diff --git a/src/Persistence/MartenTests/MultiTenancy/blue_green_version_bump_assignment.cs b/src/Persistence/MartenTests/MultiTenancy/blue_green_version_bump_assignment.cs
index 5ce04f44a..7ad55cec3 100644
--- a/src/Persistence/MartenTests/MultiTenancy/blue_green_version_bump_assignment.cs
+++ b/src/Persistence/MartenTests/MultiTenancy/blue_green_version_bump_assignment.cs
@@ -217,12 +217,11 @@ private static List<Uri> AgentsFor(IEnumerable<Uri> agents, string projectionNam
         agents.Where(uri => uri.Segments.Any(segment =>
             segment.Trim('/').Equals(projectionName, StringComparison.OrdinalIgnoreCase))).ToList();
 
-    // The (type, name, databaseId) prefix of an agent URI, mirroring the internal
-    // EventSubscriptionAgentFamily.DatabaseKeyOf that group affinity keys on.
-    private static string DatabaseKeyOf(Uri uri) =>
-        uri.Segments.Length >= 3
-            ? $"{uri.Host}/{uri.Segments[1].Trim('/')}/{uri.Segments[2].Trim('/')}"
-            : uri.AbsoluteUri;
+    // Calls the real EventSubscriptionAgentFamily.DatabaseKeyOf rather than a local copy of it (GH-3819).
+    // This used to reimplement the URI grammar verbatim, which is exactly the drift risk that made the
+    // method public: a copy asserting co-location can quietly stop matching the key the distribution
+    // really groups on, and the test would keep passing while proving nothing.
+    private static string DatabaseKeyOf(Uri uri) => EventSubscriptionAgentFamily.DatabaseKeyOf(uri);
 }
 
 public record TripStarted(Guid Id, string Description);
diff --git a/src/Wolverine/Runtime/Agents/EventSubscriptionAgentFamily.cs b/src/Wolverine/Runtime/Agents/EventSubscriptionAgentFamily.cs
index e9444d293..555358638 100644
--- a/src/Wolverine/Runtime/Agents/EventSubscriptionAgentFamily.cs
+++ b/src/Wolverine/Runtime/Agents/EventSubscriptionAgentFamily.cs
@@ -321,10 +321,17 @@ private async ValueTask<HashSet<Uri>> RetireSupersededAgentsAsync(AssignmentGrid
         return DatabaseId.TryParse(uri.Segments[2].Trim('/'), out var id) ? id : null;
     }
 
-    // Agent URIs are event-subscriptions://{type}/{name}/{databaseId}/{shard...} (see UriFor); the
-    // (type, name, databaseId) prefix identifies the shard database an agent belongs to, so grouping on it
-    // co-locates every tenant/projection agent for one database on the same node.
-    internal static string DatabaseKeyOf(Uri uri)
+    /// <summary>
+    /// The (type, name, databaseId) prefix of an event-subscription agent URI — the key group affinity
+    /// distributes on, so every tenant/projection agent belonging to one shard database is co-located on
+    /// the same node. Agent URIs are <c>event-subscriptions://{type}/{name}/{databaseId}/{shard...}</c>
+    /// (see <see cref="UriFor"/>).
+    ///
+    /// <para>Public so that callers reasoning about agent placement per database — an admin view, a
+    /// readiness probe, a test asserting co-location — can use the same key the distribution actually
+    /// uses instead of re-implementing the URI grammar and silently drifting from it. See GH-3819.</para>
+    /// </summary>
+    public static string DatabaseKeyOf(Uri uri)
         => uri.Segments.Length >= 3
             ? $"{uri.Host}/{uri.Segments[1].Trim('/')}/{uri.Segments[2].Trim('/')}"
             : uri.AbsoluteUri;
@@ -348,7 +355,7 @@ internal static string StoreKeyOf(Uri uri)
     /// else is a tenant. Returns null for a URI that doesn't parse as this scheme's grammar — such an
     /// agent is never treated as superseded.
     /// </summary>
-    internal static string? TenantNeutralKeyOf(Uri uri)
+    public static string? TenantNeutralKeyOf(Uri uri)
     {
         // Segments: "/", {storeName}, {databaseId}, {projectionName}, {shardKey}, [v{n} | tenant], [tenant]
         var segments = uri.Segments.Skip(1).Select(x => x.Trim('/')).Where(x => x.Length > 0).ToArray();