diff --git a/outlines/index/index.py b/outlines/index/index.py
index aa251ed80..525c164b3 100644
--- a/outlines/index/index.py
+++ b/outlines/index/index.py
@@ -1,5 +1,5 @@
 from dataclasses import dataclass
-from typing import NewType, Protocol, Union
+from typing import List, NewType, Optional, Protocol
 
 import torch
 
@@ -8,22 +8,11 @@
 
 @dataclass(frozen=True)
 class GenerateInstruction:
-    logits_mask: str
-    temperature: float
-    top_k: int
-    top_p: int
-
-
-@dataclass(frozen=True)
-class FillInstruction:
-    token_ids: int
-
-
-FSMInstruction = Union[GenerateInstruction, FillInstruction]
+    tokens_to_mask: List[int]
 
 
 class FSM(Protocol):
-    def next_instruction(self, state: FSMState) -> FSMInstruction:
+    def next_instruction(self, state: FSMState) -> GenerateInstruction:
         ...
 
     def next_state(self, state: FSMState, token_id: torch.Tensor) -> FSMState:
@@ -31,3 +20,33 @@ def next_state(self, state: FSMState, token_id: torch.Tensor) -> FSMState:
 
     def is_final_state(self, state: FSMState) -> bool:
         ...
+
+
+class StopAtTokenFSM:
+    def __init__(self, stop_token_id: int, max_tokens: Optional[int] = None):
+        self.stop_token_id = stop_token_id
+        self.max_tokens = max_tokens
+        self.num_tokens_generated = 0
+
+    def next_instructions(self, _: FSMState) -> GenerateInstruction:
+        return GenerateInstruction([])
+
+    def next_state(self, state: FSMState, token_id: torch.Tensor) -> FSMState:
+        self.num_tokens_generated += 1
+
+        if token_id == self.stop_token_id:
+            return FSMState(1)
+        else:
+            return FSMState(0)
+
+    def is_final_state(self, state: FSMState) -> bool:
+        # Stop if the maximum number of tokens has been generated
+        # regardless of whether the stop token id has been found.
+        if self.max_tokens is not None:
+            if self.num_tokens_generated == self.max_tokens:
+                return True
+
+        if state == 1:
+            return True
+        else:
+            return False