diff --git a/src/ModelContextProtocol.Core/Protocol/Tool.cs b/src/ModelContextProtocol.Core/Protocol/Tool.cs
index 8abbfd88c..ce17d9f7d 100644
--- a/src/ModelContextProtocol.Core/Protocol/Tool.cs
+++ b/src/ModelContextProtocol.Core/Protocol/Tool.cs
@@ -64,6 +64,7 @@ public sealed class Tool : IBaseMetadata
     /// </para>
     /// </remarks>
     [JsonPropertyName("inputSchema")]
+    [JsonRequired]
     public JsonElement InputSchema
     {
         get => field;
diff --git a/tests/ModelContextProtocol.Tests/Protocol/ToolTests.cs b/tests/ModelContextProtocol.Tests/Protocol/ToolTests.cs
index 5b2160571..d8335c055 100644
--- a/tests/ModelContextProtocol.Tests/Protocol/ToolTests.cs
+++ b/tests/ModelContextProtocol.Tests/Protocol/ToolTests.cs
@@ -105,6 +105,14 @@ public static void ToolInputSchema_HasValidDefaultSchema()
         Assert.Equal("object", typeElement.GetString());
     }
 
+    [Fact]
+    public static void ToolInputSchema_DeserializationRejectsMissingInputSchema()
+    {
+        const string json = """{"name":"test"}""";
+
+        Assert.Throws<JsonException>(() => JsonSerializer.Deserialize<Tool>(json, McpJsonUtilities.DefaultOptions));
+    }
+
     [Theory]
     [InlineData("null")]
     [InlineData("false")]