Build Agent Skill

Building WinUI

AI Agent Quick Start

# Always wrap commands with .\initrun.ps1 — it sets up the build environment automatically.
# Default flavor is amd64chk. Override with -Flavor.

.\initrun.ps1 .\build.cmd /q                            # full repo build (product + tests) — USE THIS BY DEFAULT
.\initrun.ps1 .\build.cmd /q product                    # product code only (no tests)
.\initrun.ps1 .\build.cmd /q mux                        # MUX only (Microsoft.UI.Xaml.dll)
.\initrun.ps1 msb /q "path\to\project.vcxproj"          # build a single project
.\initrun.ps1 -Flavor arm64fre .\build.cmd /q           # build for a different flavor

Prefer bt for inner-loop builds

If the only changes are to source files (.cpp, .h, .idl, .xaml,

.appxmanifest), use the bt-build skill instead of MSBuild. bt skips

MSBuild entirely, replaying only the dirty compile/link steps in seconds.

Use MSBuild (this skill) when any of these are true:

• .vcxproj / .vcxitems files were added, removed, or edited
• .props / .targets files were changed
• NuGet package dependencies changed
• WinRT runtime classes were added or removed
• Packaging, signing, or AppX bundling is needed
• First build (no binlog exists yet)
• You are unsure whether bt covers the change

Rules:

• Always prefix with .\initrun.ps1
• Always pass /q for quiet output (errors only)
• Set initial_wait to at least 300 seconds — builds take 1-10+ minutes
• When the user asks to "build the repo" or just "build" without specifying a target, use .\initrun.ps1 .\build.cmd /q (full build).

Only use mux or a single project when the user asks for a specific component or when you know exactly which files changed.

First-Time Setup

A full init must be run once per flavor to download tools and NuGet packages.

initrun.ps1 will fail with "Run a full init first" if this hasn't been done.

When you see that error, run a full init for the needed flavor:

.\init.ps1                # default: amd64chk
.\init.ps1 amd64fre       # specific flavor

Set initial_wait to at least 300 seconds — the first init downloads tools and restores NuGet packages.

Flavors: amd64chk, amd64fre, x86chk, x86fre, arm64chk, arm64fre (chk = debug, fre = release)

After init completes, retry the original initrun.ps1 build command.

If you get build errors that seem to indicate missing dependencies, try running init again.

Commands

| Command | What it builds | Time |

|---------|---------------|------|

| .\initrun.ps1 .\build.cmd /q | Everything (product + tests) | 10+ min |

| .\initrun.ps1 .\build.cmd /q mux | Microsoft.UI.Xaml.dll only | 1-6 min |

| .\initrun.ps1 .\build.cmd /q product | Product code (no tests) | 5-10 min |

| .\initrun.ps1 .\build.cmd /q /c | Clean + full rebuild | 15+ min |

| .\initrun.ps1 msb /q "<project>" | Single .vcxproj | 5s - 5 min |

Flags (for .\build.cmd)

| Flag | Effect |

|------|--------|

| /q | Quiet — errors only, plus elapsed time |

| /b | Reduced parallelism (/m:2) — prevents PCH virtual memory exhaustion on limited-memory machines |

| /c | Clean build — deletes BuildOutput first. Use on first build or when switching flavors |

| /restore | NuGet restore before building |

| /nomock | Skip mock package. Use if you're only updating product and test code underdxaml/ and don't need to run MUXControls or sample tests.) |

| /fake | Dry run — print commands without executing |

What to Build After a Code Change

| Files changed in | Build command |

|---|---|

| dxaml/xcp/ (source only) | bt:** bt build · MSBuild: .\initrun.ps1 msb /q "dxaml\xcp\dxaml\dllsrv\winrt\native\Microsoft.ui.xaml.vcxproj" |

| controls/dev/ or controls/idl/ (source only) | bt: bt build · MSBuild: .\initrun.ps1 msb /q "controls\dev\dll\Microsoft.UI.Xaml.Controls.vcxproj" |

| dxaml/test/native/external/<area>/ (source only) | bt:** bt build · MSBuild: .\initrun.ps1 msb /q "dxaml\test\native\external\<area>\Microsoft.UI.Xaml.Tests.External.<Area>.vcxproj" |

| .vcxproj, .vcxitems, .props, .targets, NuGet deps | .\initrun.ps1 .\build.cmd /q (MSBuild only — do NOT use bt) |

| Multiple areas or unsure | .\initrun.ps1 .\build.cmd /q |

Test areas: controls, foundation, framework, automation

Terminology

MUX = Microsoft.UI.Xaml.dll (core XAML runtime). This is NOT Microsoft.UI.Xaml.Controls.dll.

Troubleshooting

error C3859: Failed to create virtual memory for PCH / error C1076: compiler limit: internal heap limit reached

Symptom: Build fails with dozens of PCH (precompiled header) virtual memory errors across multiple .cpp files.

This typically happens when building with the default /m:4 parallelism on machines with limited memory.

Root Cause: Multiple parallel cl.exe compiler instances each try to allocate large PCH memory regions, exhausting the process address space.

Fix:

• Use the /b flag in build.cmd which sets /m:2 (2 parallel processes):

   .\initrun.ps1 .\build.cmd /q /b

• If /b still fails, close other memory-intensive applications (browsers, VS instances, etc.).
• If it keeps failing, stale PCH files from a previous build with a different compiler version may be the cause. Do a clean build:

   .\initrun.ps1 .\build.cmd /q /c /b

error C1853: precompiled header file is from a different version of the compiler

Symptom: Build fails saying the .pch file is from a different compiler version.

Root Cause: Stale precompiled header files remain from a previous build with a different compiler (e.g., after a VS update).

Fix: Do a clean build with /c:

.\initrun.ps1 .\build.cmd /q /c /b

Missing Spectre mitigation libs

Symptom: Build errors about missing Spectre mitigation libraries from Visual Studio.

Fix: Import the .vsconfig file from the repo root via Visual Studio Installer:

• Open Visual Studio Installer
• Click "More" → "Import configuration"
• Select <repo-root>\.vsconfig
• Install the missing components

DevEnvDir environment variable not set

Symptom: This message appears at the start of every initrun.ps1 command.

Root Cause: This is informational, not an error. initrun.ps1 automatically runs DevCmd.cmd to set up the VS environment.

Fix: No fix needed — this is normal behavior.

NuGet restore fails with authentication errors

Symptom: init.ps1 fails during NuGet package restore with 401/403 errors.

Root Cause: Missing or expired Azure DevOps credentials for internal NuGet feeds.

Fix:

• Ensure Azure Artifacts Credential Provider is installed (init.ps1 should do this automatically)
• If it persists, manually authenticate:

   dotnet nuget update source OSClient --username "your-alias" --password "your-PAT"

• Or use nuget.exe sources update with a Personal Access Token from https://dev.azure.com/microsoft/_usersSettings/tokens

dotnet-install fails to download SDK

Symptom: init.ps1 fails while downloading the .NET SDK.

Root Cause: Network connectivity issue or the download URL has changed.

Fix:

• Check your internet connection and VPN
• Retry — transient network errors are common
• If the URL has changed, check Version.props for the expected SDK version and install it manually from https://dotnet.microsoft.com/download