diff --git a/.gitignore b/.gitignore
index 83cfc3f..dd76b50 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,6 +5,7 @@ wandb/
 outputs/
 multirun/
 metrics/
+adjacency_debug.txt
 
 # Python-generated files
 __pycache__/
diff --git a/README.md b/README.md
index 572fd73..e9cabc6 100644
--- a/README.md
+++ b/README.md
@@ -13,6 +13,21 @@ To set up the UV module, you can run the following command:
 uv sync --frozen
 ```
 
+## Repository Structure
+
+```text
+.
+├── configs/                # Hydra configuration files (YAML)
+├── docs/                   # Comprehensive documentation and API guides
+├── runs/                   # Default output directory for Hydra and training artifacts
+├── scripts/                # High-level entrypoints for training, simulation, and evaluation
+├── src/
+│   └── brittle_star_project/ # Core library and environment logic
+│       ├── evaluation/     # Checkpoint evaluation, rollout logic, and metrics persistence
+│       └── trainers/       # Training loop implementations (e.g., PPO)
+└── tests/                  # Unit and integration tests
+```
+
 ## Usage
 
 For detailed instructions on how to use the project, please refer to the **[API Documentation](docs/README.md)**.
diff --git a/configs/centralized-final.yaml b/configs/centralized-final.yaml
new file mode 100644
index 0000000..f40570d
--- /dev/null
+++ b/configs/centralized-final.yaml
@@ -0,0 +1,71 @@
+# Custom Main Configuration
+#
+# Use with:
+#   uv run python scripts/train.py --config-name main_config_custom
+#
+# This keeps the project defaults intact while giving you a single custom
+# training entrypoint you can edit freely.
+
+defaults:
+  - brittle_star_config
+  - experiment: base
+  - logging: default
+  - evaluation: default
+  - ppo: default
+  - architecture: centralized
+  - morphology: 5_arms_full
+  - arena: default
+  - environment: directed_locomotion
+  - simulation: default
+  - _self_
+
+morphology:
+  morph_mode: CENTRALIZED
+
+experiment:
+  exp_name: "final-models/centralized/"
+  seed: 42
+  torch_deterministic: true
+  cuda: true
+
+logging:
+  track: true
+  save_model: true
+  save_checkpoints: true
+  upload_final_model: true
+  upload_checkpoints: true
+  checkpoint_frequency: 20
+  wandb_project_name: "final-models"
+
+evaluation:
+  evaluate_checkpoints: true
+  eval_max_steps: 2000
+  eval_seed: 0
+
+ppo:
+  learning_rate: 0.0001
+  total_timesteps: 16384000
+  num_envs: 128
+  num_steps: 64
+  anneal_lr: true
+  gamma: 0.99
+  gae_lambda: 0.95
+  num_minibatches: 32
+  update_epochs: 4
+  norm_adv: true
+  clip_coef: 0.2
+  clip_vloss: true
+  ent_coef: 0.001
+  vf_coef: 1.0
+  max_grad_norm: 0.5
+  target_kl: 0.02
+
+environment:
+  simulation_time: 100000.0
+  target_distance: 3.0
+
+hydra:
+  job:
+    chdir: true
+  run:
+    dir: ${experiment.base_run_dir}/${experiment.exp_name}/${now:%Y-%m-%d}/${now:%H-%M-%S}
diff --git a/configs/environment/directed_locomotion.yaml b/configs/environment/directed_locomotion.yaml
index 465d80e..b664ca1 100644
--- a/configs/environment/directed_locomotion.yaml
+++ b/configs/environment/directed_locomotion.yaml
@@ -2,11 +2,11 @@
 # Baseline task setting.
 
 task: DIRECTED_LOCOMOTION
-simulation_time: 5000.0
+simulation_time: 100000.0
 num_physics_steps_per_control_step: 10
 time_scale: 2
 camera_ids: [0, 1]
 render_size: [480, 640]
 joint_randomization_noise_scale: 0.0
-target_distance: 0.6
+target_distance: 3.0
 light_perlin_noise_scale: 0
diff --git a/configs/environment/light_escape.yaml b/configs/environment/light_escape.yaml
index c79cfb9..ade85ba 100644
--- a/configs/environment/light_escape.yaml
+++ b/configs/environment/light_escape.yaml
@@ -2,7 +2,7 @@
 # Advanced task requiring movement away from light source.
 
 task: LIGHT_ESCAPE
-simulation_time: 5.0
+simulation_time: 100000.0
 num_physics_steps_per_control_step: 10
 time_scale: 2
 camera_ids: [0, 1]
diff --git a/configs/evaluation/default.yaml b/configs/evaluation/default.yaml
index ac49e46..a7c2a00 100644
--- a/configs/evaluation/default.yaml
+++ b/configs/evaluation/default.yaml
@@ -3,6 +3,6 @@
 
 evaluate_checkpoints: false
 # Max number of control steps during evaluation rollout.
-eval_max_steps: 5000
+eval_max_steps: 2000
 # Seed for deterministic evaluation reset.
 eval_seed: 0
diff --git a/configs/evaluation/poster.yaml b/configs/evaluation/poster.yaml
new file mode 100644
index 0000000..0777cd1
--- /dev/null
+++ b/configs/evaluation/poster.yaml
@@ -0,0 +1,23 @@
+# @package evaluation
+# Configuration for the models used in the poster comparison.
+
+# Standard evaluation settings
+evaluate_checkpoints: false
+eval_max_steps: 5000
+eval_seed: 0
+
+# Cross-model comparison settings
+# We use 10 episodes to get a more robust average for the final poster results.
+comparison_base_seed: 0
+comparison_num_episodes: 2
+comparison_output_csv: "runs/evaluation/comparison.csv"
+
+# Paths to the .cleanrl_model files to be compared (relative to workspace root).
+comparison_models:
+  - "runs/input-space-2-arms/2026-05-02/08-14-58/final_model.flax"
+
+# Path to the morphologies to evaluate against.
+comparison_morphologies:
+  - "configs/morphology/5_arms_full.yaml"
+  - "configs/morphology/3_arms.yaml"
+  - "configs/morphology/2_arms.yaml"
diff --git a/configs/fully-connected-final.yaml b/configs/fully-connected-final.yaml
new file mode 100644
index 0000000..8d60451
--- /dev/null
+++ b/configs/fully-connected-final.yaml
@@ -0,0 +1,74 @@
+# Custom Main Configuration
+#
+# Use with:
+#   uv run python scripts/train.py --config-name main_config_custom
+#
+# This keeps the project defaults intact while giving you a single custom
+# training entrypoint you can edit freely.
+
+defaults:
+  - brittle_star_config
+  - experiment: base
+  - logging: default
+  - evaluation: default
+  - ppo: default
+  - architecture: decentralized
+  - morphology: 5_arms_full
+  - arena: default
+  - environment: directed_locomotion
+  - simulation: default
+  - _self_
+
+architecture:
+  topology_type: "fully_connected"
+
+morphology:
+  morph_mode: FULLY_CONNECTED
+
+experiment:
+  exp_name: "final-models/fully-connected/"
+  seed: 42
+  torch_deterministic: true
+  cuda: true
+
+logging:
+  track: true
+  save_model: true
+  save_checkpoints: true
+  upload_final_model: true
+  upload_checkpoints: true
+  checkpoint_frequency: 20
+  wandb_project_name: "final-models"
+
+evaluation:
+  evaluate_checkpoints: true
+  eval_max_steps: 2000
+  eval_seed: 0
+
+ppo:
+  learning_rate: 0.0001
+  total_timesteps: 16384000
+  num_envs: 128
+  num_steps: 64
+  anneal_lr: true
+  gamma: 0.99
+  gae_lambda: 0.95
+  num_minibatches: 32
+  update_epochs: 4
+  norm_adv: true
+  clip_coef: 0.2
+  clip_vloss: true
+  ent_coef: 0.001
+  vf_coef: 1.0
+  max_grad_norm: 0.5
+  target_kl: 0.02
+
+environment:
+  simulation_time: 100000.0
+  target_distance: 3.0
+
+hydra:
+  job:
+    chdir: true
+  run:
+    dir: ${experiment.base_run_dir}/${experiment.exp_name}/${now:%Y-%m-%d}/${now:%H-%M-%S}
diff --git a/configs/ring-final.yaml b/configs/ring-final.yaml
new file mode 100644
index 0000000..ffba64f
--- /dev/null
+++ b/configs/ring-final.yaml
@@ -0,0 +1,74 @@
+# Custom Main Configuration
+#
+# Use with:
+#   uv run python scripts/train.py --config-name main_config_custom
+#
+# This keeps the project defaults intact while giving you a single custom
+# training entrypoint you can edit freely.
+
+defaults:
+  - brittle_star_config
+  - experiment: base
+  - logging: default
+  - evaluation: default
+  - ppo: default
+  - architecture: decentralized
+  - morphology: 5_arms_full
+  - arena: default
+  - environment: directed_locomotion
+  - simulation: default
+  - _self_
+
+architecture:
+  topology_type: "ring"
+
+morphology:
+  morph_mode: RING
+
+experiment:
+  exp_name: "final-models/ring/"
+  seed: 42
+  torch_deterministic: true
+  cuda: true
+
+logging:
+  track: true
+  save_model: true
+  save_checkpoints: true
+  upload_final_model: true
+  upload_checkpoints: true
+  checkpoint_frequency: 20
+  wandb_project_name: "final-models"
+
+evaluation:
+  evaluate_checkpoints: true
+  eval_max_steps: 2000
+  eval_seed: 0
+
+ppo:
+  learning_rate: 0.0001
+  total_timesteps: 16384000
+  num_envs: 128
+  num_steps: 64
+  anneal_lr: true
+  gamma: 0.99
+  gae_lambda: 0.95
+  num_minibatches: 32
+  update_epochs: 4
+  norm_adv: true
+  clip_coef: 0.2
+  clip_vloss: true
+  ent_coef: 0.001
+  vf_coef: 1.0
+  max_grad_norm: 0.5
+  target_kl: 0.02
+
+environment:
+  simulation_time: 100000.0
+  target_distance: 3.0
+
+hydra:
+  job:
+    chdir: true
+  run:
+    dir: ${experiment.base_run_dir}/${experiment.exp_name}/${now:%Y-%m-%d}/${now:%H-%M-%S}
diff --git a/docs/api/evaluation.md b/docs/api/evaluation.md
new file mode 100644
index 0000000..e5f91d6
--- /dev/null
+++ b/docs/api/evaluation.md
@@ -0,0 +1,56 @@
+# Checkpoint & Model Evaluation
+
+This guide covers how to evaluate trained brittle star models, with a focus on measuring defect tolerance (amputations) across different controller architectures.
+
+## Checkpoint Evaluation (During Training)
+
+The `PPOTrainer` can automatically evaluate every saved checkpoint using the fast MJX backend. This is enabled via configuration.
+
+### Configuration
+
+In your experiment config or via CLI:
+```bash
+python scripts/train.py evaluation.evaluate_checkpoints=true evaluation.eval_max_steps=5000
+```
+
+Results are saved to `runs/<run_dir>/metrics/checkpoint_evaluation.csv` and synced to Weights & Biases if enabled.
+
+## Cross-Model & Defect Tolerance Analysis
+
+To measure how well different controllers handle damage (amputations), use `scripts/compare_models.py`. This script performs a grid search over models x morphologies.
+
+1. Create or update a YAML file in `configs/evaluation`.
+2. Run the benchmark:
+
+```bash
+python scripts/compare_models.py evaluation=poster
+```
+
+The script will evaluate every combination of model and morphology for the specified number of episodes.
+
+The results are saved to a CSV (default: `metrics/model_comparison.csv`).
+
+### CSV Schema
+
+| Column                | Description                                                  |
+|-----------------------|--------------------------------------------------------------|
+| `model_path`          | Path to the trained weights.                                 |
+| `architecture`        | The `morph_mode` of the model (e.g., `CENTRALIZED`, `RING`). |
+| `arm_0` ... `arm_4`   | Number of segments in each arm slot (0 = amputated).         |
+| `num_active_arms`     | Total number of arms with segments > 0.                      |
+| `seed`                | The episode seed.                                            |
+| `eval_return`         | Accumulated shaped reward.                                   |
+| `approx_max_velocity` | Average velocity: `(initial_dist - final_dist) / steps`.     |
+| `reached_target`      | Whether the robot finished within the success radius.        |
+
+## Post-hoc Checkpoint Scanning
+
+If you need to re-evaluate every saved checkpoint in a run (e.g., to generate a learning curve with different metrics):
+
+```bash
+python scripts/evaluate_checkpoints.py \
+    simulation.model_path=runs/<run_id>/final_model.flax \
+    evaluation.eval_max_steps=2000
+```
+
+This script scans the `checkpoints/` directory of the specified run and evaluates every `.flax` file it finds using the model's training morphology.
diff --git a/docs/api/simulation.md b/docs/api/simulation.md
index 0a0e041..1e288b9 100644
--- a/docs/api/simulation.md
+++ b/docs/api/simulation.md
@@ -37,3 +37,5 @@ uv run scripts/simulate.py \
 
 Videos and evaluation metadata are stored in timestamped folders alongside the model:
 `runs/your_run/final_model_evaluations/eval_<timestamp>/simulation.mp4`
+
+For batch evaluation and cross-model comparison, see the **[Evaluation Guide](./evaluation.md)**.
diff --git a/docs/api/training.md b/docs/api/training.md
index a9678bb..d3e6c51 100644
--- a/docs/api/training.md
+++ b/docs/api/training.md
@@ -38,12 +38,18 @@ To run with your custom experiment file:
 uv run python scripts/train.py experiment=my_experiment
 ```
 
-### Command-Line Overrides
-
-You can override any parameter directly from the command line using Hydra's dot notation. This is useful for quick tests:
-
 ```bash
 uv run python scripts/train.py ppo.learning_rate=0.001 ppo.num_envs=32 logging.track=true
 ```
 
+## Evaluation During Training
+
+By default, the trainer saves checkpoints but does not evaluate them. To enable automatic headless evaluation of every saved checkpoint, set `evaluation.evaluate_checkpoints=true`:
+
+```bash
+uv run python scripts/train.py evaluation.evaluate_checkpoints=true
+```
+
+For more details on evaluation metrics and comparison tools, see [Evaluation](./evaluation.md).
+
 For more details on tracking your experiments, see [Tracking & Monitoring](./tracking.md).
diff --git a/scripts/compare_models.py b/scripts/compare_models.py
new file mode 100644
index 0000000..b3e8338
--- /dev/null
+++ b/scripts/compare_models.py
@@ -0,0 +1,182 @@
+"""Compare multiple trained policies across shared evaluation conditions.
+
+For each model listed in evaluation.comparison_models, this script runs
+`comparison_num_episodes` headless rollouts (seeded sequentially from
+`comparison_base_seed`) and writes a results CSV to `comparison_output_csv`.
+
+Results include two metrics per episode:
+- `eval_return`    — shaped reward (same function used during training)
+- `max_velocity`  — approximated as initial_xy_dist / steps taken
+
+Usage:
+    # With the default evaluation config
+    python scripts/compare_models.py evaluation=poster
+
+    # Override the output path on the fly
+    python scripts/compare_models.py evaluation=poster \\
+        evaluation.comparison_output_csv=metrics/quick_comparison.csv
+"""
+
+from __future__ import annotations
+
+import csv
+import logging
+import time
+from pathlib import Path
+
+import hydra
+from omegaconf import DictConfig, OmegaConf
+
+from brittle_star_project.configs.main_config import BrittleStarConfig
+from brittle_star_project.configs.register_configs import register_configs
+from brittle_star_project.evaluation import build_eval_env
+from brittle_star_project.evaluation.checkpoint import load_metadata, metadata_to_configs
+from brittle_star_project.evaluation.rollout import rollout_headless
+
+_FIELDNAMES = [
+    "model_path",
+    "architecture",
+    "arm_0",
+    "arm_1",
+    "arm_2",
+    "arm_3",
+    "arm_4",
+    "num_active_arms",
+    "seed",
+    "reached_target",
+    "episode_length",
+    "eval_return",
+    "initial_target_distance",
+    "final_xy_dist",
+    "approx_max_velocity",
+]
+
+
+def _approx_max_velocity(result) -> float | None:
+    """Approximate max velocity as distance covered per step.
+
+    This is a rough upper bound: (initial_dist - final_dist) / steps.
+    """
+    if result.initial_target_distance is None or result.final_xy_dist is None or result.length <= 0:
+        return None
+    dist_covered = result.initial_target_distance - result.final_xy_dist
+    return dist_covered / result.length
+
+
+@hydra.main(config_path="../configs", config_name="main_config", version_base="1.3")
+def main(dict_cfg: DictConfig) -> None:
+    logging.basicConfig(level=logging.INFO, format="%(levelname)s: %(message)s")
+    logger = logging.getLogger(__name__)
+
+    cfg: BrittleStarConfig = OmegaConf.to_object(
+        OmegaConf.merge(OmegaConf.structured(BrittleStarConfig), dict_cfg)
+    )
+    eval_cfg = cfg.evaluation
+
+    model_paths = [str(p) for p in eval_cfg.comparison_models]
+    if not model_paths:
+        raise ValueError(
+            "evaluation.comparison_models is empty. "
+            "Add at least one model path in your evaluation config."
+        )
+
+    base_seed = int(eval_cfg.comparison_base_seed)
+    num_episodes = int(eval_cfg.comparison_num_episodes)
+    max_steps = int(eval_cfg.eval_max_steps)
+
+    seeds = list(range(base_seed, base_seed + num_episodes))
+
+    output_path = Path(hydra.utils.to_absolute_path(eval_cfg.comparison_output_csv))
+    output_path.parent.mkdir(parents=True, exist_ok=True)
+
+    logger.info(
+        f"Comparing {len(model_paths)} models over {num_episodes} episodes "
+        f"(seeds {seeds[0]}–{seeds[-1]})."
+    )
+    logger.info(f"Results will be written to: {output_path}")
+
+    with open(output_path, "w", newline="") as csv_file:
+        writer = csv.DictWriter(csv_file, fieldnames=_FIELDNAMES)
+        writer.writeheader()
+
+        for model_path_str in model_paths:
+            model_path = Path(hydra.utils.to_absolute_path(model_path_str))
+            logger.info(f"Evaluating model: {model_path.name}")
+
+            try:
+                metadata = load_metadata(model_path)
+            except FileNotFoundError as e:
+                logger.warning(f"Skipping model — {e}")
+                continue
+
+            training = metadata_to_configs(metadata)
+
+            # Determine morphologies to evaluate
+            # If comparison_morphologies is empty, use the model's training morphology
+            morphologies = [None]
+            if eval_cfg.comparison_morphologies:
+                morphologies = [
+                    Path(hydra.utils.to_absolute_path(m)) for m in eval_cfg.comparison_morphologies
+                ]
+
+            for morph_path in morphologies:
+                morph_label = morph_path.name if morph_path else "training"
+                logger.info(f"  Morphology: {morph_label}")
+
+                bundle = build_eval_env(
+                    model_path=model_path,
+                    training=training,
+                    metadata=metadata,
+                    morphology_override_path=morph_path,
+                )
+
+                for seed in seeds:
+                    t0 = time.time()
+                    result = rollout_headless(
+                        env=bundle.env,
+                        policy=bundle.policy,
+                        seed=seed,
+                        max_steps=max_steps,
+                        action_low=bundle.action_low,
+                        action_high=bundle.action_high,
+                        action_mask=bundle.action_mask,
+                    )
+                    elapsed = time.time() - t0
+
+                    velocity = _approx_max_velocity(result)
+
+                    logger.debug(
+                        f"    seed={seed:3d} | "
+                        f"reached={str(result.reached_target):<5} | "
+                        f"return={result.return_:+8.3f} | "
+                        f"steps={result.length:4d} | "
+                        f"({elapsed:.1f}s)"
+                    )
+
+                    row = {
+                        "model_path": model_path_str,
+                        "architecture": bundle.architecture,
+                        "num_active_arms": bundle.num_active_arms,
+                        "seed": seed,
+                        "reached_target": result.reached_target,
+                        "episode_length": result.length,
+                        "eval_return": result.return_,
+                        "initial_target_distance": result.initial_target_distance,
+                        "final_xy_dist": result.final_xy_dist,
+                        "approx_max_velocity": velocity,
+                    }
+                    # Add per-arm segments
+                    for i, segs in enumerate(bundle.segments_per_arm):
+                        row[f"arm_{i}"] = segs
+
+                    writer.writerow(row)
+                    csv_file.flush()
+
+                bundle.env.close()
+
+    logger.info(f"Done. Results saved to {output_path}")
+
+
+if __name__ == "__main__":
+    register_configs()
+    main()
diff --git a/scripts/evaluate_checkpoints.py b/scripts/evaluate_checkpoints.py
new file mode 100644
index 0000000..a63796b
--- /dev/null
+++ b/scripts/evaluate_checkpoints.py
@@ -0,0 +1,264 @@
+"""Re-evaluate saved checkpoints from a completed training run using MJX.
+
+This script scans the checkpoint directory of a training run (the `checkpoints/`
+folder inside a Hydra output directory), loads each `.flax` checkpoint, runs
+one deterministic evaluation episode with `build_eval_rollout_fn`, and appends
+the result to the run's `metrics/checkpoint_evaluation.csv`.
+
+It is intended for post-training analysis when per-checkpoint evaluation was not
+enabled during training (`evaluate_checkpoints: false`).
+
+Usage:
+    python scripts/evaluate_checkpoints.py \
+        simulation.model_path=runs/2024-01-01/12-00-00/final_model.flax \
+        evaluation.eval_max_steps=5000 \
+        evaluation.eval_seed=0
+
+The script resolves the run directory from `simulation.model_path`, discovers
+all `*.flax` checkpoints under `checkpoints/`, and evaluates them in order.
+"""
+
+from __future__ import annotations
+from brittle_star_project.MLPs.mlps import (
+    Actor,
+    GenericDenseLayersWithActivation,
+    MessagePasser,
+)
+from brittle_star_project.MLPs.adjancency_builder import build_adjacency
+from brittle_star_project.environment import MorphMode
+from brittle_star_project.MLPs.routing import apply_per_node
+import logging
+import re
+from pathlib import Path
+
+import hydra
+import jax
+import numpy as np
+import jax.numpy as jnp
+
+from omegaconf import DictConfig, OmegaConf
+
+from brittle_star_project.configs.main_config import BrittleStarConfig
+from brittle_star_project.configs.register_configs import register_configs
+from brittle_star_project.environment.BrittleStarJaxEnvWrapper import BrittleStarJaxEnvWrapper
+from brittle_star_project.environment.obs_processing import create_obs_processor
+from brittle_star_project.environment.padded_obs_wrapper import compute_padding_masks
+from brittle_star_project.evaluation.checkpoint import (
+    load_metadata,
+    load_params,
+    metadata_to_configs,
+)
+from brittle_star_project.evaluation.evaluate_mjx import (
+    append_checkpoint_eval_row,
+    build_eval_rollout_fn,
+    evaluate_checkpoint_mjx,
+)
+from brittle_star_project.trainers.PPOTrainer import reward_fn
+
+
+def _parse_iteration(checkpoint_path: Path) -> int:
+    """Parse the iteration number from a checkpoint filename like `checkpoint_0042.flax`."""
+    match = re.search(r"(\d+)", checkpoint_path.stem)
+    return int(match.group(1)) if match else -1
+
+
+@hydra.main(config_path="../configs", config_name="main_config", version_base="1.3")
+def main(dict_cfg: DictConfig) -> None:
+    logging.basicConfig(level=logging.INFO, format="%(levelname)s: %(message)s")
+    logger = logging.getLogger(__name__)
+
+    cfg: BrittleStarConfig = OmegaConf.to_object(
+        OmegaConf.merge(OmegaConf.structured(BrittleStarConfig), dict_cfg)
+    )
+    sim_cfg = cfg.simulation
+    eval_cfg = cfg.evaluation
+
+    # --- Resolve the model path to find the run directory ---
+    model_path_str = sim_cfg.model_path
+    if model_path_str is None:
+        raise ValueError(
+            "simulation.model_path must point to the final_model.flax of a training run."
+        )
+
+    model_path = Path(hydra.utils.to_absolute_path(model_path_str))
+    run_dir = model_path.parent
+
+    checkpoints_dir = run_dir / "checkpoints"
+    if not checkpoints_dir.exists():
+        raise FileNotFoundError(
+            f"No checkpoints/ directory found in run directory: {run_dir}\n"
+            "Make sure simulation.model_path points to a completed training run."
+        )
+
+    checkpoints = sorted(checkpoints_dir.glob("*.flax"), key=_parse_iteration)
+    if not checkpoints:
+        raise FileNotFoundError(f"No .flax checkpoints found in {checkpoints_dir}")
+
+    logger.info(f"Found {len(checkpoints)} checkpoint(s) in {checkpoints_dir}")
+
+    # --- Load sidecar metadata + reconstruct training config ---
+    metadata_override = (
+        Path(hydra.utils.to_absolute_path(sim_cfg.metadata_path))
+        if sim_cfg.metadata_path is not None
+        else None
+    )
+    metadata = load_metadata(model_path, metadata_override)
+    training = metadata_to_configs(metadata)
+
+    padding_masks = compute_padding_masks(
+        segments_per_arm=training.morphology.segments_per_arm,
+        reference_segments_per_arm=training.morphology.segments_per_arm,
+    )
+
+    morph_mode = training.morphology.morph_mode
+
+    segments_per_arm = jnp.asarray(
+        training.morphology.segments_per_arm,
+        dtype=jnp.int32,
+    )
+
+    num_arms = (
+        jnp.where(
+            segments_per_arm > 0,
+            1,
+            0,
+        )
+        .sum()
+        .item()
+    )
+
+    match morph_mode:
+        case MorphMode.CENTRALIZED:
+            needed_copies = 1
+            agent_indices = [0, 1, 2, 3, 4]
+
+        case MorphMode.FULLY_CONNECTED | MorphMode.RING:
+            agent_mask = segments_per_arm > 0
+            agent_indices = jnp.where(agent_mask)[0]
+            needed_copies = num_arms
+
+        case MorphMode.SEGMENT:
+            agent_mask = segments_per_arm > 0
+            agent_indices = jnp.where(agent_mask)[0]
+
+            needed_copies = (segments_per_arm.sum() + num_arms).item()
+
+    obs_processor = create_obs_processor(
+        bounds_dict=training.obs_bounds.to_bounds_dict(),
+        padding_masks=padding_masks,
+        num_arms=num_arms,
+        needed_copies=needed_copies,
+        morph_mode=morph_mode,
+        segments_per_arm=segments_per_arm,
+        agent_indices=agent_indices,
+    )
+
+    env = BrittleStarJaxEnvWrapper(
+        morphology=training.morphology,
+        arena=training.arena,
+        env_config=training.environment,
+        num_envs=1,
+    )
+
+    action_low = np.asarray(env.single_action_space.low, dtype=np.float32)
+    action_high = np.asarray(env.single_action_space.high, dtype=np.float32)
+
+    sensor = GenericDenseLayersWithActivation(layer_sizes=[300, 300, 300])
+    actor = Actor(action_dim=env.single_action_space.shape[0])
+    sensor.apply = jax.jit(sensor.apply)
+    actor.apply = jax.jit(actor.apply)
+
+    eval_fn = build_eval_rollout_fn(
+        env=env,
+        obs_processor=obs_processor,
+        sensor_apply=sensor.apply,
+        actor_apply=actor.apply,
+        action_low=action_low,
+        action_high=action_high,
+        reward_fn=reward_fn,
+    )
+
+    morph_mode = training.morphology.morph_mode
+
+    segments_per_arm = jnp.asarray(
+        training.morphology.segments_per_arm,
+        dtype=jnp.int32,
+    )
+
+    match morph_mode:
+        case MorphMode.CENTRALIZED:
+            needed_copies = 1
+
+        case MorphMode.FULLY_CONNECTED | MorphMode.RING:
+            needed_copies = jnp.where(segments_per_arm > 0, 1, 0).sum().item()
+
+        case MorphMode.SEGMENT:
+            needed_copies = (
+                segments_per_arm.sum() + jnp.where(segments_per_arm > 0, 1, 0).sum()
+            ).item()
+
+    adj = build_adjacency(
+        training.morphology.segments_per_arm,
+        morph_mode,
+    )
+
+    sensor = GenericDenseLayersWithActivation(layer_sizes=[300, 300, 300])
+
+    actor = Actor(action_dim=env.single_action_space.shape[0] // needed_copies)
+
+    message_passer = (
+        MessagePasser(
+            hidden_dim=300,
+            num_propagation_steps=4,
+            adj_matrix=adj,
+        )
+        if morph_mode != MorphMode.CENTRALIZED
+        else None
+    )
+
+    eval_fn = build_eval_rollout_fn(
+        env=env,
+        obs_processor=obs_processor,
+        sensor_apply=lambda p, x: apply_per_node(sensor.apply, p, x),
+        actor_apply=lambda p, x: apply_per_node(actor.apply, p, x),
+        message_passer_apply=(None if message_passer is None else message_passer.apply),
+        action_low=action_low,
+        action_high=action_high,
+        reward_fn=reward_fn,
+    )
+    seed = int(eval_cfg.eval_seed)
+    max_steps = int(eval_cfg.eval_max_steps)
+
+    logger.info(f"Evaluating each checkpoint (seed={seed}, max_steps={max_steps}).")
+
+    for checkpoint_path in checkpoints:
+        iteration = _parse_iteration(checkpoint_path)
+        try:
+            params = load_params(checkpoint_path)
+        except Exception as e:
+            logger.warning(f"Could not load {checkpoint_path.name}: {e}")
+            continue
+
+        result = evaluate_checkpoint_mjx(eval_fn, params, seed=seed, max_steps=max_steps)
+        csv_path = append_checkpoint_eval_row(
+            run_dir,
+            iteration=iteration,
+            trained_timesteps=0,  # unknown without training logs
+            result=result,
+        )
+
+        logger.debug(
+            f"checkpoint={iteration:5d} | "
+            f"reached={str(result.reached_target):<5} | "
+            f"return={result.eval_return:+8.3f} | "
+            f"steps={result.steps:4d} | "
+            f"final_dist={result.final_xy_dist:.3f}"
+        )
+
+    logger.info(f"Done. CSV at: {csv_path}")
+    env.close()
+
+
+if __name__ == "__main__":
+    register_configs()
+    main()
diff --git a/scripts/simulate.py b/scripts/simulate.py
index a56e970..9f3e550 100644
--- a/scripts/simulate.py
+++ b/scripts/simulate.py
@@ -13,28 +13,20 @@ from __future__ import annotations
 from pathlib import Path
 
 import hydra
-import numpy as np
 from omegaconf import DictConfig, OmegaConf
-import yaml
 
-import jax.numpy as jnp
 
-from brittle_star_project import Backend, BrittleStarEnv, BrittleStarEnvFactory
 from brittle_star_project.configs.main_config import BrittleStarConfig
 from brittle_star_project.configs.register_configs import register_configs
-from brittle_star_project.environment.padded_obs_wrapper import compute_padding_masks
-from brittle_star_project.environment.obs_processing import create_obs_processor
-from brittle_star_project.environment.env_config import MorphMode, MorphologyConfig
 
 from brittle_star_project.evaluation.checkpoint import load_metadata, metadata_to_configs
-from brittle_star_project.evaluation.policy import PolicyAgent
+from brittle_star_project.evaluation.eval_env_builder import build_eval_env
 from brittle_star_project.evaluation.rollout import rollout_headless, rollout_viewer
 from brittle_star_project.evaluation.video import (
     record_episode,
     create_evaluation_dir,
     save_evaluation_metadata,
 )
-from brittle_star_project.MLPs.adjancency_builder import build_adjacency
 
 
 @hydra.main(config_path="../configs", config_name="main_config", version_base="1.3")
@@ -63,107 +55,28 @@ def main(dict_cfg: DictConfig) -> None:
     # 3. Reconstruct typed configs from metadata
     training = metadata_to_configs(metadata)
 
-    # 4. Determine environment morphology
-    if sim_cfg.morphology_override is not None:
-        override_path = Path(hydra.utils.to_absolute_path(sim_cfg.morphology_override))
-        if not override_path.exists():
-            raise FileNotFoundError(f"Could not find morphology override YAML at {override_path}")
-        with open(override_path, "r") as f:
-            override_dict = yaml.safe_load(f)
-        env_morphology = OmegaConf.to_object(
-            OmegaConf.merge(OmegaConf.structured(MorphologyConfig), override_dict)
-        )
-    else:
-        env_morphology = training.morphology
-
-    # 5. Build obs_processor with TRAINING morphology padding masks always
-    padding_masks = compute_padding_masks(
-        segments_per_arm=env_morphology.segments_per_arm,
-        reference_segments_per_arm=training.morphology.segments_per_arm,
-    )
-
-    segs_per_arm = jnp.array(env_morphology.segments_per_arm)
-
-    needed_copies = 0
-    agent_indices = [0, 1, 2, 3, 4]
-    match env_morphology.morph_mode:
-        case MorphMode.CENTRALIZED:
-            needed_copies = 1
-        case MorphMode.FULLY_CONNECTED | MorphMode.RING:
-            agent_mask = segs_per_arm > 0
-            agent_indices = jnp.where(agent_mask)[0]
-            needed_copies = jnp.where(segs_per_arm > 0, 1, 0).sum().item()
-        case MorphMode.SEGMENT:
-            agent_mask = segs_per_arm > 0
-            agent_indices = jnp.where(agent_mask)[0]
-            needed_copies = jnp.where(segs_per_arm > 0, 1, 0).sum().item()
-            needed_copies = (segs_per_arm.sum() + jnp.where(segs_per_arm > 0, 1, 0).sum()).item()
-
-    num_arms = jnp.where(segs_per_arm > 0, 1, 0).sum().item()
-
-    obs_processor = create_obs_processor(
-        bounds_dict=training.obs_bounds.to_bounds_dict(),
-        padding_masks=padding_masks,
-        needed_copies=needed_copies,
-        num_arms=num_arms,
-        morph_mode=env_morphology.morph_mode,
-        segments_per_arm=env_morphology.segments_per_arm,
-        agent_indices=agent_indices,
-    )
-
-    # 6. Build environment
-    backend = Backend.MJC
     seed = int(cfg.experiment.seed)
 
-    factory = BrittleStarEnvFactory()
-    raw_env = factory.create_environment(
-        backend,
-        env_morphology,
-        training.arena,
-        training.environment,
-    )
-    env = BrittleStarEnv(
-        raw_env,
-        backend=backend,
-        config=training.environment,
-        morphology_config=env_morphology,
+    # 4-7. Build evaluation environment and policy
+    override_path = None
+    if sim_cfg.morphology_override is not None:
+        override_path = Path(hydra.utils.to_absolute_path(sim_cfg.morphology_override))
+
+    bundle = build_eval_env(
+        model_path=model_path,
+        training=training,
+        metadata=metadata,
+        morphology_override_path=override_path,
     )
 
+    env = bundle.env
+    policy = bundle.policy
+    action_low = bundle.action_low
+    action_high = bundle.action_high
+    action_mask = bundle.action_mask
+
     state0 = env.reset(seed=seed)
 
-    # Calculate the action dimension the model was trained with
-    trained_action_dim = raw_env.action_space.shape[0] // needed_copies
-
-    # 7. Load policy
-    message_passing_steps = (metadata.get("architecture", {}) or {}).get("message_passing_steps")
-    if message_passing_steps is None:
-        message_passing_steps = 4
-    message_passing_steps = int(message_passing_steps)
-
-    adj_matrix = None
-    if env_morphology.morph_mode != MorphMode.CENTRALIZED:
-        adj_matrix = build_adjacency(env_morphology.segments_per_arm, env_morphology.morph_mode)
-
-    policy = PolicyAgent.from_checkpoint(
-        model_path,
-        action_dim=trained_action_dim,
-        obs_processor=obs_processor,
-        message_passing_steps=message_passing_steps,
-        adj_matrix=adj_matrix,
-    )
-
-    # Convert the JAX boolean mask to a numpy array for easy indexing
-    action_mask = np.asarray(padding_masks["mask_2x"])
-
-    # Match training's action clipping behavior.
-    action_space = getattr(raw_env, "action_space", None)
-    action_low = (
-        None if action_space is None else np.asarray(action_space.low, dtype=np.float32).ravel()
-    )
-    action_high = (
-        None if action_space is None else np.asarray(action_space.high, dtype=np.float32).ravel()
-    )
-
     # 8. Run simulation
     headless = bool(sim_cfg.headless)
     max_steps = sim_cfg.max_steps
diff --git a/src/brittle_star_project/MLPs/routing.py b/src/brittle_star_project/MLPs/routing.py
new file mode 100644
index 0000000..4033cb9
--- /dev/null
+++ b/src/brittle_star_project/MLPs/routing.py
@@ -0,0 +1,22 @@
+"""Shared JAX routing utilities for decentralized multi-agent models."""
+
+import jax
+
+
+def apply_per_node(apply_fn, params, x):
+    """Apply a Flax module independently to each node.
+
+    Args:
+        apply_fn: The module's ``apply`` method (e.g. ``sensor.apply``).
+        params: Per-node parameters with shape ``(num_nodes, ...)``.
+        x: Input tensor with shape ``(batch, num_nodes, features)``.
+
+    Returns:
+        Output tensor with shape ``(batch, num_nodes, out_features)``.
+    """
+
+    def apply_single_node(p, x_node):
+        # x_node: (batch, feat) — one node's input across the batch
+        return jax.vmap(lambda xi: apply_fn(p, xi))(x_node)
+
+    return jax.vmap(apply_single_node, in_axes=(0, 1), out_axes=1)(params, x)
diff --git a/src/brittle_star_project/configs/config_evaluation.py b/src/brittle_star_project/configs/config_evaluation.py
index 4eeadbc..c9c248b 100644
--- a/src/brittle_star_project/configs/config_evaluation.py
+++ b/src/brittle_star_project/configs/config_evaluation.py
@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 
 
 @dataclass
@@ -16,6 +16,20 @@ class EvaluationConfig:
     eval_max_steps: int = 5000
     eval_seed: int = 0
 
+    # Cross-model comparison settings.
+    # comparison_base_seed is the starting seed for generating episode seeds.
+    comparison_base_seed: int = 0
+    # comparison_num_episodes controls how many target positions to evaluate for each model.
+    comparison_num_episodes: int = 5
+    # comparison_models lists the paths (relative to workspace root) to the .cleanrl_model files.
+    comparison_models: list[str] = field(default_factory=list)
+    # Path where the comparison results CSV will be saved (relative to workspace root).
+    comparison_output_csv: str = "metrics/model_comparison.csv"
+    # Morphology override YAML paths for cross-morphology comparison.
+    # Each path points to a file in configs/morphology/ (e.g., "configs/morphology/3_arms.yaml").
+    # When empty, each model is evaluated only on its training morphology.
+    comparison_morphologies: list[str] = field(default_factory=list)
+
     def __post_init__(self) -> None:
         if self.evaluate_checkpoints and self.eval_max_steps <= 0:
             raise ValueError(
diff --git a/src/brittle_star_project/environment/obs_processing.py b/src/brittle_star_project/environment/obs_processing.py
index f38d52b..e169ee3 100644
--- a/src/brittle_star_project/environment/obs_processing.py
+++ b/src/brittle_star_project/environment/obs_processing.py
@@ -109,6 +109,7 @@ def create_obs_processor(
         joints_per_segment = 2
         joints_per_arm = segs_per_arm * joints_per_segment
         for key, arr in obs.items():
+            arr = jnp.asarray(arr)
             if arr.size == 0:
                 continue
 
@@ -121,7 +122,7 @@ def create_obs_processor(
                     idx = segment_indices[i]
                     taken = jnp.take(arr, idx, axis=0)
                     pad_len = segs_per_arm - taken.shape[0]
-                    padded = jnp.pad(taken, [(9, pad_len)] + [(0, 0)] * (taken.ndim - 1))
+                    padded = jnp.pad(taken, [(0, pad_len)] + [(0, 0)] * (taken.ndim - 1))
 
                     per_agent.append(padded.reshape(-1))
                 arr = jnp.stack(per_agent)
@@ -159,7 +160,7 @@ def create_obs_processor(
         """
         values = []
 
-        for key in ordered_keys:
+        for key in sorted(ordered_keys):
             if key not in obs:
                 continue
 
diff --git a/src/brittle_star_project/evaluation/__init__.py b/src/brittle_star_project/evaluation/__init__.py
index 2922d04..e529115 100644
--- a/src/brittle_star_project/evaluation/__init__.py
+++ b/src/brittle_star_project/evaluation/__init__.py
@@ -7,9 +7,11 @@ from .evaluate_mjx import (
     build_eval_rollout_fn,
     evaluate_checkpoint_mjx,
 )
+from .evaluate import evaluate_policy
 from .policy import PolicyAgent, ControlPolicy
 from .rollout import rollout_headless, rollout_viewer, EpisodeResult
 from .video import record_episode, create_evaluation_dir, save_evaluation_metadata
+from .eval_env_builder import EvalEnvBundle, build_eval_env
 
 __all__ = [
     # checkpoint loading
@@ -22,6 +24,8 @@ __all__ = [
     "append_checkpoint_eval_row",
     "build_eval_rollout_fn",
     "evaluate_checkpoint_mjx",
+    # CPU evaluation
+    "evaluate_policy",
     # policy
     "PolicyAgent",
     "ControlPolicy",
@@ -33,4 +37,7 @@ __all__ = [
     "record_episode",
     "create_evaluation_dir",
     "save_evaluation_metadata",
+    # env builder
+    "EvalEnvBundle",
+    "build_eval_env",
 ]
diff --git a/src/brittle_star_project/evaluation/eval_env_builder.py b/src/brittle_star_project/evaluation/eval_env_builder.py
new file mode 100644
index 0000000..6d45966
--- /dev/null
+++ b/src/brittle_star_project/evaluation/eval_env_builder.py
@@ -0,0 +1,176 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from pathlib import Path
+
+import jax.numpy as jnp
+import numpy as np
+import yaml
+from omegaconf import OmegaConf
+
+from brittle_star_project import Backend, BrittleStarEnv, BrittleStarEnvFactory
+from brittle_star_project.environment.env_config import MorphMode, MorphologyConfig
+from brittle_star_project.environment.obs_processing import create_obs_processor
+from brittle_star_project.environment.padded_obs_wrapper import compute_padding_masks
+from brittle_star_project.evaluation.checkpoint import TrainingConfig
+from brittle_star_project.evaluation.policy import PolicyAgent
+from brittle_star_project.MLPs.adjancency_builder import build_adjacency
+
+
+@dataclass
+class EvalEnvBundle:
+    """Everything needed to run a headless evaluation episode."""
+
+    env: BrittleStarEnv
+    policy: PolicyAgent
+    action_low: np.ndarray | None
+    action_high: np.ndarray | None
+    action_mask: np.ndarray | None
+    segments_per_arm: list[int]
+    num_active_arms: int
+    architecture: str
+
+
+def build_eval_env(
+    *,
+    model_path: Path,
+    training: TrainingConfig,
+    metadata: dict,
+    morphology_override_path: Path | str | None = None,
+) -> EvalEnvBundle:
+    """Build environment + policy for evaluation, optionally with a morphology override."""
+
+    # 1. Determine environment morphology
+    if morphology_override_path is not None:
+        override_path = Path(morphology_override_path)
+        if not override_path.exists():
+            raise FileNotFoundError(f"Could not find morphology override YAML at {override_path}")
+        with open(override_path, "r") as f:
+            override_dict = yaml.safe_load(f)
+        env_morphology = OmegaConf.to_object(
+            OmegaConf.merge(OmegaConf.structured(MorphologyConfig), override_dict)
+        )
+        # Force morph_mode to be inherited from training since it's baked into weights
+        env_morphology.morph_mode = training.morphology.morph_mode
+    else:
+        env_morphology = training.morphology
+
+    # 2. Build obs_processor with TRAINING morphology padding masks always
+    padding_masks = compute_padding_masks(
+        segments_per_arm=env_morphology.segments_per_arm,
+        reference_segments_per_arm=training.morphology.segments_per_arm,
+    )
+
+    training_segs_per_arm = jnp.array(training.morphology.segments_per_arm)
+
+    needed_copies = 0
+    agent_indices = [0, 1, 2, 3, 4]
+    match training.morphology.morph_mode:
+        case MorphMode.CENTRALIZED:
+            needed_copies = 1
+        case MorphMode.FULLY_CONNECTED | MorphMode.RING:
+            agent_mask = training_segs_per_arm > 0
+            agent_indices = jnp.where(agent_mask)[0].tolist()
+            needed_copies = jnp.where(training_segs_per_arm > 0, 1, 0).sum().item()
+        case MorphMode.SEGMENT:
+            agent_mask = training_segs_per_arm > 0
+            agent_indices = jnp.where(agent_mask)[0].tolist()
+            needed_copies = (
+                training_segs_per_arm.sum() + jnp.where(training_segs_per_arm > 0, 1, 0).sum()
+            ).item()
+
+    num_arms_training = jnp.where(training_segs_per_arm > 0, 1, 0).sum().item()
+
+    obs_processor = create_obs_processor(
+        bounds_dict=training.obs_bounds.to_bounds_dict(),
+        padding_masks=padding_masks,
+        needed_copies=needed_copies,
+        num_arms=num_arms_training,
+        morph_mode=training.morphology.morph_mode,
+        segments_per_arm=env_morphology.segments_per_arm,
+        agent_indices=agent_indices,
+    )
+
+    # 3. Build environment
+    backend = Backend.MJC
+    factory = BrittleStarEnvFactory()
+    raw_env = factory.create_environment(
+        backend,
+        env_morphology,
+        training.arena,
+        training.environment,
+    )
+    env = BrittleStarEnv(
+        raw_env,
+        backend=backend,
+        config=training.environment,
+        morphology_config=env_morphology,
+    )
+
+    # Calculate the action dimension the model was trained with
+    training_total_actions = sum(training.morphology.segments_per_arm) * 2
+    trained_action_dim = training_total_actions // needed_copies
+
+    # 4. Load policy
+    message_passing_steps = (metadata.get("architecture", {}) or {}).get("message_passing_steps")
+    if message_passing_steps is None:
+        message_passing_steps = 4
+    message_passing_steps = int(message_passing_steps)
+
+    adj_matrix = None
+    if training.morphology.morph_mode != MorphMode.CENTRALIZED:
+        adj_matrix = build_adjacency(
+            training.morphology.segments_per_arm, training.morphology.morph_mode
+        )
+
+        override_segs = env_morphology.segments_per_arm
+        if training.morphology.morph_mode in (MorphMode.FULLY_CONNECTED, MorphMode.RING):
+            for i, segs in enumerate(override_segs):
+                if segs == 0 and i < adj_matrix.shape[0]:
+                    adj_matrix = adj_matrix.at[i, :].set(0)
+                    adj_matrix = adj_matrix.at[:, i].set(0)
+        elif training.morphology.morph_mode == MorphMode.SEGMENT:
+            for i, segs in enumerate(override_segs):
+                if segs == 0 and i < num_arms_training:
+                    adj_matrix = adj_matrix.at[i, :].set(0)
+                    adj_matrix = adj_matrix.at[:, i].set(0)
+
+            idx = 0
+            for arm_idx, seg_count in enumerate(training.morphology.segments_per_arm):
+                if override_segs[arm_idx] == 0:
+                    for i in range(seg_count):
+                        seg_node = num_arms_training + idx + i
+                        if seg_node < adj_matrix.shape[0]:
+                            adj_matrix = adj_matrix.at[seg_node, :].set(0)
+                            adj_matrix = adj_matrix.at[:, seg_node].set(0)
+                idx += seg_count
+
+    policy = PolicyAgent.from_checkpoint(
+        model_path,
+        action_dim=trained_action_dim,
+        obs_processor=obs_processor,
+        message_passing_steps=message_passing_steps,
+        adj_matrix=adj_matrix,
+    )
+
+    # 5. Build action clipping and masks
+    action_mask = np.asarray(padding_masks["mask_2x"])
+
+    action_space = getattr(raw_env, "action_space", None)
+    action_low = (
+        None if action_space is None else np.asarray(action_space.low, dtype=np.float32).ravel()
+    )
+    action_high = (
+        None if action_space is None else np.asarray(action_space.high, dtype=np.float32).ravel()
+    )
+
+    return EvalEnvBundle(
+        env=env,
+        policy=policy,
+        action_low=action_low,
+        action_high=action_high,
+        action_mask=action_mask,
+        segments_per_arm=env_morphology.segments_per_arm,
+        num_active_arms=sum(1 for s in env_morphology.segments_per_arm if s > 0),
+        architecture=env_morphology.morph_mode.name,
+    )
diff --git a/src/brittle_star_project/evaluation/evaluate.py b/src/brittle_star_project/evaluation/evaluate.py
new file mode 100644
index 0000000..9d62880
--- /dev/null
+++ b/src/brittle_star_project/evaluation/evaluate.py
@@ -0,0 +1,58 @@
+"""MJC-based (CPU) checkpoint evaluation.
+
+This module provides the CPU-bound evaluation path using the standard MJC backend.
+It is primarily used by the `evaluate_checkpoints` CLI to compute metrics and
+render videos.
+"""
+
+from pathlib import Path
+
+import numpy as np
+
+from brittle_star_project.environment.BrittleStarJaxEnvWrapper import BrittleStarJaxEnvWrapper
+from brittle_star_project.environment.obs_processing import create_obs_processor
+from brittle_star_project.evaluation.policy import PolicyAgent
+from brittle_star_project.evaluation.rollout import EpisodeResult, rollout_headless
+
+
+def evaluate_policy(
+    env: BrittleStarJaxEnvWrapper,
+    policy_path: str | Path,
+    seed: int,
+    max_steps: int,
+) -> EpisodeResult:
+    """Evaluate a trained policy in a CPU-bound environment.
+
+    Args:
+        env: Initialised CPU environment (MJC backend).
+        policy_path: Path to the `.cleanrl_model` weights file.
+        seed: Random seed for environment reset.
+        max_steps: Maximum number of control steps.
+
+    Returns:
+        Structured result containing return, length, and distance metrics.
+    """
+    obs_processor = create_obs_processor(
+        bounds_dict=env.cfg.obs_bounds.to_bounds_dict(),
+        padding_masks=env.padding_masks,
+    )
+
+    action_dim = env.single_action_space.shape[0]
+
+    policy = PolicyAgent.from_checkpoint(
+        model_path=Path(policy_path),
+        action_dim=action_dim,
+        obs_processor=obs_processor,
+    )
+
+    action_low = np.asarray(env.single_action_space.low, dtype=np.float32)
+    action_high = np.asarray(env.single_action_space.high, dtype=np.float32)
+
+    return rollout_headless(
+        env=env,
+        policy=policy,
+        seed=seed,
+        max_steps=max_steps,
+        action_low=action_low,
+        action_high=action_high,
+    )
diff --git a/src/brittle_star_project/evaluation/evaluate_mjx.py b/src/brittle_star_project/evaluation/evaluate_mjx.py
index 317fa98..f557d2d 100644
--- a/src/brittle_star_project/evaluation/evaluate_mjx.py
+++ b/src/brittle_star_project/evaluation/evaluate_mjx.py
@@ -12,7 +12,7 @@ The key functions are:
 - `evaluate_checkpoint_mjx` — runs that function for a given set of parameters and returns a typed
   `CheckpointEvalResult`.
 - `append_checkpoint_eval_row` — persists the result to the run's
-  ``metrics/checkpoint_evaluation.csv``, migrating old schemas automatically.
+  `metrics/checkpoint_evaluation.csv`, migrating old schemas automatically.
 """
 
 from __future__ import annotations
@@ -62,20 +62,20 @@ def build_eval_rollout_fn(
     All outputs are JAX arrays. Convert to Python scalars before logging.
 
     Args:
-        env: The training environment wrapper. Must expose ``env.raw`` with
-            ``reset`` and ``step`` methods compatible with ``jax.vmap``.
+        env: The training environment wrapper. Must expose `env.raw` with
+            `reset` and `step` methods compatible with `jax.vmap`.
         obs_processor: Observation normalisation / padding callable, as
-            returned by ``create_obs_processor``.
-        sensor_apply: The sensor network's ``apply`` method (JIT-compiled).
-        actor_apply: The actor network's ``apply`` method (JIT-compiled).
+            returned by `create_obs_processor`.
+        sensor_apply: The sensor network's `apply` method (JIT-compiled).
+        actor_apply: The actor network's `apply` method (JIT-compiled).
         message_passer_apply: Optional message-passing module apply method.
             When provided, it is applied between the sensor and actor, using
-            ``params["message_passer_params"]``.
-        action_low: Per-joint action lower bound (JAX array, shape ``(action_dim,)``).
-        action_high: Per-joint action upper bound (JAX array, shape ``(action_dim,)``).
+            `params["message_passer_params"]`.
+        action_low: Per-joint action lower bound (JAX array, shape `(action_dim,)`).
+        action_high: Per-joint action upper bound (JAX array, shape `(action_dim,)`).
         reward_fn: Shaped reward function with signature
-            ``reward_fn(env_state, next_env_state) -> jnp.ndarray``.
-            Typically the module-level ``reward_fn`` from ``PPOTrainer``.
+            `reward_fn(env_state, next_env_state) -> jnp.ndarray`.
+            Typically, the module-level `reward_fn` from `PPOTrainer`.
 
     Returns:
         A JIT-compiled callable that runs one deterministic evaluation episode.
@@ -217,9 +217,9 @@ def append_checkpoint_eval_row(
     trained_timesteps: int,
     result: CheckpointEvalResult,
 ) -> Path:
-    """Append one evaluation row to ``<run_dir>/metrics/checkpoint_evaluation.csv``.
+    """Append one evaluation row to `<run_dir>/metrics/checkpoint_evaluation.csv`.
 
-    Creates the file (including the ``metrics/`` directory) if it does not yet
+    Creates the file (including the `metrics/` directory) if it does not yet
     exist. Migrates the file to the current schema if the header has changed.
 
     Args:
diff --git a/src/brittle_star_project/evaluation/policy.py b/src/brittle_star_project/evaluation/policy.py
index 5b12d07..00c1e06 100644
--- a/src/brittle_star_project/evaluation/policy.py
+++ b/src/brittle_star_project/evaluation/policy.py
@@ -7,6 +7,7 @@ import jax
 import jax.numpy as jnp
 import numpy as np
 
+from brittle_star_project.MLPs.routing import apply_per_node
 from brittle_star_project.evaluation.checkpoint import load_params
 
 
@@ -147,22 +148,12 @@ class PolicyAgent:
             obs_processor=obs_processor,
         )
 
-    def _apply_per_node(self, net, params, x):
-        # params: (nodes, ...)
-        # x: (batch, nodes, feat)
-
-        def apply_single_node(p, x_node):
-            # x_node: (batch, feat)
-            return jax.vmap(lambda xi: net.apply(p, xi))(x_node)
-
-        return jax.vmap(apply_single_node, in_axes=(0, 1), out_axes=1)(params, x)
-
     def act(self, *, observations: dict[str, Any]) -> np.ndarray:
         """Return deterministic action (actor mean, no exploration noise)."""
         batched_obs = jax.tree.map(lambda x: jnp.asarray(x)[None, ...], observations)
         obs = self._obs_processor(batched_obs)
 
-        hidden = self._apply_per_node(self._sensor, self._params["sensor_params"], obs)
+        hidden = apply_per_node(self._sensor.apply, self._params["sensor_params"], obs)
 
         if self._message_passer is not None:
             mp_params = self._params.get("message_passer_params")
@@ -172,6 +163,6 @@ class PolicyAgent:
                 )
             hidden = jax.vmap(lambda x: self._message_passer.apply(mp_params, x))(hidden)
 
-        mean, _log_std = self._apply_per_node(self._actor, self._params["actor_params"], hidden)
+        mean, _log_std = apply_per_node(self._actor.apply, self._params["actor_params"], hidden)
 
         return np.asarray(mean, dtype=np.float32).ravel()
diff --git a/src/brittle_star_project/evaluation/rollout.py b/src/brittle_star_project/evaluation/rollout.py
index 982a564..f292138 100644
--- a/src/brittle_star_project/evaluation/rollout.py
+++ b/src/brittle_star_project/evaluation/rollout.py
@@ -17,6 +17,7 @@ class EpisodeResult:
     length: int
     reached_target: bool
     final_xy_dist: float | None
+    initial_target_distance: float | None
 
 
 def _get_observations(state: Any) -> dict[str, Any] | None:
@@ -61,6 +62,7 @@ def rollout_headless(
     ep_return = 0.0
     observations = _get_observations(state)
     prev_dist = _get_xy_distance_to_target(observations) if observations else None
+    initial_target_distance = prev_dist
     reached_target = _target_reached(state=state)
 
     steps = 0
@@ -91,6 +93,7 @@ def rollout_headless(
         length=steps,
         reached_target=reached_target,
         final_xy_dist=final_dist,
+        initial_target_distance=initial_target_distance,
     )
 
 
diff --git a/src/brittle_star_project/evaluation/video.py b/src/brittle_star_project/evaluation/video.py
index 174326b..b4b44db 100644
--- a/src/brittle_star_project/evaluation/video.py
+++ b/src/brittle_star_project/evaluation/video.py
@@ -97,10 +97,10 @@ def record_episode(
     data = state.mj_data
 
     renderer = mujoco.Renderer(model, width=width, height=height)
-
     ep_return = 0.0
     observations = _get_observations(state)
     prev_dist = _get_xy_distance_to_target(observations) if observations else None
+    initial_dist = prev_dist
     reached_target = _target_reached(state=state)
 
     frames = []
@@ -145,4 +145,5 @@ def record_episode(
         length=steps,
         reached_target=reached_target,
         final_xy_dist=final_dist,
+        initial_target_distance=initial_dist,
     )
diff --git a/src/brittle_star_project/trainers/PPOTrainer.py b/src/brittle_star_project/trainers/PPOTrainer.py
index 76bb128..98d4045 100644
--- a/src/brittle_star_project/trainers/PPOTrainer.py
+++ b/src/brittle_star_project/trainers/PPOTrainer.py
@@ -23,6 +23,7 @@ from brittle_star_project.evaluation.evaluate_mjx import (
     build_eval_rollout_fn,
     evaluate_checkpoint_mjx,
 )
+from brittle_star_project.MLPs.routing import apply_per_node
 from brittle_star_project.MLPs.mlps import (
     Actor,
     AgentParams,
@@ -71,7 +72,7 @@ def _get_action_and_value_noise(
     action_high,
 ):
     # (B, n_nodes, feat)
-    hidden = apply_per_node(sensor, agent_state.params["sensor_params"], next_obs)
+    hidden = apply_per_node(sensor.apply, agent_state.params["sensor_params"], next_obs)
 
     if message_passer is not None:
         params = agent_state.params["message_passer_params"]
@@ -82,7 +83,7 @@ def _get_action_and_value_noise(
         feature_extractor, agent_state.params["feature_extractor_params"], next_obs
     )
 
-    mean, log_std = apply_per_node(actor, agent_state.params["actor_params"], hidden)
+    mean, log_std = apply_per_node(actor.apply, agent_state.params["actor_params"], hidden)
     log_std = jnp.clip(log_std, -5, 2)
     key, subkey = jax.random.split(key)
     noise = jax.random.normal(subkey, shape=mean.shape)
@@ -272,17 +273,6 @@ def _step_env_wrapped(
     )
 
 
-def apply_per_node(net, params, x):
-    # params: (nodes, ...)
-    # x: (batch, nodes, feat)
-
-    def apply_single_node(p, x_node):
-        # x_node: (batch, feat)
-        return jax.vmap(lambda xi: net.apply(p, xi))(x_node)
-
-    return jax.vmap(apply_single_node, in_axes=(0, 1), out_axes=1)(params, x)
-
-
 def apply_shared(net, params, x):
     # x: (batch, nodes, feat)
     # If the critic expects a single vector per environment:
@@ -516,10 +506,10 @@ class PPOTrainer:
         )
 
         def apply_sensor(p, x):
-            return apply_per_node(self.sensor, p, x)
+            return apply_per_node(self.sensor.apply, p, x)
 
         def apply_actor(p, x):
-            return apply_per_node(self.actor, p, x)
+            return apply_per_node(self.actor.apply, p, x)
 
         def apply_critic(p, x):
             return apply_shared(self.critic, p, x)
@@ -903,8 +893,8 @@ class PPOTrainer:
                 self._eval_fn = build_eval_rollout_fn(
                     env=self.env,
                     obs_processor=self.obs_processor,
-                    sensor_apply=lambda p, x: apply_per_node(self.sensor, p, x),
-                    actor_apply=lambda p, x: apply_per_node(self.actor, p, x),
+                    sensor_apply=lambda p, x: apply_per_node(self.sensor.apply, p, x),
+                    actor_apply=lambda p, x: apply_per_node(self.actor.apply, p, x),
                     message_passer_apply=(
                         None if self.message_passer is None else self.message_passer.apply
                     ),
diff --git a/tests/test_evaluation.py b/tests/test_evaluation.py
index bbb03f8..d8be381 100644
--- a/tests/test_evaluation.py
+++ b/tests/test_evaluation.py
@@ -14,6 +14,7 @@ from brittle_star_project.environment.env_config import (
     ArenaConfig,
     EnvConfig,
     ObservationBoundsConfig,
+    MorphMode,
 )
 from brittle_star_project.environment.env_types import Task
 
@@ -112,3 +113,106 @@ def test_load_metadata_with_override(tmp_path: Path):
     non_existent = tmp_path / "missing.yaml"
     with pytest.raises(FileNotFoundError, match="Could not find metadata YAML at"):
         load_metadata(model_path, metadata_override_path=non_existent)
+
+
+@pytest.fixture
+def mock_training_config():
+    return TrainingConfig(
+        morphology=MorphologyConfig(
+            segments_per_arm=[1, 1, 1, 1, 1], morph_mode=MorphMode.CENTRALIZED
+        ),
+        arena=ArenaConfig(),
+        environment=EnvConfig(),
+        obs_bounds=ObservationBoundsConfig(),
+    )
+
+
+@pytest.fixture
+def mock_metadata():
+    return {"architecture": {"message_passing_steps": 2}}
+
+
+def test_build_eval_env_training_morphology(tmp_path, mock_training_config, mock_metadata):
+    from brittle_star_project.evaluation.eval_env_builder import build_eval_env
+    from unittest.mock import patch
+
+    model_path = tmp_path / "model.flax"
+    patch_target = "brittle_star_project.evaluation.eval_env_builder.PolicyAgent.from_checkpoint"
+    with patch(patch_target) as mock_agent:
+        mock_agent.return_value = "mock_policy"
+        bundle = build_eval_env(
+            model_path=model_path,
+            training=mock_training_config,
+            metadata=mock_metadata,
+            morphology_override_path=None,
+        )
+        assert bundle.segments_per_arm == [1, 1, 1, 1, 1]
+        assert bundle.num_active_arms == 5
+        assert bundle.architecture == "CENTRALIZED"
+        assert bundle.policy == "mock_policy"
+
+
+def test_build_eval_env_override_morphology(tmp_path, mock_training_config, mock_metadata):
+    from brittle_star_project.evaluation.eval_env_builder import build_eval_env
+    from unittest.mock import patch
+
+    model_path = tmp_path / "model.flax"
+    override_path = tmp_path / "override.yaml"
+    override_path.write_text(yaml.dump({"segments_per_arm": [1, 0, 1, 0, 1]}))
+
+    with patch("brittle_star_project.evaluation.eval_env_builder.PolicyAgent.from_checkpoint"):
+        bundle = build_eval_env(
+            model_path=model_path,
+            training=mock_training_config,
+            metadata=mock_metadata,
+            morphology_override_path=override_path,
+        )
+        assert bundle.segments_per_arm == [1, 0, 1, 0, 1]
+        assert bundle.num_active_arms == 3
+        # Should be smaller than 5*N
+        assert sum(bundle.action_mask) < len(bundle.action_mask)
+
+
+def test_build_eval_env_action_mask_shape(tmp_path, mock_training_config, mock_metadata):
+    from brittle_star_project.evaluation.eval_env_builder import build_eval_env
+    from unittest.mock import patch
+
+    model_path = tmp_path / "model.flax"
+    override_path = tmp_path / "override.yaml"
+    override_path.write_text(yaml.dump({"segments_per_arm": [1, 0, 1, 0, 0]}))
+
+    with patch("brittle_star_project.evaluation.eval_env_builder.PolicyAgent.from_checkpoint"):
+        bundle = build_eval_env(
+            model_path=model_path,
+            training=mock_training_config,
+            metadata=mock_metadata,
+            morphology_override_path=override_path,
+        )
+        # For each segment with P-control, there's 2 actions (pitch and yaw).
+        # Total segments = 5 -> 10 actions for training.
+        assert len(bundle.action_mask) == 10
+        # Active segments = 2 -> 4 actions active.
+        assert sum(bundle.action_mask) == 4
+
+
+def test_build_eval_env_morph_mode_inherited(tmp_path, mock_training_config, mock_metadata):
+    from brittle_star_project.evaluation.eval_env_builder import build_eval_env
+    from brittle_star_project.environment.env_config import MorphMode
+    from unittest.mock import patch
+
+    model_path = tmp_path / "model.flax"
+    override_path = tmp_path / "override.yaml"
+    # No morph_mode in the override YAML
+    override_path.write_text(yaml.dump({"segments_per_arm": [1, 0, 1, 0, 1]}))
+
+    # Change training config to be RING
+    mock_training_config.morphology.morph_mode = MorphMode.RING
+
+    with patch("brittle_star_project.evaluation.eval_env_builder.PolicyAgent.from_checkpoint"):
+        bundle = build_eval_env(
+            model_path=model_path,
+            training=mock_training_config,
+            metadata=mock_metadata,
+            morphology_override_path=override_path,
+        )
+        assert bundle.architecture == "RING"
diff --git a/tests/test_network_shapes.py b/tests/test_network_shapes.py
index 7bec744..ce189f4 100644
--- a/tests/test_network_shapes.py
+++ b/tests/test_network_shapes.py
@@ -36,12 +36,14 @@ def test_centralized_forward_pass_with_padding():
     )
     global_state = obs_processor(amputated_obs)
 
-    # 40 + 40 + 20 + padding = 145 dimensions
-    assert global_state.shape == (batch_size, 1, 145), (
-        f"Expected global state shape (2, 1, 145), got {global_state.shape}"
+    # joint_position: 5 arms × 8 joints (padded) = 40
+    # joint_velocity:  5 arms × 8 joints (padded) = 40
+    # segment_contact: 5 arms × 4 segs  (padded) = 20
+    # Total = 100 (no disk or direction keys supplied)
+    assert global_state.shape == (batch_size, 1, 100), (
+        f"Expected global state shape (2, 1, 100), got {global_state.shape}"
     )
 
-    # 4. Initialize dummy networks (40 actuators for the max morphology output)
     actor = Actor(action_dim=40)
     critic = OneDenseLayerMLP()  # Acts as the centralized critic
 
diff --git a/tests/test_obs_processor.py b/tests/test_obs_processor.py
index 17c5c8f..7008648 100644
--- a/tests/test_obs_processor.py
+++ b/tests/test_obs_processor.py
@@ -7,17 +7,14 @@ from brittle_star_project.environment.env_config import MorphMode, ObservationBo
 
 obs_bounds = ObservationBoundsConfig().to_bounds_dict()
 
-"""
-Test for obs_processor.
-
-Centralized: 40 features per agent:
-    disk_z_tilt              → scalar → reshaped to (1,)  → 1 feat
-    joint_actuator_force     → 8 joints padded to 8       → 8 feat
-    joint_position           → 8 joints padded to 8       → 8 feat
-    joint_velocity           → 8 joints padded to 8       → 8 feat
-    robot_direction_to_target→ (x, y)                     → 2 feat
-    segment_contact          → 4 segs, pre-padded by 9    → 13 feat (9 leading + 4)
-"""
+# Features per decentralized agent (one arm's data):
+#   disk_z_tilt              → scalar                    → 1 feat
+#   joint_actuator_force     → 4 segs × 2 joints         → 8 feat
+#   joint_position           → 4 segs × 2 joints         → 8 feat
+#   joint_velocity           → 4 segs × 2 joints         → 8 feat
+#   robot_direction_to_target→ (x, y)                    → 2 feat
+#   segment_contact          → 4 segs                    → 4 feat
+# Total per agent: 1+8+8+8+2+4 = 31
 
 NUM_ARMS = 5
 SEGS_PER_ARM = 4  # healthy segments per arm
@@ -28,7 +25,17 @@ SEGS_DAMAGED = [4, 4, 4, 4, 0]  # arm 4 fully disabled
 SEGS_DAMAGED_2 = [4, 0, 4, 2, 4]  # arm 3 fully disabled
 AGENT_INDICES = [0, 1, 2, 3, 4]
 
-FEAT_PER_AGENT = 1 + 8 + 8 + 8 + 2 + 13  # = 40
+FEAT_PER_AGENT = 1 + 8 + 8 + 8 + 2 + 4  # = 31
+
+# Centralized flattening (needed_copies=1, one copy of global features):
+#   disk_z_tilt              → repeated once              →  1 feat
+#   joint_actuator_force     → 5 arms × 8 joints          → 40 feat
+#   joint_position           → 5 arms × 8 joints          → 40 feat
+#   joint_velocity           → 5 arms × 8 joints          → 40 feat
+#   robot_direction_to_target→ repeated once              →  2 feat
+#   segment_contact          → 5 arms × 4 segs            → 20 feat
+# Total: 1+40+40+40+2+20 = 143
+FEAT_CENTRALIZED = 1 + 40 + 40 + 40 + 2 + 20  # = 143
 
 
 def make_obs(segs_per_arm: list[int]) -> dict:
@@ -73,10 +80,8 @@ def test_centralized_no_damage():
     obs = batch_obs(obs)
     global_state = proc(obs)
 
-    # shape test
-    assert global_state.shape == (1, 1, 188)
-
-    # TODO: more?
+    # Centralized: 5 agents flattened into 1 → shape (1, 1, 155)
+    assert global_state.shape == (1, 1, FEAT_CENTRALIZED)
 
 
 def test_centralized_damaged_1_arm():
@@ -86,7 +91,7 @@ def test_centralized_damaged_1_arm():
     global_state = proc(obs)
 
     # shape test
-    assert global_state.shape == (1, 1, 188)
+    assert global_state.shape == (1, 1, FEAT_CENTRALIZED)
 
 
 def test_centralized_damaged_2_arms():
@@ -96,7 +101,7 @@ def test_centralized_damaged_2_arms():
     global_state = proc(obs)
 
     # shape test
-    assert global_state.shape == (1, 1, 188)
+    assert global_state.shape == (1, 1, FEAT_CENTRALIZED)
 
 
 def test_decentralized_fully_connected_no_damage():