feat(annotate): phase 0 — derive canonical vocabulary from sample episodes

The pipeline previously emitted near-unique subtask + memory phrasings
per episode (free-form LLM rephrasing). On the downstream low-level
policy that collapses the action expert's conditioning to noise: every
episode pairs a different paraphrase with similar motions, so the
expert learns a flat scene-prior that ignores the subtask string —
then at inference the high-level head invents *yet another* paraphrase
and the expert produces tiny "uncertain hover" chunks.

Add a vocabulary-discovery phase (phase 0) that runs once per dataset:

  - watches the first ``vocabulary.sample_episodes`` (default 3)
    episode videos as one Qwen-VL prompt,
  - asks the VLM to derive ~``n_subtask_target`` canonical imperative
    subtask labels and ~``n_memory_target`` first-person past-tense
    memory milestones that recur across the demos,
  - persists them to ``meta/canonical_vocabulary.json`` (human-
    inspectable, hand-editable), and
  - wires the resulting ``Vocabulary`` into the ``plan`` module so
    every per-episode subtask + memory call is constrained to those
    exact strings (both as prompt-side instructions *and* post-VLM
    validation: paraphrases snap to the closest canonical entry via
    token-set overlap; below a 0.5 Jaccard floor the subtask is
    dropped rather than warped into something semantically wrong).

Operator workflow:

  - first run discovers the vocabulary, writes the JSON, and runs
    the ``plan`` module against it,
  - subsequent runs reuse the on-disk file (``reuse_existing=True``
    default) so hand-edits stick,
  - set ``--vocabulary.enabled=False`` to fall back to free-form
    generation (the original behaviour).

The discovery prompt forbids gerunds / third-person / adverbs and
caps the lists to the requested counts, matching the Hi-Robot /
π0.6-MEM convention of small per-environment vocabularies. The
``plan`` module's subtask + memory prompts grow a conditional
``{vocabulary_block}`` slot rendered only when a vocabulary is
present; without one the templates collapse to their previous
free-form form.

Tests: 11 new unit tests under tests/annotations/test_vocabulary.py
cover the on-disk round-trip, discovery against the fixture dataset,
``reuse_existing`` short-circuit, paraphrase canonicalisation, off-
vocab subtask dropping, and the no-vocabulary pass-through path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

docs/source/annotation_pipeline.mdx

@@ -7,7 +7,8 @@
 
 ## What the pipeline produces
 
-Three modules write into a per-episode staging tree, then a single writer
+A vocabulary-discovery phase derives a small canonical wording, then three
+modules write into a per-episode staging tree, then a single writer
 rewrites the data shards in place:
 
 | Style / atom                                | Column                | Module         |
@@ -20,6 +21,20 @@ rewrites the data shards in place:
 | speech tool-call atom (`style=null`, `say`) | `language_events`     | `interjections`|
 | `vqa` (user / assistant pair)               | `language_events`     | `vqa`          |
 
+The `plan` module is constrained to a **canonical vocabulary** discovered
+once per dataset by the `vocabulary` module (phase 0). It watches a few
+sample episode videos (`--vocabulary.sample_episodes`, default `3`) and
+asks the VLM to derive a small set of imperative subtask labels
+(~`--vocabulary.n_subtask_target`, default `10`) and first-person memory
+milestones (~`--vocabulary.n_memory_target`, default `6`) that recur
+across the demos. The result lands at
+`meta/canonical_vocabulary.json` (human-readable / hand-editable) and is
+reused on every subsequent run. The `plan` module then constrains both
+subtask + memory generation to those exact strings — the downstream
+low-level policy sees a small, repeatable target distribution instead of
+thousands of LLM paraphrases. Disable with `--vocabulary.enabled=False`
+to fall back to free-form generation.
+
 The writer does **not** add a `tools` column to the parquet — the tool
 catalog lives at `meta/info.json["tools"]` instead (see
 [Tools](./tools)). After every annotation run the pipeline ensures the

src/lerobot/annotations/steerable_pipeline/__init__.py

@@ -26,11 +26,25 @@ outputs are staged per-episode before a final parquet rewrite:
 
 from .config import AnnotationPipelineConfig
 from .validator import StagingValidator, ValidationReport
+from .vocabulary import (
+    VOCABULARY_FILENAME,
+    Vocabulary,
+    VocabularyDiscoveryModule,
+    load_vocabulary,
+    save_vocabulary,
+    vocabulary_path,
+)
 from .writer import LanguageColumnsWriter
 
 __all__ = [
+    "VOCABULARY_FILENAME",
     "AnnotationPipelineConfig",
     "LanguageColumnsWriter",
     "StagingValidator",
     "ValidationReport",
+    "Vocabulary",
+    "VocabularyDiscoveryModule",
+    "load_vocabulary",
+    "save_vocabulary",
+    "vocabulary_path",
 ]

src/lerobot/annotations/steerable_pipeline/config.py

@@ -21,6 +21,42 @@ from pathlib import Path
 from typing import Any
 
 
+@dataclass
+class VocabularyConfig:
+    """Phase 0 — dataset-level canonical vocabulary discovery.
+
+    Watches the first ``sample_episodes`` episode videos and asks the VLM
+    to derive a small canonical vocabulary (~``n_subtask_target`` subtask
+    labels + ~``n_memory_target`` memory milestones) that every episode
+    in the dataset will reuse. The output lands at
+    ``meta/canonical_vocabulary.json`` and feeds phase 1's subtask +
+    memory generation as both a prompt-side constraint and a post-VLM
+    validation gate.
+
+    Why this exists: free-form LLM rephrasing per episode produces near-
+    unique subtask strings, which makes the downstream low-level policy's
+    conditioning effectively noise — at inference the policy generates a
+    *new* paraphrase the action expert has never seen and produces tiny
+    cautious actions. Forcing every episode onto the same small set of
+    canonical strings gives the action expert dense supervision per
+    string and a small target distribution to learn against.
+
+    Set ``enabled=False`` to fall back to free-form generation (original
+    behaviour). ``reuse_existing=True`` keeps a hand-edited vocabulary
+    file from being clobbered on re-runs.
+    """
+
+    enabled: bool = True
+    sample_episodes: int = 3
+    n_subtask_target: int = 10
+    n_memory_target: int = 6
+    max_video_frames_per_episode: int = 32
+    # When True (default), an existing meta/canonical_vocabulary.json is
+    # loaded as-is and no VLM call is made — lets operators hand-edit the
+    # file. Set False to always rediscover from the sample episodes.
+    reuse_existing: bool = True
+
+
 @dataclass
 class PlanConfig:
     """``plan`` module: plan + subtasks + memory + task augmentation.
@@ -186,6 +222,7 @@ class AnnotationPipelineConfig:
 
     seed: int = 1729
 
+    vocabulary: VocabularyConfig = field(default_factory=VocabularyConfig)
     plan: PlanConfig = field(default_factory=PlanConfig)
     interjections: InterjectionsConfig = field(default_factory=InterjectionsConfig)
     vqa: VqaConfig = field(default_factory=VqaConfig)

src/lerobot/annotations/steerable_pipeline/executor.py

@@ -15,8 +15,14 @@
 # limitations under the License.
 """In-process executor that runs the annotation phases.
 
-The executor plans **six phases** in the dependency order from the plan:
+The executor plans **seven phases** in the dependency order from the plan:
 
+    phase 0: vocabulary discovery — derive a small canonical vocabulary
+             from the first few sample-episode videos (subtask labels +
+             memory milestones) and persist it next to the dataset; the
+             ``plan`` module then constrains every per-episode generation
+             to those strings, so the downstream policy sees a small,
+             repeatable conditioning distribution
     phase 1: ``plan`` module (plan + subtasks + memory)
     phase 2: ``interjections`` module (interjections + speech)
     phase 3: ``plan`` plan-update pass — re-runs plan emission at every
@@ -88,6 +94,7 @@ class Executor:
     vqa: Any  # GeneralVqaModule
     writer: LanguageColumnsWriter
     validator: StagingValidator
+    vocabulary: Any = None  # VocabularyDiscoveryModule | None
 
     def run(self, root: Path) -> PipelineRunSummary:
         records = list(iter_episodes(root, only_episodes=self.config.only_episodes))
@@ -102,6 +109,10 @@ class Executor:
 
         phases: list[PhaseResult] = []
 
+        # Phase 0: vocabulary discovery. Mutates ``self.plan.vocabulary``
+        # so subsequent per-episode plan calls see the canonical labels.
+        phases.append(self._run_vocabulary_phase(records, root))
+
         # Phase 1: ``plan`` module (plan + subtasks + memory)
         phases.append(self._run_module_phase("plan", records, staging_dir, self.plan))
         # Phase 2: ``interjections`` module (interjections + speech). It
@@ -172,6 +183,62 @@ class Executor:
                 flush=True,
             )
 
+    def _run_vocabulary_phase(
+        self, records: list[EpisodeRecord], root: Path
+    ) -> PhaseResult:
+        """Discover (or load) the canonical vocabulary, wire it into ``self.plan``.
+
+        Returns a ``PhaseResult`` whose ``episodes_processed`` is the number
+        of sample episodes consulted (0 when disabled or no VLM call was
+        needed); ``episodes_skipped`` is always ``0`` because vocabulary is
+        a once-per-dataset artifact, not a per-episode product.
+        """
+        from .vocabulary import load_vocabulary, save_vocabulary  # noqa: PLC0415
+
+        if self.vocabulary is None or not getattr(self.vocabulary, "enabled", False):
+            print(
+                "[annotate] phase=vocabulary skipped (module disabled or unset)",
+                flush=True,
+            )
+            return PhaseResult(name="vocabulary", episodes_processed=0, episodes_skipped=0)
+
+        existing = load_vocabulary(root)
+        if existing is not None and self.config.vocabulary.reuse_existing:
+            print(
+                f"[annotate] phase=vocabulary reusing {root / 'meta' / 'canonical_vocabulary.json'} "
+                f"({len(existing.subtasks)} subtask labels, "
+                f"{len(existing.memory_milestones)} memory milestones)",
+                flush=True,
+            )
+            self.plan.vocabulary = existing
+            return PhaseResult(name="vocabulary", episodes_processed=0, episodes_skipped=0)
+
+        sample_n = max(1, min(int(self.config.vocabulary.sample_episodes), len(records)))
+        print(
+            f"[annotate] phase=vocabulary discovering from {sample_n} sample episode(s)...",
+            flush=True,
+        )
+        t0 = time.time()
+        vocab = self.vocabulary.discover(records[:sample_n], existing=existing)
+        if vocab is None:
+            print(
+                "[annotate] phase=vocabulary returned no vocabulary — "
+                "plan module will fall back to free-form generation",
+                flush=True,
+            )
+            return PhaseResult(name="vocabulary", episodes_processed=0, episodes_skipped=0)
+
+        save_path = save_vocabulary(root, vocab)
+        print(
+            f"[annotate] phase=vocabulary wrote {save_path} "
+            f"({len(vocab.subtasks)} subtask labels, "
+            f"{len(vocab.memory_milestones)} memory milestones) in "
+            f"{time.time() - t0:.1f}s",
+            flush=True,
+        )
+        self.plan.vocabulary = vocab
+        return PhaseResult(name="vocabulary", episodes_processed=sample_n, episodes_skipped=0)
+
     def _run_module_phase(
         self,
         name: str,

src/lerobot/annotations/steerable_pipeline/modules/plan_subtasks_memory.py

@@ -17,6 +17,7 @@
 
 from __future__ import annotations
 
+import logging
 from collections.abc import Sequence
 from dataclasses import dataclass, field
 from pathlib import Path
@@ -34,6 +35,9 @@ from ..prompts import load as load_prompt
 from ..reader import EpisodeRecord, reconstruct_subtask_spans, snap_to_frame
 from ..staging import EpisodeStaging
 from ..vlm_client import VlmClient
+from ..vocabulary import Vocabulary
+
+logger = logging.getLogger(__name__)
 
 
 @dataclass
@@ -54,6 +58,11 @@ class PlanSubtasksMemoryModule:
     vlm: VlmClient
     config: PlanConfig
     frame_provider: FrameProvider = field(default_factory=null_provider)
+    vocabulary: Vocabulary | None = None
+    """When set, the module constrains subtask + memory generation to the
+    canonical strings in ``vocabulary``. Phase 0 (vocabulary discovery)
+    populates this once per dataset; ``None`` falls back to free-form
+    generation (original behaviour)."""
 
     @property
     def enabled(self) -> bool:
@@ -311,6 +320,7 @@ class PlanSubtasksMemoryModule:
             min_subtask_seconds=self.config.min_subtask_seconds,
             max_steps=self.config.plan_max_steps,
             episode_duration=f"{episode_duration:.3f}",
+            vocabulary_block=self._subtask_vocabulary_block(),
         )
         spans = self._vlm_field(self._video_message(record, prompt), "subtasks")
         if not spans:
@@ -330,12 +340,100 @@ class PlanSubtasksMemoryModule:
             end = max(t0, min(end, t_last))
             if end < start:
                 start, end = end, start
+            if not text:
+                continue
+            text = self._canonicalize_subtask(text)
             if not text:
                 continue
             cleaned.append({"text": text, "start": start, "end": end})
         cleaned.sort(key=lambda s: s["start"])
         return cleaned
 
+    # ------------------------------------------------------------------
+    # Canonical-vocabulary helpers
+    # ------------------------------------------------------------------
+
+    def _subtask_vocabulary_block(self) -> str:
+        """Bullet-list of canonical subtasks the VLM must pick from.
+
+        Returns an empty string when no vocabulary is configured —
+        ``module_1_subtasks.txt`` then falls back to its free-form
+        rules (original behaviour).
+        """
+        if self.vocabulary is None or not self.vocabulary.subtasks:
+            return ""
+        bullets = "\n".join(f"- {s}" for s in self.vocabulary.subtasks)
+        return (
+            "You MUST choose each subtask label verbatim from this canonical "
+            "vocabulary — pick the closest match for each phase of the demo, "
+            "and reuse the SAME string every time that phase recurs. The "
+            "low-level policy is conditioned on these exact strings; any "
+            "novel paraphrase you invent will make its conditioning OOD.\n"
+            "Canonical subtask labels:\n"
+            f"{bullets}\n\n"
+        )
+
+    def _memory_vocabulary_block(self) -> str:
+        """Bullet-list of canonical memory milestones the VLM must pick from."""
+        if self.vocabulary is None or not self.vocabulary.memory_milestones:
+            return ""
+        bullets = "\n".join(f"- {m}" for m in self.vocabulary.memory_milestones)
+        return (
+            "Compose the memory by picking ONLY from this canonical milestone "
+            "list — append a milestone (or rewrite the running memory to "
+            "compress past ones) using these exact phrases. Do not invent new "
+            "wording: every paraphrase weakens the downstream conditioning.\n"
+            "Canonical memory milestones:\n"
+            f"{bullets}\n\n"
+        )
+
+    def _canonicalize_subtask(self, text: str) -> str:
+        """Snap ``text`` to the closest canonical subtask string, or drop it.
+
+        Without a vocabulary, the original text passes through. With a
+        vocabulary, an exact case-insensitive match wins; failing that,
+        the best Jaccard overlap on the word set is used as a tolerant
+        fuzzy match (handles articles / minor reorderings). If nothing
+        clears the floor, the subtask is dropped — better to skip a
+        phase than to feed the action expert an off-distribution string.
+        """
+        if self.vocabulary is None or not self.vocabulary.subtasks:
+            return text.strip()
+        candidates = self.vocabulary.subtasks
+        cleaned = text.strip()
+        lowered = cleaned.lower()
+        for candidate in candidates:
+            if candidate.lower() == lowered:
+                return candidate
+        # Jaccard fallback: token-set overlap, drop articles + adverbs.
+        ignore = {"the", "a", "an", "to", "into", "from", "of", "on", "over", "at"}
+        words = {w for w in lowered.replace(",", " ").split() if w and w not in ignore}
+        if not words:
+            return ""
+        best: tuple[float, str] | None = None
+        for candidate in candidates:
+            cand_words = {
+                w for w in candidate.lower().replace(",", " ").split() if w and w not in ignore
+            }
+            if not cand_words:
+                continue
+            inter = len(words & cand_words)
+            union = len(words | cand_words)
+            score = inter / union if union else 0.0
+            if best is None or score > best[0]:
+                best = (score, candidate)
+        # Floor: require at least ~half the tokens to overlap. Below that
+        # the VLM is hallucinating a novel phrase; drop rather than warp
+        # it into something semantically wrong.
+        if best is None or best[0] < 0.5:
+            logger.warning(
+                "subtask %r did not match any canonical label (best=%s) — dropping",
+                cleaned,
+                best,
+            )
+            return ""
+        return best[1]
+
     def _generate_plan(
         self,
         record: EpisodeRecord,  # noqa: ARG002  (kept for signature stability)
@@ -397,6 +495,7 @@ class PlanSubtasksMemoryModule:
             prior_memory=prior_memory or "(none)",
             completed_subtask=completed,
             remaining_subtasks=", ".join(remaining) if remaining else "(none)",
+            vocabulary_block=self._memory_vocabulary_block(),
         )
         memory = self._vlm_field(self._text_message(prompt), "memory")
         return memory.strip() if isinstance(memory, str) else ""

src/lerobot/annotations/steerable_pipeline/prompts/module_0_vocabulary.txt

@@ -0,0 +1,46 @@
+You are inspecting {n_episodes} sample episode video(s) from a teleoperated
+robot dataset. Every episode in the dataset performs the SAME task; the
+user originally asked: "{episode_task}".
+
+Watch all the clips and produce a SHORT canonical vocabulary that every
+episode in this dataset will reuse. The downstream low-level policy is
+conditioned on these strings — duplicate phrasings (e.g. "grasp blue
+cube" vs "pick up the blue cube") would destroy the conditioning, so
+pick one wording per concept and reuse it everywhere.
+
+You output two lists:
+
+1. `subtasks`: imperative, telegraphic commands the robot can execute.
+   - Verb-first. Drop articles, adverbs, qualifiers.
+   - Consistent object nouns (if the task says "cube", every subtask says
+     "cube" — never "block" / "object").
+   - Atomic — one skill per subtask (gripper-open events, contact, regrasps,
+     transitions all become cut points).
+   - Aim for ~{n_subtask_target} labels. Fewer is better than more.
+   - Good: "move to blue cube", "grasp blue cube", "lift blue cube",
+     "place blue cube in box", "release blue cube", "retract arm".
+   - Bad: "the robot arm moves towards the blue cube" (third person,
+     too long), "carefully pick up the cube" (adverb, article),
+     "carrying the yellow cube over the green basket" (gerund — should
+     be imperative "transport yellow cube to green basket").
+
+2. `memory_milestones`: first-person past-tense sentences the running
+   memory composes from. Each subtask phase that produces a lasting
+   change should have a milestone; transient motions (move, retract)
+   should NOT.
+   - First person, past tense. Start with "I".
+   - One sentence. Functional outcome only — no grasp / motion detail.
+   - Aim for ~{n_memory_target} milestones.
+   - Good: "I picked up the blue cube.", "I placed the blue cube in
+     the green box.", "I wiped the counter."
+   - Bad: "The robot arm grasped the blue cube." (third person),
+     "I carefully grasped the blue cube with the parallel gripper."
+     (irrelevant detail), "I moved towards the blue cube." (transient
+     motion — should be omitted, not memorialised).
+
+Output strictly valid JSON of shape:
+
+  {{
+    "subtasks": ["<verb phrase>", ...],
+    "memory_milestones": ["I <past-tense sentence>.", ...]
+  }}

src/lerobot/annotations/steerable_pipeline/prompts/module_1_memory.txt

@@ -13,7 +13,7 @@ Previous memory: {prior_memory}
 Just-completed subtask: "{completed_subtask}"
 Remaining subtasks (for relevance judgement only): {remaining_subtasks}
 
-Write the memory as a short FIRST-PERSON, PAST-TENSE narrative of what the
+{vocabulary_block}Write the memory as a short FIRST-PERSON, PAST-TENSE narrative of what the
 robot has accomplished so far — the running story it would tell itself.
 
 Authoring rules:

src/lerobot/annotations/steerable_pipeline/prompts/module_1_subtasks.txt

@@ -4,9 +4,9 @@ The user originally asked: "{episode_task}"
 
 You are shown the entire demonstration as a single video. Watch the
 whole clip, then segment it into a list of consecutive atomic subtasks
-the robot performs. Write short, telegraphic action labels.
+the robot performs.
 
-Authoring rules — Hi Robot atom granularity, pi0.7-style short prompts:
+{vocabulary_block}Authoring rules — Hi Robot atom granularity, pi0.7-style short prompts:
 
 - Each subtask = one atomic skill the low-level policy can execute.
 - Write each subtask as an IMPERATIVE COMMAND, starting with a verb:

src/lerobot/annotations/steerable_pipeline/vocabulary.py

@@ -0,0 +1,224 @@
+#!/usr/bin/env python
+
+# Copyright 2026 The HuggingFace Inc. team. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Dataset-level canonical vocabulary discovery (Phase 0).
+
+The downstream consumer of these annotations is a low-level action expert
+conditioned on the ``subtask`` string. Free-form per-episode LLM rephrasing
+gives near-unique strings per occurrence, which collapses the action
+expert's conditioning to noise and makes runtime subtask-paraphrase drift
+catastrophic. The Hi-Robot / π0.6-MEM recipe ships a small canonical
+vocabulary per environment (~10 strings) that every episode reuses; this
+module derives that vocabulary automatically from the first few episode
+videos and persists it next to the dataset.
+
+Pipeline-level flow:
+
+    Phase 0 (here): watch N sample episodes → produce vocabulary.json
+    Phase 1 (plan module): reuse vocabulary on every episode, both as
+                           prompt-side constraint *and* post-VLM validation
+
+The vocabulary is JSON, lives at ``<root>/meta/canonical_vocabulary.json``,
+and is human-inspectable / hand-editable — if the discovered set is wrong,
+operators edit the file and re-run the pipeline without phase 0.
+"""
+
+from __future__ import annotations
+
+import json
+import logging
+from collections.abc import Sequence
+from dataclasses import dataclass, field
+from pathlib import Path
+from typing import Any
+
+from .config import VocabularyConfig
+from .frames import FrameProvider, null_provider, to_video_block
+from .prompts import load as load_prompt
+from .reader import EpisodeRecord
+from .vlm_client import VlmClient
+
+logger = logging.getLogger(__name__)
+
+VOCABULARY_FILENAME = "canonical_vocabulary.json"
+
+
+@dataclass
+class Vocabulary:
+    """Canonical phrasings shared across every episode of one dataset.
+
+    Both lists are strict: per-episode subtask + memory generation pick
+    from these strings only; the downstream policy then has a small,
+    repeatable target distribution to learn instead of thousands of
+    LLM paraphrases.
+    """
+
+    subtasks: tuple[str, ...]
+    """Imperative subtask labels — what the low-level policy is conditioned
+    on. Verb-first, telegraphic, consistent object nouns. Example:
+    ``("move to blue cube", "grasp blue cube", "lift blue cube",
+       "place blue cube in box", "retract arm")``.
+    """
+
+    memory_milestones: tuple[str, ...]
+    """First-person past-tense milestone sentences — building blocks for
+    the running memory string. Example: ``("I picked up the blue cube.",
+    "I placed the blue cube in the green box.")``. Each milestone maps
+    1:1 onto a completed subtask phase; ``memory_at_step_k`` is the
+    concatenation of milestones for completed phases.
+    """
+
+    def to_json(self) -> dict[str, list[str]]:
+        return {
+            "subtasks": list(self.subtasks),
+            "memory_milestones": list(self.memory_milestones),
+        }
+
+    @classmethod
+    def from_json(cls, payload: dict[str, Any]) -> Vocabulary:
+        subtasks = tuple(
+            str(s).strip() for s in (payload.get("subtasks") or []) if str(s).strip()
+        )
+        memory_milestones = tuple(
+            str(s).strip() for s in (payload.get("memory_milestones") or []) if str(s).strip()
+        )
+        return cls(subtasks=subtasks, memory_milestones=memory_milestones)
+
+    def is_empty(self) -> bool:
+        return not self.subtasks and not self.memory_milestones
+
+
+def vocabulary_path(root: Path) -> Path:
+    """Return the canonical on-disk location for the vocabulary file."""
+    return root / "meta" / VOCABULARY_FILENAME
+
+
+def load_vocabulary(root: Path) -> Vocabulary | None:
+    """Read ``<root>/meta/canonical_vocabulary.json`` if present.
+
+    Returns ``None`` when the file does not exist — callers fall back to
+    free-form (unconstrained) subtask + memory generation, preserving the
+    pipeline's behaviour on datasets that never ran phase 0.
+    """
+    path = vocabulary_path(root)
+    if not path.exists():
+        return None
+    try:
+        payload = json.loads(path.read_text(encoding="utf-8"))
+    except (OSError, json.JSONDecodeError) as exc:
+        logger.warning("could not read %s: %s — proceeding without vocabulary", path, exc)
+        return None
+    if not isinstance(payload, dict):
+        logger.warning("%s is not a JSON object — ignoring", path)
+        return None
+    vocab = Vocabulary.from_json(payload)
+    if vocab.is_empty():
+        return None
+    return vocab
+
+
+def save_vocabulary(root: Path, vocab: Vocabulary) -> Path:
+    """Atomically persist ``vocab`` to ``<root>/meta/canonical_vocabulary.json``."""
+    path = vocabulary_path(root)
+    path.parent.mkdir(parents=True, exist_ok=True)
+    tmp = path.with_suffix(path.suffix + ".tmp")
+    tmp.write_text(
+        json.dumps(vocab.to_json(), indent=2, ensure_ascii=False) + "\n",
+        encoding="utf-8",
+    )
+    tmp.replace(path)
+    return path
+
+
+@dataclass
+class VocabularyDiscoveryModule:
+    """Derive a dataset-level canonical vocabulary from sample episodes.
+
+    Phase 0 of the executor: pulls ``config.sample_episodes`` episode
+    videos, packs them into one Qwen-VL multi-video prompt, and asks the
+    model to enumerate the small set of canonical subtask labels +
+    memory milestones that recur across them. The output is persisted
+    to ``meta/canonical_vocabulary.json`` and consumed by phase 1.
+    """
+
+    vlm: VlmClient
+    config: VocabularyConfig
+    frame_provider: FrameProvider = field(default_factory=null_provider)
+
+    @property
+    def enabled(self) -> bool:
+        return self.config.enabled
+
+    def discover(
+        self,
+        records: Sequence[EpisodeRecord],
+        *,
+        existing: Vocabulary | None = None,
+    ) -> Vocabulary | None:
+        """Run vocabulary discovery against the first N sample episodes.
+
+        ``existing`` short-circuits the VLM call when ``config.reuse_existing``
+        is True and an on-disk vocabulary is already present — keeps re-runs
+        cheap and lets operators hand-edit the file without it getting
+        overwritten.
+        """
+        if existing is not None and self.config.reuse_existing:
+            logger.info(
+                "vocabulary: reusing existing (%d subtasks, %d memory milestones)",
+                len(existing.subtasks),
+                len(existing.memory_milestones),
+            )
+            return existing
+
+        sample = list(records[: max(1, int(self.config.sample_episodes))])
+        if not sample:
+            return None
+
+        task_hint = next((r.episode_task for r in sample if r.episode_task), "")
+        prompt = load_prompt("module_0_vocabulary").format(
+            episode_task=task_hint or "(unspecified)",
+            n_episodes=len(sample),
+            n_subtask_target=int(self.config.n_subtask_target),
+            n_memory_target=int(self.config.n_memory_target),
+        )
+        # Pack one video block per sample episode so the VLM sees the
+        # variation across episodes (different starting poses, different
+        # object placements) rather than overfitting to one trajectory.
+        content: list[dict[str, Any]] = []
+        for record in sample:
+            video_frames = self.frame_provider.video_for_episode(
+                record, int(self.config.max_video_frames_per_episode)
+            )
+            if video_frames:
+                content.extend(to_video_block(video_frames))
+        content.append({"type": "text", "text": prompt})
+        messages = [{"role": "user", "content": content}]
+
+        result = self.vlm.generate_json([messages])[0]
+        if not isinstance(result, dict):
+            logger.warning("vocabulary: VLM did not return a JSON object — skipping")
+            return None
+
+        vocab = Vocabulary.from_json(result)
+        if vocab.is_empty():
+            logger.warning("vocabulary: VLM returned an empty vocabulary — skipping")
+            return None
+        logger.info(
+            "vocabulary: discovered %d subtask labels + %d memory milestones from %d episodes",
+            len(vocab.subtasks),
+            len(vocab.memory_milestones),
+            len(sample),
+        )
+        return vocab

src/lerobot/scripts/lerobot_annotate.py

@@ -40,6 +40,7 @@ from lerobot.annotations.steerable_pipeline.modules import (
 )
 from lerobot.annotations.steerable_pipeline.validator import StagingValidator
 from lerobot.annotations.steerable_pipeline.vlm_client import make_vlm_client
+from lerobot.annotations.steerable_pipeline.vocabulary import VocabularyDiscoveryModule
 from lerobot.annotations.steerable_pipeline.writer import LanguageColumnsWriter
 from lerobot.configs import parser
 
@@ -88,6 +89,9 @@ def annotate(cfg: AnnotationPipelineConfig) -> None:
         vlm=vlm, config=cfg.interjections, seed=cfg.seed, frame_provider=frame_provider
     )
     vqa = GeneralVqaModule(vlm=vlm, config=cfg.vqa, seed=cfg.seed, frame_provider=frame_provider)
+    vocabulary = VocabularyDiscoveryModule(
+        vlm=vlm, config=cfg.vocabulary, frame_provider=frame_provider
+    )
     writer = LanguageColumnsWriter()
     validator = StagingValidator(
         dataset_camera_keys=tuple(getattr(frame_provider, "camera_keys", []) or []) or None,
@@ -98,6 +102,7 @@ def annotate(cfg: AnnotationPipelineConfig) -> None:
         plan=plan,
         interjections=interjections,
         vqa=vqa,
+        vocabulary=vocabulary,
         writer=writer,
         validator=validator,
     )

tests/annotations/test_vocabulary.py

@@ -0,0 +1,271 @@
+#!/usr/bin/env python
+
+# Copyright 2026 The HuggingFace Inc. team. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Vocabulary-discovery phase (phase 0) tests."""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+from lerobot.annotations.steerable_pipeline.config import (
+    PlanConfig,
+    VocabularyConfig,
+)
+from lerobot.annotations.steerable_pipeline.modules import PlanSubtasksMemoryModule
+from lerobot.annotations.steerable_pipeline.reader import iter_episodes
+from lerobot.annotations.steerable_pipeline.staging import EpisodeStaging
+from lerobot.annotations.steerable_pipeline.vocabulary import (
+    Vocabulary,
+    VocabularyDiscoveryModule,
+    load_vocabulary,
+    save_vocabulary,
+    vocabulary_path,
+)
+
+from ._helpers import make_canned_responder
+
+
+_CANONICAL_SUBTASKS = (
+    "grasp blue cube",
+    "place blue cube in box",
+    "retract arm",
+)
+_CANONICAL_MEMORY = (
+    "I picked up the blue cube.",
+    "I placed the blue cube in the box.",
+)
+
+
+# ---------------------------------------------------------------------------
+# Vocabulary dataclass + on-disk round-trip
+# ---------------------------------------------------------------------------
+
+
+def test_vocabulary_roundtrip(tmp_path: Path) -> None:
+    vocab = Vocabulary(
+        subtasks=_CANONICAL_SUBTASKS, memory_milestones=_CANONICAL_MEMORY
+    )
+    save_path = save_vocabulary(tmp_path, vocab)
+    assert save_path == vocabulary_path(tmp_path)
+    assert save_path.exists()
+
+    loaded = load_vocabulary(tmp_path)
+    assert loaded is not None
+    assert loaded.subtasks == _CANONICAL_SUBTASKS
+    assert loaded.memory_milestones == _CANONICAL_MEMORY
+
+
+def test_vocabulary_load_missing_returns_none(tmp_path: Path) -> None:
+    assert load_vocabulary(tmp_path) is None
+
+
+def test_vocabulary_load_malformed_returns_none(tmp_path: Path) -> None:
+    path = vocabulary_path(tmp_path)
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text("{ not valid json", encoding="utf-8")
+    assert load_vocabulary(tmp_path) is None
+
+
+def test_vocabulary_load_empty_payload_returns_none(tmp_path: Path) -> None:
+    path = vocabulary_path(tmp_path)
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(json.dumps({"subtasks": [], "memory_milestones": []}), encoding="utf-8")
+    assert load_vocabulary(tmp_path) is None
+
+
+# ---------------------------------------------------------------------------
+# Discovery module
+# ---------------------------------------------------------------------------
+
+
+def test_vocabulary_discovery_calls_vlm_and_returns_vocab(
+    fixture_dataset_root: Path,
+) -> None:
+    vlm = make_canned_responder(
+        {
+            "canonical vocabulary": {
+                "subtasks": list(_CANONICAL_SUBTASKS),
+                "memory_milestones": list(_CANONICAL_MEMORY),
+            }
+        }
+    )
+    module = VocabularyDiscoveryModule(vlm=vlm, config=VocabularyConfig(sample_episodes=2))
+    records = list(iter_episodes(fixture_dataset_root))
+    vocab = module.discover(records)
+    assert vocab is not None
+    assert vocab.subtasks == _CANONICAL_SUBTASKS
+    assert vocab.memory_milestones == _CANONICAL_MEMORY
+
+
+def test_vocabulary_discovery_reuses_existing(fixture_dataset_root: Path) -> None:
+    """``reuse_existing=True`` short-circuits the VLM call entirely."""
+
+    def _explode(_messages):  # pragma: no cover - must not be called
+        raise AssertionError("VLM should not be invoked when reusing existing vocabulary")
+
+    from lerobot.annotations.steerable_pipeline.vlm_client import StubVlmClient
+
+    vlm = StubVlmClient(responder=_explode)
+    module = VocabularyDiscoveryModule(
+        vlm=vlm, config=VocabularyConfig(reuse_existing=True)
+    )
+    records = list(iter_episodes(fixture_dataset_root))
+    existing = Vocabulary(subtasks=("a", "b"), memory_milestones=("I a.",))
+    vocab = module.discover(records, existing=existing)
+    assert vocab is existing
+
+
+def test_vocabulary_discovery_empty_payload_returns_none(
+    fixture_dataset_root: Path,
+) -> None:
+    vlm = make_canned_responder({"canonical vocabulary": {"subtasks": [], "memory_milestones": []}})
+    module = VocabularyDiscoveryModule(vlm=vlm, config=VocabularyConfig())
+    records = list(iter_episodes(fixture_dataset_root))
+    assert module.discover(records) is None
+
+
+# ---------------------------------------------------------------------------
+# PlanSubtasksMemoryModule consumes the vocabulary
+# ---------------------------------------------------------------------------
+
+
+def test_plan_module_inlines_vocab_into_subtask_prompt(
+    fixture_dataset_root: Path, tmp_path: Path
+) -> None:
+    captured: list[str] = []
+
+    def responder(messages):
+        # Find the last user text block and stash it for inspection.
+        for message in messages:
+            content = message.get("content")
+            if isinstance(content, list):
+                for block in content:
+                    if isinstance(block, dict) and block.get("type") == "text":
+                        captured.append(block.get("text", ""))
+        # Return canned subtasks; pick the first two canonical strings so
+        # the validator accepts them.
+        return {
+            "subtasks": [
+                {"text": "grasp blue cube", "start": 0.0, "end": 0.4},
+                {"text": "place blue cube in box", "start": 0.4, "end": 0.9},
+            ]
+        }
+
+    from lerobot.annotations.steerable_pipeline.vlm_client import StubVlmClient
+
+    vlm = StubVlmClient(responder=responder)
+    vocab = Vocabulary(subtasks=_CANONICAL_SUBTASKS, memory_milestones=_CANONICAL_MEMORY)
+    module = PlanSubtasksMemoryModule(
+        vlm=vlm,
+        config=PlanConfig(n_task_rephrasings=0),
+        vocabulary=vocab,
+    )
+    record = next(iter_episodes(fixture_dataset_root))
+    staging = EpisodeStaging(tmp_path / "stage", record.episode_index)
+    module.run_episode(record, staging)
+    # The subtask prompt (and the memory prompt) carries the canonical
+    # bullet list so the VLM can't paraphrase them away.
+    assert any("Canonical subtask labels:" in t for t in captured)
+    assert any("grasp blue cube" in t for t in captured)
+
+
+def test_plan_module_canonicalizes_paraphrased_subtask(
+    fixture_dataset_root: Path, tmp_path: Path
+) -> None:
+    """Off-vocab paraphrase with high token overlap snaps to canonical form."""
+    from lerobot.annotations.steerable_pipeline.vlm_client import StubVlmClient
+
+    def responder(_messages):
+        return {
+            "subtasks": [
+                # paraphrase of "grasp blue cube" — overlapping tokens
+                {"text": "grasp the blue cube", "start": 0.0, "end": 0.4},
+                # paraphrase of "place blue cube in box" — high overlap
+                {"text": "place the blue cube into the box", "start": 0.4, "end": 0.9},
+            ]
+        }
+
+    vlm = StubVlmClient(responder=responder)
+    vocab = Vocabulary(subtasks=_CANONICAL_SUBTASKS, memory_milestones=_CANONICAL_MEMORY)
+    module = PlanSubtasksMemoryModule(
+        vlm=vlm,
+        config=PlanConfig(n_task_rephrasings=0),
+        vocabulary=vocab,
+    )
+    record = next(iter_episodes(fixture_dataset_root))
+    staging = EpisodeStaging(tmp_path / "stage", record.episode_index)
+    module.run_episode(record, staging)
+    rows = staging.read("plan")
+    subtask_texts = [r["content"] for r in rows if r["style"] == "subtask"]
+    # Both paraphrases snapped onto canonical strings.
+    assert subtask_texts == ["grasp blue cube", "place blue cube in box"]
+
+
+def test_plan_module_drops_off_vocab_subtask(
+    fixture_dataset_root: Path, tmp_path: Path
+) -> None:
+    """A subtask with low overlap to every canonical label is dropped."""
+    from lerobot.annotations.steerable_pipeline.vlm_client import StubVlmClient
+
+    def responder(_messages):
+        return {
+            "subtasks": [
+                # in-vocab
+                {"text": "grasp blue cube", "start": 0.0, "end": 0.4},
+                # off-vocab hallucination — no token overlap above the
+                # Jaccard floor; should be dropped.
+                {"text": "perform a fancy macarena dance", "start": 0.4, "end": 0.9},
+            ]
+        }
+
+    vlm = StubVlmClient(responder=responder)
+    vocab = Vocabulary(subtasks=_CANONICAL_SUBTASKS, memory_milestones=_CANONICAL_MEMORY)
+    module = PlanSubtasksMemoryModule(
+        vlm=vlm,
+        config=PlanConfig(n_task_rephrasings=0),
+        vocabulary=vocab,
+    )
+    record = next(iter_episodes(fixture_dataset_root))
+    staging = EpisodeStaging(tmp_path / "stage", record.episode_index)
+    module.run_episode(record, staging)
+    rows = staging.read("plan")
+    subtask_texts = [r["content"] for r in rows if r["style"] == "subtask"]
+    assert subtask_texts == ["grasp blue cube"]
+
+
+def test_plan_module_without_vocab_passes_through(
+    fixture_dataset_root: Path, tmp_path: Path
+) -> None:
+    """No vocabulary configured → original free-form behavior is preserved."""
+    from lerobot.annotations.steerable_pipeline.vlm_client import StubVlmClient
+
+    def responder(_messages):
+        return {
+            "subtasks": [
+                {"text": "any free-form text the VLM wants", "start": 0.0, "end": 1.0},
+            ]
+        }
+
+    vlm = StubVlmClient(responder=responder)
+    module = PlanSubtasksMemoryModule(
+        vlm=vlm, config=PlanConfig(n_task_rephrasings=0)
+    )
+    record = next(iter_episodes(fixture_dataset_root))
+    staging = EpisodeStaging(tmp_path / "stage", record.episode_index)
+    module.run_episode(record, staging)
+    rows = staging.read("plan")
+    subtask_texts = [r["content"] for r in rows if r["style"] == "subtask"]
+    assert subtask_texts == ["any free-form text the VLM wants"]