feat(data): add recipe-driven language supervision

docs/source/language_and_recipes.mdx

@@ -141,6 +141,17 @@ sample["target_message_indices"]
 
 The renderer does not apply a tokenizer chat template. Policy processors decide how to serialize the messages for their backbone, which keeps the same dataset usable across SmolVLA, Pi0.5, and any future VLM that expects OpenAI-style chat messages.
 
+## Blends
+
+Blend recipes select one weighted sub-recipe deterministically from the sample index.
+`recipes/subtask_mem.yaml` trains the compact core blend — high-level subtask prediction, low-level execution, and memory. `recipes/subtask_mem_vqa_speech.yaml` is the fuller variant that also adds VQA and spoken interjection responses.
+
+A message recipe with a supervised assistant turn on the `low_level` stream trains
+the π0.5 paper's joint sequence instead of a blend: the target span gets text CE
+while also conditioning the action losses in the same forward.
+`recipes/subtask_joint.yaml` is the provided example; pair it with
+`--policy.joint_subtask_conditioning=true` at inference.
+
 ## Graceful absence
 
 If both language columns are missing, `None`, or empty, `RenderMessagesStep` is a no-op.

src/lerobot/configs/default.py

@@ -33,6 +33,8 @@ class DatasetConfig:
     # looked up under $HF_LEROBOT_HOME/repo_id and Hub downloads use a revision-safe cache under $HF_LEROBOT_HOME/hub.
     root: str | None = None
     episodes: list[int] | None = None
+    # Episode indices to drop (e.g. corrupt or heterogeneous ones). Applied on top of `episodes`.
+    exclude_episodes: list[int] | None = None
     image_transforms: ImageTransformsConfig = field(default_factory=ImageTransformsConfig)
     revision: str | None = None
     use_imagenet_stats: bool = True
@@ -62,6 +64,10 @@ class DatasetConfig:
             if len(self.episodes) != len(set(self.episodes)):
                 duplicates = sorted({ep for ep in self.episodes if self.episodes.count(ep) > 1})
                 raise ValueError(f"Episode indices contain duplicates: {duplicates}")
+        if self.exclude_episodes is not None and any(ep < 0 for ep in self.exclude_episodes):
+            raise ValueError(
+                f"exclude_episodes must be non-negative, got: {[ep for ep in self.exclude_episodes if ep < 0]}"
+            )
 
 
 @dataclass

src/lerobot/configs/recipe.py

@@ -78,7 +78,7 @@ class MessageTurn:
             raise ValueError(f"Unsupported message stream: {self.stream!r}")
         if self.content is None and self.tool_calls_from is None:
             raise ValueError("MessageTurn.content is required unless tool_calls_from is set.")
-        if self.content is not None and not isinstance(self.content, (str, list)):
+        if self.content is not None and not isinstance(self.content, str | list):
             raise TypeError("MessageTurn.content must be a string, a list of HF-style blocks, or None.")
         if isinstance(self.content, list):
             for block in self.content:
@@ -147,7 +147,7 @@ class TrainingRecipe:
         return cls.from_dict(data)
 
     def _validate_message_recipe(self) -> None:
-        """Ensure every templated binding is known and at least one turn is a target."""
+        """Validate bindings and require text or low-level action supervision."""
         assert self.messages is not None
         known_bindings = set(DEFAULT_BINDINGS) | set(self.bindings or {}) | {"task"}
 
@@ -156,8 +156,14 @@ class TrainingRecipe:
             if missing:
                 raise ValueError(f"MessageTurn references unknown binding(s): {sorted(missing)}")
 
-        if not any(turn.target for turn in self.messages):
+        has_target = any(turn.target for turn in self.messages)
-            raise ValueError("Message recipes must contain at least one target turn.")
+        has_low_level = any(turn.stream == "low_level" for turn in self.messages)
+        if not (has_target or has_low_level):
+            raise ValueError(
+                "Message recipes must contain at least one supervised turn — "
+                "either ``target: true`` (text CE) or ``stream: low_level`` "
+                "(flow/action loss)."
+            )
 
     def _validate_blend_recipe(self) -> None:
         """Ensure each blend component is a non-empty, weighted message recipe."""

src/lerobot/configs/recipes/subtask.yaml

@@ -0,0 +1,16 @@
+# Predicts subtasks from tasks and trains subtask-conditioned action flow without memory or plans.
+# Requires `subtask` annotations; samples with missing `if_present` bindings do not render.
+
+blend:
+
+  high_level_subtask:
+    weight: 0.30
+    messages:
+      - {role: user, content: "${task}", stream: high_level}
+      - {role: assistant, content: "${subtask}", stream: high_level, target: true, if_present: subtask}
+
+  low_level_execution:
+    weight: 0.70
+    messages:
+      # The low-level stream trains action flow on the generated or annotated subtask.
+      - {role: user, content: "${subtask}", stream: low_level, if_present: subtask}

src/lerobot/configs/recipes/subtask_joint.yaml

@@ -0,0 +1,13 @@
+# Paper-style joint sequence (pi0.5 §IV-B): one sample supervises the subtask
+# text with CE and, because the assistant turn is part of the prefix, conditions
+# the FAST and flow action losses on the same annotated subtask in one forward.
+# The supervised span is attended causally; the action losses see task + subtask.
+#
+# Pair with `--policy.joint_subtask_conditioning=true` at inference so the flow
+# prefix reproduces this layout (task turn with state + causal generated subtask).
+# Samples without a `subtask` annotation fall back to a plain task-prompt
+# low-level sample via `if_present`.
+
+messages:
+  - {role: user, content: "${task}", stream: low_level}
+  - {role: assistant, content: "${subtask}", stream: low_level, target: true, if_present: subtask}

src/lerobot/configs/recipes/subtask_mem.yaml

@@ -0,0 +1,30 @@
+# Trains subtask prediction, subtask-conditioned action flow, and memory updates without plans.
+# Requires `subtask` and `memory`; missing `if_present` bindings skip the affected sub-recipe.
+
+blend:
+
+  high_level_subtask:
+    weight: 0.25
+    messages:
+      - {role: user, content: "${task}", stream: high_level}
+      - {role: assistant, content: "${subtask}", stream: high_level, target: true, if_present: subtask}
+
+  low_level_execution:
+    weight: 0.60
+    messages:
+      # The low-level stream trains action flow on the generated or annotated subtask.
+      - {role: user, content: "${subtask}", stream: low_level, if_present: subtask}
+
+  memory_update:
+    # `active_at` densifies sparse boundaries while preserving the prior-memory/subtask mapping.
+    # Inference controls update timing through `subtask_change` events.
+    weight: 0.15
+    bindings:
+      prior_memory: "nth_prev(style=memory, offset=1)"
+      current_memory: "active_at(t, style=memory)"
+      completed_subtask: "nth_prev(style=subtask, offset=1)"
+    messages:
+      - {role: user, content: "${task}", stream: high_level}
+      - {role: assistant, content: "Previous memory: ${prior_memory}", stream: high_level, if_present: prior_memory}
+      - {role: user, content: "Completed subtask: ${completed_subtask}", stream: high_level, if_present: completed_subtask}
+      - {role: assistant, content: "${current_memory}", stream: high_level, target: true, if_present: current_memory}

src/lerobot/configs/recipes/subtask_mem_vqa_speech.yaml

@@ -0,0 +1,70 @@
+# Adds memory, spoken interjection responses, and camera-grounded VQA to subtask/action training.
+# Missing optional annotations skip only their sub-recipe; `say` tool calls tokenize as `<say>...</say>`.
+
+blend:
+
+  high_level_subtask:
+    weight: 0.25
+    messages:
+      - {role: user, content: "${task}", stream: high_level}
+      - {role: assistant, content: "${subtask}", stream: high_level, target: true, if_present: subtask}
+
+  low_level_execution:
+    weight: 0.40
+    messages:
+      # The low-level stream trains action flow on the generated or annotated subtask.
+      - {role: user, content: "${subtask}", stream: low_level, if_present: subtask}
+
+  memory_update:
+    # `active_at` densifies sparse boundaries while preserving the prior-memory/subtask mapping.
+    # Inference controls update timing through `subtask_change` events.
+    weight: 0.10
+    bindings:
+      prior_memory: "nth_prev(style=memory, offset=1)"
+      current_memory: "active_at(t, style=memory)"
+      completed_subtask: "nth_prev(style=subtask, offset=1)"
+    messages:
+      - {role: user, content: "${task}", stream: high_level}
+      - {role: assistant, content: "Previous memory: ${prior_memory}", stream: high_level, if_present: prior_memory}
+      - {role: user, content: "Completed subtask: ${completed_subtask}", stream: high_level, if_present: completed_subtask}
+      - {role: assistant, content: "${current_memory}", stream: high_level, target: true, if_present: current_memory}
+
+  user_interjection_response:
+    weight: 0.10
+    bindings:
+      interjection: "emitted_at(t, style=interjection)"
+      speech: "emitted_at(t, role=assistant, tool_name=say)"
+    messages:
+      - {role: user, content: "${task}", stream: high_level}
+      - {role: user, content: "${interjection}", stream: high_level, if_present: interjection}
+      # The assistant target is a `say` tool call flattened to a `<say>...</say>` marker.
+      - {role: assistant, stream: high_level, target: true, if_present: speech, tool_calls_from: speech}
+
+  # Each camera uses a separate VQA sub-recipe for view-specific binding.
+  ask_vqa_top:
+    weight: 0.075
+    bindings:
+      vqa_query: "emitted_at(t, style=vqa, role=user, camera=observation.images.front)"
+      vqa: "emitted_at(t, style=vqa, role=assistant, camera=observation.images.front)"
+    messages:
+      - role: user
+        stream: high_level
+        if_present: vqa_query
+        content:
+          - {type: image, feature: observation.images.front}
+          - {type: text, text: "${vqa_query}"}
+      - {role: assistant, content: "${vqa}", stream: high_level, target: true, if_present: vqa}
+
+  ask_vqa_wrist:
+    weight: 0.075
+    bindings:
+      vqa_query: "emitted_at(t, style=vqa, role=user, camera=observation.images.wrist)"
+      vqa: "emitted_at(t, style=vqa, role=assistant, camera=observation.images.wrist)"
+    messages:
+      - role: user
+        stream: high_level
+        if_present: vqa_query
+        content:
+          - {type: image, feature: observation.images.wrist}
+          - {type: text, text: "${vqa_query}"}
+      - {role: assistant, content: "${vqa}", stream: high_level, target: true, if_present: vqa}

src/lerobot/datasets/dataset_reader.py

@@ -163,10 +163,40 @@ class DatasetReader:
     def _load_hf_dataset(self) -> datasets.Dataset:
         """hf_dataset contains all the observations, states, actions, rewards, etc."""
         features = get_hf_features_from_features(self._meta.features)
+        # Annotated datasets may have language columns absent from metadata.
+        # Extend the schema before the strict Parquet cast.
+        features = self._extend_features_with_language_columns(features)
         hf_dataset = load_nested_dataset(self.root / "data", features=features, episodes=self.episodes)
         hf_dataset.set_transform(hf_transform_to_torch)
         return hf_dataset
 
+    def _extend_features_with_language_columns(self, features: datasets.Features) -> datasets.Features:
+        """Register language columns found in Parquet but missing from metadata."""
+        # Leave empty datasets to fail through the normal loading path.
+        try:
+            sample = next((self.root / "data").glob("*/*.parquet"))
+        except StopIteration:
+            return features
+
+        from pyarrow import parquet as _pq  # noqa: PLC0415
+
+        schema_names = set(_pq.read_schema(sample).names)
+        from .language import (  # noqa: PLC0415
+            LANGUAGE_EVENTS,
+            LANGUAGE_PERSISTENT,
+            language_events_column_feature,
+            language_persistent_column_feature,
+        )
+
+        extra: dict[str, object] = {}
+        if LANGUAGE_PERSISTENT in schema_names and LANGUAGE_PERSISTENT not in features:
+            extra[LANGUAGE_PERSISTENT] = language_persistent_column_feature()
+        if LANGUAGE_EVENTS in schema_names and LANGUAGE_EVENTS not in features:
+            extra[LANGUAGE_EVENTS] = language_events_column_feature()
+        if not extra:
+            return features
+        return datasets.Features({**features, **extra})
+
     def _check_cached_episodes_sufficient(self) -> bool:
         """Check if the cached dataset contains all requested episodes and their video files."""
         if self.hf_dataset is None or len(self.hf_dataset) == 0:

src/lerobot/datasets/factory.py

@@ -66,6 +66,17 @@ def resolve_delta_timestamps(
     return delta_timestamps
 
 
+def _resolve_episodes(
+    episodes: list[int] | None, exclude_episodes: list[int] | None, total_episodes: int
+) -> list[int] | None:
+    """Apply an episode exclusion list on top of an optional allowlist."""
+    if not exclude_episodes:
+        return episodes
+    base = episodes if episodes is not None else list(range(total_episodes))
+    excluded = set(exclude_episodes)
+    return [episode for episode in base if episode not in excluded]
+
+
 def make_dataset(cfg: TrainPipelineConfig) -> LeRobotDataset | MultiLeRobotDataset:
     """Handles the logic of setting up delta timestamps and image transforms before creating a dataset.
 
@@ -87,11 +98,14 @@ def make_dataset(cfg: TrainPipelineConfig) -> LeRobotDataset | MultiLeRobotDatas
             cfg.dataset.repo_id, root=cfg.dataset.root, revision=cfg.dataset.revision
         )
         delta_timestamps = resolve_delta_timestamps(cfg.trainable_config, ds_meta)
+        episodes = _resolve_episodes(
+            cfg.dataset.episodes, cfg.dataset.exclude_episodes, ds_meta.total_episodes
+        )
         if not cfg.dataset.streaming:
             dataset = LeRobotDataset(
                 cfg.dataset.repo_id,
                 root=cfg.dataset.root,
-                episodes=cfg.dataset.episodes,
+                episodes=episodes,
                 delta_timestamps=delta_timestamps,
                 image_transforms=image_transforms,
                 revision=cfg.dataset.revision,
@@ -104,7 +118,7 @@ def make_dataset(cfg: TrainPipelineConfig) -> LeRobotDataset | MultiLeRobotDatas
             dataset = StreamingLeRobotDataset(
                 cfg.dataset.repo_id,
                 root=cfg.dataset.root,
-                episodes=cfg.dataset.episodes,
+                episodes=episodes,
                 delta_timestamps=delta_timestamps,
                 image_transforms=image_transforms,
                 revision=cfg.dataset.revision,

src/lerobot/datasets/language_render.py

@@ -162,14 +162,28 @@ def render_sample(
     task: str | None = None,
     dataset_ctx: Any | None = None,
 ) -> RenderedMessages | None:
-    """Render the chat-style messages for a single dataset sample.
+    """Resolve one sample's bindings and render its message recipe.
 
-    Resolves the recipe's bindings against ``persistent`` and ``events`` rows
+    Returns ``None`` when no text or low-level action supervision applies.
-    at frame timestamp ``t``, then expands the recipe's message templates.
-    Returns ``None`` if the resolved sample contains no target message.
     """
     persistent_rows = _normalize_rows(persistent or [])
     event_rows = _normalize_rows(events or [])
+
+    # Route sparse VQA frames to a matching view-specific component before weighted selection.
+    # This avoids dropping annotated frames or selecting VQA without annotations.
+    if recipe.blend is not None:
+        vqa_rendered = _render_vqa_if_present(
+            recipe,
+            persistent=persistent_rows,
+            events=event_rows,
+            t=t,
+            sample_idx=sample_idx,
+            task=task,
+            dataset_ctx=dataset_ctx,
+        )
+        if vqa_rendered is not None:
+            return vqa_rendered
+
     selected_recipe = _select_recipe(recipe, sample_idx)
     bindings = _resolve_bindings(
         selected_recipe,
@@ -183,6 +197,55 @@ def render_sample(
     return _render_message_recipe(selected_recipe, bindings)
 
 
+def _render_vqa_if_present(
+    recipe: TrainingRecipe,
+    *,
+    persistent: Sequence[LanguageRow],
+    events: Sequence[LanguageRow],
+    t: float,
+    sample_idx: int,
+    task: str | None,
+    dataset_ctx: Any | None,
+) -> RenderedMessages | None:
+    """Render a matching VQA component, or return ``None`` for normal selection.
+
+    Multiple matching views are selected deterministically by relative weight.
+    """
+    assert recipe.blend is not None
+    renderable: list[tuple[float, RenderedMessages]] = []
+    for name, component in recipe.blend.items():
+        if not name.startswith("ask_vqa"):
+            continue
+        bindings = _resolve_bindings(
+            component,
+            persistent=persistent,
+            events=events,
+            t=t,
+            sample_idx=sample_idx,
+            task=task,
+            dataset_ctx=dataset_ctx,
+        )
+        rendered = _render_message_recipe(component, bindings)
+        if rendered is not None:
+            renderable.append((float(component.weight or 0.0), rendered))
+
+    if not renderable:
+        return None
+    if len(renderable) == 1:
+        return renderable[0][1]
+
+    # Choose among matching cameras by relative weight, or uniformly when all weights are zero.
+    total = sum(w for w, _ in renderable) or float(len(renderable))
+    digest = hashlib.blake2b(f"vqa:{sample_idx}".encode(), digest_size=8).digest()
+    draw = int.from_bytes(digest, "big") / 2**64 * total
+    cumulative = 0.0
+    for w, rendered in renderable:
+        cumulative += w or (total / len(renderable))
+        if draw < cumulative:
+            return rendered
+    return renderable[-1][1]
+
+
 def _select_recipe(recipe: TrainingRecipe, sample_idx: int) -> TrainingRecipe:
     """Pick a deterministic blend component for ``sample_idx`` (or return ``recipe``)."""
     if recipe.blend is None:
@@ -346,7 +409,9 @@ def _render_message_recipe(
         if turn.target:
             target_indices.append(message_idx)
 
-    if not target_indices:
+    # Keep samples with either text targets or low-level action supervision.
+    has_low_level = any(stream == "low_level" for stream in streams)
+    if not target_indices and not has_low_level:
         return None
 
     rendered = {
@@ -403,14 +468,12 @@ def _validate_rendered(rendered: RenderedMessages) -> None:
 
     if len(streams) != len(messages):
         raise ValueError("message_streams must be aligned with messages.")
-    if not target_indices:
+    # Require text or low-level action supervision.
-        raise ValueError("Rendered samples must contain at least one target message.")
+    if not target_indices and not any(s == "low_level" for s in streams):
+        raise ValueError("Rendered samples must contain a target message or a low_level-stream message.")
     for idx in target_indices:
         if idx < 0 or idx >= len(messages):
             raise ValueError(f"Target message index {idx} is out of bounds.")
-    # ``stream`` is enforced non-None at MessageTurn construction time
-    # (see ``MessageTurn.__post_init__``), so a missing stream here would
-    # mean the dataclass invariant was bypassed; no need to re-check.
 
 
 def _nth_relative(

src/lerobot/processor/batch_processor.py

@@ -175,9 +175,6 @@ class AddBatchDimensionComplementaryDataStep(ComplementaryDataProcessorStep):
             if isinstance(task_index_value, Tensor) and task_index_value.dim() == 0:
                 complementary_data["task_index"] = task_index_value.unsqueeze(0)
 
-        complementary_data.pop("language_persistent", None)
-        complementary_data.pop("language_events", None)
-
         if "messages" in complementary_data:
             messages = complementary_data["messages"]
             if isinstance(messages, list) and (not messages or isinstance(messages[0], dict)):

src/lerobot/processor/pipeline.py

@@ -41,7 +41,7 @@ from pathlib import Path
 from typing import Any, TypedDict, TypeVar, cast
 
 import torch
-from huggingface_hub import hf_hub_download
+from huggingface_hub import hf_hub_download, snapshot_download
 from safetensors.torch import load_file, save_file
 
 from lerobot.configs import PipelineFeatureType, PolicyFeature
@@ -205,6 +205,10 @@ class ProcessorStep(ABC):
         """
         return None
 
+    def save_artifacts(self, save_directory: Path) -> dict[str, str]:
+        """Save non-tensor assets and map constructor arguments to relative paths."""
+        return {}
+
     def reset(self) -> None:
         """Resets the internal state of the processor step, if any."""
         return None
@@ -549,6 +553,22 @@ class DataProcessorPipeline[TInput, TOutput](HubMixin):
         pipeline_config = self.get_config()
         pipeline_state_dict = self.state_dict()
 
+        for processor_step, step_entry in zip(self.steps, pipeline_config["steps"], strict=True):
+            artifacts = processor_step.save_artifacts(save_directory)
+            if artifacts:
+                for config_key, relative_path in artifacts.items():
+                    artifact_path = Path(relative_path)
+                    if artifact_path.is_absolute() or ".." in artifact_path.parts:
+                        raise ValueError(
+                            f"Processor artifact path must be relative to the checkpoint: {relative_path!r}"
+                        )
+                    if not (save_directory / artifact_path).exists():
+                        raise FileNotFoundError(
+                            f"Processor step did not save declared artifact '{relative_path}'"
+                        )
+                    step_entry["config"][config_key] = artifact_path.as_posix()
+                step_entry["artifacts"] = artifacts
+
         for state_key, step_state_dict in pipeline_state_dict.items():
             state_filename = f"{state_key}.safetensors"
             save_file(step_state_dict, save_directory / state_filename)
@@ -733,7 +753,13 @@ class DataProcessorPipeline[TInput, TOutput](HubMixin):
 
         # 3. Build steps with overrides
         steps, validated_overrides = cls._build_steps_with_overrides(
-            loaded_config, overrides or {}, model_id, base_path, hub_download_kwargs, is_local_source
+            loaded_config,
+            overrides or {},
+            model_id,
+            base_path,
+            config_filename,
+            hub_download_kwargs,
+            is_local_source,
         )
 
         # 4. Validate that all overrides were used
@@ -922,6 +948,7 @@ class DataProcessorPipeline[TInput, TOutput](HubMixin):
         overrides: dict[str, Any],
         model_id: str,
         base_path: Path | None,
+        config_filename: str,
         hub_download_kwargs: dict[str, Any],
         is_local_source: bool = False,
     ) -> tuple[list[ProcessorStep], set[str]]:
@@ -976,15 +1003,68 @@ class DataProcessorPipeline[TInput, TOutput](HubMixin):
             ImportError: If a step class cannot be imported or found in registry
             ValueError: If a step cannot be instantiated with its configuration
         """
+        loaded_config = deepcopy(loaded_config)
+        cls._resolve_artifact_paths(
+            loaded_config,
+            model_id,
+            base_path,
+            config_filename,
+            hub_download_kwargs,
+        )
         steps, remaining_override_keys = cls._build_steps_from_config(loaded_config, overrides)
 
         for step_instance, step_entry in zip(steps, loaded_config["steps"], strict=True):
             cls._load_step_state(
-                step_instance, step_entry, model_id, base_path, hub_download_kwargs, is_local_source
+                step_instance,
+                step_entry,
+                model_id,
+                base_path,
+                config_filename,
+                hub_download_kwargs,
+                is_local_source,
             )
 
         return steps, remaining_override_keys
 
+    @classmethod
+    def _resolve_artifact_paths(
+        cls,
+        loaded_config: dict[str, Any],
+        model_id: str,
+        base_path: Path | None,
+        config_filename: str,
+        hub_download_kwargs: dict[str, Any],
+    ) -> None:
+        """Resolve declared relative processor artifacts before step construction."""
+        is_local = Path(model_id).is_dir() or Path(model_id).is_file()
+
+        for step_entry in loaded_config["steps"]:
+            artifacts = step_entry.get("artifacts", {})
+            for config_key, relative_path in artifacts.items():
+                artifact_path = Path(relative_path)
+                if artifact_path.is_absolute() or ".." in artifact_path.parts:
+                    raise ValueError(
+                        f"Processor artifact path must be relative to the checkpoint: {relative_path!r}"
+                    )
+
+                resolved_path = base_path / artifact_path if base_path is not None else artifact_path
+                if not resolved_path.exists() and not is_local:
+                    repository_path = Path(config_filename).parent / artifact_path
+                    snapshot_download(
+                        repo_id=model_id,
+                        repo_type="model",
+                        allow_patterns=f"{repository_path.as_posix()}/**",
+                        **hub_download_kwargs,
+                    )
+
+                if not resolved_path.exists():
+                    step_name = step_entry.get("registry_name", step_entry.get("class", "unknown"))
+                    raise FileNotFoundError(
+                        f"Missing processor artifact '{relative_path}' for step '{step_name}' "
+                        f"next to '{config_filename}'. Checkpoint artifacts are incomplete."
+                    )
+                step_entry["config"][config_key] = str(resolved_path)
+
     @classmethod
     def _build_steps_from_config(
         cls,
@@ -1144,6 +1224,7 @@ class DataProcessorPipeline[TInput, TOutput](HubMixin):
         step_entry: dict[str, Any],
         model_id: str,
         base_path: Path | None,
+        config_filename: str,
         hub_download_kwargs: dict[str, Any],
         is_local_source: bool = False,
     ) -> None:
@@ -1209,7 +1290,7 @@ class DataProcessorPipeline[TInput, TOutput](HubMixin):
             # Download from Hub
             state_path = hf_hub_download(
                 repo_id=model_id,
-                filename=state_filename,
+                filename=(Path(config_filename).parent / state_filename).as_posix(),
                 repo_type="model",
                 **hub_download_kwargs,
             )

src/lerobot/processor/render_messages_processor.py

@@ -16,7 +16,7 @@
 
 from __future__ import annotations
 
-from dataclasses import dataclass
+from dataclasses import asdict, dataclass
 from typing import Any
 
 from lerobot.configs import PipelineFeatureType, PolicyFeature
@@ -32,17 +32,18 @@ from .pipeline import ProcessorStep, ProcessorStepRegistry
 @dataclass
 @ProcessorStepRegistry.register(name="render_messages_processor")
 class RenderMessagesStep(ProcessorStep):
-    """Processor step that turns raw language columns into rendered chat messages.
+    """Render language columns into recipe-defined messages and supervision metadata."""
-
-    Reads ``language_persistent`` and ``language_events`` from the transition's
-    complementary data, renders them through ``recipe`` at the sample timestamp,
-    and replaces the raw columns with the resulting ``messages`` /
-    ``message_streams`` / ``target_message_indices`` keys.
-    """
 
     recipe: TrainingRecipe
     dataset_ctx: Any | None = None
 
+    def __post_init__(self) -> None:
+        if isinstance(self.recipe, dict):
+            self.recipe = TrainingRecipe.from_dict(self.recipe)
+
+    def get_config(self) -> dict[str, Any]:
+        return {"recipe": asdict(self.recipe)}
+
     def __call__(self, transition: EnvTransition) -> EnvTransition | None:
         """Render messages for a single transition; return ``None`` to drop it."""
         complementary_data = transition.get(TransitionKey.COMPLEMENTARY_DATA) or {}
@@ -50,7 +51,17 @@ class RenderMessagesStep(ProcessorStep):
         events = complementary_data.get(LANGUAGE_EVENTS) or []
 
         if not persistent and not events:
-            return transition
+            rendered = _fallback_low_level_render(complementary_data.get("task"))
+            if rendered is None:
+                return transition
+            new_transition = transition.copy()
+            new_complementary_data = dict(new_transition.get(TransitionKey.COMPLEMENTARY_DATA) or {})
+            new_complementary_data.update(rendered)
+            new_transition[TransitionKey.COMPLEMENTARY_DATA] = new_complementary_data
+            return new_transition
+
+        if _is_batched_language(persistent) or _is_batched_language(events):
+            return self._call_batch(transition, complementary_data, persistent, events)
 
         timestamp = complementary_data.get("timestamp")
         if timestamp is None:
@@ -67,18 +78,147 @@ class RenderMessagesStep(ProcessorStep):
             dataset_ctx=self.dataset_ctx,
         )
         if rendered is None:
-            return None
+            rendered = _fallback_low_level_render(complementary_data.get("task"))
+            if rendered is None:
+                return None
 
         new_transition = transition.copy()
-        new_complementary_data = dict(complementary_data)
+        new_complementary_data = dict(new_transition.get(TransitionKey.COMPLEMENTARY_DATA) or {})
         new_complementary_data.pop(LANGUAGE_PERSISTENT, None)
         new_complementary_data.pop(LANGUAGE_EVENTS, None)
         new_complementary_data.update(rendered)
         new_transition[TransitionKey.COMPLEMENTARY_DATA] = new_complementary_data
         return new_transition
 
+    def _call_batch(
+        self,
+        transition: EnvTransition,
+        complementary_data: dict[str, Any],
+        persistent_batch: list,
+        events_batch: list,
+    ) -> EnvTransition | None:
+        timestamp = complementary_data.get("timestamp")
+        if timestamp is None:
+            raise KeyError("RenderMessagesStep requires sample timestamp in complementary data.")
+
+        batch_size = max(len(persistent_batch), len(events_batch))
+        messages: list[list[dict[str, Any]]] = []
+        message_streams: list[list[str | None]] = []
+        target_message_indices: list[list[int]] = []
+        keep_indices: list[int] = []
+
+        for i in range(batch_size):
+            rendered = render_sample(
+                recipe=self.recipe,
+                persistent=persistent_batch[i] if i < len(persistent_batch) else [],
+                events=events_batch[i] if i < len(events_batch) else [],
+                t=_batch_value(timestamp, i),
+                sample_idx=int(_batch_value(complementary_data.get("index", 0), i)),
+                task=_batch_value(complementary_data.get("task"), i),
+                dataset_ctx=self.dataset_ctx,
+            )
+            if rendered is None:
+                rendered = _fallback_low_level_render(_batch_value(complementary_data.get("task"), i))
+                if rendered is None:
+                    continue
+            keep_indices.append(i)
+            messages.append(rendered["messages"])
+            message_streams.append(rendered["message_streams"])
+            target_message_indices.append(rendered["target_message_indices"])
+
+        if not messages:
+            return None
+
+        new_transition = (
+            _select_batch_indices(transition, keep_indices)
+            if len(keep_indices) != batch_size
+            else transition.copy()
+        )
+        new_complementary_data = dict(new_transition.get(TransitionKey.COMPLEMENTARY_DATA) or {})
+        new_complementary_data.pop(LANGUAGE_PERSISTENT, None)
+        new_complementary_data.pop(LANGUAGE_EVENTS, None)
+        new_complementary_data["messages"] = messages
+        new_complementary_data["message_streams"] = message_streams
+        new_complementary_data["target_message_indices"] = target_message_indices
+        new_transition[TransitionKey.COMPLEMENTARY_DATA] = new_complementary_data
+        return new_transition
+
     def transform_features(
         self, features: dict[PipelineFeatureType, dict[str, PolicyFeature]]
     ) -> dict[PipelineFeatureType, dict[str, PolicyFeature]]:
         """Pass features through unchanged; rendering only touches complementary data."""
         return features
+
+
+def _scalar(value: Any) -> float | int:
+    """Unwrap a tensor/array/single-element list into a Python scalar."""
+    if hasattr(value, "item"):
+        return value.item()
+    if isinstance(value, list):
+        if len(value) != 1:
+            raise ValueError(f"Expected a scalar, got list of length {len(value)}: {value!r}")
+        return _scalar(value[0])
+    return value
+
+
+def _is_batched_language(value: Any) -> bool:
+    return isinstance(value, list) and bool(value) and isinstance(value[0], list)
+
+
+def _batch_value(value: Any, index: int) -> Any:
+    if value is None:
+        return None
+    if isinstance(value, list):
+        return value[index]
+    if hasattr(value, "ndim") and value.ndim > 0:
+        return _scalar(value[index])
+    return _scalar(value)
+
+
+def _select_batch_indices(transition: EnvTransition, indices: list[int]) -> EnvTransition:
+    selected = transition.copy()
+    for key in (TransitionKey.OBSERVATION, TransitionKey.COMPLEMENTARY_DATA):
+        data = selected.get(key)
+        if isinstance(data, dict):
+            selected[key] = {k: _select_value(v, indices) for k, v in data.items()}
+    action = selected.get(TransitionKey.ACTION)
+    if action is not None:
+        selected[TransitionKey.ACTION] = _select_value(action, indices)
+    return selected
+
+
+def _select_value(value: Any, indices: list[int]) -> Any:
+    if isinstance(value, list) and len(value) >= len(indices):
+        return [value[i] for i in indices]
+    if hasattr(value, "index_select") and hasattr(value, "new_tensor") and getattr(value, "ndim", 0) > 0:
+        return value.index_select(0, value.new_tensor(indices).long())
+    return value
+
+
+def _fallback_low_level_render(task: Any) -> dict[str, Any] | None:
+    """Keep action-only samples trainable when no recipe branch matches."""
+    if hasattr(task, "item"):
+        task = task.item()
+    if isinstance(task, list):
+        messages = []
+        message_streams = []
+        target_message_indices = []
+        for t in task:
+            rendered = _fallback_low_level_render(t)
+            if rendered is None:
+                return None
+            messages.append(rendered["messages"])
+            message_streams.append(rendered["message_streams"])
+            target_message_indices.append(rendered["target_message_indices"])
+        return {
+            "messages": messages,
+            "message_streams": message_streams,
+            "target_message_indices": target_message_indices,
+        }
+    if not isinstance(task, str) or not task:
+        return None
+    return {
+        "messages": [{"role": "user", "content": task}],
+        "message_streams": ["low_level"],
+        "target_message_indices": [],
+    }

src/lerobot/processor/tokenizer_processor.py

@@ -25,6 +25,7 @@ from __future__ import annotations
 
 import logging
 from dataclasses import dataclass, field
+from pathlib import Path
 from typing import TYPE_CHECKING, Any
 
 import torch
@@ -32,6 +33,7 @@ import torch
 from lerobot.configs import FeatureType, PipelineFeatureType, PolicyFeature
 from lerobot.types import EnvTransition, RobotObservation, TransitionKey
 from lerobot.utils.constants import (
+    ACTION_CODE_TOKEN_MASK,
     ACTION_TOKEN_MASK,
     ACTION_TOKENS,
     OBS_LANGUAGE_ATTENTION_MASK,
@@ -136,7 +138,7 @@ class TokenizerProcessorStep(ObservationProcessorStep):
         # Standardize to a list of strings for the tokenizer
         if isinstance(task, str):
             return [task]
-        elif isinstance(task, (list, tuple)) and all(isinstance(t, str) for t in task):
+        elif isinstance(task, list | tuple) and all(isinstance(t, str) for t in task):
             return list(task)
 
         return None
@@ -349,6 +351,7 @@ class ActionTokenizerProcessorStep(ActionProcessorStep):
     max_action_tokens: int = 256
     fast_skip_tokens: int = 128
     paligemma_tokenizer_name: str = "google/paligemma-3b-pt-224"
+    allow_truncation: bool = True
     # Internal tokenizer instance (not part of the config)
     action_tokenizer: Any = field(default=None, init=False, repr=False)
     _paligemma_tokenizer: Any = field(default=None, init=False, repr=False)
@@ -412,14 +415,15 @@ class ActionTokenizerProcessorStep(ActionProcessorStep):
             # During inference, no action is available, skip tokenization
             return new_transition
 
-        # Tokenize and get both tokens and mask
+        # Tokenize and get masks for the full formatted sequence and the discrete action codes.
-        tokens, mask = self._tokenize_action(action)
+        tokens, mask, code_mask = self._tokenize_action(action)
 
         # Store mask in complementary data
         complementary_data = new_transition.get(TransitionKey.COMPLEMENTARY_DATA, {})
         if complementary_data is None:
             complementary_data = {}
         complementary_data[ACTION_TOKEN_MASK] = mask
+        complementary_data[ACTION_CODE_TOKEN_MASK] = code_mask
         complementary_data[ACTION_TOKENS] = tokens
         new_transition[TransitionKey.COMPLEMENTARY_DATA] = complementary_data
         return new_transition
@@ -430,7 +434,7 @@ class ActionTokenizerProcessorStep(ActionProcessorStep):
         """
         return self._paligemma_tokenizer.vocab_size - 1 - self.fast_skip_tokens - tokens
 
-    def _tokenize_action(self, action: torch.Tensor) -> tuple[torch.Tensor, torch.Tensor]:
+    def _tokenize_action(self, action: torch.Tensor) -> tuple[torch.Tensor, torch.Tensor, torch.Tensor]:
         """
         Tokenizes the action tensor and creates a mask.
 
@@ -459,6 +463,7 @@ class ActionTokenizerProcessorStep(ActionProcessorStep):
         # The fast tokenizer expects action data and returns token IDs
         tokens_list = []
         masks_list = []
+        code_masks_list = []
 
         for i in range(batch_size):
             # Tokenize single action (move to CPU first as tokenizer uses scipy which requires numpy)
@@ -476,65 +481,82 @@ class ActionTokenizerProcessorStep(ActionProcessorStep):
             if tokens.dim() > 1:
                 tokens = tokens.flatten()
 
+            action_code_tokens = self._act_tokens_to_paligemma_tokens(tokens)
             bos_id = self._paligemma_tokenizer.bos_token_id
-            # add bos
+            prompt_tokens = torch.tensor(
+                self._paligemma_tokenizer.encode("Action: ", add_special_tokens=False),
+                device=action.device,
+            )
+            end_tokens = torch.tensor(self._paligemma_tokenizer.encode("|"), device=action.device)
+
+            code_start = 1 + len(prompt_tokens)
+            code_end = code_start + len(action_code_tokens)
             tokens = torch.cat(
                 [
                     torch.tensor([bos_id], device=action.device),
-                    torch.tensor(
+                    prompt_tokens,
-                        self._paligemma_tokenizer.encode("Action: ", add_special_tokens=False),
+                    action_code_tokens,
-                        device=action.device,
+                    end_tokens,
-                    ),
-                    self._act_tokens_to_paligemma_tokens(tokens),
-                    torch.tensor(self._paligemma_tokenizer.encode("|"), device=action.device),
                 ]
             )
+            code_mask = torch.zeros(len(tokens), dtype=torch.bool, device=action.device)
+            code_mask[code_start:code_end] = True
 
             # Truncate or pad to max_action_tokens
             if len(tokens) > self.max_action_tokens:
+                if not self.allow_truncation:
+                    raise ValueError(
+                        f"FAST action sequence has {len(tokens)} tokens, exceeding "
+                        f"max_action_tokens={self.max_action_tokens}."
+                    )
                 logging.warning(
                     f"Token length ({len(tokens)}) exceeds max length ({self.max_action_tokens}), truncating. "
                     "Consider increasing the `max_action_tokens` in your model config if this happens frequently."
                 )
                 tokens = tokens[: self.max_action_tokens]
+                code_mask = code_mask[: self.max_action_tokens]
                 mask = torch.ones(self.max_action_tokens, dtype=torch.bool, device=action.device)
             else:
+                pad_len = self.max_action_tokens - len(tokens)
                 mask = torch.cat(
                     [
                         torch.ones(len(tokens), dtype=torch.bool, device=action.device),
-                        torch.zeros(
+                        torch.zeros(pad_len, dtype=torch.bool, device=action.device),
-                            self.max_action_tokens - len(tokens), dtype=torch.bool, device=action.device
-                        ),
                     ]
                 )
+                code_mask = torch.nn.functional.pad(code_mask, (0, pad_len), value=False)
                 # Pad tokens with zeros
-                tokens = torch.nn.functional.pad(tokens, (0, self.max_action_tokens - len(tokens)), value=0)
+                tokens = torch.nn.functional.pad(tokens, (0, pad_len), value=0)
 
             tokens_list.append(tokens)
             masks_list.append(mask)
+            code_masks_list.append(code_mask)
 
         # Stack into batched tensors
         tokens_batch = torch.stack(tokens_list, dim=0)  # (B, max_action_tokens)
         masks_batch = torch.stack(masks_list, dim=0)  # (B, max_action_tokens)
+        code_masks_batch = torch.stack(code_masks_list, dim=0)  # (B, max_action_tokens)
 
         # Remove batch dimension if input was single sample
         if single_sample:
             tokens_batch = tokens_batch.squeeze(0)
             masks_batch = masks_batch.squeeze(0)
+            code_masks_batch = code_masks_batch.squeeze(0)
 
         # Move to the same device as the input
         if device is not None:
             tokens_batch = tokens_batch.to(device)
             masks_batch = masks_batch.to(device)
+            code_masks_batch = code_masks_batch.to(device)
 
-        return tokens_batch, masks_batch
+        return tokens_batch, masks_batch, code_masks_batch
 
     def action(self, action: torch.Tensor) -> torch.Tensor:
         """
         This method is not used since we override __call__.
         Required by ActionProcessorStep ABC.
         """
-        tokens, _ = self._tokenize_action(action)
+        tokens, _, _ = self._tokenize_action(action)
         return tokens
 
     def get_config(self) -> dict[str, Any]:
@@ -550,6 +572,9 @@ class ActionTokenizerProcessorStep(ActionProcessorStep):
         config = {
             "trust_remote_code": self.trust_remote_code,
             "max_action_tokens": self.max_action_tokens,
+            "fast_skip_tokens": self.fast_skip_tokens,
+            "paligemma_tokenizer_name": self.paligemma_tokenizer_name,
+            "allow_truncation": self.allow_truncation,
         }
 
         # Only save tokenizer_name if it was used to create the tokenizer
@@ -558,6 +583,14 @@ class ActionTokenizerProcessorStep(ActionProcessorStep):
 
         return config
 
+    def save_artifacts(self, save_directory: Path) -> dict[str, str]:
+        artifact_path = Path("action_tokenizer")
+        save_pretrained = getattr(self.action_tokenizer, "save_pretrained", None)
+        if save_pretrained is None:
+            raise TypeError("Action tokenizer must implement save_pretrained() to save a portable pipeline.")
+        save_pretrained(save_directory / artifact_path)
+        return {"action_tokenizer_name": artifact_path.as_posix()}
+
     def transform_features(
         self, features: dict[PipelineFeatureType, dict[str, PolicyFeature]]
     ) -> dict[PipelineFeatureType, dict[str, PolicyFeature]]:

src/lerobot/utils/collate.py

@@ -22,7 +22,7 @@ from torch.utils.data._utils.collate import default_collate
 
 from lerobot.datasets.language import LANGUAGE_COLUMNS
 
-_PYTHON_LIST_KEYS = {"messages", "message_streams", "target_message_indices"}
+_PYTHON_LIST_KEYS = {"messages", "message_streams", "target_message_indices", *LANGUAGE_COLUMNS}
 
 
 def lerobot_collate_fn(batch: list[dict[str, Any] | None]) -> dict[str, Any] | None:

src/lerobot/utils/constants.py

@@ -26,6 +26,7 @@ OBS_IMAGES = OBS_IMAGE + "s"
 OBS_LANGUAGE = OBS_STR + ".language"
 OBS_LANGUAGE_TOKENS = OBS_LANGUAGE + ".tokens"
 OBS_LANGUAGE_ATTENTION_MASK = OBS_LANGUAGE + ".attention_mask"
+OBS_LANGUAGE_CAUSAL_MARKS = OBS_LANGUAGE + ".causal_marks"
 OBS_LANGUAGE_SUBTASK = OBS_STR + ".subtask"
 OBS_LANGUAGE_SUBTASK_TOKENS = OBS_LANGUAGE_SUBTASK + ".tokens"
 OBS_LANGUAGE_SUBTASK_ATTENTION_MASK = OBS_LANGUAGE_SUBTASK + ".attention_mask"
@@ -34,6 +35,7 @@ ACTION = "action"
 ACTION_PREFIX = ACTION + "."
 ACTION_TOKENS = ACTION + ".tokens"
 ACTION_TOKEN_MASK = ACTION + ".token_mask"
+ACTION_CODE_TOKEN_MASK = ACTION + ".code_token_mask"
 REWARD = "next.reward"
 TRUNCATED = "next.truncated"
 DONE = "next.done"

tests/configs/test_recipe.py

@@ -29,6 +29,13 @@ def test_message_recipe_validates_unknown_binding():
         )
 
 
+def test_canonical_recipe_loads():
+    """The canonical PI052 blend YAML loads + validates."""
+    recipe = TrainingRecipe.from_yaml(Path("src/lerobot/configs/recipes/subtask_mem_vqa_speech.yaml"))
+    assert recipe.blend is not None
+    assert sum(c.weight for c in recipe.blend.values()) == pytest.approx(1.0)
+
+
 def test_message_turn_requires_a_stream():
     """Every turn must declare a stream — None is rejected at construction.
 

tests/datasets/test_language_render.py

@@ -343,6 +343,84 @@ def test_resolve_task_explicit_override_beats_rephrasings():
     assert rendered["messages"][0]["content"] == "explicit override wins"
 
 
+def test_flow_only_low_level_recipe_renders_without_target():
+    """Regression: a flow-only ``low_level`` recipe has no ``target`` turn —
+    its supervision is the action-expert flow loss, not text-CE. It must
+    still render (not ``None``), otherwise every blend draw of it is dropped
+    and the action expert never receives a flow loss."""
+    recipe = TrainingRecipe(
+        messages=[
+            MessageTurn(
+                role="user",
+                content="${subtask}",
+                stream="low_level",
+                if_present="subtask",
+            ),
+        ],
+        bindings={"subtask": "active_at(t, style=subtask)"},
+    )
+
+    rendered = render_sample(
+        recipe=recipe,
+        persistent=PERSISTENT,
+        events=[],
+        t=0.5,
+        sample_idx=0,
+        task="clean kitchen",
+    )
+
+    assert rendered is not None
+    assert rendered["messages"] == [{"role": "user", "content": "subtask 0"}]
+    assert rendered["message_streams"] == ["low_level"]
+    assert rendered["target_message_indices"] == []
+
+
+def test_vqa_frame_is_consumed_over_the_weighted_blend():
+    """A frame carrying a VQA annotation renders the ``ask_vqa*`` sub-recipe
+    even when its blend weight is tiny — VQA annotations are sparse and must
+    never be wasted on a subtask/action draw."""
+    recipe = TrainingRecipe(
+        blend={
+            "high_level_subtask": TrainingRecipe(
+                weight=0.99,
+                messages=[
+                    MessageTurn(role="user", content="${task}", stream="high_level"),
+                    MessageTurn(role="assistant", content="a subtask", stream="high_level", target=True),
+                ],
+            ),
+            "ask_vqa_top": TrainingRecipe(
+                weight=0.01,
+                bindings={
+                    "vqa_query": "emitted_at(t, style=vqa, role=user, camera=observation.images.top)",
+                    "vqa": "emitted_at(t, style=vqa, role=assistant, camera=observation.images.top)",
+                },
+                messages=[
+                    MessageTurn(
+                        role="user", content="${vqa_query}", stream="high_level", if_present="vqa_query"
+                    ),
+                    MessageTurn(
+                        role="assistant",
+                        content="${vqa}",
+                        stream="high_level",
+                        target=True,
+                        if_present="vqa",
+                    ),
+                ],
+            ),
+        }
+    )
+    # A frame WITH a vqa event renders VQA on every sample_idx, despite the
+    # ask_vqa weight being only 0.01.
+    for sample_idx in range(20):
+        rendered = render_sample(
+            recipe=recipe, persistent=PERSISTENT, events=EVENTS_AT_1, t=1.0, sample_idx=sample_idx, task="x"
+        )
+        assert rendered["messages"][-1]["content"] == '{"count": 2}', sample_idx
+    # A frame WITHOUT a vqa event falls back to the normal weighted blend.
+    rendered = render_sample(recipe=recipe, persistent=PERSISTENT, events=[], t=1.0, sample_idx=0, task="x")
+    assert rendered["messages"][-1]["content"] == "a subtask"
+
+
 def test_emitted_at_persistent_tolerates_small_timestamp_drift():
     """Persistent ``emitted_at`` should match within EMITTED_AT_TOLERANCE_S
     so callers that derive ``t`` arithmetically (``frame_idx / fps``) still

tests/datasets/test_sampler.py

@@ -25,7 +25,7 @@ from datasets import Dataset  # noqa: E402
 from lerobot.datasets.io_utils import (
     hf_transform_to_torch,
 )
-from lerobot.datasets.sampler import EpisodeAwareSampler
+from lerobot.datasets.sampler import EpisodeAwareSampler, compute_sampler_state
 
 
 def calculate_episode_data_index(hf_dataset: Dataset) -> dict[str, torch.Tensor]:
@@ -154,8 +154,6 @@ def test_partial_episode_drop_warns(caplog):
 
 # --- seeded (seed, epoch) shuffling, resume, and state ---
 
-from lerobot.datasets.sampler import compute_sampler_state  # noqa: E402
-
 EPISODE_BOUNDS = ([0, 2, 3], [2, 3, 6])  # episodes of 2, 1 and 3 frames
 
 

tests/processor/test_render_messages_processor.py

@@ -12,7 +12,9 @@ from lerobot.processor.render_messages_processor import RenderMessagesStep  # no
 from lerobot.types import TransitionKey  # noqa: E402
 
 
-def test_render_messages_step_noops_without_language_columns():
+def test_render_messages_step_renders_task_fallback_without_language_columns():
+    """No language columns + a task string → low-level task fallback render,
+    matching what the policy sees at eval time on unannotated observations."""
     recipe = TrainingRecipe(
         messages=[
             MessageTurn(role="user", content="${task}", stream="high_level"),
@@ -21,6 +23,24 @@ def test_render_messages_step_noops_without_language_columns():
     )
     transition = create_transition(complementary_data={"task": "do it"})
 
+    out = RenderMessagesStep(recipe)(transition)
+    data = out[TransitionKey.COMPLEMENTARY_DATA]
+
+    assert data["messages"] == [{"role": "user", "content": "do it"}]
+    assert data["message_streams"] == ["low_level"]
+    assert data["target_message_indices"] == []
+    assert data["task"] == "do it"
+
+
+def test_render_messages_step_noops_without_language_columns_or_task():
+    recipe = TrainingRecipe(
+        messages=[
+            MessageTurn(role="user", content="${task}", stream="high_level"),
+            MessageTurn(role="assistant", content="${subtask}", stream="low_level", target=True),
+        ]
+    )
+    transition = create_transition(complementary_data={})
+
     assert RenderMessagesStep(recipe)(transition) == transition
 
 
@@ -58,3 +78,70 @@ def test_render_messages_step_renders_and_drops_raw_language():
     assert data["messages"][-1]["content"] == "reach carefully"
     assert data["message_streams"] == ["high_level", "low_level"]
     assert data["target_message_indices"] == [1]
+
+
+def test_render_messages_step_falls_back_to_low_level_task_when_recipe_misses():
+    recipe = TrainingRecipe(
+        messages=[
+            MessageTurn(
+                role="assistant",
+                content="${subtask}",
+                stream="high_level",
+                target=True,
+                if_present="subtask",
+            ),
+        ]
+    )
+    transition = create_transition(
+        complementary_data={
+            "task": "pick the cube",
+            "timestamp": torch.tensor(0.0),
+            "index": torch.tensor(7),
+            "language_persistent": [],
+            "language_events": [{"style": "unmatched", "timestamp": 0.0}],
+        }
+    )
+
+    out = RenderMessagesStep(recipe)(transition)
+    data = out[TransitionKey.COMPLEMENTARY_DATA]
+
+    assert data["messages"] == [{"role": "user", "content": "pick the cube"}]
+    assert data["message_streams"] == ["low_level"]
+    assert data["target_message_indices"] == []
+
+
+def test_render_messages_step_falls_back_per_sample_in_batched_language():
+    recipe = TrainingRecipe(
+        messages=[
+            MessageTurn(
+                role="assistant",
+                content="${subtask}",
+                stream="high_level",
+                target=True,
+                if_present="subtask",
+            ),
+        ]
+    )
+    transition = create_transition(
+        action=torch.arange(4).reshape(2, 2),
+        complementary_data={
+            "task": ["pick the cube", "open the drawer"],
+            "timestamp": torch.tensor([0.0, 1.0]),
+            "index": torch.tensor([7, 8]),
+            "language_persistent": [[], []],
+            "language_events": [
+                [{"style": "unmatched", "timestamp": 0.0}],
+                [{"style": "unmatched", "timestamp": 1.0}],
+            ],
+        },
+    )
+
+    out = RenderMessagesStep(recipe)(transition)
+    data = out[TransitionKey.COMPLEMENTARY_DATA]
+
+    assert data["messages"] == [
+        [{"role": "user", "content": "pick the cube"}],
+        [{"role": "user", "content": "open the drawer"}],
+    ]
+    assert data["message_streams"] == [["low_level"], ["low_level"]]
+    assert data["target_message_indices"] == [[], []]

tests/processor/test_tokenizer_processor.py

@@ -25,7 +25,7 @@ import pytest
 import torch
 
 from lerobot.configs.types import FeatureType, PipelineFeatureType, PolicyFeature
-from lerobot.processor import DataProcessorPipeline, TokenizerProcessorStep
+from lerobot.processor import ActionTokenizerProcessorStep, DataProcessorPipeline, TokenizerProcessorStep
 from lerobot.processor.converters import create_transition, identity_transition
 from lerobot.types import TransitionKey
 from lerobot.utils.constants import (
@@ -88,6 +88,46 @@ class MockTokenizer:
         return result
 
 
+def test_action_tokenizer_config_preserves_token_mapping():
+    processor = object.__new__(ActionTokenizerProcessorStep)
+    processor.trust_remote_code = True
+    processor.max_action_tokens = 384
+    processor.fast_skip_tokens = 64
+    processor.paligemma_tokenizer_name = "custom/paligemma"
+    processor.allow_truncation = False
+    processor.action_tokenizer_name = "custom/fast"
+    processor.action_tokenizer_input_object = None
+
+    assert processor.get_config() == {
+        "trust_remote_code": True,
+        "max_action_tokens": 384,
+        "fast_skip_tokens": 64,
+        "paligemma_tokenizer_name": "custom/paligemma",
+        "allow_truncation": False,
+        "action_tokenizer_name": "custom/fast",
+    }
+
+
+def test_action_tokenizer_can_reject_truncated_sequences():
+    processor = object.__new__(ActionTokenizerProcessorStep)
+    processor.max_action_tokens = 4
+    processor.fast_skip_tokens = 128
+    processor.allow_truncation = False
+    processor.action_tokenizer = lambda _actions: [1, 2, 3]
+    processor._paligemma_tokenizer = type(
+        "Tokenizer",
+        (),
+        {
+            "vocab_size": 1000,
+            "bos_token_id": 2,
+            "encode": lambda _self, text, **_kwargs: [10, 11] if text == "Action: " else [12, 1],
+        },
+    )()
+
+    with pytest.raises(ValueError, match="max_action_tokens=4"):
+        processor._tokenize_action(torch.zeros(1, 2, 1))
+
+
 @pytest.fixture
 def mock_tokenizer():
     """Provide a mock tokenizer for testing."""