diff --git a/Directory.Packages.props b/Directory.Packages.props
index 7706c9bb..7811596e 100644
--- a/Directory.Packages.props
+++ b/Directory.Packages.props
@@ -22,8 +22,8 @@
              IEventDatabase.QueryDeadLetterEventsAsync (dead-letter row drill-in), DeadLetterEvent.TenantId,
              and the tenant-aware daemon surface. Polecat implements the dead-letter row query + per-tenant
              FetchDeadLetterCountsAsync below. -->
-        <PackageVersion Include="JasperFx" Version="2.20.0" />
-        <PackageVersion Include="JasperFx.Events" Version="2.20.0" />
+        <PackageVersion Include="JasperFx" Version="2.23.0" />
+        <PackageVersion Include="JasperFx.Events" Version="2.23.0" />
         <!-- Pin the sibling JasperFx packages for parity with Marten 9's matrix
              even though Polecat doesn't currently reference them directly —
              keeps the lockstep matrix coherent when transitive resolution
diff --git a/src/Polecat.Tests/rebuild_concurrency_cap_resolution.cs b/src/Polecat.Tests/rebuild_concurrency_cap_resolution.cs
new file mode 100644
index 00000000..d7941f25
--- /dev/null
+++ b/src/Polecat.Tests/rebuild_concurrency_cap_resolution.cs
@@ -0,0 +1,82 @@
+using JasperFx;
+using JasperFx.Events;
+using Microsoft.Data.SqlClient;
+
+namespace Polecat.Tests;
+
+/// <summary>
+/// jasperfx#420 / marten#4710 companion: resolution of the per-database rebuild
+/// concurrency cap surfaced through <c>IEventStore.MaxConcurrentRebuildsPerDatabase</c>.
+/// Pure unit tests — the pool-size derivation parses the connection string via
+/// <see cref="SqlConnectionStringBuilder"/> without opening a connection.
+/// </summary>
+public class rebuild_concurrency_cap_resolution
+{
+    private const string DummyConnectionString =
+        "Server=localhost;Database=rebuild_cap;Integrated Security=true;TrustServerCertificate=true";
+
+    private static DocumentStore buildStore(string connectionString, Action<StoreOptions>? configure = null)
+    {
+        var options = new StoreOptions
+        {
+            ConnectionString = connectionString,
+            AutoCreateSchemaObjects = AutoCreate.None,
+        };
+        configure?.Invoke(options);
+        return new DocumentStore(options);
+    }
+
+    [Fact]
+    public void configured_value_wins_over_derived_default()
+    {
+        using var store = buildStore(DummyConnectionString,
+            opts => opts.DaemonSettings.MaxConcurrentRebuildsPerDatabase = 3);
+        ((IEventStore)store).MaxConcurrentRebuildsPerDatabase.ShouldBe(3);
+    }
+
+    [Fact]
+    public void non_positive_configured_value_disables_the_cap()
+    {
+        using var store = buildStore(DummyConnectionString,
+            opts => opts.DaemonSettings.MaxConcurrentRebuildsPerDatabase = 0);
+        ((IEventStore)store).MaxConcurrentRebuildsPerDatabase.ShouldBeNull();
+    }
+
+    [Fact]
+    public void derived_default_is_pool_size_over_eight_with_floor_of_one()
+    {
+        var connectionString = new SqlConnectionStringBuilder(DummyConnectionString)
+        {
+            MaxPoolSize = 64
+        }.ConnectionString;
+
+        using var store = buildStore(connectionString);
+        ((IEventStore)store).MaxConcurrentRebuildsPerDatabase.ShouldBe(8);
+    }
+
+    [Fact]
+    public void derived_default_floors_at_one_for_tiny_pools()
+    {
+        var connectionString = new SqlConnectionStringBuilder(DummyConnectionString)
+        {
+            MaxPoolSize = 5
+        }.ConnectionString;
+
+        using var store = buildStore(connectionString);
+        ((IEventStore)store).MaxConcurrentRebuildsPerDatabase.ShouldBe(1);
+    }
+
+    [Fact]
+    public async Task usage_descriptor_carries_the_effective_cap()
+    {
+        // jasperfx#434: CritterWatch#309's rebuild dispatcher reads the effective cap
+        // off the EventStoreUsage descriptor rather than guessing.
+        using var store = buildStore(DummyConnectionString,
+            opts => opts.DaemonSettings.MaxConcurrentRebuildsPerDatabase = 6);
+
+        var usage = await ((IEventStore)store).TryCreateUsage(CancellationToken.None);
+
+        usage.ShouldNotBeNull();
+        usage!.MaxConcurrentRebuildsPerDatabase.ShouldBe(6);
+    }
+}
diff --git a/src/Polecat/DocumentStore.EventStore.cs b/src/Polecat/DocumentStore.EventStore.cs
index e3c92ddf..ab90a1be 100644
--- a/src/Polecat/DocumentStore.EventStore.cs
+++ b/src/Polecat/DocumentStore.EventStore.cs
@@ -62,6 +62,35 @@ public partial class DocumentStore : IEventStore<IDocumentSession, IQuerySession
     // Type stays the provider ("SqlServer"). See polecat#207.
     EventStoreIdentity IEventStore.Identity => new(Options.StoreName.ToLowerInvariant(), "SqlServer");
 
+    // jasperfx#420 / marten#4710 companion: the resolved per-database rebuild concurrency cap read
+    // by the CLI rebuild path (ProjectionHost.TryRebuildShardsAsync). Explicit configuration on
+    // StoreOptions.DaemonSettings wins; otherwise derive max(1, MaxPoolSize / 8) from the SqlClient
+    // connection pool so a wide store cannot exhaust the pool during a rebuild while leaving
+    // headroom for application traffic. Falls back to null (the historical unbounded fan-out)
+    // when no pool signal is reachable.
+    int? IEventStore.MaxConcurrentRebuildsPerDatabase
+    {
+        get
+        {
+            if (Options.DaemonSettings.MaxConcurrentRebuildsPerDatabase.HasValue)
+            {
+                var configured = Options.DaemonSettings.MaxConcurrentRebuildsPerDatabase.Value;
+                return configured > 0 ? configured : null;
+            }
+
+            try
+            {
+                using var connection = Database.CreateStorageConnection();
+                var poolSize = new SqlConnectionStringBuilder(connection.ConnectionString).MaxPoolSize;
+                return Math.Max(1, poolSize / 8);
+            }
+            catch (Exception)
+            {
+                return null;
+            }
+        }
+    }
+
     Uri IEventStore.Subject => Database.DatabaseUri;
 
     // Store-agnostic accessor for every IEventDatabase backing this store (jasperfx#387).
@@ -213,6 +242,10 @@ Task IEventStore.CompactStreamAsync(string streamKey, CancellationToken token)
         daemon.AddValue(nameof(Options.DaemonSettings.DaemonLockId), Options.DaemonSettings.DaemonLockId);
         usage.Children["DaemonSettings"] = daemon;
 
+        // jasperfx#434: surface the effective rebuild cap so monitoring tools (CritterWatch#309's
+        // rebuild dispatcher) can size their orchestration off the wire.
+        usage.MaxConcurrentRebuildsPerDatabase = ((IEventStore)this).MaxConcurrentRebuildsPerDatabase;
+
         // OpenTelemetry child
         var otel = new OptionsDescription { Subject = "Polecat.OpenTelemetryOptions" };
         otel.AddValue(nameof(Options.OpenTelemetry.TrackConnections), Options.OpenTelemetry.TrackConnections);