CaffeinatedCoder · Aug 16, 2026 · Aug 16, 2026 · Aug 16, 2026 · Aug 16, 2026
diff --git a/CLAUDE.md b/CLAUDE.md
@@ -103,7 +103,7 @@ below exist because ordinary review does not catch it.
 | Test class | Guards |
 |---|---|
 | `ChangelogConsistencyTests` | The changelog lives in four files (root README + one per package). Asserts the shipped version is documented, no README runs ahead of `Directory.Build.props`, package changelogs are a subset of the root's, and sections are newest-first. |
-| `PackagingConventionTests` | Every package ships its own README as `PackageReadmeFile`; `.targets` ship to both `build/` and `buildTransitive/`, reference a real `IDesignTimeServices` in their own assembly, and set `ForProvider` on satellites but not on core. |
+| `PackagingConventionTests` | Every package ships its own README as `PackageReadmeFile`; `.targets` ship to both `build/` and `buildTransitive/`, reference a real `IDesignTimeServices` in their own assembly, and set `ForProvider` on satellites but not on core. Package validation is enabled and its baseline is the shipped version or the release before it, never older — the baseline is what `dotnet pack` diffs the public surface against (CP0002 on a removed member), and one left behind stops seeing API added since it. |
 | `ClaudeMdConsistencyTests` | This file. Prose cannot be asserted, so it checks the falsifiable parts: cited paths and file names exist, annotation keys under a prefix this repo owns are declared somewhere, `Type.Member` references resolve, and the stated size of the Npgsql whitelist matches it. Those are what a rename rots silently — and the count claim had already gone stale by two. |
 | `BuilderApiParityTests` | Every key in a satellite's annotation whitelist is reachable from a builder method. `SqlServer:DataCompression` sat whitelisted with no API for a full release; this catches that class of drift by invoking every builder extension and diffing the keys it sets. |
 | `SecurityPolicyConsistencyTests` | SECURITY.md's supported-versions table names the minor being shipped, and its `< x.y` row meets it. The table is prose a version bump forgets — the sibling repository shipped 6.2.0 with the table still saying 6.1.x. |

diff --git a/Directory.Build.props b/Directory.Build.props
@@ -43,6 +43,29 @@
         a clean checkout where bin/ is empty, which is how CI found it.
     -->
 
+    <!--
+        Every pack is checked against the last release for API breaks.
+
+        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. Until now a removed public
+        member packed cleanly, and whether a release broke anyone rested on reading the diff.
+
+        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 — 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>5.0.3</PackageValidationBaselineVersion>
+    </PropertyGroup>
+
     <!--
         Debugging support for consumers.
 

diff --git a/test/EFCore.ComplexIndexes.Tests/PackagingConventionTests.cs b/test/EFCore.ComplexIndexes.Tests/PackagingConventionTests.cs
@@ -131,6 +131,45 @@ public void Sbom_filter_covers_every_private_reference()
           + "consumers never receive.");
     }
 
+    /// <summary>
+    /// Package validation compares each pack against <c>PackageValidationBaselineVersion</c>. A
+    /// baseline left behind stops seeing API added since it — a member introduced in 5.1 and
+    /// removed in 5.2 is invisible to a 5.0 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(DisplayName = "The package-validation baseline is the shipped version or the release before it")]
+    public void Package_validation_baseline_is_the_shipped_version_or_the_release_before_it()
+    {
+        var props = XDocument.Load(RepositoryLayout.BuildProps);
+
+        Assert.AreEqual(
+            "true", props.Descendants("EnablePackageValidation").SingleOrDefault()?.Value.Trim(),
+            "Directory.Build.props does not enable package validation, so a removed public member "
+          + "packs cleanly and whether a release breaks anyone rests on reading the diff.");
+
+        var shipped  = Version.Parse(props.Descendants("Version").Single().Value);
+        var previous = ChangelogVersions().Where(version => version < shipped).DefaultIfEmpty().Max();
+        var baseline = props.Descendants("PackageValidationBaselineVersion").SingleOrDefault()?.Value.Trim();
+
+        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} "
+          + $"and the release before it is {previous?.ToString() ?? "none"}. The baseline must be one of "
+          + "those two — move it to the release just superseded when bumping Version, or API added "
+          + "since the old baseline goes unvalidated.");
+    }
+
+    // The root README's "## What changed in x.y.z" headings — the same source ChangelogConsistencyTests reads.
+    private static IEnumerable<Version> ChangelogVersions() =>
+        Regex.Matches(File.ReadAllText(RepositoryLayout.RootReadme), @"^## What changed in (\d+\.\d+\.\d+)\s*$", RegexOptions.Multiline)
+             .Select(match => Version.Parse(match.Groups[1].Value));
+
     private static string ReleaseWorkflow =>
         Path.Combine(RepositoryLayout.Root, ".github", "workflows", "release.yml");