diff --git a/PolyPilot.Tests/SessionPersistenceTests.cs b/PolyPilot.Tests/SessionPersistenceTests.cs
index 134da166..b87e2505 100644
--- a/PolyPilot.Tests/SessionPersistenceTests.cs
+++ b/PolyPilot.Tests/SessionPersistenceTests.cs
@@ -405,4 +405,81 @@ public void Merge_DeletedMultiAgentSessions_InClosedIds_Excluded()
         Assert.Single(result);
         Assert.Equal("regular-session", result[0].SessionId);
     }
+
+    // --- SweepOrphanedTempSessionDirs tests ---
+
+    [Fact]
+    public void Sweep_OrphanedDirs_AreDeleted()
+    {
+        var tempBase = Path.Combine(Path.GetTempPath(), $"polypilot-sweep-test-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(tempBase);
+        var orphanDir = Directory.CreateDirectory(Path.Combine(tempBase, "orphan1")).FullName;
+        try
+        {
+            CopilotService.SweepOrphanedTempSessionDirs(tempBase, Array.Empty<string?>());
+
+            Assert.False(Directory.Exists(orphanDir));
+        }
+        finally
+        {
+            if (Directory.Exists(tempBase)) Directory.Delete(tempBase, true);
+        }
+    }
+
+    [Fact]
+    public void Sweep_PersistedDirs_AreKept()
+    {
+        var tempBase = Path.Combine(Path.GetTempPath(), $"polypilot-sweep-test-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(tempBase);
+        var persistedDir = Directory.CreateDirectory(Path.Combine(tempBase, "persisted1")).FullName;
+        var orphanDir = Directory.CreateDirectory(Path.Combine(tempBase, "orphan1")).FullName;
+        try
+        {
+            CopilotService.SweepOrphanedTempSessionDirs(tempBase, new[] { persistedDir });
+
+            Assert.True(Directory.Exists(persistedDir));
+            Assert.False(Directory.Exists(orphanDir));
+        }
+        finally
+        {
+            if (Directory.Exists(tempBase)) Directory.Delete(tempBase, true);
+        }
+    }
+
+    [Fact]
+    public void Sweep_WhenTempBaseDoesNotExist_DoesNotThrow()
+    {
+        var nonExistentBase = Path.Combine(Path.GetTempPath(), $"polypilot-nonexistent-{Guid.NewGuid():N}");
+        var ex = Record.Exception(() =>
+            CopilotService.SweepOrphanedTempSessionDirs(nonExistentBase, Array.Empty<string?>()));
+        Assert.Null(ex);
+    }
+
+    [Fact]
+    public void Sweep_NullEntriesInPersistedList_AreIgnored()
+    {
+        var tempBase = Path.Combine(Path.GetTempPath(), $"polypilot-sweep-test-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(tempBase);
+        var orphanDir = Directory.CreateDirectory(Path.Combine(tempBase, "orphan1")).FullName;
+        try
+        {
+            // Passing nulls/empties should not throw and should treat them as not-persisted
+            CopilotService.SweepOrphanedTempSessionDirs(tempBase, new string?[] { null, "", null });
+
+            Assert.False(Directory.Exists(orphanDir));
+        }
+        finally
+        {
+            if (Directory.Exists(tempBase)) Directory.Delete(tempBase, true);
+        }
+    }
+
+    [Fact]
+    public void TempSessionsBase_IsIsolatedInTests()
+    {
+        // SetBaseDirForTesting should redirect TempSessionsBase away from the real temp dir
+        var realTempBase = Path.Combine(Path.GetTempPath(), "polypilot-sessions");
+        Assert.NotEqual(realTempBase, CopilotService.TempSessionsBase, StringComparer.OrdinalIgnoreCase);
+        Assert.StartsWith(TestSetup.TestBaseDir, CopilotService.TempSessionsBase, StringComparison.OrdinalIgnoreCase);
+    }
 }
diff --git a/PolyPilot/Services/CopilotService.Persistence.cs b/PolyPilot/Services/CopilotService.Persistence.cs
index 3dc2a92b..c1d51db7 100644
--- a/PolyPilot/Services/CopilotService.Persistence.cs
+++ b/PolyPilot/Services/CopilotService.Persistence.cs
@@ -142,6 +142,26 @@ internal static List<ActiveSessionEntry> MergeSessionEntries(
         return merged;
     }
 
+    /// <summary>
+    /// Delete temp session directories under <paramref name="tempBase"/> that are
+    /// not referenced by any persisted session's WorkingDirectory.
+    /// Called on startup to clean up directories left behind by crashed sessions.
+    /// </summary>
+    internal static void SweepOrphanedTempSessionDirs(string tempBase, IEnumerable<string?> persistedWorkingDirs)
+    {
+        if (!Directory.Exists(tempBase)) return;
+
+        var keepDirs = new HashSet<string>(
+            persistedWorkingDirs.Where(d => !string.IsNullOrEmpty(d)).Select(d => d!),
+            StringComparer.OrdinalIgnoreCase);
+
+        foreach (var dir in Directory.GetDirectories(tempBase))
+        {
+            if (!keepDirs.Contains(dir))
+                try { Directory.Delete(dir, true); } catch { }
+        }
+    }
+
     /// <summary>
     /// Load and resume all previously active sessions
     /// </summary>
@@ -153,6 +173,10 @@ public async Task RestorePreviousSessionsAsync(CancellationToken cancellationTok
             {
                 var json = await File.ReadAllTextAsync(ActiveSessionsFile, cancellationToken);
                 var entries = JsonSerializer.Deserialize<List<ActiveSessionEntry>>(json);
+
+                // Sweep orphaned temp session directories from crashed/killed sessions
+                SweepOrphanedTempSessionDirs(TempSessionsBase, entries?.Select(e => e.WorkingDirectory) ?? []);
+
                 if (entries != null && entries.Count > 0)
                 {
                     Debug($"Restoring {entries.Count} previous sessions...");
diff --git a/PolyPilot/Services/CopilotService.cs b/PolyPilot/Services/CopilotService.cs
index 74989aba..9c44cd09 100644
--- a/PolyPilot/Services/CopilotService.cs
+++ b/PolyPilot/Services/CopilotService.cs
@@ -128,8 +128,12 @@ internal static void SetBaseDirForTesting(string path)
         Volatile.Write(ref _copilotBaseDir, null);
         _sessionStatePath = null;
         _pendingOrchestrationFile = null;
+        _tempSessionsBase = Path.Combine(path, "polypilot-sessions");
     }
 
+    private static string? _tempSessionsBase;
+    internal static string TempSessionsBase => _tempSessionsBase ??= Path.Combine(Path.GetTempPath(), "polypilot-sessions");
+
     private static string? _projectDir;
     private static string ProjectDir => _projectDir ??= FindProjectDir();
 
@@ -1304,7 +1308,17 @@ public async Task<AgentSessionInfo> CreateSessionAsync(string name, string? mode
 
         var sessionModel = Models.ModelHelper.NormalizeToSlug(model ?? DefaultModel);
         if (string.IsNullOrEmpty(sessionModel)) sessionModel = DefaultModel;
-        var sessionDir = string.IsNullOrWhiteSpace(workingDirectory) ? ProjectDir : workingDirectory;
+        string sessionDir;
+        if (string.IsNullOrWhiteSpace(workingDirectory))
+        {
+            var tempDir = Path.Combine(TempSessionsBase, Guid.NewGuid().ToString("N"));
+            try { Directory.CreateDirectory(tempDir); sessionDir = tempDir; }
+            catch (Exception ex) { Debug($"Failed to create temp session dir '{tempDir}': {ex.Message}"); sessionDir = ProjectDir; }
+        }
+        else
+        {
+            sessionDir = workingDirectory;
+        }
 
         // Build system message with critical relaunch instructions
         // Note: The CLI automatically loads .github/copilot-instructions.md from the working directory,
@@ -2162,6 +2176,12 @@ public async Task<bool> CloseSessionAsync(string name)
         if (state.Session is not null)
             try { await state.Session.DisposeAsync(); } catch { /* session may already be disposed */ }
 
+        // Clean up auto-created temp session directory
+        var closedWorkingDir = state.Info.WorkingDirectory;
+        if (!string.IsNullOrEmpty(closedWorkingDir) &&
+            string.Equals(Path.GetDirectoryName(closedWorkingDir), TempSessionsBase, StringComparison.OrdinalIgnoreCase))
+            try { Directory.Delete(closedWorkingDir, true); } catch { }
+
         if (_activeSessionName == name)
         {
             _activeSessionName = _sessions.Keys.FirstOrDefault();