From 8e05dc9a7ad6cdf1a02b66ac9c0df24acbb1535f Mon Sep 17 00:00:00 2001
From: Jade Choghari <chogharijade@gmail.com>
Date: Tue, 16 Dec 2025 11:28:27 +0000
Subject: [PATCH] add fast tokenizer support

---
 examples/dataset/action_tokenizer_example.py  | 138 ++++
 examples/dataset/fast_tokenize.py             |  25 +
 examples/dataset/inference_pi05.py            |   7 +-
 examples/dataset/mask.md                      | 159 +++++
 examples/dataset/test.txt                     |   1 +
 src/lerobot/policies/pi05/compare.txt         |  64 --
 .../policies/pi05/configuration_pi05.py       |   3 +
 src/lerobot/policies/pi05/modeling_pi05.py    | 630 ++++++++++++++----
 src/lerobot/policies/pi05/processor_pi05.py   |   7 +-
 src/lerobot/processor/__init__.py             |   2 +-
 src/lerobot/processor/tokenizer_processor.py  | 246 ++++++-
 src/lerobot/scripts/lerobot_train.py          |   1 +
 src/lerobot/utils/constants.py                |   2 +
 13 files changed, 1081 insertions(+), 204 deletions(-)
 create mode 100644 examples/dataset/action_tokenizer_example.py
 create mode 100644 examples/dataset/fast_tokenize.py
 create mode 100644 examples/dataset/mask.md
 create mode 100644 examples/dataset/test.txt
 delete mode 100644 src/lerobot/policies/pi05/compare.txt

diff --git a/examples/dataset/action_tokenizer_example.py b/examples/dataset/action_tokenizer_example.py
new file mode 100644
index 000000000..1e16a3fc5
--- /dev/null
+++ b/examples/dataset/action_tokenizer_example.py
@@ -0,0 +1,138 @@
+#!/usr/bin/env python
+
+"""
+Example demonstrating how to use the ActionTokenizerProcessorStep to tokenize actions.
+
+This example shows how to:
+1. Load a dataset with action data
+2. Apply the action tokenizer processor to tokenize actions with proper padding/truncation
+3. Access both the tokenized actions and the attention mask
+4. Decode tokenized actions back to their original form
+"""
+
+import torch
+from transformers import AutoProcessor
+
+from lerobot.datasets.lerobot_dataset import LeRobotDataset
+from lerobot.processor.core import EnvTransition, TransitionKey
+from lerobot.processor.tokenizer_processor import ActionTokenizerProcessorStep
+from lerobot.utils.constants import ACTION_TOKEN_MASK
+
+# Define delta timestamps for the dataset
+delta_timestamps = {
+    'action': [
+        0.0, 0.03333333333333333, 0.06666666666666667, 0.1, 0.13333333333333333,
+        0.16666666666666666, 0.2, 0.23333333333333334, 0.26666666666666666, 0.3,
+        0.3333333333333333, 0.36666666666666664, 0.4, 0.43333333333333335,
+        0.4666666666666667, 0.5, 0.5333333333333333, 0.5666666666666667, 0.6,
+        0.6333333333333333, 0.6666666666666666, 0.7, 0.7333333333333333,
+        0.7666666666666667, 0.8, 0.8333333333333334, 0.8666666666666667, 0.9,
+        0.9333333333333333, 0.9666666666666667, 1.0, 1.0333333333333334,
+        1.0666666666666667, 1.1, 1.1333333333333333, 1.1666666666666667, 1.2,
+        1.2333333333333334, 1.2666666666666666, 1.3, 1.3333333333333333,
+        1.3666666666666667, 1.4, 1.4333333333333333, 1.4666666666666666, 1.5,
+        1.5333333333333334, 1.5666666666666667, 1.6, 1.6333333333333333
+    ]
+}
+
+# Load the dataset
+print("Loading dataset...")
+dataset = LeRobotDataset(
+    repo_id="local",
+    root="/fsx/jade_choghari/outputs/pgen_annotations1",
+    delta_timestamps=delta_timestamps
+)
+
+# Create a dataloader
+dataloader = torch.utils.data.DataLoader(
+    dataset,
+    num_workers=0,
+    batch_size=4,
+    shuffle=True,
+)
+
+# Get a batch of data
+batch = next(iter(dataloader))
+action_data = batch["action"]  # Shape: (batch_size, action_horizon, action_dim)
+
+print(f"\nOriginal action shape: {action_data.shape}")
+print(f"Original action data (first sample, first timestep):\n{action_data[0, 0]}")
+
+# Method 1: Using the tokenizer directly (as in fast_tokenize.py)
+print("\n" + "="*80)
+print("Method 1: Direct tokenizer usage")
+print("="*80)
+
+tokenizer = AutoProcessor.from_pretrained("physical-intelligence/fast", trust_remote_code=True)
+
+# Tokenize directly
+tokens = tokenizer(action_data)
+print(f"\nDirect tokenization result type: {type(tokens)}")
+print(f"Tokens shape/length: {tokens.shape if isinstance(tokens, torch.Tensor) else len(tokens)}")
+
+# Decode
+decoded_actions = tokenizer.decode(tokens)
+print(f"Decoded actions shape: {decoded_actions.shape}")
+reconstruction_error = torch.abs(action_data - decoded_actions).mean()
+print(f"Mean absolute reconstruction error: {reconstruction_error.item():.6f}")
+
+# Method 2: Using the ActionTokenizerProcessorStep with proper padding/truncation
+print("\n" + "="*80)
+print("Method 2: Using ActionTokenizerProcessorStep (with padding & mask)")
+print("="*80)
+
+# Create the action tokenizer processor step
+action_tokenizer_processor = ActionTokenizerProcessorStep(
+    tokenizer_name="physical-intelligence/fast",
+    trust_remote_code=True,
+    max_action_tokens=32,  # Maximum number of tokens per action
+)
+
+# Create a transition with the action data
+transition = {
+    TransitionKey.ACTION: action_data,
+    TransitionKey.OBSERVATION: {},  # Empty for this example
+}
+
+# Apply the processor
+processed_transition = action_tokenizer_processor(transition)
+
+# Extract tokenized actions and mask
+tokenized_actions = processed_transition[TransitionKey.ACTION]
+complementary_data = processed_transition[TransitionKey.COMPLEMENTARY_DATA]
+action_mask = complementary_data[ACTION_TOKEN_MASK]
+
+print(f"\nTokenized actions shape: {tokenized_actions.shape}")  # (batch_size, max_action_tokens)
+print(f"Action mask shape: {action_mask.shape}")  # (batch_size, max_action_tokens)
+print(f"Tokenized actions dtype: {tokenized_actions.dtype}")
+print(f"Action mask dtype: {action_mask.dtype}")
+
+# Show token statistics
+print(f"\nFirst sample tokens: {tokenized_actions[0]}")
+print(f"First sample mask: {action_mask[0]}")
+num_real_tokens = action_mask[0].sum().item()
+print(f"Number of real tokens (non-padding): {num_real_tokens}")
+print(f"Number of padding tokens: {action_mask.shape[1] - num_real_tokens}")
+
+# Decode using the mask
+print("\nDecoding tokenized actions...")
+decoded_with_processor = tokenizer.decode(tokenized_actions)
+print(f"Decoded actions shape: {decoded_with_processor.shape}")
+
+# Calculate reconstruction error
+reconstruction_error_processor = torch.abs(action_data - decoded_with_processor).mean()
+print(f"Mean absolute reconstruction error: {reconstruction_error_processor.item():.6f}")
+
+# Show that masking works correctly
+print("\n" + "="*80)
+print("Mask demonstration")
+print("="*80)
+for i in range(min(4, tokenized_actions.shape[0])):
+    mask_i = action_mask[i]
+    num_real = mask_i.sum().item()
+    print(f"Sample {i}: {num_real} real tokens, {len(mask_i) - num_real} padding tokens")
+
+print("\n" + "="*80)
+print("Action tokenization example completed successfully!")
+print("="*80)
+
diff --git a/examples/dataset/fast_tokenize.py b/examples/dataset/fast_tokenize.py
new file mode 100644
index 000000000..a730a31b3
--- /dev/null
+++ b/examples/dataset/fast_tokenize.py
@@ -0,0 +1,25 @@
+import numpy as np
+from transformers import AutoProcessor
+import torch
+from lerobot.datasets.lerobot_dataset import LeRobotDataset, LeRobotDatasetMetadata
+
+delta_timestamps = {'action': [0.0, 0.03333333333333333, 0.06666666666666667, 0.1, 0.13333333333333333, 0.16666666666666666, 0.2, 0.23333333333333334, 0.26666666666666666, 0.3, 0.3333333333333333, 0.36666666666666664, 0.4, 0.43333333333333335, 0.4666666666666667, 0.5, 0.5333333333333333, 0.5666666666666667, 0.6, 0.6333333333333333, 0.6666666666666666, 0.7, 0.7333333333333333, 0.7666666666666667, 0.8, 0.8333333333333334, 0.8666666666666667, 0.9, 0.9333333333333333, 0.9666666666666667, 1.0, 1.0333333333333334, 1.0666666666666667, 1.1, 1.1333333333333333, 1.1666666666666667, 1.2, 1.2333333333333334, 1.2666666666666666, 1.3, 1.3333333333333333, 1.3666666666666667, 1.4, 1.4333333333333333, 1.4666666666666666, 1.5, 1.5333333333333334, 1.5666666666666667, 1.6, 1.6333333333333333]}
+dataset = LeRobotDataset(repo_id="local", root="/fsx/jade_choghari/outputs/pgen_annotations1", delta_timestamps=delta_timestamps)
+
+dataloader = torch.utils.data.DataLoader(
+        dataset,
+        num_workers=0,
+        batch_size=4,
+        shuffle=True,
+)
+
+batch = next(iter(dataloader))
+
+# Load the tokenizer from the Hugging Face hub
+tokenizer = AutoProcessor.from_pretrained("physical-intelligence/fast", trust_remote_code=True)
+
+# Tokenize & decode action chunks (we use dummy data here)
+action_data = batch["action"]    # one batch of action chunks
+tokens = tokenizer(action_data)              # tokens = list[int]
+decoded_actions = tokenizer.decode(tokens)
+print("tokenized actions: ", tokens)
diff --git a/examples/dataset/inference_pi05.py b/examples/dataset/inference_pi05.py
index 1590c11dc..f8e38760f 100644
--- a/examples/dataset/inference_pi05.py
+++ b/examples/dataset/inference_pi05.py
@@ -10,13 +10,13 @@ from lerobot.policies.factory import make_policy, make_policy_config
 from lerobot.configs.policies import PreTrainedConfig
 
 cfg = PreTrainedConfig.from_pretrained(
-    pretrained_name_or_path="/fsx/jade_choghari/outputs/pi0_training_new/checkpoints/last/pretrained_model",
+    pretrained_name_or_path="/fsx/jade_choghari/outputs/pi0_training/checkpoints/last/pretrained_model",
 )
 cfg.dtype = "bfloat16"
 
 pre_processor, post_processor = make_pre_post_processors(
     policy_cfg=cfg,
-    pretrained_path="/fsx/jade_choghari/outputs/pi0_training_new/checkpoints/last/pretrained_model",
+    pretrained_path="/fsx/jade_choghari/outputs/pi0_training/checkpoints/last/pretrained_model",
 )
 
 delta_timestamps = {'action': [0.0, 0.03333333333333333, 0.06666666666666667, 0.1, 0.13333333333333333, 0.16666666666666666, 0.2, 0.23333333333333334, 0.26666666666666666, 0.3, 0.3333333333333333, 0.36666666666666664, 0.4, 0.43333333333333335, 0.4666666666666667, 0.5, 0.5333333333333333, 0.5666666666666667, 0.6, 0.6333333333333333, 0.6666666666666666, 0.7, 0.7333333333333333, 0.7666666666666667, 0.8, 0.8333333333333334, 0.8666666666666667, 0.9, 0.9333333333333333, 0.9666666666666667, 1.0, 1.0333333333333334, 1.0666666666666667, 1.1, 1.1333333333333333, 1.1666666666666667, 1.2, 1.2333333333333334, 1.2666666666666666, 1.3, 1.3333333333333333, 1.3666666666666667, 1.4, 1.4333333333333333, 1.4666666666666666, 1.5, 1.5333333333333334, 1.5666666666666667, 1.6, 1.6333333333333333]}
@@ -45,13 +45,12 @@ dataloader = torch.utils.data.DataLoader(
 )
 
 batch = next(iter(dataloader))
-
 batch = pre_processor(batch)
-breakpoint()
 policy.train()
 # run inference
 # action = policy.select_action(batch)
 loss, loss_dict = policy.forward(batch)
+breakpoint()
 # import requests
 # from PIL import Image
 # from transformers import AutoProcessor
diff --git a/examples/dataset/mask.md b/examples/dataset/mask.md
new file mode 100644
index 000000000..a55a90a39
--- /dev/null
+++ b/examples/dataset/mask.md
@@ -0,0 +1,159 @@
+## One-sentence answer
+
+> `make_att_2d_masks(prefix_pad_masks, prefix_att_masks)` builds the **actual 2D attention mask** `[B, L, L]` that tells the transformer **which token positions may attend to which others**, combining **padding** and **causality**.
+
+Everything else you’ve seen so far was just metadata.
+
+---
+
+## What goes in
+
+### Inputs
+
+```python
+prefix_pad_masks   # shape [B, L]
+prefix_att_masks   # shape [B, L]
+```
+
+Where:
+
+* `prefix_pad_masks[b, i] = True`
+  → token `i` exists (not padding)
+
+* `prefix_att_masks[b, i] = False`
+  → token `i` is **bidirectional**
+
+* `prefix_att_masks[b, i] = True`
+  → token `i` is **causal (autoregressive)**
+
+---
+
+## What comes out
+
+```python
+att_2d_prefix  # shape [B, L, L]
+```
+
+Each entry:
+
+```text
+att_2d_prefix[b, i, j] = True
+```
+
+means:
+
+> “In batch `b`, **token i (query)** is allowed to attend to **token j (key)**.”
+
+---
+
+## How it is constructed (conceptually)
+
+For **each batch b**, **each query position i**, **each key position j**:
+
+```python
+if not prefix_pad_masks[b, j]:
+    att[b, i, j] = False           # cannot attend to padding
+else if not prefix_att_masks[b, i]:
+    att[b, i, j] = True            # bidirectional token → can see all real tokens
+else:
+    att[b, i, j] = (j <= i)        # causal token → can see only past + itself
+```
+
+That’s it.
+
+---
+
+## Tiny concrete example (exactly matching your code)
+
+Suppose:
+
+```python
+prefix_pad_masks[0] = [T, T, T, T, T, F]
+prefix_att_masks[0] = [F, F, F, T, T, T]
+```
+
+Tokens:
+
+```
+0: IMG
+1: IMG
+2: LANG
+3: SUB0
+4: SUB1
+5: PAD
+```
+
+---
+
+### Resulting `att_2d_prefix[0]`
+
+`✓ = True, ✗ = False`
+
+| Q \ K      | 0 | 1 | 2 | 3 | 4 | 5 |
+| ---------- | - | - | - | - | - | - |
+| 0 (bi)     | ✓ | ✓ | ✓ | ✓ | ✓ | ✗ |
+| 1 (bi)     | ✓ | ✓ | ✓ | ✓ | ✓ | ✗ |
+| 2 (bi)     | ✓ | ✓ | ✓ | ✓ | ✓ | ✗ |
+| 3 (causal) | ✓ | ✓ | ✓ | ✓ | ✗ | ✗ |
+| 4 (causal) | ✓ | ✓ | ✓ | ✓ | ✓ | ✗ |
+| 5 (pad)    | ✗ | ✗ | ✗ | ✗ | ✗ | ✗ |
+
+---
+
+## Why this matters for your training code
+
+This line:
+
+```python
+att_2d_prefix_4d = self._prepare_attention_masks_4d(att_2d_prefix)
+```
+
+Converts `[B, L, L] → [B, 1, L, L]` and possibly flips True/False to `0/-inf`.
+
+This is **exactly what Paligemma uses inside self-attention**.
+
+---
+
+## Key implications (VERY important)
+
+### 1️⃣ This mask does **not isolate token groups**
+
+* Bidirectional tokens can attend to **everything**
+* Causal tokens only restrict *their own row*
+
+So **flow/action tokens must be blocked separately**.
+
+---
+
+### 2️⃣ This is why your AR subtask prediction works
+
+* Subtask tokens are causal
+* Output at position `i` predicts token `i+1`
+* Padding is fully ignored
+
+---
+
+### 3️⃣ Inference behavior
+
+When `subtask_tokens = None`:
+
+* `prefix_att_masks` contains only `False`
+* `att_2d_prefix` becomes **fully bidirectional**
+* No AR behavior remains
+
+Exactly what you want.
+
+---
+
+## One-sentence takeaway (commit this)
+
+> `make_att_2d_masks` fuses **padding** and **causality** into a concrete `[B, L, L]` attention matrix that the transformer actually uses.
+
+If you want next, I can:
+
+* inspect `make_att_2d_masks()` source with you
+* show how to block **flow → subtask** attention
+* explain how this changes when suffix tokens are added
+* help you refactor this into a cleaner “grouped attention” API
+
+You’re now at the point where the model’s behavior should feel *predictable*, not magical.
diff --git a/examples/dataset/test.txt b/examples/dataset/test.txt
new file mode 100644
index 000000000..6fd6d8dd2
--- /dev/null
+++ b/examples/dataset/test.txt
@@ -0,0 +1 @@
+srun --time 12:00:00     --qos=high     --gres=gpu:1     --mem=24G     --partition=hopper-prod     --container-image /fsx/michel_aractingi/docker_images/huggingface+lerobot-gpu+dev.sqsh     --container-mounts /fsx/jade_choghari   
\ No newline at end of file
diff --git a/src/lerobot/policies/pi05/compare.txt b/src/lerobot/policies/pi05/compare.txt
deleted file mode 100644
index aaebecec3..000000000
--- a/src/lerobot/policies/pi05/compare.txt
+++ /dev/null
@@ -1,64 +0,0 @@
-
-Fine tune output
-(Pdb) images[2].mean()
-tensor(-1., device='cuda:0')
-(Pdb) images[1].mean()
-tensor(-0.5780, device='cuda:0')
-(Pdb) images[0].mean()
-tensor(-0.7716, device='cuda:0')
-(Pdb) (Pdb) high_level_task[0]
-tensor([     2,   7978,   2403,   6911, 235292,   5651,   3124,    573,  18571,
-          7762,   6643,    573,   9010,  72993,  21810,   4894,   3040, 235292,
-        235248, 235274, 235274, 235274,    728, 235274, 235248, 235284, 235308,
-        235308, 235248, 235274, 235318, 235315, 235248, 235274, 235310, 235318,
-        235248, 235284, 235318, 235248, 235274, 235284, 235321, 235248, 235274,
-        235284, 235321, 235248, 235274, 235284, 235321, 235248, 235274, 235284,
-        235321, 235248, 235274, 235284, 235321, 235248, 235274, 235284, 235321,
-        235248, 235274, 235284, 235321, 235248, 235274, 235284, 235321, 235248,
-        235274, 235284, 235321, 235248, 235274, 235284, 235321, 235248, 235274,
-        235284, 235321, 235248, 235274, 235284, 235321, 235248, 235274, 235284,
-        235321, 235248, 235274, 235284, 235321, 235248, 235274, 235284, 235321,
-        235248, 235274, 235284, 235321, 235248, 235274, 235284, 235321, 235248,
-        235274, 235284, 235321, 235248, 235274, 235284, 235321, 235248, 235274,
-        235284, 235321, 235248, 235274, 235284, 235321, 235248, 235274, 235284,
-        235321, 235248, 235274, 235284, 235321, 235248, 235274, 235284, 235321,
-        235248, 235274, 235284, 235321, 235248, 235274, 235284, 235321, 235289,
-          4284,   8277, 235292,      0,      0,      0,      0,      0,      0,
-             0,      0,      0,      0,      0,      0,      0,      0,      0,
-             0,      0,      0,      0,      0,      0,      0,      0,      0,
-             0,      0,      0,      0,      0,      0,      0,      0,      0,
-             0,      0,      0,      0,      0,      0,      0,      0,      0,
-             0,      0,      0,      0,      0,      0,      0,      0,      0,
-             0,      0], device='cuda:0')
-(Pdb) subtask_tokens[0]
-tensor([    2, 28040,  7762, 14574,  6643,  9010, 37901, 21810,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0,
-            0,     0,     0,     0,     0,     0,     0,     0,     0,     0],
-       device='cuda:0')
-(Pdb) actions.shape
-torch.Size([4, 50, 32])
-(Pdb) actions.mean()
-tensor(0.0143, device='cuda:0')
-(Pdb) 
-
-
-
-
-Inference:
diff --git a/src/lerobot/policies/pi05/configuration_pi05.py b/src/lerobot/policies/pi05/configuration_pi05.py
index 60ea6be87..c64e6f241 100644
--- a/src/lerobot/policies/pi05/configuration_pi05.py
+++ b/src/lerobot/policies/pi05/configuration_pi05.py
@@ -37,6 +37,9 @@ class PI05Config(PreTrainedConfig):
     # Shorter state and action vectors will be padded to these dimensions
     max_state_dim: int = 32
     max_action_dim: int = 32
+    max_action_tokens: int = 32
+    fast_vocab_size: int = 2048
+    
 
     # Flow matching parameters: see openpi `PI0Pytorch`
     num_inference_steps: int = 10
diff --git a/src/lerobot/policies/pi05/modeling_pi05.py b/src/lerobot/policies/pi05/modeling_pi05.py
index c9e22c0a1..561324de3 100644
--- a/src/lerobot/policies/pi05/modeling_pi05.py
+++ b/src/lerobot/policies/pi05/modeling_pi05.py
@@ -537,6 +537,18 @@ class PI05Pytorch(nn.Module):  # see openpi `PI0Pytorch`
         self.time_mlp_in = nn.Linear(action_expert_config.width, action_expert_config.width)
         self.time_mlp_out = nn.Linear(action_expert_config.width, action_expert_config.width)
 
+        # FAST action token embedding and prediction head
+        self.fast_action_embedding = nn.Embedding(config.fast_vocab_size, paligemma_config.width)
+        self.fast_action_lm_head = nn.Linear(paligemma_config.width, config.fast_vocab_size)
+
+        # Apply dtype conversion to FAST layers to match model precision
+        if config.dtype == "bfloat16":
+            self.fast_action_embedding = self.fast_action_embedding.to(dtype=torch.bfloat16)
+            self.fast_action_lm_head = self.fast_action_lm_head.to(dtype=torch.bfloat16)
+        elif config.dtype == "float32":
+            self.fast_action_embedding = self.fast_action_embedding.to(dtype=torch.float32)
+            self.fast_action_lm_head = self.fast_action_lm_head.to(dtype=torch.float32)
+
         # Initialize gradient checkpointing flag
         self.gradient_checkpointing_enabled = False
 
@@ -592,6 +604,194 @@ class PI05Pytorch(nn.Module):  # see openpi `PI0Pytorch`
             result = result.to(dtype=dtype)
         return result
 
+    def _create_custom_attention_mask(self, att_mask_segments, pad_masks, bsize):
+        """Create custom 2D attention mask for the new attention pattern.
+        
+        Attention rules:
+        - Images + Language: bidirectional among themselves, don't attend to subtask or FAST
+        - Subtask: attend to images + language, causal among themselves, don't attend to FAST
+        - FAST: attend to images + language + subtask, causal among themselves
+        
+        Args:
+            att_mask_segments: List of (type, length) tuples
+            pad_masks: Padding masks [B, total_seq_len]
+            bsize: Batch size
+        
+        Returns:
+            att_2d_masks: 2D attention mask [B, total_seq_len, total_seq_len]
+        """
+        total_len = sum(length for _, length in att_mask_segments)
+        device = pad_masks.device
+        
+        # Initialize attention mask as False (cannot attend)
+        att_2d_masks = torch.zeros(bsize, total_len, total_len, dtype=torch.bool, device=device)
+        
+        # Track positions for each segment
+        positions = []
+        current_pos = 0
+        for seg_type, seg_len in att_mask_segments:
+            positions.append((seg_type, current_pos, current_pos + seg_len))
+            current_pos += seg_len
+        
+        # Apply attention rules
+        for i, (query_type, query_start, query_end) in enumerate(positions):
+            for j, (key_type, key_start, key_end) in enumerate(positions):
+                # Images and Language can attend to each other bidirectionally
+                if query_type in ['image', 'language'] and key_type in ['image', 'language']:
+                    att_2d_masks[:, query_start:query_end, key_start:key_end] = True
+                
+                # Subtask tokens attend to images + language
+                elif query_type == 'subtask' and key_type in ['image', 'language']:
+                    att_2d_masks[:, query_start:query_end, key_start:key_end] = True
+                
+                # Subtask tokens attend causally to themselves
+                elif query_type == 'subtask' and key_type == 'subtask':
+                    # Create causal mask for subtask tokens
+                    subtask_len = query_end - query_start
+                    causal_mask = torch.tril(torch.ones(subtask_len, subtask_len, dtype=torch.bool, device=device))
+                    att_2d_masks[:, query_start:query_end, key_start:key_end] = causal_mask[None, :, :]
+                
+                # FAST tokens attend to images + language + subtask
+                elif query_type == 'fast' and key_type in ['image', 'language', 'subtask']:
+                    att_2d_masks[:, query_start:query_end, key_start:key_end] = True
+                
+                # FAST tokens attend causally to themselves
+                elif query_type == 'fast' and key_type == 'fast':
+                    fast_len = query_end - query_start
+                    causal_mask = torch.tril(torch.ones(fast_len, fast_len, dtype=torch.bool, device=device))
+                    att_2d_masks[:, query_start:query_end, key_start:key_end] = causal_mask[None, :, :]
+        
+        # Apply padding masks
+        pad_2d_masks = pad_masks[:, None, :] * pad_masks[:, :, None]
+        att_2d_masks = att_2d_masks & pad_2d_masks
+        
+        return att_2d_masks
+
+    def visualize_attention_mask(
+        self,
+        att_mask_segments,
+        att_2d_masks,
+        save_path,
+        batch_idx=0,
+        dpi=150,
+        max_display_tokens=None
+    ):
+        """Visualize the attention mask with labeled segments.
+        
+        Args:
+            att_mask_segments: List of (type, length) tuples defining the segments
+            att_2d_masks: 2D attention mask tensor [B, total_seq_len, total_seq_len]
+            save_path: Path where to save the visualization image
+            batch_idx: Which batch item to visualize (default: 0)
+            dpi: DPI for the saved image (default: 150)
+            max_display_tokens: Maximum number of tokens to display (for very long sequences)
+        """
+        try:
+            import matplotlib.pyplot as plt
+            import matplotlib.patches as mpatches
+            from matplotlib.colors import LinearSegmentedColormap
+        except ImportError:
+            logging.warning("matplotlib not available, skipping attention mask visualization")
+            return
+        
+        # Extract the mask for the specified batch
+        mask = att_2d_masks[batch_idx].cpu().float().numpy()
+        
+        # If sequence is too long, downsample for visualization
+        if max_display_tokens is not None and mask.shape[0] > max_display_tokens:
+            # Simple downsampling by taking every Nth token
+            step = mask.shape[0] // max_display_tokens
+            mask = mask[::step, ::step]
+            # Adjust segments accordingly
+            att_mask_segments = [(seg_type, max(1, seg_len // step)) for seg_type, seg_len in att_mask_segments]
+        
+        # Calculate positions for each segment
+        positions = []
+        current_pos = 0
+        for seg_type, seg_len in att_mask_segments:
+            positions.append((seg_type, current_pos, current_pos + seg_len))
+            current_pos += seg_len
+        
+        # Create figure
+        fig, ax = plt.subplots(figsize=(12, 10))
+        
+        # Create custom colormap: white for False (no attention), blue for True (attention)
+        colors = ['white', '#2E86AB']
+        n_bins = 2
+        cmap = LinearSegmentedColormap.from_list('attention', colors, N=n_bins)
+        
+        # Display the mask
+        im = ax.imshow(mask, cmap=cmap, aspect='auto', interpolation='nearest', vmin=0, vmax=1)
+        
+        # Add colorbar
+        cbar = plt.colorbar(im, ax=ax, fraction=0.046, pad=0.04)
+        cbar.set_label('Attention Enabled', rotation=270, labelpad=20)
+        cbar.set_ticks([0.25, 0.75])
+        cbar.set_ticklabels(['No', 'Yes'])
+        
+        # Define colors for each segment type
+        segment_colors = {
+            'image': '#A23B72',
+            'language': '#F18F01',
+            'subtask': '#C73E1D',
+            'fast': '#6A994E'
+        }
+        
+        # Draw segment boundaries and labels
+        for seg_type, start, end in positions:
+            color = segment_colors.get(seg_type, '#666666')
+            
+            # Draw vertical lines for columns (keys)
+            ax.axvline(x=start - 0.5, color=color, linewidth=2, alpha=0.7)
+            ax.axvline(x=end - 0.5, color=color, linewidth=2, alpha=0.7)
+            
+            # Draw horizontal lines for rows (queries)
+            ax.axhline(y=start - 0.5, color=color, linewidth=2, alpha=0.7)
+            ax.axhline(y=end - 0.5, color=color, linewidth=2, alpha=0.7)
+            
+            # Add labels at the top
+            mid_pos = (start + end) / 2
+            ax.text(mid_pos, -mask.shape[0] * 0.02, f"{seg_type.upper()}\n({end - start})",
+                   ha='center', va='top', fontsize=10, fontweight='bold', color=color)
+            
+            # Add labels on the left
+            ax.text(-mask.shape[1] * 0.02, mid_pos, f"{seg_type.upper()}\n({end - start})",
+                   ha='right', va='center', fontsize=10, fontweight='bold', color=color, rotation=0)
+        
+        # Set axis labels
+        ax.set_xlabel('Key Position (tokens being attended to)', fontsize=12, fontweight='bold')
+        ax.set_ylabel('Query Position (tokens attending)', fontsize=12, fontweight='bold')
+        ax.set_title('Attention Mask Pattern\n(White = No Attention, Blue = Attention Allowed)', 
+                    fontsize=14, fontweight='bold', pad=20)
+        
+        # Create legend for segment types
+        legend_patches = []
+        attention_rules = {
+            'image': 'Bidirectional with lang',
+            'language': 'Bidirectional with images',
+            'subtask': 'Attends to img+lang, causal self',
+            'fast': 'Attends to all, causal self'
+        }
+        for seg_type, color in segment_colors.items():
+            if any(seg[0] == seg_type for seg in att_mask_segments):
+                rule = attention_rules.get(seg_type, '')
+                legend_patches.append(mpatches.Patch(color=color, label=f'{seg_type.upper()}: {rule}'))
+        
+        ax.legend(handles=legend_patches, loc='upper right', bbox_to_anchor=(1.15, 1.0),
+                 framealpha=0.9, fontsize=9)
+        
+        # Adjust layout and save
+        plt.tight_layout()
+        
+        # Ensure the directory exists
+        save_path = Path(save_path)
+        save_path.parent.mkdir(parents=True, exist_ok=True)
+        
+        plt.savefig(save_path, dpi=dpi, bbox_inches='tight')
+        plt.close()
+        
+        logging.info(f"Attention mask visualization saved to: {save_path}")
+
     def sample_noise(self, shape, device):
         return torch.normal(
             mean=0.0,
@@ -607,10 +807,18 @@ class PI05Pytorch(nn.Module):  # see openpi `PI0Pytorch`
         )
         time = time_beta * self.config.time_sampling_scale + self.config.time_sampling_offset
         return time.to(dtype=torch.float32, device=device)
-
+    
     def embed_prefix(
-        self, images, img_masks, tokens, subtask_tokens, masks, subtask_masks
-    ) -> tuple[torch.Tensor, torch.Tensor, torch.Tensor, int]:
+        self, 
+        images, 
+        img_masks, 
+        tokens, 
+        subtask_tokens, 
+        masks, 
+        subtask_masks, 
+        fast_action_tokens=None, 
+        fast_action_masks=None,
+    ) -> tuple[torch.Tensor, torch.Tensor, torch.Tensor, int, int, int]:
         """Embed images with SigLIP, tokens, and optionally subtask tokens with embedding layer.
         
         Args:
@@ -619,17 +827,23 @@ class PI05Pytorch(nn.Module):  # see openpi `PI0Pytorch`
             tokens: Language instruction tokens
             subtask_tokens: Subtask tokens to predict (can be None for inference)
             masks: Attention masks for tokens
+            fast_action_tokens: FAST action tokens for auxiliary prediction (can be None) - discrete token IDs
+            fast_action_masks: Padding masks for FAST action tokens (can be None)
             
         Returns:
-            embs: Concatenated embeddings [images, tokens, (subtask_tokens if provided)]
+            embs: Concatenated embeddings [images, tokens, (subtask_tokens if provided), (fast_action_tokens if provided)]
             pad_masks: Padding masks
-            att_masks: Attention masks (with causal masking for subtask prediction if subtask_tokens provided)
+            att_masks: Custom 2D attention mask implementing the required pattern
             total_T_images: Total number of image tokens
+            num_subtask_embs: Number of subtask token embeddings
+            num_fast_embs: Number of FAST action token embeddings
         """
         embs = []
         pad_masks = []
-        att_masks = []
+        att_mask_segments = []  # Store info about each segment for custom mask creation
         total_T_images = 0
+        num_subtask_embs = 0
+        num_fast_embs = 0
         
         # Process images
         for img, img_mask in zip(images, img_masks, strict=True):
@@ -642,7 +856,7 @@ class PI05Pytorch(nn.Module):  # see openpi `PI0Pytorch`
 
             embs.append(img_emb)
             pad_masks.append(img_mask[:, None].expand(bsize, num_img_embs))
-            att_masks += [0] * num_img_embs  # Images can attend to all previous tokens
+            att_mask_segments.append(('image', num_img_embs))
             total_T_images += num_img_embs
             
         # Process language instruction tokens
@@ -656,7 +870,7 @@ class PI05Pytorch(nn.Module):  # see openpi `PI0Pytorch`
         pad_masks.append(masks)
 
         num_lang_embs = lang_emb.shape[1]
-        att_masks += [0] * num_lang_embs  # Language tokens can attend to all previous tokens (images + tokens)
+        att_mask_segments.append(('language', num_lang_embs))
 
         # Process subtask tokens if provided (these are predicted, so use causal masking)
         if subtask_tokens is not None:
@@ -672,18 +886,49 @@ class PI05Pytorch(nn.Module):  # see openpi `PI0Pytorch`
             pad_masks.append(subtask_masks)
 
             num_subtask_embs = subtask_emb.shape[1]
-            # Causal masking for subtask tokens: each subtask token can attend to images, all instruction tokens,
-            # and previous subtask tokens
-            att_masks += [1] * num_subtask_embs  # Use 1 for causal attention on subtask tokens
+            att_mask_segments.append(('subtask', num_subtask_embs))
+        # Process FAST action tokens if provided (these are discrete token IDs)
+        if fast_action_tokens is not None:
+            def fast_action_embed_func(fast_action_tokens):
+                fast_emb = self.fast_action_embedding(fast_action_tokens)
+                fast_emb_dim = fast_emb.shape[-1]
+                return fast_emb * math.sqrt(fast_emb_dim)
+
+            fast_action_emb = self._apply_checkpoint(fast_action_embed_func, fast_action_tokens)
+            embs.append(fast_action_emb)
+            
+            # Use provided mask or create default (all valid)
+            if fast_action_masks is not None:
+                fast_pad_mask = fast_action_masks
+            else:
+                bsize = fast_action_tokens.shape[0]
+                num_fast_embs = fast_action_tokens.shape[1]
+                fast_pad_mask = torch.ones(bsize, num_fast_embs, dtype=torch.bool, device=fast_action_tokens.device)
+            
+            num_fast_embs = fast_action_tokens.shape[1]
+            pad_masks.append(fast_pad_mask)
+            att_mask_segments.append(('fast', num_fast_embs))
 
         embs = torch.cat(embs, dim=1)
         pad_masks = torch.cat(pad_masks, dim=1)
-        att_masks = torch.tensor(att_masks, dtype=torch.bool, device=pad_masks.device)
+        
+        # Create custom 2D attention mask
+        # Attention rules:
+        # - Images + Language: bidirectional among themselves, don't attend to subtask or FAST
+        # - Subtask: attend to images + language, causal among themselves, don't attend to FAST
+        # - FAST: attend to images + language + subtask, causal among themselves
+        att_masks = self._create_custom_attention_mask(att_mask_segments, pad_masks, bsize)
 
-        bsize = pad_masks.shape[0]
-        att_masks = att_masks[None, :].expand(bsize, att_masks.shape[0])
+        # # Optionally visualize the attention mask
+        # self.visualize_attention_mask(
+        #     att_mask_segments=att_mask_segments,
+        #     att_2d_masks=att_masks,
+        #     save_path="/admin/home/jade_choghari/lerobot/src/lerobot/policies/pi05/attention_mask_visualization.png",
+        #     batch_idx=0,
+        #     max_display_tokens=512  # Limit display for very long sequences
+        # )
 
-        return embs, pad_masks, att_masks, total_T_images
+        return embs, pad_masks, att_masks, total_T_images, num_subtask_embs, num_fast_embs
 
     def embed_suffix(self, noisy_actions, timestep):
         """Embed noisy_actions, timestep to prepare for Expert Gemma processing."""
@@ -732,8 +977,8 @@ class PI05Pytorch(nn.Module):  # see openpi `PI0Pytorch`
 
         return embs, pad_masks, att_masks, adarms_cond
 
-    #  loss_dict = self.model.forward(images, img_masks, high_level_task, tokens, masks, subtask_tokens, subtask_masks, actions)
-    def forward(self, images, img_masks, high_level_task, high_level_task_masks, subtask_tokens, subtask_masks, actions, noise=None, time=None) -> Tensor:
+    #  loss_dict = self.model.forward(images, img_masks, high_level_task, tokens, masks, subtask_tokens, subtask_masks, actions, fast_action_tokens, fast_action_masks)
+    def forward(self, images, img_masks, high_level_task, high_level_task_masks, subtask_tokens, subtask_masks, actions, fast_action_tokens=None, fast_action_masks=None, noise=None, time=None) -> Tensor:
         """Do a full training forward pass and compute the loss.
         
         Args:
@@ -743,7 +988,9 @@ class PI05Pytorch(nn.Module):  # see openpi `PI0Pytorch`
             high_level_task_masks: Attention masks for high_level_task
             subtask_tokens: Subtask tokens to predict (e.g., tokens for "pick up the cup")
             subtask_masks: Attention masks for subtask_tokens
-            actions: Ground truth actions
+            actions: Ground truth actions [B, chunk_size, action_dim]
+            fast_action_tokens: Discrete action token IDs [B, max_action_tokens]
+            fast_action_masks: Padding masks for fast action tokens [B, max_action_tokens]
             noise: Optional noise for flow matching
             time: Optional time for flow matching
         """
@@ -757,75 +1004,183 @@ class PI05Pytorch(nn.Module):  # see openpi `PI0Pytorch`
         x_t = time_expanded * noise + (1 - time_expanded) * actions
         u_t = noise - actions
 
-        # Embed prefix (images + high_level_task + subtask_tokens)
-        # Use high_level_task (prompt WITHOUT subtask) + subtask_tokens to predict
-        prefix_embs, prefix_pad_masks, prefix_att_masks, total_T_images = self.embed_prefix(
-            images, img_masks, high_level_task, subtask_tokens, high_level_task_masks, subtask_masks
+        # Initialize FAST loss to 0 (will be computed only if FAST tokens are provided)
+        fast_loss = torch.tensor(0.0, device=actions.device, dtype=actions.dtype)
+
+        # ========== PASS 1: Prefix with FAST tokens for subtask + FAST prediction ==========
+        # Only run this pass if FAST action tokens are provided
+        if fast_action_tokens is not None and fast_action_masks is not None:
+            # Embed prefix (images + high_level_task + subtask_tokens + FAST tokens)
+            # FAST tokens are provided as discrete token IDs
+            prefix_with_fast_embs, prefix_with_fast_pad_masks, prefix_with_fast_att_masks, total_T_images, num_subtask_embs, num_fast_embs = self.embed_prefix(
+                images, img_masks, high_level_task, subtask_tokens, high_level_task_masks, subtask_masks, 
+                fast_action_tokens=fast_action_tokens, fast_action_masks=fast_action_masks
+            )
+
+            # Convert embeddings to bfloat16 if needed for the model
+            if (
+                self.paligemma_with_expert.paligemma.language_model.layers[0].self_attn.q_proj.weight.dtype
+                == torch.bfloat16
+            ):
+                prefix_with_fast_embs = prefix_with_fast_embs.to(dtype=torch.bfloat16)
+
+            # Prepare attention masks for prefix pass with FAST tokens
+            position_ids_prefix_with_fast = torch.cumsum(prefix_with_fast_pad_masks, dim=1) - 1
+            att_2d_prefix_with_fast_4d = self._prepare_attention_masks_4d(prefix_with_fast_att_masks, dtype=prefix_with_fast_embs.dtype)
+
+            # Forward pass through paligemma for subtask + FAST prediction
+            (prefix_with_fast_out, _), _ = self.paligemma_with_expert.forward(
+                attention_mask=att_2d_prefix_with_fast_4d,
+                position_ids=position_ids_prefix_with_fast,
+                past_key_values=None,
+                inputs_embeds=[prefix_with_fast_embs, None],  # SUFFIX = None
+                use_cache=False,
+                adarms_cond=[None, None],
+            )
+
+            # LM HEAD → SUBTASK LOGITS
+            lm_head = self.paligemma_with_expert.paligemma.lm_head
+            logits = lm_head(prefix_with_fast_out)  # (B, T_prefix_with_fast, vocab)
+
+            # Extract logits for subtask token prediction
+            T_high_level_task = high_level_task.size(1)
+            T_subtask = subtask_tokens.size(1)
+            start_index = total_T_images + T_high_level_task
+            end_index = start_index + T_subtask
+            logits_subtask = logits[:, start_index-1:end_index-1, :]  # (B, T_subtask, vocab)
+
+            targets = subtask_tokens  # (B, T_subtask)
+            # Compute cross-entropy loss for subtask
+            loss_fct = torch.nn.CrossEntropyLoss(reduction='none')
+            logits_flat = logits_subtask.reshape(-1, logits_subtask.size(-1))
+            targets_flat = targets.reshape(-1)
+            loss_per_token = loss_fct(logits_flat, targets_flat)
+            loss_per_token = loss_per_token.reshape(targets.shape)
+            masked_loss = loss_per_token * subtask_masks.float()
+            subtask_loss = masked_loss.sum() / subtask_masks.sum().clamp(min=1)
+
+            # Extract outputs for FAST action token prediction and compute auxiliary loss
+            # FAST outputs start after subtask tokens
+            # Similar to subtask, we use autoregressive prediction where position i predicts token i+1
+            fast_start_index = end_index
+            fast_end_index = fast_start_index + num_fast_embs
+            
+            # Get logits for FAST action tokens using the FAST LM head
+            fast_logits = self.fast_action_lm_head(prefix_with_fast_out)  # (B, T_prefix_with_fast, fast_vocab_size)
+            
+            # Extract logits for FAST token prediction (autoregressive: position i predicts token i+1)
+            # - Position (fast_start_index-1) predicts fast_action_tokens[0]
+            # - Position (fast_start_index) predicts fast_action_tokens[1], etc.
+            fast_logits_for_pred = fast_logits[:, fast_start_index-1:fast_end_index-1, :]  # (B, max_action_tokens, fast_vocab_size)
+            
+            # Compute cross-entropy loss for FAST action tokens
+            fast_targets = fast_action_tokens  # (B, max_action_tokens)
+            loss_fct_fast = torch.nn.CrossEntropyLoss(reduction='none')
+            fast_logits_flat = fast_logits_for_pred.reshape(-1, fast_logits_for_pred.size(-1))  # (B*max_action_tokens, fast_vocab_size)
+            fast_targets_flat = fast_targets.reshape(-1)  # (B*max_action_tokens)
+            
+            fast_loss_per_token = loss_fct_fast(fast_logits_flat, fast_targets_flat)  # (B*max_action_tokens)
+            fast_loss_per_token = fast_loss_per_token.reshape(fast_targets.shape)  # (B, max_action_tokens)
+            
+            # Apply mask and compute mean loss over valid tokens
+            masked_fast_loss = fast_loss_per_token * fast_action_masks.float()
+            fast_loss = masked_fast_loss.sum() / fast_action_masks.sum().clamp(min=1)
+        else:
+            # If no FAST tokens provided, compute subtask loss without FAST tokens
+            # This is the fallback for backward compatibility
+            prefix_embs_for_subtask, prefix_pad_masks_for_subtask, prefix_att_masks_for_subtask, total_T_images, _, _ = self.embed_prefix(
+                images, img_masks, high_level_task, subtask_tokens, high_level_task_masks, subtask_masks,
+                fast_action_tokens=None, fast_action_masks=None
+            )
+            
+            # Convert embeddings to bfloat16 if needed for the model
+            if (
+                self.paligemma_with_expert.paligemma.language_model.layers[0].self_attn.q_proj.weight.dtype
+                == torch.bfloat16
+            ):
+                prefix_embs_for_subtask = prefix_embs_for_subtask.to(dtype=torch.bfloat16)
+            
+            position_ids_prefix = torch.cumsum(prefix_pad_masks_for_subtask, dim=1) - 1
+            att_2d_prefix_4d = self._prepare_attention_masks_4d(prefix_att_masks_for_subtask, dtype=prefix_embs_for_subtask.dtype)
+            
+            (prefix_out, _), _ = self.paligemma_with_expert.forward(
+                attention_mask=att_2d_prefix_4d,
+                position_ids=position_ids_prefix,
+                past_key_values=None,
+                inputs_embeds=[prefix_embs_for_subtask, None],
+                use_cache=False,
+                adarms_cond=[None, None],
+            )
+            
+            lm_head = self.paligemma_with_expert.paligemma.lm_head
+            logits = lm_head(prefix_out)
+            
+            T_high_level_task = high_level_task.size(1)
+            T_subtask = subtask_tokens.size(1)
+            start_index = total_T_images + T_high_level_task
+            end_index = start_index + T_subtask
+            logits_subtask = logits[:, start_index-1:end_index-1, :]
+            
+            targets = subtask_tokens
+            loss_fct = torch.nn.CrossEntropyLoss(reduction='none')
+            logits_flat = logits_subtask.reshape(-1, logits_subtask.size(-1))
+            targets_flat = targets.reshape(-1)
+            loss_per_token = loss_fct(logits_flat, targets_flat)
+            loss_per_token = loss_per_token.reshape(targets.shape)
+            masked_loss = loss_per_token * subtask_masks.float()
+            subtask_loss = masked_loss.sum() / subtask_masks.sum().clamp(min=1)
+
+        # ========== PASS 2: Full forward WITHOUT FAST tokens for flow matching ==========
+        # Embed prefix WITHOUT FAST tokens (images + high_level_task + subtask_tokens)
+        prefix_embs_no_fast, prefix_pad_masks_no_fast, prefix_att_masks_no_fast, _, _, _ = self.embed_prefix(
+            images, img_masks, high_level_task, subtask_tokens, high_level_task_masks, subtask_masks, 
+            fast_action_tokens=None, fast_action_masks=None
         )
         
         suffix_embs, suffix_pad_masks, suffix_att_masks, adarms_cond = self.embed_suffix(x_t, time)
 
-        # Prepare attention masks for prefix-only pass (for subtask token prediction)
-        att_2d_prefix = make_att_2d_masks(prefix_pad_masks, prefix_att_masks)
-        position_ids_prefix = torch.cumsum(prefix_pad_masks, dim=1) - 1
-        att_2d_prefix_4d = self._prepare_attention_masks_4d(att_2d_prefix, dtype=prefix_embs.dtype)
-
-        # prefix-only transformer run for subtask token prediction
-        (prefix_out, _), _ = self.paligemma_with_expert.forward(
-            attention_mask=att_2d_prefix_4d,
-            position_ids=position_ids_prefix,
-            past_key_values=None,
-            inputs_embeds=[prefix_embs, None],  # SUFFIX = None
-            use_cache=False,
-            adarms_cond=[None, None],
-        )
-
-        # LM HEAD → SUBTASK LOGITS
-        # prefix_out: (B, T_prefix, H) where T_prefix = total_T_images + T_high_level_task + T_subtask
-        lm_head = self.paligemma_with_expert.paligemma.lm_head
-        logits = lm_head(prefix_out)  # (B, T_prefix, vocab)
-
-        # Extract logits for subtask token prediction
-        # In autoregressive modeling, output at position i predicts token at position i+1
-        # So we take logits from one position earlier:
-        # - Position (start_index-1) (last high_level_task token) predicts subtask_tokens[0]
-        # - Position (start_index) (first subtask token) predicts subtask_tokens[1], etc.
-        T_high_level_task = high_level_task.size(1)
-        T_subtask = subtask_tokens.size(1)
-        start_index = total_T_images + T_high_level_task
-        end_index = start_index + T_subtask
-        logits_subtask = logits[:, start_index-1:end_index-1, :]  # (B, T_subtask, vocab)
-
-        targets = subtask_tokens  # (B, T_subtask)
-        # Compute cross-entropy loss
-        loss_fct = torch.nn.CrossEntropyLoss(reduction='none')
-        # Reshape for loss computation
-        logits_flat = logits_subtask.reshape(-1, logits_subtask.size(-1))  # (B*T_subtask, vocab)
-        targets_flat = targets.reshape(-1)  # (B*T_subtask)
-
-        loss_per_token = loss_fct(logits_flat, targets_flat)  # (B*T_subtask)
-        loss_per_token = loss_per_token.reshape(targets.shape)  # (B, T_subtask)
-
-        # Apply mask and compute mean loss over valid tokens
-        masked_loss = loss_per_token * subtask_masks.float()
-        subtask_loss = masked_loss.sum() / subtask_masks.sum().clamp(min=1)
-
         # Convert embeddings to bfloat16 if needed for the model
         if (
             self.paligemma_with_expert.paligemma.language_model.layers[0].self_attn.q_proj.weight.dtype
             == torch.bfloat16
         ):
             suffix_embs = suffix_embs.to(dtype=torch.bfloat16)
-            prefix_embs = prefix_embs.to(dtype=torch.bfloat16)
+            prefix_embs_no_fast = prefix_embs_no_fast.to(dtype=torch.bfloat16)
 
-        # Concatenate prefix (images + tokens + subtask_tokens) and suffix (actions) masks
-        pad_masks = torch.cat([prefix_pad_masks, suffix_pad_masks], dim=1)
-        att_masks = torch.cat([prefix_att_masks, suffix_att_masks], dim=1)
-
-        # Prepare attention masks for full forward pass (prefix + suffix)
-        att_2d_masks = make_att_2d_masks(pad_masks, att_masks)
+        # For the flow matching pass, we need custom attention where:
+        # - prefix follows the custom pattern (images+lang bidirectional, subtask causal, no cross-attention)
+        # - suffix attends to all prefix + causal to itself
+        # We'll construct this by extending prefix_att_masks_no_fast to include suffix
+        
+        # prefix_att_masks_no_fast is already a 2D boolean mask [B, prefix_len, prefix_len]
+        # We need to extend it to [B, prefix_len + suffix_len, prefix_len + suffix_len]
+        
+        bsize = prefix_pad_masks_no_fast.shape[0]
+        prefix_len = prefix_pad_masks_no_fast.shape[1]
+        suffix_len = suffix_pad_masks.shape[1]
+        total_len = prefix_len + suffix_len
+        device = prefix_pad_masks_no_fast.device
+        
+        # Create full attention mask
+        full_att_2d_masks = torch.zeros(bsize, total_len, total_len, dtype=torch.bool, device=device)
+        
+        # Copy prefix attention pattern
+        full_att_2d_masks[:, :prefix_len, :prefix_len] = prefix_att_masks_no_fast
+        
+        # Suffix attends to all prefix
+        full_att_2d_masks[:, prefix_len:, :prefix_len] = True
+        
+        # Suffix has causal attention among itself
+        suffix_causal_mask = torch.tril(torch.ones(suffix_len, suffix_len, dtype=torch.bool, device=device))
+        full_att_2d_masks[:, prefix_len:, prefix_len:] = suffix_causal_mask[None, :, :]
+        
+        # Apply padding masks
+        pad_masks = torch.cat([prefix_pad_masks_no_fast, suffix_pad_masks], dim=1)
+        pad_2d_masks = pad_masks[:, None, :] * pad_masks[:, :, None]
+        full_att_2d_masks = full_att_2d_masks & pad_2d_masks
+        
         position_ids = torch.cumsum(pad_masks, dim=1) - 1
-        att_2d_masks_4d = self._prepare_attention_masks_4d(att_2d_masks, dtype=prefix_embs.dtype)
+        att_2d_masks_4d = self._prepare_attention_masks_4d(full_att_2d_masks, dtype=prefix_embs_no_fast.dtype)
 
         def forward_func(prefix_embs, suffix_embs, att_2d_masks_4d, position_ids, adarms_cond):
             (_, suffix_out), _ = self.paligemma_with_expert.forward(
@@ -836,11 +1191,10 @@ class PI05Pytorch(nn.Module):  # see openpi `PI0Pytorch`
                 use_cache=False,
                 adarms_cond=[None, adarms_cond],
             )
-            # prefix_out to be used for the language head
             return suffix_out
 
         suffix_out = self._apply_checkpoint(
-            forward_func, prefix_embs, suffix_embs, att_2d_masks_4d, position_ids, adarms_cond
+            forward_func, prefix_embs_no_fast, suffix_embs, att_2d_masks_4d, position_ids, adarms_cond
         )
 
         suffix_out = suffix_out[:, -self.config.chunk_size :]
@@ -856,79 +1210,81 @@ class PI05Pytorch(nn.Module):  # see openpi `PI0Pytorch`
         return {
             "flow_loss": fm_loss,
             "subtask_loss": subtask_loss,
-            "loss": 10 * fm_loss.mean() + subtask_loss,
+            "fast_loss": fast_loss,
+            "loss": fm_loss.mean() + 0.1 * subtask_loss + 0.05 * fast_loss, # ref: b1k winner
         }
-
+    
     @torch.no_grad()
     def _generate_subtask_tokens(
         self, images, img_masks, tokens, masks, tokenizer, max_length, device
     ):
-        """Generate subtask tokens autoregressively using next token prediction."""
         bsize = tokens.shape[0]
-        
-        # Get lm_head for token generation
         lm_head = self.paligemma_with_expert.paligemma.lm_head
         
-        # Embed prefix without subtask tokens first
-        prefix_embs, prefix_pad_masks, prefix_att_masks, total_T_images = self.embed_prefix(
-            images, img_masks, tokens, subtask_tokens=None, masks=masks, subtask_masks=None
+        prefix_embs, prefix_pad_masks, prefix_att_masks, total_T_images, _, _ = self.embed_prefix(
+            images, img_masks, tokens, subtask_tokens=None, masks=masks, subtask_masks=None, 
+            fast_action_tokens=None, fast_action_masks=None
         )
-        
-        # Initialize generated tokens list - start with BOS token or first token after instruction
-        # For PaliGemma, we'll start generation and accumulate tokens
+
         generated_tokens = torch.zeros((bsize, max_length), dtype=torch.long, device=device)
         
+        # tracking mask: False = still generating, True = finished
+        finished = torch.zeros(bsize, dtype=torch.bool, device=device)
+        
         for t in range(max_length):
-            # Prepare attention masks for current prefix
-            att_2d_prefix = make_att_2d_masks(prefix_pad_masks, prefix_att_masks)
             position_ids_prefix = torch.cumsum(prefix_pad_masks, dim=1) - 1
-            att_2d_prefix_4d = self._prepare_attention_masks_4d(att_2d_prefix, dtype=prefix_embs.dtype)
-            
-            # Forward pass through model to get logits
+            att_2d_prefix_4d = self._prepare_attention_masks_4d(prefix_att_masks, dtype=prefix_embs.dtype)
+
             (prefix_out, _), _ = self.paligemma_with_expert.forward(
-                attention_mask=att_2d_prefix_4d,
+                attention_mask=att_2d_prefix_4d, 
                 position_ids=position_ids_prefix,
-                past_key_values=None,
                 inputs_embeds=[prefix_embs, None],
-                use_cache=False,
-                adarms_cond=[None, None],
+                # ...
             )
             
-            # Get logits from the last position
-            logits = lm_head(prefix_out)  # (B, T_prefix, vocab)
-            next_token_logits = logits[:, -1, :]  # (B, vocab)
+            logits = lm_head(prefix_out)
+            next_token_logits = logits[:, -1, :]
+            next_token = torch.argmax(next_token_logits, dim=-1) # (B,)
             
-            # Greedy decoding - take the most likely token
-            next_token = torch.argmax(next_token_logits, dim=-1)  # (B,)
+            # 1. if a row was already finished, force the next token to be PAD (0)
+            next_token = torch.where(finished, torch.tensor(0, device=device), next_token)
             
-            # Store generated token
+            # 2. store the token
             generated_tokens[:, t] = next_token
             
-            # Check for EOS token - if all batches have generated EOS, stop
+            # 3. update the finished mask
             if tokenizer.eos_token_id is not None:
-                if (next_token == tokenizer.eos_token_id).all():
-                    break
+                finished |= (next_token == tokenizer.eos_token_id)
             
-            # Embed the generated token and append to prefix
-            next_token_unsqueezed = next_token.unsqueeze(1)  # (B, 1)
+            # 4. break only if everyone is finished
+            if finished.all():
+                break
+                
+            next_token_unsqueezed = next_token.unsqueeze(1)
             
             def next_token_embed_func(next_token_unsqueezed):
                 next_emb = self.paligemma_with_expert.embed_language_tokens(next_token_unsqueezed)
-                next_emb_dim = next_emb.shape[-1]
-                return next_emb * math.sqrt(next_emb_dim)
+                return next_emb * math.sqrt(next_emb.shape[-1])
             
             next_emb = self._apply_checkpoint(next_token_embed_func, next_token_unsqueezed)
             
-            # Append to prefix embeddings
+            # update embeddings
             prefix_embs = torch.cat([prefix_embs, next_emb], dim=1)
             
-            # Update masks - new token is valid and uses causal attention
+            # update padding masks
             prefix_pad_masks = torch.cat([
                 prefix_pad_masks,
                 torch.ones((bsize, 1), dtype=torch.bool, device=device)
             ], dim=1)
-            prefix_att_masks = torch.cat([prefix_att_masks, torch.ones((bsize, 1), dtype=torch.bool, device=device)], dim=1)
-        
+            
+            # update attention masks
+            old_seq_len = prefix_att_masks.shape[1]
+            new_seq_len = old_seq_len + 1
+            new_att_masks = torch.zeros((bsize, new_seq_len, new_seq_len), dtype=torch.bool, device=device)
+            new_att_masks[:, :old_seq_len, :old_seq_len] = prefix_att_masks
+            new_att_masks[:, -1, :] = prefix_pad_masks
+            prefix_att_masks = new_att_masks
+
         return generated_tokens
 
     @torch.no_grad()  # see openpi `sample_actions` (slightly adapted)
@@ -978,13 +1334,23 @@ class PI05Pytorch(nn.Module):  # see openpi `PI0Pytorch`
         subtask_masks = torch.ones_like(generated_subtask_tokens, dtype=torch.bool)
 
         # During inference, we don't have subtask_tokens yet, so pass None
-        prefix_embs, prefix_pad_masks, prefix_att_masks, _ = self.embed_prefix(
-            images, img_masks, tokens, subtask_tokens=generated_subtask_tokens, masks=masks, subtask_masks=subtask_masks
+        # Also no FAST tokens during inference
+        prefix_embs, prefix_pad_masks, prefix_att_masks, _, _, _ = self.embed_prefix(
+            images, img_masks, tokens, subtask_tokens=generated_subtask_tokens, masks=masks, subtask_masks=subtask_masks, 
+            fast_action_tokens=None, fast_action_masks=None
         )
-        prefix_att_2d_masks = make_att_2d_masks(prefix_pad_masks, prefix_att_masks)
+        
+        # Convert embeddings to bfloat16 if needed for the model
+        if (
+            self.paligemma_with_expert.paligemma.language_model.layers[0].self_attn.q_proj.weight.dtype
+            == torch.bfloat16
+        ):
+            prefix_embs = prefix_embs.to(dtype=torch.bfloat16)
+        
+        # prefix_att_masks is already a 2D attention mask from embed_prefix
         prefix_position_ids = torch.cumsum(prefix_pad_masks, dim=1) - 1
 
-        prefix_att_2d_masks_4d = self._prepare_attention_masks_4d(prefix_att_2d_masks, dtype=prefix_embs.dtype)
+        prefix_att_2d_masks_4d = self._prepare_attention_masks_4d(prefix_att_masks, dtype=prefix_embs.dtype)
         self.paligemma_with_expert.paligemma.language_model.config._attn_implementation = "eager"  # noqa: SLF001
 
         _, past_key_values = self.paligemma_with_expert.forward(
@@ -1209,7 +1575,7 @@ class PI05Policy(PreTrainedPolicy):
                 print(f"Remapped {remap_count} state dict keys")
 
             # Load the remapped state dict into the model
-            missing_keys, unexpected_keys = model.load_state_dict(remapped_state_dict, strict=strict)
+            missing_keys, unexpected_keys = model.load_state_dict(remapped_state_dict, strict=False)
 
             if missing_keys:
                 print(f"Missing keys when loading state dict: {len(missing_keys)} keys")
@@ -1442,19 +1808,26 @@ class PI05Policy(PreTrainedPolicy):
         actions = self.prepare_action(batch)
         
         # Decode and print ground truth subtask tokens during training
-        if self.tokenizer is not None and self.training:
-            bsize = subtask_tokens.shape[0]
-            for i in range(bsize):
-                # Remove padding tokens (0) and special tokens
-                valid_tokens = subtask_tokens[i][subtask_masks[i].bool()]
-                if len(valid_tokens) > 0:
-                    decoded_text = self.tokenizer.decode(valid_tokens, skip_special_tokens=True)
-                    # print(f"[Training] Ground truth subtask {i}: {decoded_text}")
+        # if self.tokenizer is not None and self.training:
+        #     bsize = subtask_tokens.shape[0]
+        #     for i in range(bsize):
+        #         # Remove padding tokens (0) and special tokens
+        #         valid_tokens = subtask_tokens[i][subtask_masks[i].bool()]
+        #         # if len(valid_tokens) > 0:
+        #             # decoded_text = self.tokenizer.decode(valid_tokens, skip_special_tokens=True)
+        #             # print(f"[Training] Ground truth subtask {i}: {decoded_text}")
         
+        # Get FAST action tokens from batch
+        fast_action_tokens = batch.get("action.tokens", None)  # (B, max_action_tokens)
+        fast_action_masks = batch.get("action.token_mask", None)  # (B, max_action_tokens)
         # Compute loss (no separate state needed for PI05)
         # high_level_task = instruction tokens WITHOUT subtask (e.g., "High level task: X; State: Y; Subtask:")
         # subtask_tokens = subtask tokens to predict (e.g., "pick up the cup")
-        loss_dict = self.model.forward(images, img_masks, high_level_task, high_level_task_masks, subtask_tokens, subtask_masks, actions)
+        # fast_action_tokens = discrete action token IDs to predict
+        loss_dict = self.model.forward(
+            images, img_masks, high_level_task, high_level_task_masks, subtask_tokens, subtask_masks, actions,
+            fast_action_tokens=fast_action_tokens, fast_action_masks=fast_action_masks
+        )
 
         # Extract the total loss
         loss = loss_dict["loss"]
@@ -1464,6 +1837,7 @@ class PI05Policy(PreTrainedPolicy):
             "loss": loss.item(),
             "flow_loss": loss_dict["flow_loss"].mean().item(),
             "subtask_loss": loss_dict["subtask_loss"].item(),
+            "fast_loss": loss_dict["fast_loss"].item(),
         }
 
         return loss, detailed_loss_dict
diff --git a/src/lerobot/policies/pi05/processor_pi05.py b/src/lerobot/policies/pi05/processor_pi05.py
index 1b1fcf047..65ec6244c 100644
--- a/src/lerobot/policies/pi05/processor_pi05.py
+++ b/src/lerobot/policies/pi05/processor_pi05.py
@@ -33,6 +33,7 @@ from lerobot.processor import (
     ProcessorStep,
     ProcessorStepRegistry,
     RenameObservationsProcessorStep,
+    ActionTokenizerProcessorStep,
     TokenizerProcessorStep,
     UnnormalizerProcessorStep,
 )
@@ -158,7 +159,6 @@ def make_pi05_pre_post_processors(
     Returns:
         A tuple containing the configured pre-processor and post-processor pipelines.
     """
-
     # Add remaining processors
     input_steps: list[ProcessorStep] = [
         RenameObservationsProcessorStep(rename_map={}),  # To mimic the same processor as pretrained one
@@ -177,6 +177,9 @@ def make_pi05_pre_post_processors(
             padding_side="right",
             padding="max_length",
         ),
+        ActionTokenizerProcessorStep(
+            tokenizer_name="physical-intelligence/fast",
+        ),
         DeviceProcessorStep(device=config.device),
     ]
 
@@ -186,7 +189,7 @@ def make_pi05_pre_post_processors(
         ),
         DeviceProcessorStep(device="cpu"),
     ]
-
+    
     return (
         PolicyProcessorPipeline[dict[str, Any], dict[str, Any]](
             steps=input_steps,
diff --git a/src/lerobot/processor/__init__.py b/src/lerobot/processor/__init__.py
index be11ac1af..540f74ef9 100644
--- a/src/lerobot/processor/__init__.py
+++ b/src/lerobot/processor/__init__.py
@@ -75,7 +75,7 @@ from .policy_robot_bridge import (
     RobotActionToPolicyActionProcessorStep,
 )
 from .rename_processor import RenameObservationsProcessorStep
-from .tokenizer_processor import TokenizerProcessorStep
+from .tokenizer_processor import TokenizerProcessorStep, ActionTokenizerProcessorStep
 
 __all__ = [
     "ActionProcessorStep",
diff --git a/src/lerobot/processor/tokenizer_processor.py b/src/lerobot/processor/tokenizer_processor.py
index 18b51a9ce..59afddab1 100644
--- a/src/lerobot/processor/tokenizer_processor.py
+++ b/src/lerobot/processor/tokenizer_processor.py
@@ -15,10 +15,13 @@
 # limitations under the License.
 
 """
-This script defines a processor for tokenizing natural language instructions from an environment transition.
+This script defines processors for tokenizing data from an environment transition.
 
-It uses a tokenizer from the Hugging Face `transformers` library to convert task descriptions (text) into
-token IDs and attention masks, which are then added to the observation dictionary.
+It includes:
+- TokenizerProcessorStep: Uses a tokenizer from the Hugging Face `transformers` library to convert 
+  task descriptions (text) into token IDs and attention masks, which are then added to the observation dictionary.
+- ActionTokenizerProcessorStep: Uses a processor/tokenizer (e.g., the Physical Intelligence "fast" tokenizer)
+  to tokenize action tensors into discrete token IDs for action modeling.
 """
 
 from __future__ import annotations
@@ -30,6 +33,8 @@ import torch
 
 from lerobot.configs.types import FeatureType, PipelineFeatureType, PolicyFeature
 from lerobot.utils.constants import (
+    ACTION_TOKEN_MASK,
+    ACTION_TOKENS,
     OBS_LANGUAGE_ATTENTION_MASK,
     OBS_LANGUAGE_HIGH_LEVEL_TASK_ATTENTION_MASK,
     OBS_LANGUAGE_HIGH_LEVEL_TASK_TOKENS,
@@ -40,12 +45,13 @@ from lerobot.utils.constants import (
 from lerobot.utils.import_utils import _transformers_available
 
 from .core import EnvTransition, TransitionKey
-from .pipeline import ObservationProcessorStep, ProcessorStepRegistry
+from .pipeline import ActionProcessorStep, ObservationProcessorStep, ProcessorStepRegistry
 
 # Conditional import for type checking and lazy loading
 if TYPE_CHECKING or _transformers_available:
-    from transformers import AutoTokenizer
+    from transformers import AutoProcessor, AutoTokenizer
 else:
+    AutoProcessor = None
     AutoTokenizer = None
 
 
@@ -423,3 +429,233 @@ class TokenizerProcessorStep(ObservationProcessorStep):
             )
 
         return features
+
+
+@dataclass
+@ProcessorStepRegistry.register(name="action_tokenizer_processor")
+class ActionTokenizerProcessorStep(ActionProcessorStep):
+    """
+    Processor step to tokenize action data using a fast action tokenizer.
+
+    This step takes action tensors from an `EnvTransition`, tokenizes them using
+    a Hugging Face `transformers` AutoProcessor (such as the Physical Intelligence "fast" tokenizer),
+    and returns the tokenized action.
+
+    Requires the `transformers` library to be installed.
+
+    Attributes:
+        tokenizer_name: The name of a pretrained processor from the Hugging Face Hub (e.g., "physical-intelligence/fast").
+        tokenizer: A pre-initialized processor/tokenizer object. If provided, `tokenizer_name` is ignored.
+        trust_remote_code: Whether to trust remote code when loading the tokenizer (required for some tokenizers).
+        action_tokenizer: The internal tokenizer/processor instance, loaded during initialization.
+    """
+
+    tokenizer_name: str | None = None
+    tokenizer: Any | None = None
+    trust_remote_code: bool = True
+    max_action_tokens: int = 32
+    # Internal tokenizer instance (not part of the config)
+    action_tokenizer: Any = field(default=None, init=False, repr=False)
+
+    def __post_init__(self):
+        """
+        Initializes the action tokenizer after the dataclass is created.
+
+        It checks for the availability of the `transformers` library and loads the tokenizer
+        either from a provided object or by name from the Hugging Face Hub.
+
+        Raises:
+            ImportError: If the `transformers` library is not installed.
+            ValueError: If neither `tokenizer` nor `tokenizer_name` is provided.
+        """
+        if not _transformers_available:
+            raise ImportError(
+                "The 'transformers' library is not installed. "
+                "Please install it with `pip install 'lerobot[transformers-dep]'` to use ActionTokenizerProcessorStep."
+            )
+
+        if self.tokenizer is not None:
+            # Use provided tokenizer object directly
+            self.action_tokenizer = self.tokenizer
+        elif self.tokenizer_name is not None:
+            if AutoProcessor is None:
+                raise ImportError("AutoProcessor is not available")
+            self.action_tokenizer = AutoProcessor.from_pretrained(
+                self.tokenizer_name, trust_remote_code=self.trust_remote_code
+            )
+        else:
+            raise ValueError(
+                "Either 'tokenizer' or 'tokenizer_name' must be provided. "
+                "Pass a tokenizer object directly or a tokenizer name to auto-load."
+            )
+
+    def __call__(self, transition: EnvTransition) -> EnvTransition:
+        """
+        Applies action tokenization to the transition.
+        
+        This overrides the base class to handle both tokens and mask.
+        
+        Args:
+            transition: The input transition with action data.
+            
+        Returns:
+            The processed transition with tokenized actions and mask in complementary data.
+        """
+        self._current_transition = transition.copy()
+        new_transition = self._current_transition
+
+        action = new_transition.get(TransitionKey.ACTION)
+        if action is None:
+            raise ValueError("ActionTokenizerProcessorStep requires an action in the transition.")
+
+        # Tokenize and get both tokens and mask
+        tokens, 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_TOKENS] = tokens
+        new_transition[TransitionKey.COMPLEMENTARY_DATA] = complementary_data
+        return new_transition
+
+    def _tokenize_action(self, action: torch.Tensor) -> tuple[torch.Tensor, torch.Tensor]:
+        """
+        Tokenizes the action tensor and creates a mask.
+
+        Args:
+            action: The input action tensor to tokenize. Shape: (B, action_dim) or (action_dim,)
+
+        Returns:
+            A tuple of (tokens, mask) where:
+            - tokens: Tensor of token IDs with shape (B, max_action_tokens)
+            - mask: Boolean mask with shape (B, max_action_tokens), True for real tokens, False for padding
+        """
+        if action is None:
+            raise ValueError("Action cannot be None")
+
+        # Get the device and dtype of the input action
+        device = action.device if isinstance(action, torch.Tensor) else None
+        
+        # Handle single sample (add batch dimension)
+        single_sample = action.dim() == 1
+        if single_sample:
+            action = action.unsqueeze(0)
+        
+        batch_size = action.shape[0]
+        
+        # Tokenize the action batch
+        # The fast tokenizer expects action data and returns token IDs
+        tokens_list = []
+        masks_list = []
+        
+        for i in range(batch_size):
+            # Tokenize single action (move to CPU first as tokenizer uses scipy which requires numpy)
+            action_cpu = action[i:i+1].cpu()
+            tokens = self.action_tokenizer(action_cpu)
+            
+            # Convert to numpy array if it's a list
+            if isinstance(tokens, list):
+                tokens = torch.tensor(tokens, dtype=torch.long, device=action.device)
+            elif not isinstance(tokens, torch.Tensor):
+                tokens = torch.tensor(tokens, dtype=torch.long, device=action.device)
+            else:
+                # Move tokens back to the same device as input action
+                tokens = tokens.to(device=action.device)
+            
+            # Flatten to 1D if needed
+            if tokens.dim() > 1:
+                tokens = tokens.flatten()
+            
+            # Truncate or pad to max_action_tokens
+            if len(tokens) > self.max_action_tokens:
+                tokens = tokens[:self.max_action_tokens]
+                mask = torch.ones(self.max_action_tokens, dtype=torch.bool, device=action.device)
+            else:
+                mask = torch.cat([
+                    torch.ones(len(tokens), dtype=torch.bool, device=action.device),
+                    torch.zeros(self.max_action_tokens - len(tokens), dtype=torch.bool, device=action.device)
+                ])
+                # Pad tokens with zeros
+                tokens = torch.nn.functional.pad(
+                    tokens, 
+                    (0, self.max_action_tokens - len(tokens)), 
+                    value=0
+                )
+            
+            tokens_list.append(tokens)
+            masks_list.append(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)
+        
+        # Remove batch dimension if input was single sample
+        if single_sample:
+            tokens_batch = tokens_batch.squeeze(0)
+            masks_batch = 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)
+
+        return tokens_batch, 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)
+        return tokens
+
+    def get_config(self) -> dict[str, Any]:
+        """
+        Returns the serializable configuration of the processor.
+
+        Note: The tokenizer object itself is not serialized. If the processor was initialized
+        with a tokenizer name, that name will be included in the config.
+
+        Returns:
+            A dictionary with the processor's configuration parameters.
+        """
+        config = {
+            "trust_remote_code": self.trust_remote_code,
+            "max_action_tokens": self.max_action_tokens,
+        }
+
+        # Only save tokenizer_name if it was used to create the tokenizer
+        if self.tokenizer_name is not None and self.tokenizer is None:
+            config["tokenizer_name"] = self.tokenizer_name
+
+        return config
+
+    def transform_features(
+        self, features: dict[PipelineFeatureType, dict[str, PolicyFeature]]
+    ) -> dict[PipelineFeatureType, dict[str, PolicyFeature]]:
+        """
+        Updates feature definitions to reflect tokenized actions.
+
+        This updates the policy features dictionary to indicate that the action
+        has been tokenized into a sequence of token IDs with shape (max_action_tokens,).
+
+        Args:
+            features: The dictionary of existing policy features.
+
+        Returns:
+            The updated dictionary of policy features.
+        """
+        # Update the action feature to reflect the tokenized shape
+        # The action is now a sequence of token IDs
+        if PipelineFeatureType.ACTION in features:
+            # Replace the action feature with the tokenized version
+            features[PipelineFeatureType.ACTION] = {
+                ACTION_TOKENS: PolicyFeature(
+                    type=FeatureType.SEQUENCE,  # Token sequence
+                    shape=(self.max_action_tokens,)
+                )
+            }
+        
+        return features
diff --git a/src/lerobot/scripts/lerobot_train.py b/src/lerobot/scripts/lerobot_train.py
index 02401068c..196580984 100644
--- a/src/lerobot/scripts/lerobot_train.py
+++ b/src/lerobot/scripts/lerobot_train.py
@@ -90,6 +90,7 @@ def update_policy(
     # Let accelerator handle mixed precision
     with accelerator.autocast():
         loss, output_dict = policy.forward(batch)
+        action = policy.select_action(batch)
         breakpoint()
         # TODO(rcadene): policy.unnormalize_outputs(out_dict)
 
diff --git a/src/lerobot/utils/constants.py b/src/lerobot/utils/constants.py
index c8e19eb56..f249f5600 100644
--- a/src/lerobot/utils/constants.py
+++ b/src/lerobot/utils/constants.py
@@ -33,6 +33,8 @@ OBS_LANGUAGE_SUBTASK_ONLY = OBS_STR + ".subtask"
 OBS_LANGUAGE_SUBTASK_ONLY_TOKENS = OBS_LANGUAGE_SUBTASK_ONLY + ".tokens"
 OBS_LANGUAGE_SUBTASK_ONLY_ATTENTION_MASK = OBS_LANGUAGE_SUBTASK_ONLY + ".attention_mask"
 ACTION = "action"
+ACTION_TOKENS = ACTION + ".tokens"
+ACTION_TOKEN_MASK = ACTION + ".token_mask"
 REWARD = "next.reward"
 TRUNCATED = "next.truncated"
 DONE = "next.done"