diff --git a/src/EventSourcingTests/Bugs/Bug_4197_fetch_for_writing_natural_key.cs b/src/EventSourcingTests/Bugs/Bug_4197_fetch_for_writing_natural_key.cs
new file mode 100644
index 0000000000..1d18149a40
--- /dev/null
+++ b/src/EventSourcingTests/Bugs/Bug_4197_fetch_for_writing_natural_key.cs
@@ -0,0 +1,95 @@
+using System;
+using System.Threading.Tasks;
+using JasperFx.Events.Aggregation;
+using Marten;
+using Marten.Events.Projections;
+using Marten.Testing.Harness;
+using Shouldly;
+using Xunit;
+
+namespace EventSourcingTests.Bugs;
+
+public class Bug_4197_fetch_for_writing_natural_key : OneOffConfigurationsContext
+{
+    // Types matching the user's repro
+    public sealed record Bug4197AggregateKey(string Value);
+
+    public sealed record Bug4197AggregateCreatedEvent(Guid Id, string Key);
+
+    public sealed class Bug4197Aggregate
+    {
+        public Guid Id { get; set; }
+
+        [NaturalKey]
+        public Bug4197AggregateKey Key { get; set; }
+
+        [NaturalKeySource]
+        public void Apply(Bug4197AggregateCreatedEvent e)
+        {
+            Id = e.Id;
+            Key = new Bug4197AggregateKey(e.Key);
+        }
+    }
+
+    [Fact]
+    public async Task fetch_for_writing_with_natural_key_without_explicit_projection_registration()
+    {
+        // This matches the user's repro: no explicit projection registration,
+        // just a self-aggregating type with [NaturalKey] and [NaturalKeySource].
+        // Marten should auto-discover the aggregate and its natural key.
+        StoreOptions(opts =>
+        {
+            // No explicit projection registration - relying on auto-discovery
+        });
+
+        // First call FetchForWriting to trigger auto-discovery, then apply schema
+        await using var session1 = theStore.LightweightSession();
+        var preCheck = await session1.Events.FetchForWriting<Bug4197Aggregate, Bug4197AggregateKey>(
+            new Bug4197AggregateKey("nonexistent"));
+        preCheck.Aggregate.ShouldBeNull();
+
+        // Now the projection is auto-registered, apply the schema
+        await theStore.Storage.ApplyAllConfiguredChangesToDatabaseAsync();
+
+        await using var session = theStore.LightweightSession();
+
+        var aggregateId = Guid.NewGuid();
+        var aggregateKey = new Bug4197AggregateKey("randomkeyvalue");
+        var e = new Bug4197AggregateCreatedEvent(aggregateId, aggregateKey.Value);
+
+        session.Events.StartStream<Bug4197Aggregate>(aggregateId, e);
+        await session.SaveChangesAsync();
+
+        // This should NOT throw: InvalidOperationException: Invalid identifier type for aggregate
+        var stream = await session.Events.FetchForWriting<Bug4197Aggregate, Bug4197AggregateKey>(aggregateKey);
+
+        stream.ShouldNotBeNull();
+        stream.Aggregate.ShouldNotBeNull();
+        stream.Aggregate.Key.ShouldBe(aggregateKey);
+    }
+
+    [Fact]
+    public async Task fetch_for_writing_with_natural_key_with_inline_snapshot()
+    {
+        // This is the working pattern from the existing tests
+        StoreOptions(opts =>
+        {
+            opts.Projections.Snapshot<Bug4197Aggregate>(SnapshotLifecycle.Inline);
+        });
+
+        await using var session = theStore.LightweightSession();
+
+        var aggregateId = Guid.NewGuid();
+        var aggregateKey = new Bug4197AggregateKey("randomkeyvalue");
+        var e = new Bug4197AggregateCreatedEvent(aggregateId, aggregateKey.Value);
+
+        session.Events.StartStream<Bug4197Aggregate>(aggregateId, e);
+        await session.SaveChangesAsync();
+
+        var stream = await session.Events.FetchForWriting<Bug4197Aggregate, Bug4197AggregateKey>(aggregateKey);
+
+        stream.ShouldNotBeNull();
+        stream.Aggregate.ShouldNotBeNull();
+        stream.Aggregate.Key.ShouldBe(aggregateKey);
+    }
+}
diff --git a/src/Marten/Events/EventStore.FetchForWriting.cs b/src/Marten/Events/EventStore.FetchForWriting.cs
index 54c11ce275..c6122439c2 100644
--- a/src/Marten/Events/EventStore.FetchForWriting.cs
+++ b/src/Marten/Events/EventStore.FetchForWriting.cs
@@ -2,6 +2,8 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Linq;
+using System.Reflection;
 using System.Threading;
 using System.Threading.Tasks;
 using ImTools;
@@ -11,6 +13,8 @@
 using JasperFx.Events;
 using JasperFx.Events.Aggregation;
 using JasperFx.Events.Projections;
+using Marten.Events.Fetching;
+using Marten.Events.Projections;
 using Marten.Internal;
 using Marten.Internal.Sessions;
 using Marten.Internal.Storage;
@@ -246,6 +250,10 @@ private IAggregateFetchPlan<TDoc, TId> determineFetchPlan<TDoc, TId>(StoreOption
         // before attempting the cast to IEventIdentityStrategy<TId>
         if (typeof(TId) != typeof(Guid) && typeof(TId) != typeof(string))
         {
+            // Auto-discover natural key from [NaturalKey] attribute on the aggregate type
+            // BEFORE iterating planners, so the projection is registered and available
+            tryAutoRegisterNaturalKeyProjection<TDoc, TId>(options);
+
             foreach (var planner in options.Projections.allPlanners())
             {
                 // Pass null identity - natural key planners don't use it
@@ -269,6 +277,34 @@ private IAggregateFetchPlan<TDoc, TId> determineFetchPlan<TDoc, TId>(StoreOption
         throw new InvalidOperationException(
             $"Unable to determine a fetch plan for aggregate {typeof(TDoc).FullNameInCode()}. Is there a valid single stream aggregation projection for this type?");
     }
+
+    /// <summary>
+    /// Auto-discovers a natural key from [NaturalKey] attribute on the aggregate type
+    /// and registers an Inline snapshot projection if no projection exists yet.
+    /// This enables FetchForWriting with natural keys on self-aggregating types
+    /// without requiring explicit projection registration.
+    /// </summary>
+    private static void tryAutoRegisterNaturalKeyProjection<TDoc, TId>(StoreOptions options)
+        where TDoc : class where TId : notnull
+    {
+        // Skip if a projection is already registered for this aggregate type
+        if (options.Projections.TryFindAggregate(typeof(TDoc), out _))
+        {
+            return;
+        }
+
+        var naturalKeyProp = typeof(TDoc).GetProperties(BindingFlags.Public | BindingFlags.Instance)
+            .FirstOrDefault(p => p.GetCustomAttribute<NaturalKeyAttribute>() != null);
+
+        if (naturalKeyProp == null || naturalKeyProp.PropertyType != typeof(TId))
+        {
+            return;
+        }
+
+        // Register an Inline snapshot projection so the natural key infrastructure
+        // (natural key table, inline projection, NaturalKeyFetchPlanner) all activate
+        options.Projections.Snapshot<TDoc>(SnapshotLifecycle.Inline);
+    }
 }
 
 public interface IAggregateFetchPlan<TDoc, in TId> where TDoc : notnull