diff --git a/src/metrics.zig b/src/metrics.zig
index 38beb22..1be570c 100644
--- a/src/metrics.zig
+++ b/src/metrics.zig
@@ -479,3 +479,122 @@ test "token bucket never partially consumes on a failed acquire" {
     try std.testing.expectApproxEqAbs(@as(f64, 5), b.availableAt(0), 1e-9);
 }
 
+test "sliding window limiter smooths across a window boundary" {
+    const windowNs: u64 = std.time.ns_per_s;
+    var lim = SlidingWindowLimiter.init(10, windowNs, 0);
+
+    var i: usize = 0;
+    while (i < 10) : (i += 1) try std.testing.expect(lim.allowAt(0));
+    try std.testing.expect(!lim.allowAt(0)); // exhausted current window
+
+    // Halfway into the next window: previous window's 10 requests are
+    // weighted at 50% overlap (5), so only 5 more should be allowed before
+    // the weighted total reaches the limit of 10.
+    const halfwayNextWindow = windowNs + windowNs / 2;
+    var allowed: usize = 0;
+    i = 0;
+    while (i < 20) : (i += 1) {
+        if (lim.allowAt(halfwayNextWindow)) allowed += 1;
+    }
+    try std.testing.expectEqual(@as(usize, 5), allowed);
+}
+
+test "sliding window limiter resets fully after a gap of more than one window" {
+    const windowNs: u64 = std.time.ns_per_s;
+    var lim = SlidingWindowLimiter.init(3, windowNs, 0);
+    try std.testing.expect(lim.allowAt(0));
+    try std.testing.expect(lim.allowAt(0));
+    try std.testing.expect(lim.allowAt(0));
+    try std.testing.expect(!lim.allowAt(0));
+
+    // Jump 10 windows ahead: previous window count must not leak in.
+    const farFuture = 10 * windowNs;
+    try std.testing.expect(lim.allowAt(farFuture));
+    try std.testing.expect(lim.allowAt(farFuture));
+    try std.testing.expect(lim.allowAt(farFuture));
+    try std.testing.expect(!lim.allowAt(farFuture));
+}
+
+test "registry exports counters gauges and histograms as sorted prometheus text" {
+    var c = MetricCounter{};
+    c.incrBy(7);
+    var g = Gauge{};
+    g.set(2.5);
+    const bounds = [_]f64{ 5, 10 };
+    var h = try Histogram.init(std.testing.allocator, &bounds);
+    defer h.deinit();
+    h.observe(3);
+    h.observe(8);
+
+    var reg = Registry.init(std.testing.allocator);
+    defer reg.deinit();
+    try reg.registerCounter("requests_total", &c);
+    try reg.registerGauge("temperature", &g);
+    try reg.registerHistogram("latency", &h);
+    try std.testing.expectEqual(@as(usize, 3), reg.count());
+
+    var buf = std.ArrayList(u8).init(std.testing.allocator);
+    defer buf.deinit();
+    try reg.exportText(std.testing.allocator, buf.writer());
+
+    const text = buf.items;
+    // sorted alphabetically: latency < requests_total < temperature
+    try std.testing.expect(std.mem.indexOf(u8, text, "latency_bucket{le=\"5\"} 1\n") != null);
+    try std.testing.expect(std.mem.indexOf(u8, text, "latency_bucket{le=\"10\"} 2\n") != null);
+    try std.testing.expect(std.mem.indexOf(u8, text, "latency_bucket{le=\"+Inf\"} 2\n") != null);
+    try std.testing.expect(std.mem.indexOf(u8, text, "latency_sum 11") != null);
+    try std.testing.expect(std.mem.indexOf(u8, text, "latency_count 2\n") != null);
+    try std.testing.expect(std.mem.indexOf(u8, text, "requests_total 7\n") != null);
+    try std.testing.expect(std.mem.indexOf(u8, text, "temperature 2.5\n") != null);
+
+    const latencyPos = std.mem.indexOf(u8, text, "latency_bucket").?;
+    const requestsPos = std.mem.indexOf(u8, text, "requests_total").?;
+    const temperaturePos = std.mem.indexOf(u8, text, "temperature").?;
+    try std.testing.expect(latencyPos < requestsPos);
+    try std.testing.expect(requestsPos < temperaturePos);
+}
+
+test "registry re-registering the same name overwrites rather than duplicating" {
+    var c1 = MetricCounter{};
+    c1.incrBy(1);
+    var c2 = MetricCounter{};
+    c2.incrBy(99);
+
+    var reg = Registry.init(std.testing.allocator);
+    defer reg.deinit();
+    try reg.registerCounter("x", &c1);
+    try reg.registerCounter("x", &c2);
+    try std.testing.expectEqual(@as(usize, 1), reg.count());
+
+    var buf = std.ArrayList(u8).init(std.testing.allocator);
+    defer buf.deinit();
+    try reg.exportText(std.testing.allocator, buf.writer());
+    try std.testing.expectEqualStrings("x 99\n", buf.items);
+}
+
+test "formatDbStats renders levels up to the deepest populated one" {
+    var s = stats_mod.Stats{};
+    s.memtableEntries = 3;
+    s.memtableBytes = 128;
+    s.sstableCount = 2;
+    s.cacheHits = 9;
+    s.cacheMisses = 1;
+    s.levels[0].tables = 1;
+    s.levels[0].entries = 10;
+    s.levels[2].tables = 4;
+    s.levels[2].entries = 40;
+
+    var buf = std.ArrayList(u8).init(std.testing.allocator);
+    defer buf.deinit();
+    try formatDbStats(buf.writer(), "cobalt", s);
+
+    const text = buf.items;
+    try std.testing.expect(std.mem.indexOf(u8, text, "cobalt_memtable_entries 3\n") != null);
+    try std.testing.expect(std.mem.indexOf(u8, text, "cobalt_cache_hit_rate 0.900000\n") != null);
+    try std.testing.expect(std.mem.indexOf(u8, text, "cobalt_level_tables{level=\"0\"} 1\n") != null);
+    try std.testing.expect(std.mem.indexOf(u8, text, "cobalt_level_tables{level=\"2\"} 4\n") != null);
+    // level 1 is empty but must still appear since deepestLevel is 2
+    try std.testing.expect(std.mem.indexOf(u8, text, "cobalt_level_tables{level=\"1\"} 0\n") != null);
+    // level 3+ must not appear at all
+    try std.testing.expect(std.mem.indexOf(u8, text, "level=\"3\"") == null);
+}