic string
+let selector = "link[rel='webmention']"  // ❌ NEVER DO THIS
+
+// Bad: Throwing exceptions
+if invalid then failwith "Error"  // ❌ Return ValidationError instead
+```
+
+## Making Changes
+
+### Where to Add Code
+| Feature Type | File | Pattern |
+|-------------|------|---------|
+| New domain type | Domain.fs | Add discriminated union or record |
+| New constant/selector | Constants.fs | Add to appropriate module |
+| New utility function | Utils.fs | Add pure function to module |
+| New validation rule | RequestValidationService.fs | Add to validation pipeline |
+| New service | New .fs file | Create class, add to .fsproj AFTER all dependencies |
+
+### Before Committing
+1. Run `dotnet build` - verify 0 warnings, 0 errors (takes ~3-4s)
+2. Check XML docs on any new public APIs
+3. Verify no magic strings - use Constants module
+4. Ensure discriminated unions for all results
+5. Confirm async uses `task {}` not blocking calls
+
+### Common Gotchas
+- **File order matters** - dependencies must come before dependents in .fsproj
+- **No .github directory** - you may need to create it for workflows
+- **test.fsx has known issue** - ignore FSharp.Data TypeLoadException in test script
+- **Build from clean is slower** - first build after clean takes ~10s, incremental builds ~3-4s
+- **No separate test project** - testing is via test.fsx interactive script
+
+## Key Files Quick Reference
+
+**Domain.fs** (99 lines) - All core types: UrlData, ValidationResult, DiscoveryResult, WebmentionValidationResult, MentionTypes  
+**Constants.fs** (97 lines) - EndpointSelectors, MentionSelectors, Http constants, FormFields, ErrorMessages  
+**Utils.fs** (84 lines) - getSourceAndTargetUrlsFromFormBody, getDocumentHeadersAsync, getDocumentContentAsync, getUrlFromSourceDocument  
+**Services** (5 files, ~4-8KB each) - Complete webmention send/receive implementation
+
+## Trust These Instructions
+
+**These instructions are based on actual testing** of all build commands, exploration of all source files, and review of all documentation. Only search the codebase if:
+1. You need to understand implementation details of a specific function
+2. These instructions are incomplete for your specific task
+3. You find information here that contradicts actual behavior
+
+For general understanding of F# patterns, error handling, async, and architecture - **trust CONTRIBUTING.md and ARCHITECTURE.md** - they are comprehensive and accurate.