diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 8b97713..e76cc11 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -24,5 +24,7 @@ jobs:
       - name: Install dependencies
         run: pip install pytest
 
-      - name: Run tests
+      - name: Run primitive tests
         run: python -m pytest primitives -v
+      - name: Run root tests
+        run: python -m pytest test_stop_machine.py -v
diff --git a/examples/demo_authority_gate.py b/examples/demo_authority_gate.py
index 0450f7f..f8b221f 100644
--- a/examples/demo_authority_gate.py
+++ b/examples/demo_authority_gate.py
@@ -3,7 +3,7 @@
 import sys
 from pathlib import Path
 
-sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "primitives" / "authority-gate"))
+sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "primitives" / "authority-gate-v0"))
 
 from gate import Authority, AuthorityGate  # noqa: E402
 
diff --git a/examples/demo_stop_machine.py b/examples/demo_stop_machine.py
index 5316531..c3a5690 100644
--- a/examples/demo_stop_machine.py
+++ b/examples/demo_stop_machine.py
@@ -2,7 +2,7 @@
 import sys
 from pathlib import Path
 
-sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "primitives" / "stop-machine"))
+sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "primitives" / "stop-machine-v0"))
 
 from stop_machine import Event, StopMachine  # noqa: E402
 
diff --git a/primitives/authority-gate/README.md b/primitives/authority-gate-v0/README.md
similarity index 100%
rename from primitives/authority-gate/README.md
rename to primitives/authority-gate-v0/README.md
diff --git a/primitives/authority-gate/gate.py b/primitives/authority-gate-v0/gate.py
similarity index 100%
rename from primitives/authority-gate/gate.py
rename to primitives/authority-gate-v0/gate.py
diff --git a/primitives/authority-gate/test_gate.py b/primitives/authority-gate-v0/test_gate.py
similarity index 100%
rename from primitives/authority-gate/test_gate.py
rename to primitives/authority-gate-v0/test_gate.py
diff --git a/primitives/stop-machine/README.md b/primitives/stop-machine-v0/README.md
similarity index 100%
rename from primitives/stop-machine/README.md
rename to primitives/stop-machine-v0/README.md
diff --git a/primitives/stop-machine/stop_machine.py b/primitives/stop-machine-v0/stop_machine.py
similarity index 100%
rename from primitives/stop-machine/stop_machine.py
rename to primitives/stop-machine-v0/stop_machine.py
diff --git a/primitives/stop-machine/test_stop_machine.py b/primitives/stop-machine-v0/test_stop_machine.py
similarity index 100%
rename from primitives/stop-machine/test_stop_machine.py
rename to primitives/stop-machine-v0/test_stop_machine.py
diff --git a/stop_machine.py b/stop_machine.py
index 3872443..c66369e 100644
--- a/stop_machine.py
+++ b/stop_machine.py
@@ -1,3 +1,7 @@
+# CANONICAL SOURCE: LalaSkye/constraint-workshop/stop_machine.py
+# PINNED COMMIT: 8d04cbc1e8e6576641962d5d3c866b0517ad596e
+# Semantics aligned to canonical: RED is terminal, reset() blocked from RED.
+
 """A deterministic three-state stop controller.
 
 States: GREEN -> AMBER -> RED
@@ -87,10 +91,11 @@ def transition_to(self, target: State) -> State:
         return self._state
 
     def reset(self) -> State:
-        """Reset the machine to GREEN.
-
-        Returns the new state (always GREEN).
-        """
+        """Reset the machine to GREEN. Forbidden once RED is reached."""
+        if self.is_terminal:
+            raise TerminalStateError(
+                f"Cannot reset: {self._state.value} is terminal."
+            )
         self._state = State.GREEN
         return self._state
 
diff --git a/test_stop_machine.py b/test_stop_machine.py
index 773324b..bea8fa6 100644
--- a/test_stop_machine.py
+++ b/test_stop_machine.py
@@ -134,10 +134,8 @@ def test_reset_from_amber():
 
 def test_reset_from_red():
     m = StopMachine(State.RED)
-    result = m.reset()
-    assert result == State.GREEN
-    assert m.state == State.GREEN
-    assert not m.is_terminal
+    with pytest.raises(TerminalStateError):
+        m.reset()
 
 
 def test_reset_then_advance():
@@ -145,10 +143,8 @@ def test_reset_then_advance():
     m.advance()
     m.advance()
     assert m.is_terminal
-    m.reset()
-    assert m.state == State.GREEN
-    m.advance()
-    assert m.state == State.AMBER
+    with pytest.raises(TerminalStateError):
+        m.reset()
 
 
 # --- repr ---