diff --git a/data_only_viz/hand_features.py b/data_only_viz/hand_features.py
index 470e5ea..4602aca 100644
--- a/data_only_viz/hand_features.py
+++ b/data_only_viz/hand_features.py
@@ -10,10 +10,18 @@ from collections import deque
 
 WRIST, THUMB_TIP, MIDDLE_MCP, PINKY_TIP = 0, 4, 9, 20
 
-NEUTRAL_HAND = {"cx": 0.5, "cy": 0.5, "openness": 0.0, "speed": 0.0}
+# index/middle/ring/pinky tips + their MCP knuckles, for the "all fingers
+# curled" fist metric (thumb excluded — a fist curls the four fingers).
+_FINGER_TIPS = (8, 12, 16, 20)
+_FINGER_MCPS = (5, 9, 13, 17)
+
+NEUTRAL_HAND = {"cx": 0.5, "cy": 0.5, "openness": 0.0, "speed": 0.0, "fist": 0.0}
 
 # openness calibration: (span/size) maps fist~0.3 -> 0, open~2.0 -> 1
 _OPEN_LO, _OPEN_HI = 0.5, 2.0
+# fist calibration: per-finger tip->MCP distance / hand size. curled ~0.25,
+# extended ~0.75. closedness = 1 at/below LO, 0 at/above HI.
+_EXT_LO, _EXT_HI = 0.25, 0.75
 
 
 def _finite(v: float, fallback: float) -> float:
@@ -51,7 +59,15 @@ class HandFeatureExtractor:
                           ys[THUMB_TIP] - ys[PINKY_TIP])
         openness = _clamp((span / size - _OPEN_LO) / (_OPEN_HI - _OPEN_LO),
                           0.0, 1.0)
-        return {"cx": cx, "cy": cy, "openness": openness}
+        # fist = 1.0 only when ALL four fingers are curled to the palm (min over
+        # fingers): one extended finger drops it toward 0, so a true fist is
+        # required rather than just a small thumb-pinky span.
+        fist = 1.0
+        for tip, mcp in zip(_FINGER_TIPS, _FINGER_MCPS):
+            ext = math.hypot(xs[tip] - xs[mcp], ys[tip] - ys[mcp]) / size
+            curled = _clamp(1.0 - (ext - _EXT_LO) / (_EXT_HI - _EXT_LO), 0.0, 1.0)
+            fist = min(fist, curled)
+        return {"cx": cx, "cy": cy, "openness": openness, "fist": fist}
 
     def _speed(self, slot: int, cx: float, cy: float) -> float:
         buf = self._buf[slot]
diff --git a/data_only_viz/tests/test_hand_features.py b/data_only_viz/tests/test_hand_features.py
index 19b894b..e3aa540 100644
--- a/data_only_viz/tests/test_hand_features.py
+++ b/data_only_viz/tests/test_hand_features.py
@@ -100,3 +100,34 @@ def test_numpy_row_landmarks_finite_features():
     assert math.isfinite(hand["cy"])
     assert math.isfinite(hand["openness"])
     assert hand["openness"] > 0.8  # span/size = 2.0, same as test_open_hand
+
+
+def _fist_hand(tip_ext):
+    """All 4 finger MCPs at center; each fingertip placed tip_ext*size away
+    (so the per-finger tip->MCP ratio == tip_ext). size (wrist->mcp9) = 0.10."""
+    cx, cy = 0.5, 0.5
+    pts = [LM(cx, cy) for _ in range(21)]
+    pts[0] = LM(cx, cy + 0.10)                # wrist -> size 0.10
+    for mcp in (5, 9, 13, 17):
+        pts[mcp] = LM(cx, cy)
+    for tip in (8, 12, 16, 20):
+        pts[tip] = LM(cx + tip_ext * 0.10, cy)
+    pts[4] = LM(cx - 0.05, cy)                # thumb
+    return pts
+
+
+def test_fist_all_fingers_curled():
+    out = HandFeatureExtractor().step([_fist_hand(0.20)])   # tips at knuckles
+    assert (out["L"] or out["R"])["fist"] > 0.9
+
+
+def test_open_hand_is_not_a_fist():
+    out = HandFeatureExtractor().step([_fist_hand(0.80)])   # fingers extended
+    assert (out["L"] or out["R"])["fist"] < 0.1
+
+
+def test_one_extended_finger_breaks_the_fist():
+    h = _fist_hand(0.20)                                    # all curled...
+    h[8] = LM(0.5 + 0.80 * 0.10, 0.5)                       # ...except the index
+    out = HandFeatureExtractor().step([h])
+    assert (out["L"] or out["R"])["fist"] < 0.1            # min over fingers