diff --git a/src/CryptoExchanges.Net.Bybit/Streaming/BybitStreamProtocol.cs b/src/CryptoExchanges.Net.Bybit/Streaming/BybitStreamProtocol.cs
index eaf4353..57325fd 100644
--- a/src/CryptoExchanges.Net.Bybit/Streaming/BybitStreamProtocol.cs
+++ b/src/CryptoExchanges.Net.Bybit/Streaming/BybitStreamProtocol.cs
@@ -143,8 +143,7 @@ public StreamFrame Classify(ReadOnlySpan<byte> frame)
     {
         StreamKind.Ticker => $"tickers.{request.WireSymbol}",
         StreamKind.Trade => $"publicTrade.{request.WireSymbol}",
-        StreamKind.OrderBook when request.Depth.HasValue => $"orderbook.{request.Depth}.{request.WireSymbol}",
-        StreamKind.OrderBook => $"orderbook.{DefaultOrderBookDepth}.{request.WireSymbol}",
+        StreamKind.OrderBook => $"orderbook.{MapOrderBookDepth(request.Depth)}.{request.WireSymbol}",
         StreamKind.Kline when request.Interval is not null =>
             $"kline.{MapInterval(request.Interval)}.{request.WireSymbol}",
         StreamKind.Kline => $"kline.1.{request.WireSymbol}",
@@ -152,6 +151,19 @@ public StreamFrame Classify(ReadOnlySpan<byte> frame)
             $"Unsupported stream kind: {request.Kind}")
     };
 
+    // Bybit v5 spot publishes order books only at tiers 1/50/200/1000; round each request UP to
+    // the nearest tier, throwing above 1000 (deepest tier) rather than under-delivering silently.
+    private static int MapOrderBookDepth(int? requested) => requested switch
+    {
+        null => DefaultOrderBookDepth,
+        <= 1 => 1,
+        <= 50 => 50,
+        <= 200 => 200,
+        <= 1000 => 1000,
+        _ => throw new ArgumentOutOfRangeException(nameof(requested), requested,
+            $"Bybit v5 spot supports order-book depths 1, 50, 200, or 1000; requested {requested}.")
+    };
+
     private static string MapInterval(string intervalToken) => intervalToken switch
     {
         nameof(KlineInterval.OneMinute) => "1",
diff --git a/tests/CryptoExchanges.Net.Bybit.Tests.Unit/Streaming/BybitStreamProtocolTests.cs b/tests/CryptoExchanges.Net.Bybit.Tests.Unit/Streaming/BybitStreamProtocolTests.cs
index 891d377..9118be4 100644
--- a/tests/CryptoExchanges.Net.Bybit.Tests.Unit/Streaming/BybitStreamProtocolTests.cs
+++ b/tests/CryptoExchanges.Net.Bybit.Tests.Unit/Streaming/BybitStreamProtocolTests.cs
@@ -235,7 +235,7 @@ public void BuildSubscribe_Trade_ProducesCorrectTopic()
     [Fact]
     public void BuildSubscribe_OrderBook_DefaultDepth50()
     {
-        // Confirmed default depth = 50 (Bybit v5 spot available levels: 1, 50, 200).
+        // Confirmed default depth = 50 (Bybit v5 spot available levels: 1, 50, 200, 1000).
         var protocol = MakeProtocol();
         var request = new StreamRequest(StreamKind.OrderBook, "BTCUSDT");
 
@@ -269,6 +269,73 @@ public void BuildSubscribe_OrderBook_ExplicitDepth200()
         doc.RootElement.GetProperty("args")[0].GetString().Should().Be("orderbook.200.BTCUSDT");
     }
 
+    [Theory]
+    [InlineData(20, "orderbook.50.BTCUSDT")]
+    [InlineData(5, "orderbook.50.BTCUSDT")]
+    [InlineData(1, "orderbook.1.BTCUSDT")]
+    [InlineData(50, "orderbook.50.BTCUSDT")]
+    [InlineData(200, "orderbook.200.BTCUSDT")]
+    [InlineData(500, "orderbook.1000.BTCUSDT")]
+    [InlineData(1000, "orderbook.1000.BTCUSDT")]
+    public void BuildSubscribe_OrderBook_RoundsDepthUpToSupportedTier(int requestedDepth, string expectedTopic)
+    {
+        // Bybit v5 spot publishes order books only at depths 1/50/200/1000; a non-tier depth
+        // would yield an invalid topic the venue rejects, so each request rounds UP to a tier.
+        var protocol = MakeProtocol();
+        var request = new StreamRequest(StreamKind.OrderBook, "BTCUSDT", Depth: requestedDepth);
+
+        var wire = protocol.BuildSubscribe(request);
+        using var doc = JsonDocument.Parse(wire);
+
+        doc.RootElement.GetProperty("args")[0].GetString().Should().Be(expectedTopic);
+    }
+
+    [Theory]
+    [InlineData(1001)]
+    [InlineData(5000)]
+    public void BuildSubscribe_OrderBook_DepthAbove1000_Throws(int requestedDepth)
+    {
+        // 1000 is the deepest Bybit v5 spot tier; a deeper request is unsatisfiable and must
+        // throw rather than silently clamp DOWN to a shallower book than asked for.
+        var protocol = MakeProtocol();
+        var request = new StreamRequest(StreamKind.OrderBook, "BTCUSDT", Depth: requestedDepth);
+
+        var act = () => protocol.BuildSubscribe(request);
+
+        act.Should().Throw<ArgumentOutOfRangeException>();
+    }
+
+    [Fact]
+    public void BuildSubscribe_OrderBook_NullDepth_MapsToDefaultTier50()
+    {
+        var protocol = MakeProtocol();
+        var request = new StreamRequest(StreamKind.OrderBook, "BTCUSDT", Depth: null);
+
+        var wire = protocol.BuildSubscribe(request);
+        using var doc = JsonDocument.Parse(wire);
+
+        doc.RootElement.GetProperty("args")[0].GetString().Should().Be("orderbook.50.BTCUSDT");
+    }
+
+    [Fact]
+    public void RoutingKeyFor_OrderBook_NonTierDepth_MatchesBuildSubscribeTopic()
+    {
+        // Depth 20 is not a Bybit tier; both RoutingKeyFor and the subscribe topic must map it
+        // to the same tier (50) or data frames would never route back to the subscription.
+        var protocol = MakeProtocol();
+        var request = new StreamRequest(StreamKind.OrderBook, "BTCUSDT", Depth: 20);
+
+        var routingKey = protocol.RoutingKeyFor(request);
+        var wire = protocol.BuildSubscribe(request);
+        using var doc = JsonDocument.Parse(wire);
+        var subscribeTopic = doc.RootElement.GetProperty("args")[0].GetString();
+
+        routingKey.Should().Be("orderbook.50.BTCUSDT");
+        subscribeTopic.Should().Be("orderbook.50.BTCUSDT");
+        routingKey.Should().Be(subscribeTopic,
+            "RoutingKeyFor and BuildSubscribe must share one mapped keyspace so frames reach their subscription");
+    }
+
     [Fact]
     public void BuildSubscribe_Kline_OneMinute()
     {