diff --git a/tests/Trax.Core.Tests.Meta/Tests/DependencyDirectionTests.cs b/tests/Trax.Core.Tests.Meta/Tests/DependencyDirectionTests.cs
new file mode 100644
index 0000000..e141f54
--- /dev/null
+++ b/tests/Trax.Core.Tests.Meta/Tests/DependencyDirectionTests.cs
@@ -0,0 +1,136 @@
+namespace Trax.Core.Tests.Meta.Tests;
+
+/// <summary>
+/// Enforces the Trax dependency chain (CLAUDE.md): a repo may take a <c>Trax.*</c> <c>PackageReference</c>
+/// only on its own family or an allowed upstream family, never a downstream or parallel one. The map below
+/// is the whole chain; the test self-detects which repo it runs in from the <c>.slnx</c> at the repo root
+/// and applies that repo's rule, so the identical file lives in every repo's Meta project. Intra-repo
+/// dependencies use <c>ProjectReference</c> and are exempt (they are not PackageReferences).
+///
+/// <para>This catches an upstream repo pulling in a downstream one, for example a test that references
+/// Trax.Api.GraphQL from inside Trax.Effect. That only compiles against the locally-packed feed and hides a
+/// real layering inversion. Sibling <see cref="CrossRepoPackageReferenceTests"/> checks that a legitimate
+/// cross-repo reference is centrally managed; this one checks it points the right way in the first place.</para>
+/// </summary>
+[TestFixture]
+public class DependencyDirectionTests
+{
+    // For each repo family, the full set of upstream families it may PackageReference. Its own family is
+    // always allowed. Empirically matches the current graph and the chain: Core is the root; Effect -> Core;
+    // Mediator -> Effect; Scheduler -> Mediator; Api -> Scheduler; Dashboard -> Api; Cli -> Scheduler;
+    // Samples sits at the bottom and may reference everything upstream.
+    private static readonly IReadOnlyDictionary<string, string[]> AllowedUpstream = new Dictionary<
+        string,
+        string[]
+    >(StringComparer.Ordinal)
+    {
+        ["Trax.Core"] = Array.Empty<string>(),
+        ["Trax.Effect"] = new[] { "Trax.Core" },
+        ["Trax.Mediator"] = new[] { "Trax.Core", "Trax.Effect" },
+        ["Trax.Scheduler"] = new[] { "Trax.Core", "Trax.Effect", "Trax.Mediator" },
+        ["Trax.Api"] = new[] { "Trax.Core", "Trax.Effect", "Trax.Mediator", "Trax.Scheduler" },
+        ["Trax.Dashboard"] = new[]
+        {
+            "Trax.Core",
+            "Trax.Effect",
+            "Trax.Mediator",
+            "Trax.Scheduler",
+            "Trax.Api",
+        },
+        ["Trax.Cli"] = new[] { "Trax.Core", "Trax.Effect", "Trax.Mediator", "Trax.Scheduler" },
+        ["Trax.Samples"] = new[]
+        {
+            "Trax.Core",
+            "Trax.Effect",
+            "Trax.Mediator",
+            "Trax.Scheduler",
+            "Trax.Dashboard",
+            "Trax.Api",
+            "Trax.Cli",
+        },
+    };
+
+    private static readonly string[] Families =
+    {
+        "Trax.Core",
+        "Trax.Effect",
+        "Trax.Mediator",
+        "Trax.Scheduler",
+        "Trax.Dashboard",
+        "Trax.Api",
+        "Trax.Cli",
+        "Trax.Samples",
+    };
+
+    [Test]
+    public void No_project_references_a_downstream_or_parallel_Trax_package()
+    {
+        var repo = DetectRepo();
+        repo.Should()
+            .NotBeNull(
+                "the current repo family should be detectable from the .slnx at the repo root"
+            );
+
+        var allowed = new HashSet<string>(AllowedUpstream[repo!], StringComparer.Ordinal) { repo! };
+        var offenders = new List<string>();
+
+        foreach (var csproj in SourceFiles.Projects())
+        {
+            XDocument doc;
+            try
+            {
+                doc = XDocument.Load(csproj);
+            }
+            catch (Exception ex)
+            {
+                Assert.Fail($"Failed to parse {RepoRoot.Relative(csproj)}: {ex.Message}");
+                return;
+            }
+
+            foreach (var pkg in doc.Descendants("PackageReference"))
+            {
+                var include = pkg.Attribute("Include")?.Value;
+                if (
+                    string.IsNullOrEmpty(include)
+                    || !include.StartsWith("Trax.", StringComparison.Ordinal)
+                )
+                    continue;
+
+                // A Trax package outside the eight core-repo families (e.g. a standalone Trax.Runner.*)
+                // is out of scope for this chain guard; skip it rather than flag it.
+                var family = FamilyOf(include);
+                if (family is null)
+                    continue;
+                if (!allowed.Contains(family))
+                    offenders.Add($"{RepoRoot.Relative(csproj)} -> {include} [{family}]");
+            }
+        }
+
+        offenders
+            .Should()
+            .BeEmpty(
+                $"{repo} may PackageReference only its own family and its allowed upstream "
+                    + $"[{string.Join(", ", AllowedUpstream[repo!])}] per the Trax dependency chain. A "
+                    + "reference to any other Trax family points the wrong way (downstream or parallel); use "
+                    + "a ProjectReference for intra-repo dependencies. Offenders:\n  "
+                    + string.Join("\n  ", offenders)
+            );
+    }
+
+    private static string? FamilyOf(string package) =>
+        Families.FirstOrDefault(f =>
+            package.Equals(f, StringComparison.Ordinal)
+            || package.StartsWith(f + ".", StringComparison.Ordinal)
+        );
+
+    private static string? DetectRepo()
+    {
+        var slnx = Directory
+            .EnumerateFiles(RepoRoot.Path, "*.slnx", SearchOption.TopDirectoryOnly)
+            .FirstOrDefault();
+        if (slnx is null)
+            return null;
+        var name = Path.GetFileNameWithoutExtension(slnx);
+        return AllowedUpstream.ContainsKey(name) ? name : null;
+    }
+}