diff --git a/src/RecurringThings.MongoDB/Configuration/GuidStringRepresentationConvention.cs b/src/RecurringThings.MongoDB/Configuration/GuidStringRepresentationConvention.cs
new file mode 100644
index 0000000..7b7b11a
--- /dev/null
+++ b/src/RecurringThings.MongoDB/Configuration/GuidStringRepresentationConvention.cs
@@ -0,0 +1,47 @@
+namespace RecurringThings.MongoDB.Configuration;
+
+using System;
+using global::MongoDB.Bson.Serialization;
+using global::MongoDB.Bson.Serialization.Conventions;
+using global::MongoDB.Bson.Serialization.Serializers;
+
+/// <summary>
+/// Convention that serializes all Guid properties as strings instead of binary.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This convention applies to both <see cref="Guid"/> and <see cref="Nullable{Guid}"/> properties,
+/// ensuring consistent string representation without requiring attributes on each property.
+/// </para>
+/// <para>
+/// Uses <see cref="StringGuidSerializer"/> instead of the built-in GuidSerializer to avoid
+/// the "GuidRepresentation is Unspecified" error in newer MongoDB driver versions.
+/// </para>
+/// </remarks>
+internal sealed class GuidStringRepresentationConvention : IMemberMapConvention
+{
+    private static readonly StringGuidSerializer GuidSerializer = new();
+
+    /// <summary>
+    /// Gets the name of this convention.
+    /// </summary>
+    public string Name => "GuidStringRepresentation";
+
+    /// <summary>
+    /// Applies the convention to a member map.
+    /// </summary>
+    /// <param name="memberMap">The member map to apply the convention to.</param>
+    public void Apply(BsonMemberMap memberMap)
+    {
+        ArgumentNullException.ThrowIfNull(memberMap);
+
+        if (memberMap.MemberType == typeof(Guid))
+        {
+            memberMap.SetSerializer(GuidSerializer);
+        }
+        else if (memberMap.MemberType == typeof(Guid?))
+        {
+            memberMap.SetSerializer(new NullableSerializer<Guid>(GuidSerializer));
+        }
+    }
+}
diff --git a/src/RecurringThings.MongoDB/Configuration/MongoDbInitializer.cs b/src/RecurringThings.MongoDB/Configuration/MongoDbInitializer.cs
new file mode 100644
index 0000000..1227173
--- /dev/null
+++ b/src/RecurringThings.MongoDB/Configuration/MongoDbInitializer.cs
@@ -0,0 +1,62 @@
+namespace RecurringThings.MongoDB.Configuration;
+
+using System;
+using global::MongoDB.Bson.Serialization.Conventions;
+
+/// <summary>
+/// Initializes MongoDB conventions when the assembly is loaded.
+/// </summary>
+/// <remarks>
+/// This ensures that conventions are registered before any serialization occurs,
+/// regardless of whether <see cref="RecurringThingsBuilderExtensions.UseMongoDb"/> is called.
+/// </remarks>
+internal static class MongoDbInitializer
+{
+    private static bool _initialized;
+    private static readonly object _lock = new();
+
+    /// <summary>
+    /// Registers MongoDB conventions for RecurringThings documents.
+    /// </summary>
+    /// <remarks>
+    /// This method is idempotent and can be called multiple times safely.
+    /// It registers the following conventions:
+    /// <list type="bullet">
+    /// <item><description>NamedIdMemberConvention: Maps "Id" property to MongoDB "_id" field</description></item>
+    /// <item><description>CamelCaseElementNameConvention: Converts property names to camelCase</description></item>
+    /// <item><description>IgnoreIfNullConvention: Excludes null properties from serialization</description></item>
+    /// <item><description>GuidStringRepresentationConvention: Serializes Guid properties as strings</description></item>
+    /// </list>
+    /// </remarks>
+    internal static void EnsureInitialized()
+    {
+        if (_initialized)
+        {
+            return;
+        }
+
+        lock (_lock)
+        {
+            if (_initialized)
+            {
+                return;
+            }
+
+            var conventionPack = new ConventionPack
+            {
+                new NamedIdMemberConvention("Id"),
+                new CamelCaseElementNameConvention(),
+                new IgnoreIfNullConvention(true),
+                new GuidStringRepresentationConvention()
+            };
+
+            ConventionRegistry.Register(
+                "RecurringThingsConventions",
+                conventionPack,
+                t => t.FullName?.StartsWith("RecurringThings", StringComparison.Ordinal) == true);
+
+            _initialized = true;
+        }
+    }
+
+}
diff --git a/src/RecurringThings.MongoDB/Configuration/RecurringThingsBuilderExtensions.cs b/src/RecurringThings.MongoDB/Configuration/RecurringThingsBuilderExtensions.cs
index 11738cf..754a93f 100644
--- a/src/RecurringThings.MongoDB/Configuration/RecurringThingsBuilderExtensions.cs
+++ b/src/RecurringThings.MongoDB/Configuration/RecurringThingsBuilderExtensions.cs
@@ -1,7 +1,6 @@
 namespace RecurringThings.MongoDB.Configuration;
 
 using System;
-using global::MongoDB.Bson.Serialization.Conventions;
 using global::MongoDB.Driver;
 using Microsoft.Extensions.DependencyInjection;
 using RecurringThings.Configuration;
@@ -44,12 +43,8 @@ public static RecurringThingsBuilder UseMongoDb(
         configure(options);
         options.Validate();
 
-        // Register camelCase naming convention for RecurringThings documents
-        var conventionPack = new ConventionPack { new CamelCaseElementNameConvention() };
-        ConventionRegistry.Register(
-            "RecurringThingsCamelCase",
-            conventionPack,
-            t => t.FullName?.StartsWith("RecurringThings", StringComparison.Ordinal) == true);
+        // Ensure MongoDB conventions are registered (idempotent)
+        MongoDbInitializer.EnsureInitialized();
 
         // Mark MongoDB as configured
         builder.MongoDbConfigured = true;
diff --git a/src/RecurringThings.MongoDB/Configuration/StringGuidSerializer.cs b/src/RecurringThings.MongoDB/Configuration/StringGuidSerializer.cs
new file mode 100644
index 0000000..2473a57
--- /dev/null
+++ b/src/RecurringThings.MongoDB/Configuration/StringGuidSerializer.cs
@@ -0,0 +1,37 @@
+namespace RecurringThings.MongoDB.Configuration;
+
+using System;
+using global::MongoDB.Bson;
+using global::MongoDB.Bson.Serialization;
+using global::MongoDB.Bson.Serialization.Serializers;
+
+/// <summary>
+/// A custom GUID serializer that stores GUIDs as strings in MongoDB.
+/// </summary>
+/// <remarks>
+/// This serializer is used instead of <see cref="GuidSerializer"/> with <see cref="BsonType.String"/>
+/// because the standard serializer throws "GuidRepresentation is Unspecified" in newer MongoDB drivers.
+/// This implementation directly serializes/deserializes GUIDs as strings without the GuidRepresentation validation.
+/// </remarks>
+internal sealed class StringGuidSerializer : StructSerializerBase<Guid>
+{
+    /// <inheritdoc/>
+    public override Guid Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
+    {
+        var bsonType = context.Reader.GetCurrentBsonType();
+
+        return bsonType switch
+        {
+            BsonType.String => Guid.Parse(context.Reader.ReadString()),
+            BsonType.Binary => context.Reader.ReadBinaryData().ToGuid(),
+            BsonType.Null => throw new FormatException("Cannot deserialize null to Guid"),
+            _ => throw new FormatException($"Cannot deserialize BsonType {bsonType} to Guid")
+        };
+    }
+
+    /// <inheritdoc/>
+    public override void Serialize(BsonSerializationContext context, BsonSerializationArgs args, Guid value)
+    {
+        context.Writer.WriteString(value.ToString());
+    }
+}
diff --git a/src/RecurringThings.MongoDB/Documents/RecurringThingDocument.cs b/src/RecurringThings.MongoDB/Documents/RecurringThingDocument.cs
index 22f8b44..e45f3d7 100644
--- a/src/RecurringThings.MongoDB/Documents/RecurringThingDocument.cs
+++ b/src/RecurringThings.MongoDB/Documents/RecurringThingDocument.cs
@@ -2,8 +2,6 @@ namespace RecurringThings.MongoDB.Documents;
 
 using System;
 using System.Collections.Generic;
-using global::MongoDB.Bson;
-using global::MongoDB.Bson.Serialization.Attributes;
 using RecurringThings.Domain;
 using RecurringThings.Options;
 
@@ -19,16 +17,18 @@ namespace RecurringThings.MongoDB.Documents;
 /// - "override": A modified occurrence from a recurrence
 /// </para>
 /// <para>
-/// Fields are mapped to camelCase for MongoDB conventions.
+/// Serialization is configured via conventions (no attributes required):
+/// - The Id property is mapped as the document _id using NamedIdMemberConvention
+/// - All Guid properties are serialized as strings using GuidStringRepresentationConvention
+/// - Fields are mapped to camelCase using CamelCaseElementNameConvention
+/// - Null properties are automatically ignored using IgnoreIfNullConvention
 /// </para>
 /// </remarks>
 public sealed class RecurringThingDocument
 {
     /// <summary>
-    /// Gets or sets the unique identifier.
+    /// Gets or sets the unique identifier (maps to MongoDB _id field).
     /// </summary>
-    [BsonId]
-    [BsonRepresentation(BsonType.String)]
     public Guid Id { get; set; }
 
     /// <summary>
@@ -55,7 +55,6 @@ public sealed class RecurringThingDocument
     /// <remarks>
     /// Present on recurrences and occurrences. Null for exceptions and overrides.
     /// </remarks>
-    [BsonIgnoreIfNull]
     public string? Type { get; set; }
 
     /// <summary>
@@ -64,7 +63,6 @@ public sealed class RecurringThingDocument
     /// <remarks>
     /// Present on recurrences, occurrences, and overrides. Null for exceptions.
     /// </remarks>
-    [BsonIgnoreIfNull]
     public DateTime? StartTime { get; set; }
 
     /// <summary>
@@ -74,7 +72,6 @@ public sealed class RecurringThingDocument
     /// Present on occurrences and overrides. Null for recurrences and exceptions.
     /// Computed as StartTime + Duration.
     /// </remarks>
-    [BsonIgnoreIfNull]
     public DateTime? EndTime { get; set; }
 
     /// <summary>
@@ -84,7 +81,6 @@ public sealed class RecurringThingDocument
     /// Stored as milliseconds for efficient MongoDB operations.
     /// Present on recurrences, occurrences, and overrides. Null for exceptions.
     /// </remarks>
-    [BsonIgnoreIfNull]
     public long? DurationMs { get; set; }
 
     /// <summary>
@@ -93,7 +89,6 @@ public sealed class RecurringThingDocument
     /// <remarks>
     /// Present on recurrences and occurrences. Null for exceptions and overrides.
     /// </remarks>
-    [BsonIgnoreIfNull]
     public string? TimeZone { get; set; }
 
     /// <summary>
@@ -102,7 +97,6 @@ public sealed class RecurringThingDocument
     /// <remarks>
     /// Present only on recurrences.
     /// </remarks>
-    [BsonIgnoreIfNull]
     public DateTime? RecurrenceEndTime { get; set; }
 
     /// <summary>
@@ -111,7 +105,6 @@ public sealed class RecurringThingDocument
     /// <remarks>
     /// Present only on recurrences.
     /// </remarks>
-    [BsonIgnoreIfNull]
     public string? RRule { get; set; }
 
     /// <summary>
@@ -126,7 +119,6 @@ public sealed class RecurringThingDocument
     /// monthly patterns with day &lt;= 28, or patterns where no months are affected).
     /// </para>
     /// </remarks>
-    [BsonIgnoreIfNull]
     public string? MonthDayBehavior { get; set; }
 
     /// <summary>
@@ -135,8 +127,6 @@ public sealed class RecurringThingDocument
     /// <remarks>
     /// Present on exceptions and overrides.
     /// </remarks>
-    [BsonRepresentation(BsonType.String)]
-    [BsonIgnoreIfNull]
     public Guid? RecurrenceId { get; set; }
 
     /// <summary>
@@ -145,7 +135,6 @@ public sealed class RecurringThingDocument
     /// <remarks>
     /// Present on exceptions and overrides.
     /// </remarks>
-    [BsonIgnoreIfNull]
     public DateTime? OriginalTimeUtc { get; set; }
 
     /// <summary>
@@ -155,7 +144,6 @@ public sealed class RecurringThingDocument
     /// Denormalized from the parent recurrence at creation time.
     /// Present only on overrides.
     /// </remarks>
-    [BsonIgnoreIfNull]
     public long? OriginalDurationMs { get; set; }
 
     /// <summary>
@@ -165,13 +153,11 @@ public sealed class RecurringThingDocument
     /// Denormalized from the parent recurrence at creation time.
     /// Present only on overrides.
     /// </remarks>
-    [BsonIgnoreIfNull]
     public Dictionary<string, string>? OriginalExtensions { get; set; }
 
     /// <summary>
     /// Gets or sets the user-defined key-value metadata.
     /// </summary>
-    [BsonIgnoreIfNull]
     public Dictionary<string, string>? Extensions { get; set; }
 }
 
diff --git a/src/RecurringThings.MongoDB/Indexing/IndexManager.cs b/src/RecurringThings.MongoDB/Indexing/IndexManager.cs
index 1d7239b..6c89328 100644
--- a/src/RecurringThings.MongoDB/Indexing/IndexManager.cs
+++ b/src/RecurringThings.MongoDB/Indexing/IndexManager.cs
@@ -5,6 +5,7 @@ namespace RecurringThings.MongoDB.Indexing;
 using System.Threading;
 using System.Threading.Tasks;
 using global::MongoDB.Driver;
+using RecurringThings.MongoDB.Configuration;
 using RecurringThings.MongoDB.Documents;
 
 /// <summary>
@@ -40,6 +41,7 @@ public sealed class IndexManager
     public IndexManager(IMongoDatabase database, string collectionName = "recurring_things")
     {
         ArgumentNullException.ThrowIfNull(database);
+        MongoDbInitializer.EnsureInitialized();
         _collection = database.GetCollection<RecurringThingDocument>(collectionName);
     }
 
diff --git a/src/RecurringThings.MongoDB/Repositories/MongoOccurrenceExceptionRepository.cs b/src/RecurringThings.MongoDB/Repositories/MongoOccurrenceExceptionRepository.cs
index a4bc3c1..6f9b6ac 100644
--- a/src/RecurringThings.MongoDB/Repositories/MongoOccurrenceExceptionRepository.cs
+++ b/src/RecurringThings.MongoDB/Repositories/MongoOccurrenceExceptionRepository.cs
@@ -8,6 +8,7 @@ namespace RecurringThings.MongoDB.Repositories;
 using System.Threading.Tasks;
 using global::MongoDB.Driver;
 using RecurringThings.Domain;
+using RecurringThings.MongoDB.Configuration;
 using RecurringThings.MongoDB.Documents;
 using RecurringThings.Repository;
 using Transactional.Abstractions;
@@ -28,6 +29,7 @@ internal sealed class MongoOccurrenceExceptionRepository : IOccurrenceExceptionR
     public MongoOccurrenceExceptionRepository(IMongoDatabase database, string collectionName = "recurring_things")
     {
         ArgumentNullException.ThrowIfNull(database);
+        MongoDbInitializer.EnsureInitialized();
         _collection = database.GetCollection<RecurringThingDocument>(collectionName);
     }
 
diff --git a/src/RecurringThings.MongoDB/Repositories/MongoOccurrenceOverrideRepository.cs b/src/RecurringThings.MongoDB/Repositories/MongoOccurrenceOverrideRepository.cs
index 6b479be..a8266d6 100644
--- a/src/RecurringThings.MongoDB/Repositories/MongoOccurrenceOverrideRepository.cs
+++ b/src/RecurringThings.MongoDB/Repositories/MongoOccurrenceOverrideRepository.cs
@@ -8,6 +8,7 @@ namespace RecurringThings.MongoDB.Repositories;
 using System.Threading.Tasks;
 using global::MongoDB.Driver;
 using RecurringThings.Domain;
+using RecurringThings.MongoDB.Configuration;
 using RecurringThings.MongoDB.Documents;
 using RecurringThings.Repository;
 using Transactional.Abstractions;
@@ -28,6 +29,7 @@ internal sealed class MongoOccurrenceOverrideRepository : IOccurrenceOverrideRep
     public MongoOccurrenceOverrideRepository(IMongoDatabase database, string collectionName = "recurring_things")
     {
         ArgumentNullException.ThrowIfNull(database);
+        MongoDbInitializer.EnsureInitialized();
         _collection = database.GetCollection<RecurringThingDocument>(collectionName);
     }
 
diff --git a/src/RecurringThings.MongoDB/Repositories/MongoOccurrenceRepository.cs b/src/RecurringThings.MongoDB/Repositories/MongoOccurrenceRepository.cs
index 061937d..3f9c5c6 100644
--- a/src/RecurringThings.MongoDB/Repositories/MongoOccurrenceRepository.cs
+++ b/src/RecurringThings.MongoDB/Repositories/MongoOccurrenceRepository.cs
@@ -7,6 +7,7 @@ namespace RecurringThings.MongoDB.Repositories;
 using System.Threading.Tasks;
 using global::MongoDB.Driver;
 using RecurringThings.Domain;
+using RecurringThings.MongoDB.Configuration;
 using RecurringThings.MongoDB.Documents;
 using RecurringThings.Repository;
 using Transactional.Abstractions;
@@ -27,6 +28,7 @@ internal sealed class MongoOccurrenceRepository : IOccurrenceRepository
     public MongoOccurrenceRepository(IMongoDatabase database, string collectionName = "recurring_things")
     {
         ArgumentNullException.ThrowIfNull(database);
+        MongoDbInitializer.EnsureInitialized();
         _collection = database.GetCollection<RecurringThingDocument>(collectionName);
     }
 
diff --git a/src/RecurringThings.MongoDB/Repositories/MongoRecurrenceRepository.cs b/src/RecurringThings.MongoDB/Repositories/MongoRecurrenceRepository.cs
index 578daea..7549ee8 100644
--- a/src/RecurringThings.MongoDB/Repositories/MongoRecurrenceRepository.cs
+++ b/src/RecurringThings.MongoDB/Repositories/MongoRecurrenceRepository.cs
@@ -7,6 +7,7 @@ namespace RecurringThings.MongoDB.Repositories;
 using System.Threading.Tasks;
 using global::MongoDB.Driver;
 using RecurringThings.Domain;
+using RecurringThings.MongoDB.Configuration;
 using RecurringThings.MongoDB.Documents;
 using RecurringThings.Repository;
 using Transactional.Abstractions;
@@ -27,6 +28,7 @@ internal sealed class MongoRecurrenceRepository : IRecurrenceRepository
     public MongoRecurrenceRepository(IMongoDatabase database, string collectionName = "recurring_things")
     {
         ArgumentNullException.ThrowIfNull(database);
+        MongoDbInitializer.EnsureInitialized();
         _collection = database.GetCollection<RecurringThingDocument>(collectionName);
     }