diff --git a/backend/controller/controller.go b/backend/controller/controller.go
index 82a0a4ae3e..cac382ba58 100644
--- a/backend/controller/controller.go
+++ b/backend/controller/controller.go
@@ -60,6 +60,7 @@ type Config struct {
 	RunnerTimeout                time.Duration       `help:"Runner heartbeat timeout." default:"10s"`
 	DeploymentReservationTimeout time.Duration       `help:"Deployment reservation timeout." default:"120s"`
 	ArtefactChunkSize            int                 `help:"Size of each chunk streamed to the client." default:"1048576"`
+	IdleRunners                  int                 `help:"Number of idle runners to keep around (not supported in production)." default:"1"`
 }
 
 func (c *Config) SetDefaults() {
@@ -787,7 +788,7 @@ func (s *Service) reconcileDeployments(ctx context.Context) (time.Duration, erro
 			deploymentLogger.Debugf("Need %d more runners for %s", require, reconcile.Deployment)
 			wg.Go(func(ctx context.Context) error {
 				if err := s.deploy(ctx, deployment); err != nil {
-					deploymentLogger.Debugf("Failed to increase deployment replicas: %s", err)
+					deploymentLogger.Errorf(err, "Failed to increase deployment replicas")
 				} else {
 					deploymentLogger.Debugf("Reconciled %s to %d/%d replicas", reconcile.Deployment, reconcile.AssignedReplicas+1, reconcile.RequiredReplicas)
 					if reconcile.AssignedReplicas+1 == reconcile.RequiredReplicas {
@@ -825,7 +826,7 @@ func (s *Service) reconcileRunners(ctx context.Context) (time.Duration, error) {
 		return 0, fmt.Errorf("%s: %w", "failed to get deployments needing reconciliation", err)
 	}
 
-	totalRunners := 0
+	totalRunners := s.config.IdleRunners
 	for _, deployment := range activeDeployments {
 		totalRunners += deployment.MinReplicas
 	}
diff --git a/backend/controller/scaling/localscaling/local_scaling.go b/backend/controller/scaling/localscaling/local_scaling.go
index c60ab080f0..e3d9e33585 100644
--- a/backend/controller/scaling/localscaling/local_scaling.go
+++ b/backend/controller/scaling/localscaling/local_scaling.go
@@ -2,7 +2,6 @@ package localscaling
 
 import (
 	"context"
-	"encoding/binary"
 	"errors"
 	"fmt"
 	"net/url"
@@ -80,6 +79,7 @@ func (l *LocalScaling) SetReplicas(ctx context.Context, replicas int, idleRunner
 			Bind:               l.portAllocator.Next(),
 			ControllerEndpoint: controllerEndpoint,
 			TemplateDir:        templateDir(ctx),
+			Key:                model.NewRunnerKey(),
 		}
 
 		name := fmt.Sprintf("runner%d", i)
@@ -90,15 +90,6 @@ func (l *LocalScaling) SetReplicas(ctx context.Context, replicas int, idleRunner
 			return err
 		}
 
-		// Create a readable ULID for the runner.
-		var ulid [16]byte
-		binary.BigEndian.PutUint32(ulid[10:], uint32(len(l.runners)+1))
-		ulidStr := fmt.Sprintf("%025X", ulid)
-		err := config.Key.Scan(ulidStr)
-		if err != nil {
-			return err
-		}
-
 		runnerCtx := log.ContextWithLogger(ctx, logger.Scope(name))
 
 		runnerCtx, cancel := context.WithCancel(runnerCtx)
diff --git a/cmd/ftl/cmd_serve.go b/cmd/ftl/cmd_serve.go
index 817efa92ff..f1df245b1f 100644
--- a/cmd/ftl/cmd_serve.go
+++ b/cmd/ftl/cmd_serve.go
@@ -40,6 +40,7 @@ type serveCmd struct {
 	Background     bool          `help:"Run in the background." default:"false"`
 	Stop           bool          `help:"Stop the running FTL instance. Can be used to --background to restart the server" default:"false"`
 	StartupTimeout time.Duration `help:"Timeout for the server to start up." default:"20s"`
+	IdleRunners    int           `help:"Number of idle runners to keep around (not supported in production)." default:"1"`
 }
 
 const ftlContainerName = "ftl-db-1"
@@ -100,6 +101,7 @@ func (s *serveCmd) Run(ctx context.Context) error {
 			DSN:          dsn,
 			AllowOrigins: s.AllowOrigins,
 			NoConsole:    s.NoConsole,
+			IdleRunners:  s.IdleRunners,
 		}
 		if err := kong.ApplyDefaults(&config); err != nil {
 			return err