diff --git a/docs/platforms/frontier.rst b/docs/platforms/frontier.rst
index a57ffadd9..ef2251b32 100644
--- a/docs/platforms/frontier.rst
+++ b/docs/platforms/frontier.rst
@@ -64,10 +64,9 @@ Now grab an interactive session on one node::
 
 Then in the session run::
 
-    python run_libe_forces.py --nworkers 9
+    python run_libe_forces.py --nworkers 8
 
-This places the generator on the first worker and runs simulations on the
-others (each simulation using one GPU).
+The workers will each run simulations with one GPU each.
 
 To see GPU usage, ssh into the node you are on in another window and run::
 
diff --git a/docs/platforms/perlmutter.rst b/docs/platforms/perlmutter.rst
index a1c79703f..7f6098657 100644
--- a/docs/platforms/perlmutter.rst
+++ b/docs/platforms/perlmutter.rst
@@ -96,10 +96,9 @@ Now grab an interactive session on one node::
 Then in the session run::
 
     export LIBE_PLATFORM="perlmutter_g"
-    python run_libe_forces.py -n 5
+    python run_libe_forces.py -n 4
 
-This places the generator on the first worker and runs simulations on the
-others (each simulation using one GPU).
+The workers will each run simualations with one GPU each.
 
 To see GPU usage, ssh into the node you are on in another window and run::
 
diff --git a/docs/tutorials/xopt_bayesian_gen.rst b/docs/tutorials/xopt_bayesian_gen.rst
index 9227ac8ce..14e3b50b9 100644
--- a/docs/tutorials/xopt_bayesian_gen.rst
+++ b/docs/tutorials/xopt_bayesian_gen.rst
@@ -52,7 +52,7 @@ Define the VOCS specification and set up the generator.
 
 .. code-block:: python
 
-    libE_specs = LibeSpecs(gen_on_manager=True, nworkers=4)
+    libE_specs = LibeSpecs(nworkers=4)
 
     vocs = VOCS(
         variables={"x1": [0, 1.0], "x2": [0, 10.0]},
diff --git a/examples/tutorials/gpcam_surrogate_model/gpcam.ipynb b/examples/tutorials/gpcam_surrogate_model/gpcam.ipynb
index 29616f582..53fdfd4a0 100644
--- a/examples/tutorials/gpcam_surrogate_model/gpcam.ipynb
+++ b/examples/tutorials/gpcam_surrogate_model/gpcam.ipynb
@@ -285,9 +285,9 @@
     "\n",
     "nworkers = 4\n",
     "\n",
-    "# When using gen_on_manager, nworkers is number of concurrent sims.\n",
+    "# nworkers is number of concurrent sims.\n",
     "# final_gen_send means the last evaluated points are returned to the generator to update the model.\n",
-    "libE_specs = LibeSpecs(nworkers=nworkers, gen_on_manager=True, final_gen_send=True)\n",
+    "libE_specs = LibeSpecs(nworkers=nworkers, final_gen_send=True)\n",
     "\n",
     "n = 2  # Input dimensions\n",
     "batch_size = 4\n",
@@ -400,7 +400,7 @@
     "import matplotlib\n",
     "import matplotlib.pyplot as plt\n",
     "\n",
-    "# Get \"mean_squared_error\" from generators return (worker 0 as we ran gen_on_manager)\n",
+    "# Get \"mean_squared_error\" from generators return\n",
     "mse = persis_info[0][\"mean_squared_error\"]\n",
     "niter = len(mse)\n",
     "num_sims = list(range(batch_size, (niter * batch_size) + 1, batch_size))\n",
diff --git a/examples/tutorials/xopt_bayesian_gen/xopt_EI_example.ipynb b/examples/tutorials/xopt_bayesian_gen/xopt_EI_example.ipynb
index bc27f41a2..140b961a4 100644
--- a/examples/tutorials/xopt_bayesian_gen/xopt_EI_example.ipynb
+++ b/examples/tutorials/xopt_bayesian_gen/xopt_EI_example.ipynb
@@ -98,7 +98,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "libE_specs = LibeSpecs(gen_on_manager=True, nworkers=4)\n",
+    "libE_specs = LibeSpecs(nworkers=4)\n",
     "\n",
     "vocs = VOCS(\n",
     "    variables={\"x1\": [0, 1.0], \"x2\": [0, 10.0]},\n",
diff --git a/libensemble/specs.py b/libensemble/specs.py
index 9ee04baa3..c95bdd8f8 100644
--- a/libensemble/specs.py
+++ b/libensemble/specs.py
@@ -2,6 +2,7 @@
 import warnings
 from pathlib import Path
 
+import numpy as np
 import pydantic
 from pydantic import BaseModel, ConfigDict, Field, field_validator, model_validator
 
@@ -354,6 +355,24 @@ def set_fields_from_vocs(self):
             if "_id" not in self.persis_in:
                 self.persis_in.append("_id")
 
+        # Set user["lb"]/["ub"] from VOCS continuous variables (for legacy generators
+        # that read bounds from gen_specs["user"]). Skip variables without a ``.domain``
+        # attribute (e.g., DiscreteVariable). Do not overwrite user-provided values.
+        if self.user is None:
+            self.user = {}
+        if "lb" not in self.user or "ub" not in self.user:
+            lbs, ubs = [], []
+            for _name, var in (getattr(self.vocs, "variables", None) or {}).items():
+                domain = getattr(var, "domain", None)
+                if domain is not None and len(domain) == 2:
+                    lbs.append(domain[0])
+                    ubs.append(domain[1])
+            if lbs:
+                if "lb" not in self.user:
+                    self.user["lb"] = np.array(lbs, dtype=float)
+                if "ub" not in self.user:
+                    self.user["ub"] = np.array(ubs, dtype=float)
+
         return self
 
     @model_validator(mode="after")
diff --git a/libensemble/tests/functionality_tests/test_1d_super_simple.py b/libensemble/tests/functionality_tests/test_1d_super_simple.py
index 1a178c2cf..657acd4ce 100644
--- a/libensemble/tests/functionality_tests/test_1d_super_simple.py
+++ b/libensemble/tests/functionality_tests/test_1d_super_simple.py
@@ -14,6 +14,7 @@
 # TESTSUITE_NPROCS: 2 4
 
 import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
 from libensemble.gen_funcs.sampling import latin_hypercube_sample as gen_f
@@ -38,14 +39,13 @@ def sim_f(In):
         "out": [("f", float)],
     }
 
+    vocs = VOCS(variables={"x0": [-3, 3]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": gen_f,
         "out": [("x", float, (1,))],
         "batch_size": 500,
-        "user": {
-            "lb": np.array([-3]),
-            "ub": np.array([3]),
-        },
+        "vocs": vocs,
     }
 
     exit_criteria = {"gen_max": 501}
diff --git a/libensemble/tests/functionality_tests/test_asktell_sampling.py b/libensemble/tests/functionality_tests/test_asktell_sampling.py
index f2b48547a..e4848ffa6 100644
--- a/libensemble/tests/functionality_tests/test_asktell_sampling.py
+++ b/libensemble/tests/functionality_tests/test_asktell_sampling.py
@@ -45,10 +45,6 @@ def sim_f(In):
         "out": [("x", float, (2,))],
         "initial_batch_size": 20,
         "batch_size": 10,
-        "user": {
-            "lb": np.array([-3, -2]),
-            "ub": np.array([3, 2]),
-        },
     }
 
     variables = {"x0": [-3, 3], "x1": [-2, 2]}
diff --git a/libensemble/tests/functionality_tests/test_calc_exception.py b/libensemble/tests/functionality_tests/test_calc_exception.py
index 55d99ca63..bc5f3bc4b 100644
--- a/libensemble/tests/functionality_tests/test_calc_exception.py
+++ b/libensemble/tests/functionality_tests/test_calc_exception.py
@@ -11,7 +11,7 @@
 # TESTSUITE_COMMS: mpi local tcp
 # TESTSUITE_NPROCS: 2 4
 
-import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
 from libensemble.gen_funcs.sampling import uniform_random_sample as gen_f
@@ -35,15 +35,14 @@ def six_hump_camel_err(H, persis_info, sim_specs, _):
         "out": [("f", float)],
     }
 
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": gen_f,
         "in": ["sim_id"],
         "out": [("x", float, 2)],
         "batch_size": 10,
-        "user": {
-            "lb": np.array([-3, -2]),
-            "ub": np.array([3, 2]),
-        },
+        "vocs": vocs,
     }
 
     alloc_specs = {
diff --git a/libensemble/tests/functionality_tests/test_cancel_in_alloc.py b/libensemble/tests/functionality_tests/test_cancel_in_alloc.py
index f0bcead55..3916395e4 100644
--- a/libensemble/tests/functionality_tests/test_cancel_in_alloc.py
+++ b/libensemble/tests/functionality_tests/test_cancel_in_alloc.py
@@ -18,6 +18,7 @@
 # TESTSUITE_NPROCS: 4
 
 import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
 from libensemble.gen_funcs.sampling import uniform_random_sample as gen_f
@@ -39,16 +40,15 @@
         "user": {"uniform_random_pause_ub": 10},
     }
 
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": gen_f,
         "in": ["sim_id"],
         "out": [("x", float, (2,))],
         "batch_size": 5,
         "num_active_gens": 1,
-        "user": {
-            "lb": np.array([-3, -2]),
-            "ub": np.array([3, 2]),
-        },
+        "vocs": vocs,
     }
 
     alloc_specs = {
@@ -62,7 +62,13 @@
     exit_criteria = {"sim_max": 10, "wallclock_max": 300}
 
     # Perform the run
-    H, persis_info, flag = libE(sim_specs, gen_specs, exit_criteria, libE_specs=libE_specs, alloc_specs=alloc_specs)
+    H, persis_info, flag = libE(
+        sim_specs,
+        gen_specs,
+        exit_criteria,
+        libE_specs=libE_specs,
+        alloc_specs=alloc_specs,
+    )
 
     if is_manager:
         test = np.any(H["cancel_requested"]) and np.any(H["kill_sent"])
diff --git a/libensemble/tests/functionality_tests/test_comms.py b/libensemble/tests/functionality_tests/test_comms.py
index daa9e564c..3d1c9a664 100644
--- a/libensemble/tests/functionality_tests/test_comms.py
+++ b/libensemble/tests/functionality_tests/test_comms.py
@@ -15,6 +15,7 @@
 # TESTSUITE_NPROCS: 2 4
 
 import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
 from libensemble.executors.mpi_executor import MPIExecutor  # Only used to get workerID in float_x1000
@@ -41,15 +42,14 @@
         "out": [("arr_vals", float, array_size), ("scal_val", float)],
     }
 
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": gen_f,
         "in": ["sim_id"],
         "out": [("x", float, (2,))],
         "batch_size": sim_max,
-        "user": {
-            "lb": np.array([-3, -2]),
-            "ub": np.array([3, 2]),
-        },
+        "vocs": vocs,
     }
 
     exit_criteria = {"sim_max": sim_max, "wallclock_max": 300}
diff --git a/libensemble/tests/functionality_tests/test_elapsed_time_abort.py b/libensemble/tests/functionality_tests/test_elapsed_time_abort.py
index 1396da50f..eb71fc952 100644
--- a/libensemble/tests/functionality_tests/test_elapsed_time_abort.py
+++ b/libensemble/tests/functionality_tests/test_elapsed_time_abort.py
@@ -13,7 +13,7 @@
 # TESTSUITE_COMMS: mpi local tcp
 # TESTSUITE_NPROCS: 2 4
 
-import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
 from libensemble.gen_funcs.sampling import uniform_random_sample as gen_f
@@ -34,16 +34,15 @@
         "user": {"pause_time": 2},
     }
 
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": gen_f,
         "in": ["sim_id"],
         "out": [("x", float, (2,))],
         "batch_size": 5,
         "num_active_gens": 2,
-        "user": {
-            "lb": np.array([-3, -2]),
-            "ub": np.array([3, 2]),
-        },
+        "vocs": vocs,
     }
 
     alloc_specs = {
diff --git a/libensemble/tests/functionality_tests/test_evaluate_existing_plus_gen.py b/libensemble/tests/functionality_tests/test_evaluate_existing_plus_gen.py
index 3e37bc86d..5da225427 100644
--- a/libensemble/tests/functionality_tests/test_evaluate_existing_plus_gen.py
+++ b/libensemble/tests/functionality_tests/test_evaluate_existing_plus_gen.py
@@ -15,6 +15,7 @@
 # TESTSUITE_NPROCS: 2 4
 
 import numpy as np
+from gest_api.vocs import VOCS
 
 # Import libEnsemble items for this test
 from libensemble import Ensemble
@@ -24,11 +25,8 @@
 from libensemble.specs import AllocSpecs, ExitCriteria, GenSpecs, SimSpecs
 
 
-def create_H0(gen_specs, H0_size):
+def create_H0(lb, ub, H0_size):
     """Create an H0 for give_pregenerated_sim_work"""
-    # Manually creating H0
-    ub = gen_specs["user"]["ub"]
-    lb = gen_specs["user"]["lb"]
     n = len(lb)
     b = H0_size
 
@@ -45,18 +43,19 @@ def create_H0(gen_specs, H0_size):
     sampling = Ensemble(parse_args=True)
     sampling.sim_specs = SimSpecs(sim_f=sim_f, inputs=["x"], out=[("f", float)])
 
-    gen_specs = {
-        "gen_f": gen_f,
-        "outputs": [("x", float, (2,))],
-        "batch_size": 50,
-        "user": {
-            "lb": np.array([-3, -3]),
-            "ub": np.array([3, 3]),
-        },
-    }
-    sampling.gen_specs = GenSpecs(**gen_specs)
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-3, 3]}, objectives={"f": "MINIMIZE"})
+    lb = np.array([-3, -3])
+    ub = np.array([3, 3])
+
+    sampling.gen_specs = GenSpecs(
+        gen_f=gen_f,
+        persis_in=["f"],
+        outputs=[("x", float, (2,))],
+        batch_size=50,
+        vocs=vocs,
+    )
     sampling.exit_criteria = ExitCriteria(sim_max=100)
-    sampling.H0 = create_H0(gen_specs, 50)
+    sampling.H0 = create_H0(lb, ub, 50)
     sampling.alloc_specs = AllocSpecs(alloc_f=give_sim_work_first)
     sampling.run()
 
diff --git a/libensemble/tests/functionality_tests/test_executor_forces_tutorial.py b/libensemble/tests/functionality_tests/test_executor_forces_tutorial.py
index d6b368b93..78393db20 100644
--- a/libensemble/tests/functionality_tests/test_executor_forces_tutorial.py
+++ b/libensemble/tests/functionality_tests/test_executor_forces_tutorial.py
@@ -1,8 +1,8 @@
 import os
 import sys
 
-import numpy as np
 from forces_simf import run_forces  # Sim func from current dir
+from gest_api.vocs import VOCS
 
 from libensemble import Ensemble
 from libensemble.executors import MPIExecutor
@@ -37,6 +37,8 @@
         outputs=[("energy", float)],
     )
 
+    vocs = VOCS(variables={"nparticles": [1000, 3000]}, objectives={"energy": "MINIMIZE"})
+
     ensemble.gen_specs = GenSpecs(
         gen_f=gen_f,
         inputs=[],  # No input when starting persistent generator
@@ -44,10 +46,7 @@
         outputs=[("x", float, (1,))],
         initial_batch_size=nsim_workers,
         async_return=False,
-        user={
-            "lb": np.array([1000]),  # min particles
-            "ub": np.array([3000]),  # max particles
-        },
+        vocs=vocs,
     )  # gen_specs_end_tag
 
     # Starts one persistent generator. Simulated values are returned in batch.
diff --git a/libensemble/tests/functionality_tests/test_executor_forces_tutorial_2.py b/libensemble/tests/functionality_tests/test_executor_forces_tutorial_2.py
index 2a6cda15b..a19bac4bb 100644
--- a/libensemble/tests/functionality_tests/test_executor_forces_tutorial_2.py
+++ b/libensemble/tests/functionality_tests/test_executor_forces_tutorial_2.py
@@ -1,8 +1,8 @@
 import os
 import sys
 
-import numpy as np
 from forces_simf import run_forces  # Sim func from current dir
+from gest_api.vocs import VOCS
 
 from libensemble import Ensemble, logger
 from libensemble.executors import MPIExecutor
@@ -39,6 +39,8 @@
         outputs=[("energy", float)],
     )
 
+    vocs = VOCS(variables={"nparticles": [1000, 3000]}, objectives={"energy": "MINIMIZE"})
+
     ensemble.gen_specs = GenSpecs(
         gen_f=gen_f,
         inputs=[],  # No input when starting persistent generator
@@ -46,10 +48,7 @@
         outputs=[("x", float, (1,))],
         initial_batch_size=nsim_workers,
         async_return=True,
-        user={
-            "lb": np.array([1000]),  # min particles
-            "ub": np.array([3000]),  # max particles
-        },
+        vocs=vocs,
     )  # gen_specs_end_tag
 
     # Starts one persistent generator. Simulated values are returned in batch.
diff --git a/libensemble/tests/functionality_tests/test_executor_hworld_pass_fail.py b/libensemble/tests/functionality_tests/test_executor_hworld_pass_fail.py
index a65462e0d..96058f748 100644
--- a/libensemble/tests/functionality_tests/test_executor_hworld_pass_fail.py
+++ b/libensemble/tests/functionality_tests/test_executor_hworld_pass_fail.py
@@ -13,6 +13,7 @@
 import os
 
 import numpy as np
+from gest_api.vocs import VOCS
 
 import libensemble.sim_funcs.six_hump_camel as six_hump_camel
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
@@ -73,15 +74,14 @@
         "user": {"cores": cores_per_task},
     }
 
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": gen_f,
         "in": ["sim_id"],
         "out": [("x", float, (2,))],
         "batch_size": nworkers,
-        "user": {
-            "lb": np.array([-3, -2]),
-            "ub": np.array([3, 2]),
-        },
+        "vocs": vocs,
     }
 
     # num sim_ended_count conditions in executor_hworld
diff --git a/libensemble/tests/functionality_tests/test_executor_hworld_timeout.py b/libensemble/tests/functionality_tests/test_executor_hworld_timeout.py
index 2a604007e..ef9e60618 100644
--- a/libensemble/tests/functionality_tests/test_executor_hworld_timeout.py
+++ b/libensemble/tests/functionality_tests/test_executor_hworld_timeout.py
@@ -13,6 +13,7 @@
 import os
 
 import numpy as np
+from gest_api.vocs import VOCS
 
 import libensemble.sim_funcs.six_hump_camel as six_hump_camel
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
@@ -75,15 +76,14 @@
         },
     }
 
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": gen_f,
         "in": ["sim_id"],
         "out": [("x", float, (2,))],
         "batch_size": nworkers,
-        "user": {
-            "lb": np.array([-3, -2]),
-            "ub": np.array([3, 2]),
-        },
+        "vocs": vocs,
     }
 
     alloc_specs = {
diff --git a/libensemble/tests/functionality_tests/test_mpi_runners_subnode_uneven.py b/libensemble/tests/functionality_tests/test_mpi_runners_subnode_uneven.py
index c179028ab..92b60ca58 100644
--- a/libensemble/tests/functionality_tests/test_mpi_runners_subnode_uneven.py
+++ b/libensemble/tests/functionality_tests/test_mpi_runners_subnode_uneven.py
@@ -11,7 +11,7 @@
 
 import sys
 
-import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble import logger
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
@@ -82,15 +82,14 @@
         "out": [("f", float)],
     }
 
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": gen_f,
         "in": ["sim_id"],
         "out": [("x", float, (n,))],
         "batch_size": 20,
-        "user": {
-            "lb": np.array([-3, -2]),
-            "ub": np.array([3, 2]),
-        },
+        "vocs": vocs,
     }
 
     alloc_specs = {"alloc_f": give_sim_work_first}
@@ -139,6 +138,12 @@
     }
 
     # Perform the run
-    H, _, flag = libE(sim_specs, gen_specs, exit_criteria, alloc_specs=alloc_specs, libE_specs=libE_specs)
+    H, _, flag = libE(
+        sim_specs,
+        gen_specs,
+        exit_criteria,
+        alloc_specs=alloc_specs,
+        libE_specs=libE_specs,
+    )
 
     # All asserts are in sim func
diff --git a/libensemble/tests/functionality_tests/test_mpi_runners_supernode_uneven.py b/libensemble/tests/functionality_tests/test_mpi_runners_supernode_uneven.py
index 428297d5d..f4187a670 100644
--- a/libensemble/tests/functionality_tests/test_mpi_runners_supernode_uneven.py
+++ b/libensemble/tests/functionality_tests/test_mpi_runners_supernode_uneven.py
@@ -8,7 +8,7 @@
    python test_mpi_runners_supernode_uneven.py --nworkers 5
 """
 
-import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble import logger
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
@@ -72,15 +72,14 @@
         "out": [("f", float)],
     }
 
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": gen_f,
         "in": [],
         "out": [("x", float, (n,))],
         "batch_size": 20,
-        "user": {
-            "lb": np.array([-3, -2]),
-            "ub": np.array([3, 2]),
-        },
+        "vocs": vocs,
     }
 
     exit_criteria = {"sim_max": (nsim_workers) * rounds}
@@ -134,6 +133,12 @@
     alloc_specs = {"alloc_f": give_sim_work_first}
 
     # Perform the run
-    H, _, flag = libE(sim_specs, gen_specs, exit_criteria, alloc_specs=alloc_specs, libE_specs=libE_specs)
+    H, _, flag = libE(
+        sim_specs,
+        gen_specs,
+        exit_criteria,
+        alloc_specs=alloc_specs,
+        libE_specs=libE_specs,
+    )
 
     # All asserts are in sim func
diff --git a/libensemble/tests/functionality_tests/test_mpi_warning.py b/libensemble/tests/functionality_tests/test_mpi_warning.py
index f58620b90..c2eafca37 100644
--- a/libensemble/tests/functionality_tests/test_mpi_warning.py
+++ b/libensemble/tests/functionality_tests/test_mpi_warning.py
@@ -14,7 +14,7 @@
 import os
 import time
 
-import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble import Ensemble, logger
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
@@ -33,14 +33,13 @@
     sampling = Ensemble()
     sampling.libE_specs.save_every_k_sims = 100
     sampling.sim_specs = SimSpecs(sim_f=sim_f, inputs=["x"], outputs=[("f", float)])
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2]}, objectives={"f": "MINIMIZE"})
+
     sampling.gen_specs = GenSpecs(
         gen_f=gen_f,
         outputs=[("x", float, 2)],
         batch_size=100,
-        user={
-            "lb": np.array([-3, -2]),
-            "ub": np.array([3, 2]),
-        },
+        vocs=vocs,
     )
     sampling.alloc_specs = AllocSpecs(alloc_f=give_sim_work_first)
 
diff --git a/libensemble/tests/functionality_tests/test_persistent_uniform_sampling_async.py b/libensemble/tests/functionality_tests/test_persistent_uniform_sampling_async.py
index 9dde0f620..b841c9165 100644
--- a/libensemble/tests/functionality_tests/test_persistent_uniform_sampling_async.py
+++ b/libensemble/tests/functionality_tests/test_persistent_uniform_sampling_async.py
@@ -19,6 +19,7 @@
 import sys
 
 import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble.gen_funcs.persistent_sampling import persistent_uniform as gen_f
 
@@ -44,16 +45,15 @@
         "user": {"uniform_random_pause_ub": 0.5},
     }
 
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": gen_f,
         "persis_in": ["f", "x", "sim_id"],
         "out": [("x", float, (n,))],
         "initial_batch_size": nworkers,
         "async_return": True,
-        "user": {
-            "lb": np.array([-3, -2]),
-            "ub": np.array([3, 2]),
-        },
+        "vocs": vocs,
     }
 
     exit_criteria = {"gen_max": 100, "wallclock_max": 300}
diff --git a/libensemble/tests/functionality_tests/test_sim_dirs_per_calc.py b/libensemble/tests/functionality_tests/test_sim_dirs_per_calc.py
index baa34b838..07fff2e42 100644
--- a/libensemble/tests/functionality_tests/test_sim_dirs_per_calc.py
+++ b/libensemble/tests/functionality_tests/test_sim_dirs_per_calc.py
@@ -16,7 +16,7 @@
 
 import os
 
-import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
 from libensemble.gen_funcs.sampling import uniform_random_sample as gen_f
@@ -52,14 +52,13 @@
         "out": [("f", float)],
     }
 
+    vocs = VOCS(variables={"x0": [-3, 3]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": gen_f,
         "out": [("x", float, (1,))],
         "batch_size": 20,
-        "user": {
-            "lb": np.array([-3]),
-            "ub": np.array([3]),
-        },
+        "vocs": vocs,
     }
 
     alloc_specs = {
diff --git a/libensemble/tests/functionality_tests/test_sim_dirs_per_worker.py b/libensemble/tests/functionality_tests/test_sim_dirs_per_worker.py
index 74c77252c..f75d847ad 100644
--- a/libensemble/tests/functionality_tests/test_sim_dirs_per_worker.py
+++ b/libensemble/tests/functionality_tests/test_sim_dirs_per_worker.py
@@ -16,7 +16,7 @@
 
 import os
 
-import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
 from libensemble.gen_funcs.sampling import uniform_random_sample as gen_f
@@ -51,14 +51,13 @@
         "out": [("f", float)],
     }
 
+    vocs = VOCS(variables={"x0": [-3, 3]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": gen_f,
         "out": [("x", float, (1,))],
         "batch_size": 20,
-        "user": {
-            "lb": np.array([-3]),
-            "ub": np.array([3]),
-        },
+        "vocs": vocs,
     }
 
     alloc_specs = {"alloc_f": give_sim_work_first}
diff --git a/libensemble/tests/functionality_tests/test_sim_dirs_with_exception.py b/libensemble/tests/functionality_tests/test_sim_dirs_with_exception.py
index b3189dc1a..697f4dab5 100644
--- a/libensemble/tests/functionality_tests/test_sim_dirs_with_exception.py
+++ b/libensemble/tests/functionality_tests/test_sim_dirs_with_exception.py
@@ -16,7 +16,7 @@
 
 import os
 
-import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
 from libensemble.gen_funcs.sampling import uniform_random_sample as gen_f
@@ -44,14 +44,13 @@
         "out": [("f", float)],
     }
 
+    vocs = VOCS(variables={"x0": [-3, 3]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": gen_f,
         "out": [("x", float, (1,))],
         "batch_size": 20,
-        "user": {
-            "lb": np.array([-3]),
-            "ub": np.array([3]),
-        },
+        "vocs": vocs,
     }
 
     alloc_specs = {
diff --git a/libensemble/tests/functionality_tests/test_sim_input_dir_option.py b/libensemble/tests/functionality_tests/test_sim_input_dir_option.py
index 4e58d27d6..b7b2c64f2 100644
--- a/libensemble/tests/functionality_tests/test_sim_input_dir_option.py
+++ b/libensemble/tests/functionality_tests/test_sim_input_dir_option.py
@@ -16,7 +16,7 @@
 
 import os
 
-import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
 from libensemble.gen_funcs.sampling import uniform_random_sample as gen_f
@@ -47,14 +47,13 @@
         "out": [("f", float)],
     }
 
+    vocs = VOCS(variables={"x0": [-3, 3]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": gen_f,
         "out": [("x", float, (1,))],
         "batch_size": 20,
-        "user": {
-            "lb": np.array([-3]),
-            "ub": np.array([3]),
-        },
+        "vocs": vocs,
     }
 
     exit_criteria = {"sim_max": 21}
diff --git a/libensemble/tests/functionality_tests/test_uniform_sampling.py b/libensemble/tests/functionality_tests/test_uniform_sampling.py
index 4d8a6baa7..1c7005bee 100644
--- a/libensemble/tests/functionality_tests/test_uniform_sampling.py
+++ b/libensemble/tests/functionality_tests/test_uniform_sampling.py
@@ -18,6 +18,7 @@
 import os
 
 import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
 from libensemble.gen_funcs.sampling import uniform_random_sample
@@ -45,14 +46,13 @@
     }
     # end_sim_specs_rst_tag
 
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": uniform_random_sample,  # Function generating sim_f input
         "out": [("x", float, (2,))],  # Tell libE gen_f output, type, size
         "batch_size": 500,
-        "user": {
-            "lb": np.array([-3, -2]),  # Used by this specific gen_f
-            "ub": np.array([3, 2]),  # Used by this specific gen_f
-        },
+        "vocs": vocs,
     }
     # end_gen_specs_rst_tag
 
diff --git a/libensemble/tests/functionality_tests/test_worker_exceptions.py b/libensemble/tests/functionality_tests/test_worker_exceptions.py
index efdba0ec4..296d7ead3 100644
--- a/libensemble/tests/functionality_tests/test_worker_exceptions.py
+++ b/libensemble/tests/functionality_tests/test_worker_exceptions.py
@@ -14,7 +14,7 @@
 # TESTSUITE_COMMS: mpi local tcp
 # TESTSUITE_NPROCS: 2 4
 
-import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
 from libensemble.gen_funcs.sampling import uniform_random_sample as gen_f
@@ -34,15 +34,14 @@
         "out": [("f", float)],
     }
 
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": gen_f,
         "in": [],
         "out": [("x", float, 2)],
-        "user": {
-            "lb": np.array([-3, -2]),
-            "ub": np.array([3, 2]),
-            "initial_sample": 100,
-        },
+        "vocs": vocs,
+        "user": {"initial_sample": 100},
     }
 
     libE_specs["abort_on_exception"] = False
diff --git a/libensemble/tests/functionality_tests/test_workflow_dir.py b/libensemble/tests/functionality_tests/test_workflow_dir.py
index 6502b78ed..d5ec5fc6c 100644
--- a/libensemble/tests/functionality_tests/test_workflow_dir.py
+++ b/libensemble/tests/functionality_tests/test_workflow_dir.py
@@ -16,7 +16,7 @@
 
 import os
 
-import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
 from libensemble.gen_funcs.sampling import uniform_random_sample as gen_f
@@ -49,14 +49,13 @@
         "out": [("f", float)],
     }
 
+    vocs = VOCS(variables={"x0": [-3, 3]}, objectives={"f": "MINIMIZE"})
+
     gen_specs = {
         "gen_f": gen_f,
         "out": [("x", float, (1,))],
         "batch_size": 20,
-        "user": {
-            "lb": np.array([-3]),
-            "ub": np.array([3]),
-        },
+        "vocs": vocs,
     }
 
     alloc_specs = {
diff --git a/libensemble/tests/regression_tests/test_1d_sampling.py b/libensemble/tests/regression_tests/test_1d_sampling.py
index 456b3ca60..009518e63 100644
--- a/libensemble/tests/regression_tests/test_1d_sampling.py
+++ b/libensemble/tests/regression_tests/test_1d_sampling.py
@@ -13,7 +13,7 @@
 # TESTSUITE_COMMS: mpi local threads tcp
 # TESTSUITE_NPROCS: 3 4
 
-import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble import Ensemble
 from libensemble.gen_funcs.persistent_sampling import persistent_uniform
@@ -26,15 +26,14 @@
     sampling = Ensemble(parse_args=True)
     sampling.libE_specs = LibeSpecs(save_every_k_gens=300, safe_mode=False, disable_log_files=True)
     sampling.sim_specs = SimSpecs(sim_f=sim_f, inputs=["x"], outputs=[("f", float)])
+    vocs = VOCS(variables={"x0": [-3, 3]}, objectives={"f": "MINIMIZE"})
+
     sampling.gen_specs = GenSpecs(
         gen_f=persistent_uniform,
         persis_in=["f"],
         outputs=[("x", float, (1,))],
         initial_batch_size=100,
-        user={
-            "lb": np.array([-3]),
-            "ub": np.array([3]),
-        },
+        vocs=vocs,
     )
 
     sampling.exit_criteria = ExitCriteria(sim_max=500)
diff --git a/libensemble/tests/regression_tests/test_2d_sampling.py b/libensemble/tests/regression_tests/test_2d_sampling.py
index c26a66201..27862952c 100644
--- a/libensemble/tests/regression_tests/test_2d_sampling.py
+++ b/libensemble/tests/regression_tests/test_2d_sampling.py
@@ -14,6 +14,7 @@
 # TESTSUITE_NPROCS: 2 4
 
 import numpy as np
+from gest_api.vocs import VOCS
 
 from libensemble import Ensemble
 from libensemble.alloc_funcs.give_sim_work_first import give_sim_work_first
@@ -28,14 +29,13 @@
     sampling = Ensemble(parse_args=True)
     sampling.libE_specs = LibeSpecs(save_every_k_sims=100)
     sampling.sim_specs = SimSpecs(sim_f=sim_f, inputs=["x"], outputs=[("f", float)])
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2]}, objectives={"f": "MINIMIZE"})
+
     sampling.gen_specs = GenSpecs(
         gen_f=gen_f,
         outputs=[("x", float, 2)],
         batch_size=100,
-        user={
-            "lb": np.array([-3, -2]),
-            "ub": np.array([3, 2]),
-        },
+        vocs=vocs,
     )
 
     sampling.alloc_specs = AllocSpecs(alloc_f=give_sim_work_first)
diff --git a/libensemble/tests/regression_tests/test_asktell_gpCAM.py b/libensemble/tests/regression_tests/test_asktell_gpCAM.py
index f59fc135d..ca4ca3ef9 100644
--- a/libensemble/tests/regression_tests/test_asktell_gpCAM.py
+++ b/libensemble/tests/regression_tests/test_asktell_gpCAM.py
@@ -56,10 +56,6 @@
         "persis_in": ["x", "f", "sim_id"],
         "out": [("x", float, (n,))],
         "batch_size": batch_size,
-        "user": {
-            "lb": np.array([-3, -2, -1, -1]),
-            "ub": np.array([3, 2, 1, 1]),
-        },
     }
 
     vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2], "x2": [-1, 1], "x3": [-1, 1]}, objectives={"f": "MINIMIZE"})
diff --git a/libensemble/tests/regression_tests/test_xopt_EI_initial_sample.py b/libensemble/tests/regression_tests/test_xopt_EI_initial_sample.py
index d89de9449..116ffeb13 100644
--- a/libensemble/tests/regression_tests/test_xopt_EI_initial_sample.py
+++ b/libensemble/tests/regression_tests/test_xopt_EI_initial_sample.py
@@ -39,10 +39,9 @@ def xtest_sim(H, persis_info, sim_specs, _):
 
 
 if __name__ == "__main__":
-
     batch_size = 4
 
-    libE_specs = LibeSpecs(gen_on_manager=True, nworkers=batch_size)
+    libE_specs = LibeSpecs(nworkers=batch_size)
     libE_specs.reuse_output_dir = True
 
     vocs = VOCS(
diff --git a/libensemble/tests/regression_tests/test_xopt_EI_initial_sample_instance.py b/libensemble/tests/regression_tests/test_xopt_EI_initial_sample_instance.py
index c7db6d363..28a66b076 100644
--- a/libensemble/tests/regression_tests/test_xopt_EI_initial_sample_instance.py
+++ b/libensemble/tests/regression_tests/test_xopt_EI_initial_sample_instance.py
@@ -41,10 +41,9 @@ def xtest_sim(H, persis_info, sim_specs, _):
 
 
 if __name__ == "__main__":
-
     batch_size = 4
 
-    libE_specs = LibeSpecs(gen_on_manager=True, nworkers=batch_size)
+    libE_specs = LibeSpecs(nworkers=batch_size)
     libE_specs.reuse_output_dir = True
 
     vocs = VOCS(
diff --git a/libensemble/tests/unit_tests/test_ensemble.py b/libensemble/tests/unit_tests/test_ensemble.py
index 0e5de3223..3ea2abccb 100644
--- a/libensemble/tests/unit_tests/test_ensemble.py
+++ b/libensemble/tests/unit_tests/test_ensemble.py
@@ -270,6 +270,97 @@ def test_ready_happy_path():
     assert issues == [], f"Issues should be empty but got: {issues}"
 
 
+def test_gen_specs_vocs_populates_user_bounds():
+    """GenSpecs should populate user['lb']/['ub'] from VOCS continuous variables."""
+    from gest_api.vocs import VOCS
+
+    from libensemble.specs import GenSpecs
+
+    vocs = VOCS(
+        variables={"x0": [-3, 3], "x1": [-2, 2], "x2": [-1, 1], "x3": [-1, 1]},
+        objectives={"f": "MINIMIZE"},
+    )
+    gs = GenSpecs(vocs=vocs)
+    assert "lb" in gs.user, "lb should be populated in user from VOCS"
+    assert "ub" in gs.user, "ub should be populated in user from VOCS"
+    assert isinstance(gs.user["lb"], np.ndarray), "lb should be a numpy array"
+    assert isinstance(gs.user["ub"], np.ndarray), "ub should be a numpy array"
+    assert np.array_equal(gs.user["lb"], np.array([-3, -2, -1, -1]))
+    assert np.array_equal(gs.user["ub"], np.array([3, 2, 1, 1]))
+
+
+def test_gen_specs_vocs_does_not_overwrite_user_bounds():
+    """GenSpecs should not overwrite user-provided lb/ub when vocs is also given."""
+    from gest_api.vocs import VOCS
+
+    from libensemble.specs import GenSpecs
+
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2]}, objectives={"f": "MINIMIZE"})
+    explicit_lb = np.array([0.0, 0.0])
+    explicit_ub = np.array([1.0, 1.0])
+    gs = GenSpecs(vocs=vocs, user={"lb": explicit_lb, "ub": explicit_ub})
+    assert np.array_equal(gs.user["lb"], explicit_lb), "Explicit lb should be preserved"
+    assert np.array_equal(gs.user["ub"], explicit_ub), "Explicit ub should be preserved"
+
+
+def test_gen_specs_vocs_partial_user_bounds():
+    """GenSpecs should fill in only the missing one of lb/ub if user supplies just one."""
+    from gest_api.vocs import VOCS
+
+    from libensemble.specs import GenSpecs
+
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2]}, objectives={"f": "MINIMIZE"})
+    explicit_lb = np.array([0.0, 0.0])
+    gs = GenSpecs(vocs=vocs, user={"lb": explicit_lb})
+    assert np.array_equal(gs.user["lb"], explicit_lb), "Explicit lb should be preserved"
+    assert "ub" in gs.user, "ub should be populated from VOCS"
+    assert np.array_equal(gs.user["ub"], np.array([3, 2]))
+
+
+def test_gen_specs_no_vocs_leaves_user_empty():
+    """Without VOCS, GenSpecs.user should remain empty by default."""
+    from libensemble.specs import GenSpecs
+
+    gs = GenSpecs(outputs=[("x", float, (1,))])
+    assert "lb" not in gs.user, "lb should not be auto-populated without VOCS"
+    assert "ub" not in gs.user, "ub should not be auto-populated without VOCS"
+
+
+def test_gen_specs_vocs_satisfies_legacy_user_params():
+    """VOCS-populated user bounds should satisfy legacy gen_f consumers like
+    persistent_uniform (which require lb/ub to be numpy arrays and uses len(lb)
+    for dimension)."""
+    from gest_api.vocs import VOCS
+
+    from libensemble.gen_funcs.persistent_sampling import _get_user_params
+    from libensemble.specs import GenSpecs
+
+    vocs = VOCS(variables={"x0": [-3, 3], "x1": [-2, 2]}, objectives={"f": "MINIMIZE"})
+    gs = GenSpecs(vocs=vocs, initial_batch_size=10)
+
+    # Convert to dict shape that _get_user_params expects
+    gs_dict = {"initial_batch_size": gs.initial_batch_size, "user": gs.user}
+    b, n, lb, ub = _get_user_params(gs_dict["user"], gs_dict)
+    assert b == 10
+    assert n == 2
+    assert isinstance(lb, np.ndarray) and lb.dtype == float
+    assert isinstance(ub, np.ndarray) and ub.dtype == float
+    assert np.array_equal(lb, np.array([-3.0, -2.0]))
+    assert np.array_equal(ub, np.array([3.0, 2.0]))
+
+
+def test_gen_specs_vocs_integer_domain_yields_float_array():
+    """Integer-valued VOCS domains should still produce float dtype lb/ub arrays."""
+    from gest_api.vocs import VOCS
+
+    from libensemble.specs import GenSpecs
+
+    vocs = VOCS(variables={"x0": [0, 10], "x1": [-5, 5]}, objectives={"f": "MINIMIZE"})
+    gs = GenSpecs(vocs=vocs)
+    assert gs.user["lb"].dtype == float, "lb should be float dtype even for integer-domain variables"
+    assert gs.user["ub"].dtype == float, "ub should be float dtype even for integer-domain variables"
+
+
 if __name__ == "__main__":
     test_ensemble_init()
     test_ensemble_parse_args_false()
@@ -283,3 +374,9 @@ def test_ready_happy_path():
     test_ready_missing_nworkers_local()
     test_ready_field_mismatch()
     test_ready_happy_path()
+    test_gen_specs_vocs_populates_user_bounds()
+    test_gen_specs_vocs_does_not_overwrite_user_bounds()
+    test_gen_specs_vocs_partial_user_bounds()
+    test_gen_specs_no_vocs_leaves_user_empty()
+    test_gen_specs_vocs_satisfies_legacy_user_params()
+    test_gen_specs_vocs_integer_domain_yields_float_array()