diff --git a/GitBranchStateCache.Tests/Git/GitRunnerTests.cs b/GitBranchStateCache.Tests/Git/GitRunnerTests.cs
index 4571b42..be1e8c4 100644
--- a/GitBranchStateCache.Tests/Git/GitRunnerTests.cs
+++ b/GitBranchStateCache.Tests/Git/GitRunnerTests.cs
@@ -168,6 +168,34 @@ public void BuildEnvironment_SetsTheFlagsThatKeepARunPredictable()
 		Assert.IsFalse(environment.ContainsKey("PATH"));
 	}
 
+	[TestMethod]
+	public void BuildEnvironment_WithAWorkingDirectory_StopsDiscoveryAtIt()
+	{
+		string mirror = Path.Combine(TempRoot, "github", "studio", "game");
+
+		Dictionary<string, string?> environment = GitRunner.BuildEnvironment(
+			new GitInvocation { WorkingDirectory = mirror, Arguments = ["for-each-ref"], Timeout = TimeSpan.FromSeconds(1) },
+			new GitBranchStateCacheOptions { MirrorRoot = TempRoot },
+			new Hashtable { ["GIT_CEILING_DIRECTORIES"] = "/somewhere/inherited" });
+
+		// git still finds the repository the directory itself holds, but a directory that is no longer
+		// one fails instead of being answered from an enclosing repository.
+		Assert.AreEqual(Path.GetDirectoryName(Path.GetFullPath(mirror)), environment["GIT_CEILING_DIRECTORIES"]);
+	}
+
+	[TestMethod]
+	public void BuildEnvironment_WithoutAWorkingDirectory_SetsNoCeiling()
+	{
+		Dictionary<string, string?> environment = GitRunner.BuildEnvironment(
+			new GitInvocation { Arguments = ["--version"], Timeout = TimeSpan.FromSeconds(1) },
+			new GitBranchStateCacheOptions { MirrorRoot = TempRoot },
+			new Hashtable { ["GIT_CEILING_DIRECTORIES"] = "/somewhere/inherited" });
+
+		// Dropped like every other inherited GIT_* variable, and nothing is put in its place.
+		Assert.IsTrue(environment.TryGetValue("GIT_CEILING_DIRECTORIES", out string? ceiling));
+		Assert.IsNull(ceiling);
+	}
+
 	[TestMethod]
 	public void BuildEnvironment_WithACredential_ScopesItToTheUpstream()
 	{
diff --git a/GitBranchStateCache.Tests/Git/RealRepositoryTests.cs b/GitBranchStateCache.Tests/Git/RealRepositoryTests.cs
index 1a75c3a..a264efa 100644
--- a/GitBranchStateCache.Tests/Git/RealRepositoryTests.cs
+++ b/GitBranchStateCache.Tests/Git/RealRepositoryTests.cs
@@ -109,6 +109,21 @@ public async Task RefResolver_ReadsEveryBranchWithItsTip()
 		Assert.IsTrue(branches!.Any(branch => branch.Name == "origin/feature/ui" && branch.Tip == _featureTip));
 	}
 
+	[TestMethod]
+	public async Task RefResolver_ABrokenMirrorInsideAnotherRepository_ReadsNothing()
+	{
+		// A mirror directory left holding only .last-used (the reaper racing MarkUsed), under a mirror
+		// root that sits inside a checkout. Git must not walk up and answer from the checkout.
+		string broken = Path.Combine(_workTree, "mirrors", "github", "studio", "other", "mirror.git");
+		Directory.CreateDirectory(broken);
+		await File.WriteAllTextAsync(Path.Combine(broken, ".last-used"), string.Empty);
+
+		IReadOnlyList<BranchRef>? branches = await new RefResolver(Runner, Options.Create(Settings))
+			.ListAsync(broken, CancellationToken.None);
+
+		Assert.IsNull(branches, $"Read {branches?.Count} branches from the enclosing repository.");
+	}
+
 	[TestMethod]
 	public async Task MirrorQueries_FindsTheForkPointAsTheMergeBase()
 	{
diff --git a/GitBranchStateCache/Git/GitRunner.cs b/GitBranchStateCache/Git/GitRunner.cs
index 8f4cfce..c833211 100644
--- a/GitBranchStateCache/Git/GitRunner.cs
+++ b/GitBranchStateCache/Git/GitRunner.cs
@@ -186,6 +186,18 @@ public async Task<GitResult> RunAsync(GitInvocation invocation, CancellationToke
 		// unplanned fetch in production.
 		environment["GIT_NO_LAZY_FETCH"] = "1";
 
+		// Discovery stops at the directory the run was given. Without a ceiling, a mirror directory that
+		// is no longer a repository (a partial delete, the reaper racing MarkUsed) does not fail: git
+		// walks up to the first enclosing repository and answers from that, which serves one
+		// repository's branches to a caller admitted only for another (ktsu-dev/GitBranchStateCache#104).
+		// A ceiling rather than GIT_DIR, because a clone and its staging directory run before they are
+		// repositories, and the ceiling still lets git find the repository the directory itself holds.
+		if (invocation.WorkingDirectory is not null
+			&& Path.GetDirectoryName(Path.GetFullPath(invocation.WorkingDirectory)) is { Length: > 0 } ceiling)
+		{
+			environment["GIT_CEILING_DIRECTORIES"] = ceiling;
+		}
+
 		ApplyConfigEnvironment(environment, invocation);
 		return environment;
 	}