1 files changed, 68 insertions, 1 deletions
diff --git a/libpanto/src/session_manager.zig b/libpanto/src/session_manager.zig
index 9e8e2c8..c6256e0 100644
--- a/libpanto/src/session_manager.zig
+++ b/libpanto/src/session_manager.zig
@@ -750,7 +750,13 @@ fn appendMessageToConv(
         .user => .user,
         .assistant => .assistant,
     };
-    try conv.messages.append(allocator, .{ .role = role, .content = content });
+    // Carry the recorded usage forward so compaction can size the retention
+    // window after a session is reopened (it's null for user/system).
+    try conv.messages.append(allocator, .{
+        .role = role,
+        .content = content,
+        .usage = disk_msg.usage,
+    });
 }
 
 // =============================================================================
@@ -1621,3 +1627,64 @@ test "resolveSessionId: unique prefix → match, ambiguous → error" {
 
     try testing.expectError(error.SessionNotFound, resolveSessionId(testing.allocator, io, sessions, "ffffffff"));
 }
+
+test "compaction summary round-trips through persist + resume + rebuild" {
+    const io = testing.io;
+
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    const session_file: []u8 = blk: {
+        var mgr = try SessionManager.init(testing.allocator, io, sessions, "/c");
+        defer mgr.deinit();
+
+        // System + an old turn that will be superseded.
+        const sys = try testing.allocator.alloc(DiskContentBlock, 1);
+        sys[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "you are helpful") } };
+        _ = try mgr.appendMessage(.{ .role = .system, .content = sys }, null, null);
+
+        const uo = try testing.allocator.alloc(DiskContentBlock, 1);
+        uo[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "old q") } };
+        _ = try mgr.appendMessage(.{ .role = .user, .content = uo }, "openai", "gpt-4o");
+        const ao = try testing.allocator.alloc(DiskContentBlock, 1);
+        ao[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "old a") } };
+        _ = try mgr.appendMessage(.{ .role = .assistant, .content = ao }, null, null);
+
+        // Compaction: summary message + duplicated kept suffix.
+        const cs = try testing.allocator.alloc(DiskContentBlock, 1);
+        cs[0] = .{ .compaction_summary = .{ .text = try testing.allocator.dupe(u8, "SUMMARY") } };
+        _ = try mgr.appendMessage(.{ .role = .user, .content = cs }, "openai", "gpt-4o");
+
+        const ur = try testing.allocator.alloc(DiskContentBlock, 1);
+        ur[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "recent q") } };
+        _ = try mgr.appendMessage(.{ .role = .user, .content = ur }, "openai", "gpt-4o");
+
+        break :blk try testing.allocator.dupe(u8, mgr.getSessionFile());
+    };
+    defer testing.allocator.free(session_file);
+
+    var resumed = try SessionManager.open(testing.allocator, io, session_file);
+    defer resumed.deinit();
+    var conv = try resumed.rebuildConversation();
+    defer conv.deinit();
+
+    // The compaction summary block survived as a CompactionSummary.
+    const anchor = conversation_mod.latestCompactionIndex(conv.messages.items).?;
+    try testing.expectEqualStrings(
+        "SUMMARY",
+        conv.messages.items[anchor].content.items[0].CompactionSummary.text.items,
+    );
+
+    // The active window is [summary, recent q].
+    const window = conversation_mod.activeMessageWindow(conv.messages.items);
+    try testing.expectEqual(@as(usize, 2), window.len);
+    try testing.expectEqualStrings("recent q", window[1].content.items[0].Text.items);
+
+    // System prompt survives (derived independently).
+    var sys_blocks = try conversation_mod.effectiveSystemBlocks(testing.allocator, conv.messages.items);
+    defer sys_blocks.deinit(testing.allocator);
+    try testing.expectEqual(@as(usize, 1), sys_blocks.items.len);
+    try testing.expectEqualStrings("you are helpful", sys_blocks.items[0]);
+}