CaffeinatedCoder · Aug 16, 2026 · Aug 16, 2026
diff --git a/Directory.Build.props b/Directory.Build.props
@@ -40,6 +40,29 @@
         never bit here; PackagingConventionTests keeps it out.
     -->
 
+    <!--
+        Every pack is checked against the last release for API breaks.
+
+        The README has claimed "no breaking changes" for two majors running; this makes the claim
+        mechanical. At pack time the SDK restores each package at the baseline version from
+        nuget.org and runs ApiCompat over the public surface — a removed or changed member fails the
+        pack (CP0001/CP0002/...), in CI's Pack job and in the release gate.
+
+        The baseline is the last release. When Version is bumped for the next one it stays as the
+        release being superseded, and PackagingConventionTests requires it to be that release or
+        the shipped version — never older — so it moves with every bump instead of fossilising.
+        For a release that breaks on purpose: run
+        `dotnet pack -c Release /p:ApiCompatGenerateSuppressionFile=true`, read the generated
+        CompatibilitySuppressions.xml — each entry is one breaking change and belongs in the
+        changelog's migration notes — commit it, and delete it after the release once the baseline
+        moves past it. A new package with no release yet sets PackageValidationBaselineVersion
+        empty in its own csproj until it has one.
+    -->
+    <PropertyGroup>
+        <EnablePackageValidation>true</EnablePackageValidation>
+        <PackageValidationBaselineVersion>6.2.0</PackageValidationBaselineVersion>
+    </PropertyGroup>
+
     <!-- Provenance: a consumer stepping into this code gets the exact source it is running.
          Source Link ships in the SDK, so no PackageReference is needed. -->
     <PropertyGroup>

diff --git a/test/CodoMetis.ValueRanges.Conventions.Tests/PackagingConventionTests.cs b/test/CodoMetis.ValueRanges.Conventions.Tests/PackagingConventionTests.cs
@@ -1,3 +1,4 @@
+using System.Text.RegularExpressions;
 using System.Xml.Linq;
 
 namespace CodoMetis.ValueRanges.Conventions.Tests;
@@ -136,6 +137,44 @@ public void SharedBuildProperties_DoNotSetGeneratePackageOnBuild()
           + "first and pack with --no-build instead, as the workflows do.");
     }
 
+    /// <summary>
+    /// Package validation compares each pack against <c>PackageValidationBaselineVersion</c>. A
+    /// baseline left behind stops seeing API added since it — a member introduced in 6.3 and
+    /// removed in 6.4 is invisible to a 6.2 baseline — so it may be the version being shipped
+    /// (between releases, when <c>Version</c> is the last release) or the release directly before
+    /// it (once <c>Version</c> is bumped for the next one), and never older. That lags by at most
+    /// one release and forces the move at every version bump.
+    /// </summary>
+    [TestMethod]
+    public void PackageValidationBaseline_IsTheShippedVersionOrTheReleaseBeforeIt()
+    {
+        var properties = SharedBuildProperties();
+
+        Assert.AreEqual(
+            "true", properties.GetValueOrDefault("EnablePackageValidation"),
+            "Directory.Build.props does not enable package validation, so a removed public member "
+          + "packs cleanly and the 'no breaking changes' claim rests on reading the diff.");
+
+        var shipped  = Version.Parse(RepoLayout.ShippedVersion);
+        var previous = ChangelogVersions().Where(version => version < shipped).DefaultIfEmpty().Max();
+        var baseline = properties.GetValueOrDefault("PackageValidationBaselineVersion");
+
+        Assert.IsNotNull(baseline, "Directory.Build.props sets no PackageValidationBaselineVersion.");
+
+        var allowed = new[] { shipped, previous }.Where(version => version is not null).Distinct().ToList();
+
+        Assert.IsTrue(
+            allowed.Contains(Version.Parse(baseline)),
+            $"PackageValidationBaselineVersion is {baseline}, but Directory.Build.props ships {shipped} "
+          + $"and the release before it is {previous?.ToString() ?? "none"}. The baseline must be one of "
+          + "those two — move it to the release just superseded when bumping Version, or API added "
+          + "since the old baseline goes unvalidated.");
+    }
+
+    private static IEnumerable<Version> ChangelogVersions() =>
+        Regex.Matches(File.ReadAllText(RepoLayout.RootChangelog.FullName), @"^##\s*\[(?<version>\d+\.\d+\.\d+)\]", RegexOptions.Multiline)
+             .Select(match => Version.Parse(match.Groups["version"].Value));
+
     private static Dictionary<string, string> SharedBuildProperties()
     {
         var document = XDocument.Load(Path.Combine(RepoLayout.Root.FullName, "Directory.Build.props"));