diff --git a/internal/incusplacement/starlarkexec/execute.go b/internal/incusplacement/starlarkexec/execute.go
index 4980a59b..2ddff4c5 100644
--- a/internal/incusplacement/starlarkexec/execute.go
+++ b/internal/incusplacement/starlarkexec/execute.go
@@ -53,7 +53,11 @@ type MemberSnapshot struct {
 }
 
 type InstanceSnapshot struct {
-	Name           string
+	Name string
+	// Project is the Incus project that owns the instance. Empty means the
+	// project passed to get_instances / get_instances_count, so existing
+	// fleet-worker fixtures stay in the queried project.
+	Project        string
 	MemoryLimitMiB int
 }
 
@@ -162,8 +166,9 @@ func Execute(script string, request PlacementRequest, cluster ClusterSnapshot) (
 		if err != nil {
 			return nil, err
 		}
-		instances := make([]api.Instance, 0, len(member.Instances))
-		for _, instance := range member.Instances {
+		filtered := instancesForProject(member, project)
+		instances := make([]api.Instance, 0, len(filtered))
+		for _, instance := range filtered {
 			instances = append(instances, api.Instance{
 				Name: instance.Name, Project: project, Location: location,
 				ExpandedConfig: api.ConfigMap{
@@ -185,7 +190,7 @@ func Execute(script string, request PlacementRequest, cluster ClusterSnapshot) (
 		if err != nil {
 			return nil, err
 		}
-		count := len(member.Instances)
+		count := len(instancesForProject(member, project))
 		if includePending {
 			count = member.PendingCount
 		}
@@ -269,3 +274,20 @@ func lookupMember(members map[string]MemberSnapshot, name string) (MemberSnapsho
 	}
 	return member, nil
 }
+
+func instancesForProject(member MemberSnapshot, project string) []InstanceSnapshot {
+	if project == "" {
+		return append([]InstanceSnapshot(nil), member.Instances...)
+	}
+	matched := make([]InstanceSnapshot, 0, len(member.Instances))
+	for _, instance := range member.Instances {
+		instanceProject := instance.Project
+		if instanceProject == "" {
+			instanceProject = project
+		}
+		if instanceProject == project {
+			matched = append(matched, instance)
+		}
+	}
+	return matched
+}
diff --git a/internal/incusplacement/starlarkexec/execute_test.go b/internal/incusplacement/starlarkexec/execute_test.go
index fae5e060..4e0054b4 100644
--- a/internal/incusplacement/starlarkexec/execute_test.go
+++ b/internal/incusplacement/starlarkexec/execute_test.go
@@ -121,6 +121,23 @@ func TestWrongProjectIsANoOp(t *testing.T) {
 	}
 }
 
+func TestForeignProjectInstancesDoNotConsumeFleetRAM(t *testing.T) {
+	script := renderExample(t)
+	cluster := fourEmptyMembers()
+	cluster.Members[0].Instances = []InstanceSnapshot{
+		{Name: "other-1", Project: "not-fleet", MemoryLimitMiB: 8192},
+		{Name: "other-2", Project: "not-fleet", MemoryLimitMiB: 8192},
+	}
+	cluster.Members[0].PendingCount = 0
+	outcome, err := Execute(script, eightGiBRequest(), cluster)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if outcome.Failed || outcome.Target != "gha-runner-1" {
+		t.Fatalf("other-project occupancy filled gha-fleet RAM: %#v", outcome)
+	}
+}
+
 func renderExample(t *testing.T) string {
 	t.Helper()
 	_, current, _, ok := runtime.Caller(0)