feat(policies): add joint frame transform and hardware deployment docs for MolmoAct2

Add MolmoAct2StateFrameTransformStep and MolmoAct2ActionFrameTransformStep
processor steps for cross-calibration compatibility on SO-100/101. Add
joint_signs and joint_offsets config fields. Add hardware deployment section
to molmoact2.mdx with camera naming convention, joint frame correction, and
safety guidance.

docs/source/molmoact2.mdx

@@ -386,6 +386,68 @@ These results demonstrate MolmoAct2's strong performance across diverse robotic
 manipulation tasks. To reproduce them, follow the instructions in the LIBERO
 evaluation section.
 
+## Hardware Deployment (lerobot-rollout)
+
+LeRobot-format checkpoints are available on the Hub for direct use with
+`lerobot-rollout`. Each checkpoint uses specific camera names that must
+match your robot's camera configuration.
+
+### Camera naming convention
+
+Each checkpoint expects specific `observation.images.*` keys.
+If your robot cameras have different names, use `--rename_map` to map them:
+
+| Checkpoint                    | Camera keys            | Description              |
+| ----------------------------- | ---------------------- | ------------------------ |
+| MolmoAct2-LIBERO-LeRobot      | `image`, `wrist_image` | LIBERO sim cameras       |
+| MolmoAct2-BimanualYAM-LeRobot | `top`, `left`, `right` | YAM 3-camera setup       |
+| MolmoAct2-DROID-LeRobot       | `cam0`, `cam1`         | External + wrist         |
+| MolmoAct2-SO100_101-LeRobot   | `cam0`, `cam1`         | Primary + secondary view |
+
+Example with an SO-100 robot using top and side cameras:
+
+```bash
+lerobot-rollout \
+  --policy.path=lerobot/MolmoAct2-SO100_101-LeRobot \
+  --rename_map='{"top": "cam0", "side": "cam1"}' \
+  --robot.type=so100_follower \
+  --robot.port=/dev/ttyACM0 \
+  --robot.cameras='{
+      top: {type: opencv, index_or_path: 0, width: 640, height: 480, fps: 30},
+      side: {type: opencv, index_or_path: 2, width: 640, height: 480, fps: 30}
+  }' \
+  --task="pick up the red cube" --duration=30
+```
+
+To use a wrist camera instead, just change the rename mapping:
+
+```bash
+--rename_map='{"top": "cam0", "wrist": "cam1"}'
+```
+
+### Joint frame transform (SO-100/101 zero-shot)
+
+<Tip warning={true}>
+The MolmoAct2-SO100_101 checkpoint was trained on data that uses a different
+joint calibration convention than LeRobot >= 0.5.0. Without a frame
+correction, the arm may move in the wrong direction.
+
+This affects both **zero-shot deployment** and **fine-tuning** from the
+original checkpoint. The pretrained weights expect the old convention, so
+all joint data (observations and actions) must be transformed to match.
+
+The converted LeRobot checkpoint (`lerobot/MolmoAct2-SO100_101-LeRobot`)
+already includes this correction in its processor pipeline. If you convert
+or fine-tune the checkpoint yourself, set the following in the policy config:
+
+- `joint_signs`: `[1, -1, 1, 1, 1, 1]` (flips shoulder_lift direction)
+- `joint_offsets`: `[0, 90, 90, 0, 0, 0]` (shifts shoulder_lift and elbow_flex by 90°)
+
+See the [backward compatibility guide](./backwardcomp) for details on the
+calibration change.
+
+</Tip>
+
 ## Differences From the Original Implementation
 
 This LeRobot port is intended to match MolmoAct2 behavior while using LeRobot's

src/lerobot/policies/molmoact2/configuration_molmoact2.py

@@ -79,6 +79,15 @@ class MolmoAct2Config(PreTrainedConfig):
     eval_seed: int | None = None
     rtc_config: RTCConfig | None = None
 
+    # Joint frame transform for cross-calibration compatibility.
+    # Some MolmoAct2 checkpoints were trained on data using a different joint
+    # convention than the current LeRobot calibration. Set both to apply a
+    # sign/offset correction at runtime (state before model, action after).
+    # See: https://huggingface.co/docs/lerobot/backwardcomp
+    # Default is None (no transform). Both must be set together.
+    joint_signs: list[float] | None = None
+    joint_offsets: list[float] | None = None
+
     # Default is full finetuning with gradients from the action expert flowing into the VLM.
     enable_lora_vlm: bool = False
     lora_rank: int = 64
@@ -123,6 +132,10 @@ class MolmoAct2Config(PreTrainedConfig):
 
     def __post_init__(self) -> None:
         super().__post_init__()
+        if (self.joint_signs is None) != (self.joint_offsets is None):
+            raise ValueError("joint_signs and joint_offsets must both be set or both be None.")
+        if self.joint_signs is not None and len(self.joint_signs) != len(self.joint_offsets):
+            raise ValueError("joint_signs and joint_offsets must have the same length.")
         if self.action_mode not in {"continuous", "discrete", "both"}:
             raise ValueError(
                 f"Unsupported action_mode={self.action_mode!r}. "

src/lerobot/policies/molmoact2/processor_molmoact2.py

@@ -1005,6 +1005,93 @@ class MolmoAct2PackInputsProcessorStep(ProcessorStep):
         return features
 
 
+@ProcessorStepRegistry.register(name="molmoact2_state_frame_transform")
+@dataclass
+class MolmoAct2StateFrameTransformStep(ProcessorStep):
+    """Convert robot state from arm frame to model frame before normalization.
+
+    Required for zero-shot deployment of MolmoAct2-SO100_101 on SO-100/101
+    arms calibrated with LeRobot >= 0.5.0 (v3.0 convention). The checkpoint
+    was trained on data using a different joint convention (sign flip on
+    shoulder_lift, 90 deg offset on shoulder_lift and elbow_flex).
+
+    No-op when joint_signs and joint_offsets are None (default), so this
+    step has no effect on fine-tuned models or other embodiments.
+
+    state_model = signs * arm_state + offsets
+
+    See: https://huggingface.co/docs/lerobot/backwardcomp
+    """
+
+    joint_signs: list[float] | None = None
+    joint_offsets: list[float] | None = None
+
+    def __call__(self, transition: EnvTransition) -> EnvTransition:
+        if self.joint_signs is None and self.joint_offsets is None:
+            return transition
+        observation = transition.get(TransitionKey.OBSERVATION)
+        if not isinstance(observation, dict) or OBS_STATE not in observation:
+            return transition
+        transition = transition.copy()
+        observation = observation.copy()
+        state = torch.as_tensor(observation[OBS_STATE], dtype=torch.float32)
+        n = len(self.joint_signs)
+        signs = torch.tensor(self.joint_signs, dtype=torch.float32, device=state.device)
+        offsets = torch.tensor(self.joint_offsets, dtype=torch.float32, device=state.device)
+        state[..., :n] = signs * state[..., :n] + offsets
+        observation[OBS_STATE] = state
+        transition[TransitionKey.OBSERVATION] = observation
+        return transition
+
+    def transform_features(
+        self, features: dict[PipelineFeatureType, dict[str, PolicyFeature]]
+    ) -> dict[PipelineFeatureType, dict[str, PolicyFeature]]:
+        return features
+
+    def get_config(self) -> dict[str, Any]:
+        return {"joint_signs": self.joint_signs, "joint_offsets": self.joint_offsets}
+
+
+@ProcessorStepRegistry.register(name="molmoact2_action_frame_transform")
+@dataclass
+class MolmoAct2ActionFrameTransformStep(ProcessorStep):
+    """Convert model action from model frame back to arm frame after unnormalization.
+
+    Inverse of MolmoAct2StateFrameTransformStep. Required for zero-shot
+    MolmoAct2-SO100_101 on SO-100/101 arms. No-op when both fields are None.
+
+    action_arm = signs * (model_action - offsets)
+
+    See: https://huggingface.co/docs/lerobot/backwardcomp
+    """
+
+    joint_signs: list[float] | None = None
+    joint_offsets: list[float] | None = None
+
+    def __call__(self, transition: EnvTransition) -> EnvTransition:
+        if self.joint_signs is None and self.joint_offsets is None:
+            return transition
+        action = transition.get(TransitionKey.ACTION)
+        if action is None:
+            return transition
+        transition = transition.copy()
+        action = torch.as_tensor(action, dtype=torch.float32)
+        n = len(self.joint_signs)
+        signs = torch.tensor(self.joint_signs, dtype=torch.float32, device=action.device)
+        offsets = torch.tensor(self.joint_offsets, dtype=torch.float32, device=action.device)
+        action[..., :n] = signs * (action[..., :n] - offsets)
+        transition[TransitionKey.ACTION] = action
+        return transition
+
+    def transform_features(
+        self, features: dict[PipelineFeatureType, dict[str, PolicyFeature]]
+    ) -> dict[PipelineFeatureType, dict[str, PolicyFeature]]:
+        return features
+
+    def get_config(self) -> dict[str, Any]:
+        return {"joint_signs": self.joint_signs, "joint_offsets": self.joint_offsets}
+
+
 @ProcessorStepRegistry.register(name="molmoact2_clamp_action")
 @dataclass
 class MolmoAct2ClampActionProcessorStep(ProcessorStep):
@@ -1067,6 +1154,10 @@ def make_molmoact2_pre_post_processors(
     input_steps: list[ProcessorStep] = [
         RenameObservationsProcessorStep(rename_map={}),
         AddBatchDimensionProcessorStep(),
+        MolmoAct2StateFrameTransformStep(
+            joint_signs=config.joint_signs,
+            joint_offsets=config.joint_offsets,
+        ),
         MolmoAct2MaskedNormalizerProcessorStep(
             features={**config.input_features, **config.output_features},
             norm_map=config.normalization_mapping,
@@ -1102,6 +1193,10 @@ def make_molmoact2_pre_post_processors(
             norm_map=config.normalization_mapping,
             stats=masked_dataset_stats,
         ),
+        MolmoAct2ActionFrameTransformStep(
+            joint_signs=config.joint_signs,
+            joint_offsets=config.joint_offsets,
+        ),
         DeviceProcessorStep(device="cpu"),
     ]
 

src/lerobot/rollout/context.py

@@ -320,7 +320,8 @@ def build_rollout_context(
             raise ValueError(
                 f"Visual feature mismatch between policy and robot hardware.\n"
                 f"Policy expects: {expected_visuals}\n"
-                f"Robot provides: {provided_visuals}"
+                f"Robot provides: {provided_visuals}\n"
+                "Use --rename_map to map camera names."
             )
 
     # --- 5. Dataset -------------