diff --git a/bincio/render/cli.py b/bincio/render/cli.py
index 5618499..56dd57c 100644
--- a/bincio/render/cli.py
+++ b/bincio/render/cli.py
@@ -377,6 +377,71 @@ def _link_data(site: Path, data: Path) -> None:
     console.print(f"Linked data: [cyan]{target}[/cyan] → [cyan]{public_data}[/cyan]")
 
 
+def _recompute_vam(data: Path, handle: str | None = None) -> None:
+    """Recompute climbing_vam_mh and climbing_time_s for all activities.
+
+    Reads the stored timeseries, re-runs the VAM algorithm, and patches both
+    activities/*.json and index.json in-place.  Run once after adding
+    climbing_time_s to the schema so the NerdCorner VAM chart can filter short
+    climbs and opacity-encode confidence.
+    """
+    import json
+    from bincio.extract.metrics import _VAM_SPORTS, _build_ele_1hz, _vam_from_ele_1hz
+
+    targets = [data / handle] if handle else _user_dirs(data)
+    for user_dir in targets:
+        acts_dir = user_dir / "activities"
+        index_path = user_dir / "index.json"
+        if not acts_dir.exists() or not index_path.exists():
+            continue
+        try:
+            index_data = json.loads(index_path.read_text(encoding="utf-8"))
+        except Exception:
+            continue
+
+        index_by_id = {s["id"]: s for s in index_data.get("activities", [])}
+        updated = 0
+
+        for act_path in sorted(acts_dir.glob("*.json")):
+            if act_path.stem.endswith((".timeseries", ".geojson")):
+                continue
+            ts_path = acts_dir / f"{act_path.stem}.timeseries.json"
+            if not ts_path.exists():
+                continue
+            try:
+                detail = json.loads(act_path.read_text(encoding="utf-8"))
+                if detail.get("sport") not in _VAM_SPORTS:
+                    continue
+                ts = json.loads(ts_path.read_text(encoding="utf-8"))
+                t_vals = ts.get("t", [])
+                e_vals = ts.get("elevation_m", [])
+                sparse: dict[int, float | None] = {int(t): e for t, e in zip(t_vals, e_vals)}
+                ele_1hz = _build_ele_1hz(sparse)
+                result = _vam_from_ele_1hz(ele_1hz) if ele_1hz else None
+                new_vam, new_climb_t = result if result else (None, None)
+                if (new_vam == detail.get("climbing_vam_mh") and
+                        new_climb_t == detail.get("climbing_time_s")):
+                    continue
+                detail["climbing_vam_mh"] = new_vam
+                detail["climbing_time_s"] = new_climb_t
+                act_path.write_text(
+                    json.dumps(detail, indent=2, ensure_ascii=False), encoding="utf-8"
+                )
+                summary = index_by_id.get(act_path.stem)
+                if summary is not None:
+                    summary["climbing_vam_mh"] = new_vam
+                    summary["climbing_time_s"] = new_climb_t
+                updated += 1
+            except Exception:
+                pass
+
+        if updated:
+            index_path.write_text(
+                json.dumps(index_data, indent=2, ensure_ascii=False), encoding="utf-8"
+            )
+        console.print(f"  [cyan]{user_dir.name}[/cyan]: {updated} VAM(s) recomputed")
+
+
 def _backfill_vam_summary(data: Path, handle: str | None = None) -> None:
     """Copy climbing_vam_mh from detail JSONs into index.json summaries.
 
@@ -475,6 +540,9 @@ def _backfill_speed(data: Path, handle: str | None = None) -> None:
 @click.option("--recompute-elevation", "recompute_elevation", is_flag=True,
               help="Recompute elevation_gain_m/loss_m for all activities from stored timeseries "
                    "(run once after upgrading the dropout-skip fix).")
+@click.option("--recompute-vam", "recompute_vam", is_flag=True,
+              help="Recompute climbing_vam_mh and climbing_time_s for all activities from stored "
+                   "timeseries (run once after adding climbing_time_s to the schema).")
 @click.option("--backfill-vam-summary", "backfill_vam_summary", is_flag=True,
               help="Copy climbing_vam_mh from detail JSONs into index.json summaries "
                    "(run once after the VAM curve → summary migration).")
@@ -492,6 +560,7 @@ def render(
     no_build: bool,
     recompute_climbs: bool,
     recompute_elevation: bool,
+    recompute_vam: bool,
     backfill_vam_summary: bool,
     backfill_speed: bool,
 ) -> None:
@@ -511,6 +580,10 @@ def render(
         console.print("Recomputing elevation gain/loss from timeseries…")
         _recompute_elevation(data, handle=handle)
 
+    if recompute_vam:
+        console.print("Recomputing VAM and climbing time from timeseries…")
+        _recompute_vam(data, handle=handle)
+
     if backfill_vam_summary:
         console.print("Backfilling climbing_vam_mh into summaries…")
         _backfill_vam_summary(data, handle=handle)