diff --git a/Directory.Build.props b/Directory.Build.props
index cd12396..808f39e 100644
--- a/Directory.Build.props
+++ b/Directory.Build.props
@@ -26,7 +26,12 @@
              type, and graph-level HttpGraphUsage.MiddlewareTypes — the same operator-facing pipeline
              information is available on demand via the chain's generated source code, so the descriptor
              copies were redundant payload (~30-50 KB compressed per ~163-endpoint topology). -->
-        <JasperFxVersion>2.6.1</JasperFxVersion>
+        <!-- 2.7.0: auto-assign dynamic-tenant abstraction (#413, split from #409). The auto-assign
+             IDynamicTenantSource<T>.AddTenantAsync(tenantId, CancellationToken) overload now returns
+             Task<string> — the resolved database id / partition suffix — so pooled/sharded tenancy models
+             (Marten ShardedTenancy, marten#4598) report where an auto-assigned tenant landed. Default still
+             throws NotSupportedException (additive). DynamicTenancyAdminExtensions surfaces the resolved id. -->
+        <JasperFxVersion>2.7.0</JasperFxVersion>
         <LangVersion>13</LangVersion>
         <NoWarn>1570;1571;1572;1573;1574;1587;1591;1701;1702;1711;1735;0618</NoWarn>
         <Authors>Jeremy D. Miller;Jaedyn Tonee</Authors>
diff --git a/src/CoreTests/MultiTenancy/dynamic_tenancy_admin_surface.cs b/src/CoreTests/MultiTenancy/dynamic_tenancy_admin_surface.cs
index 33f972a..a0a583c 100644
--- a/src/CoreTests/MultiTenancy/dynamic_tenancy_admin_surface.cs
+++ b/src/CoreTests/MultiTenancy/dynamic_tenancy_admin_surface.cs
@@ -20,8 +20,9 @@ public async Task auto_assign_overload_throws_by_default()
     public async Task auto_assign_overload_is_used_when_a_source_supports_it()
     {
         var source = new AutoAssignTenantSource();
-        await ((IDynamicTenantSource<string>)source).AddTenantAsync("acme");
+        var resolved = await ((IDynamicTenantSource<string>)source).AddTenantAsync("acme");
         source.AutoAssigned.ShouldBe(["acme"]);
+        resolved.ShouldBe("shard-acme"); // resolved database id / partition suffix
     }
 
     [Fact]
@@ -41,9 +42,10 @@ public async Task auto_assign_add_dispatches_to_registered_sources()
         var source = new AutoAssignTenantSource();
         var services = provider(source);
 
-        await services.AddTenantAsync("acme");
+        var resolved = await services.AddTenantAsync("acme");
 
         source.AutoAssigned.ShouldBe(["acme"]);
+        resolved.ShouldBe("shard-acme");
     }
 
     [Fact]
@@ -87,7 +89,7 @@ public async Task is_a_graceful_no_op_when_no_dynamic_source_is_registered()
 
         // None of these should throw with no source registered
         await services.AddTenantAsync("acme", "Host=db1");
-        await services.AddTenantAsync("acme");
+        (await services.AddTenantAsync("acme")).ShouldBeNull(); // no source -> no resolved assignment
         await services.DisableTenantAsync("acme");
         await services.EnableTenantAsync("acme");
         await services.RemoveTenantAsync("acme");
@@ -167,10 +169,11 @@ internal class AutoAssignTenantSource : ValueOnlyTenantSource, IDynamicTenantSou
 {
     public List<string> AutoAssigned { get; } = new();
 
-    public Task AddTenantAsync(string tenantId, CancellationToken token = default)
+    public Task<string> AddTenantAsync(string tenantId, CancellationToken token = default)
     {
         AutoAssigned.Add(tenantId);
-        return Task.CompletedTask;
+        // Simulate the assignment strategy resolving the tenant onto a shard / partition.
+        return Task.FromResult($"shard-{tenantId}");
     }
 }
 
diff --git a/src/JasperFx/MultiTenancy/DynamicTenancyAdminExtensions.cs b/src/JasperFx/MultiTenancy/DynamicTenancyAdminExtensions.cs
index a2ad8e2..a3449a3 100644
--- a/src/JasperFx/MultiTenancy/DynamicTenancyAdminExtensions.cs
+++ b/src/JasperFx/MultiTenancy/DynamicTenancyAdminExtensions.cs
@@ -32,11 +32,22 @@ public static Task AddTenantAsync(this IServiceProvider services, string tenantI
     /// Add a tenant whose connection/partition is auto-assigned by the source (sharded/partitioned style;
     /// no caller-supplied value). Dispatched to every registered dynamic tenant source — a source that
     /// requires a connection value will surface <see cref="NotSupportedException" /> from its default
-    /// <see cref="IDynamicTenantSource{T}.AddTenantAsync(string,CancellationToken)" />.
+    /// <see cref="IDynamicTenantSource{T}.AddTenantAsync(string,CancellationToken)" />. Returns the
+    /// resolved assignment (database id / partition suffix) from the first source that provisioned the
+    /// tenant, or <see langword="null" /> when no dynamic source is registered.
     /// </summary>
-    public static Task AddTenantAsync(this IServiceProvider services, string tenantId,
+    public static async Task<string?> AddTenantAsync(this IServiceProvider services, string tenantId,
         CancellationToken token = default)
-        => forEachSource(services, source => source.AddTenantAsync(tenantId, token));
+    {
+        string? resolved = null;
+        foreach (var source in services.DynamicTenantSources())
+        {
+            var assignment = await source.AddTenantAsync(tenantId, token).ConfigureAwait(false);
+            resolved ??= assignment;
+        }
+
+        return resolved;
+    }
 
     /// <summary>
     /// Disable (soft delete) a tenant across every registered dynamic tenant source.
@@ -91,7 +102,7 @@ public static Task AddTenantAsync(this IHost host, string tenantId, string conne
         => host.Services.AddTenantAsync(tenantId, connectionValue);
 
     /// <summary><inheritdoc cref="AddTenantAsync(IServiceProvider,string,CancellationToken)" /></summary>
-    public static Task AddTenantAsync(this IHost host, string tenantId, CancellationToken token = default)
+    public static Task<string?> AddTenantAsync(this IHost host, string tenantId, CancellationToken token = default)
         => host.Services.AddTenantAsync(tenantId, token);
 
     /// <summary><inheritdoc cref="DisableTenantAsync(IServiceProvider,string)" /></summary>
diff --git a/src/JasperFx/MultiTenancy/IDynamicTenantSource.cs b/src/JasperFx/MultiTenancy/IDynamicTenantSource.cs
index feaf735..acaff5d 100644
--- a/src/JasperFx/MultiTenancy/IDynamicTenantSource.cs
+++ b/src/JasperFx/MultiTenancy/IDynamicTenantSource.cs
@@ -14,15 +14,16 @@ public interface IDynamicTenantSource<T> : ITenantedSource<T>
     /// <summary>
     /// Provision a new tenant whose connection/partition is auto-assigned by the source's configured
     /// strategy — e.g. sharded-database pooling or per-tenant event partitions — rather than a
-    /// caller-supplied connection value. The default implementation throws
-    /// <see cref="NotSupportedException" />; sources that auto-assign (e.g. Marten's sharded tenancy)
-    /// override it. This is the uniform "add a tenant" entrypoint for provisioning models that own the
-    /// physical assignment, so a tool such as CritterWatch never has to sniff the concrete tenancy type.
-    /// See jasperfx#409.
+    /// caller-supplied connection value. Returns the resolved assignment: the database id (sharded pool)
+    /// or partition suffix (managed partitions) the tenant landed on, so the caller (e.g. CritterWatch)
+    /// learns where it was placed without sniffing the concrete tenancy type. The default implementation
+    /// throws <see cref="NotSupportedException" />; auto-assign sources (e.g. Marten's sharded tenancy)
+    /// override it, while caller-supplies-value sources keep
+    /// <see cref="AddTenantAsync(string,T)" />. See jasperfx#413 (split from #409).
     /// </summary>
-    Task AddTenantAsync(string tenantId, CancellationToken token = default)
+    Task<string> AddTenantAsync(string tenantId, CancellationToken token = default)
         => throw new NotSupportedException(
-            $"This tenant source ({GetType().FullName}) requires a caller-supplied connection value; call AddTenantAsync(tenantId, connectionValue) instead, or use a source that supports auto-assignment.");
+            $"This tenant source ({GetType().FullName}) does not support auto-assignment; call AddTenantAsync(tenantId, connectionValue) with a caller-supplied connection value instead.");
 
     /// <summary>
     /// Disable a tenant (soft delete). The tenant data is preserved but