Bt Build Agent Skill

bt — Fast Inner-Loop Builds

bt reads an MSBuild binary log and replays only the dirty

compile/link/MIDL/XAML/makepri steps directly — no MSBuild overhead.

Use bt for the edit-build-debug cycle on source files only.

For anything else, use the build skill (MSBuild).

Headers not yet in the tlog (newly added #includes) bt won't trigger rebuilds

until the next msbuild -bl.

Do NOT use bt when

Any of these are true → use the build skill (MSBuild) instead:

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

When in doubt, use MSBuild.

Before you start

1. Check if bt is installed

Run bt --version. If bt is missing, run the install-bt.ps1 script from

this skill's directory to download and install it. Periodically run it to

update bt as well.

2. Pick the right binlog

bt needs a binlog from a prior MSBuild build. The repo's build scripts

all write to BuildOutput\ with names like:

| Binlog | Scope |

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

| Microsoft.UI.Xaml.<flavor>.binlog | full product (incl. Controls DLL) — prefer for product code |

| MUXControls.<flavor>.binlog | test apps + infra — prefer for test code |

| Microsoft.ui.xaml.<flavor>.binlog | Microsoft.UI.Xaml.dll only — NOT Controls (mux subset) |

| Microsoft.UI.Xaml-Product.<flavor>.binlog | full product (alternate) — fallback for product code |

| Microsoft.UI.Xaml.Controls.<flavor>.binlog | Controls DLL only — narrow fallback |

| XamlCompilerPrerequisites.<flavor>.binlog | compiler tools — not used by bt |

<flavor> is amd64chk, arm64fre, etc. Single-project builds via

msb/build-clang produce <project>.<flavor>.binlog next to these.

To pick:

• Classify the change by edited paths — test (table below) / mux

(dxaml/xcp/ only) / product (everything else under controls/dev,

controls/idl, dxaml/, src/).

• Pick the freshest binlog whose scope covers that classification,

preferring broader scopes (product covers Controls, etc.).

• Infer <flavor> from the chosen filename.
• Pass --binlog BuildOutput\<chosen> to every bt invocation.

bt has no useful default to fall back to.

Caveats:

• On NTFS, Microsoft.UI.Xaml.<flavor>.binlog and

Microsoft.ui.xaml.<flavor>.binlog collide — content is whoever wrote

last; trust mtime + size, not casing.

• If product and test binlogs exist with mtimes >1h apart, warn that the

set may be inconsistent.

• If no binlog exists, run the build skill.

Test path classification

Anything under these directories is test scope:

• controls/test/MUXControls.Test/
• controls/test/MUXControlsTestApp/
• controls/test/TabViewTearOutApp/
• controls/test/TestAppCX/
• controls/test/IXMPTestApp/
• controls/test/MUXTestInfra/ *(if present)*
• Samples/AppTestAutomationHelpers/

Common workflows

| Command | What it does |

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

| bt build | Build everything dirty (always pass --binlog BuildOutput\<x>.binlog) |

| bt build MyFile.cpp | Forward walk: compile + relink anything affected by this source |

| bt build MyApp.dll | Backward walk: only what this target needs |

| bt build -c [target] | Compile only — skip link/lib |

| bt build -n | Dry run — print commands without executing |

| bt watch [--run <cmd>] | Build on file change; optionally run a command after |

| bt [--binlog some.binlog] dirty [target] | What needs building now? |

| bt bins MyHeader.h | What rebuilds when this header changes? |

| bt srcs [--headers] MyApp.dll | Sources (and optionally #included headers) feeding this target |

Run bt --help for command options (--binlog <path>),

bt <subcommand> --help for subcommand options like -j <N>,

--debounce <ms>, etc.