diff --git a/bincio/extract/writer.py b/bincio/extract/writer.py
index a37a011..3a71092 100644
--- a/bincio/extract/writer.py
+++ b/bincio/extract/writer.py
@@ -10,6 +10,18 @@ from bincio.extract.models import LapData, ParsedActivity
 from bincio.extract.simplify import build_geojson, preview_coords
 from bincio.extract.timeseries import build_timeseries
 
+# Titles that reliably identify indoor/virtual activities regardless of sub_sport metadata.
+# Strava imports from Zwift and FTP-builder platforms lose sub_sport on export.
+_INDOOR_TITLE_RE = re.compile(
+    r'\b(zwift|ftp[\s\-]builder|turbo[\s\-]?trainer|rodillo)\b',
+    re.IGNORECASE,
+)
+
+
+def _infer_indoor_title(title: str) -> bool:
+    """Return True if the title reliably identifies an indoor/virtual activity."""
+    return bool(_INDOOR_TITLE_RE.search(title))
+
 
 def make_activity_id(activity: ParsedActivity) -> str:
     """Generate a BAS activity ID from started_at + optional title slug.
@@ -278,14 +290,11 @@ def write_athlete_json(summaries: list[dict], output_dir: Path, athlete_config:
         return [[d, w] for d, w in sorted(best.items())]
 
     _INDOOR_SUB_SPORTS = {"indoor", "treadmill", "virtual"}
-    _INDOOR_TITLE_RE = re.compile(r'\bzwift\b', re.IGNORECASE)
 
     def _is_outdoor(s: dict) -> bool:
         if s.get("sub_sport") in _INDOOR_SUB_SPORTS:
             return False
-        if _INDOOR_TITLE_RE.search(s.get("title") or ""):
-            return False
-        return True
+        return not _infer_indoor_title(s.get("title") or "")
 
     all_mmps = [s["mmp"] for s in summaries if s.get("mmp") and _is_outdoor(s)]
     mmps_365 = [s["mmp"] for s in summaries if s.get("mmp") and _is_outdoor(s) and s["started_at"] >= cutoff_365]
diff --git a/bincio/render/merge.py b/bincio/render/merge.py
index 4bdefda..276de88 100644
--- a/bincio/render/merge.py
+++ b/bincio/render/merge.py
@@ -71,6 +71,7 @@ def parse_sidecar(path: Path) -> tuple[dict, str]:
 
 def apply_sidecar(detail: dict, fm: dict, body: str) -> dict:
     """Apply sidecar overrides to a detail JSON dict. Returns a modified copy."""
+    from bincio.extract.writer import _infer_indoor_title
     d = dict(detail)
     d.setdefault("custom", {})
     d["custom"] = dict(d["custom"])  # don't mutate original
@@ -81,6 +82,9 @@ def apply_sidecar(detail: dict, fm: dict, body: str) -> dict:
         d["sport"] = str(fm["sport"])
     if "sub_sport" in fm:
         d["sub_sport"] = str(fm["sub_sport"]) if fm["sub_sport"] else None
+    # Infer indoor from title when sub_sport is still absent after sidecar
+    if not d.get("sub_sport") and _infer_indoor_title(d.get("title") or ""):
+        d["sub_sport"] = "indoor"
     if "gear" in fm:
         d["gear"] = str(fm["gear"]) if fm["gear"] else d.get("gear")
     if body:
@@ -99,6 +103,7 @@ def apply_sidecar(detail: dict, fm: dict, body: str) -> dict:
 
 def _apply_sidecar_summary(summary: dict, fm: dict) -> dict:
     """Apply sidecar overrides to an index summary entry."""
+    from bincio.extract.writer import _infer_indoor_title
     s = dict(summary)
     s.setdefault("custom", {})
     s["custom"] = dict(s["custom"])
@@ -113,6 +118,9 @@ def _apply_sidecar_summary(summary: dict, fm: dict) -> dict:
         s["custom"]["highlight"] = bool(fm["highlight"])
     if "private" in fm:
         s["privacy"] = "unlisted" if fm["private"] else summary.get("privacy", "public")
+    # Infer indoor from title when sub_sport is still absent
+    if not s.get("sub_sport") and _infer_indoor_title(s.get("title") or ""):
+        s["sub_sport"] = "indoor"
 
     return s
 
@@ -156,6 +164,12 @@ def _merge_one_locked(data_dir: Path, activity_id: str) -> None:
         )
 
     needs_merge = has_sidecar or bool(image_files)
+    # Also need a real file (not symlink) when title inference would change sub_sport
+    if not needs_merge and not has_sidecar:
+        from bincio.extract.writer import _infer_indoor_title
+        _peek = json.loads(src.read_text(encoding="utf-8"))
+        if not _peek.get("sub_sport") and _infer_indoor_title(_peek.get("title") or ""):
+            needs_merge = True
 
     # Symlink the timeseries file (never merged — always points to the extract output)
     ts_src = acts_dir / f"{activity_id}.timeseries.json"
@@ -170,10 +184,13 @@ def _merge_one_locked(data_dir: Path, activity_id: str) -> None:
         dest.unlink()
 
     if needs_merge:
-        detail = json.loads(src.read_text(encoding="utf-8"))
+        detail = locals().get("_peek") or json.loads(src.read_text(encoding="utf-8"))
         if has_sidecar:
             fm, body = parse_sidecar(sidecar_path)  # type: ignore[arg-type]
             detail = apply_sidecar(detail, fm, body)
+        else:
+            # No sidecar — still apply title inference
+            detail = apply_sidecar(detail, {}, "")
         if image_files:
             detail["custom"] = dict(detail.get("custom") or {})
             detail["custom"]["images"] = image_files
@@ -195,9 +212,8 @@ def _merge_one_locked(data_dir: Path, activity_id: str) -> None:
     activities = []
     for s in index.get("activities", []):
         aid = s.get("id", "")
-        if aid in all_sidecars:
-            fm, _ = all_sidecars[aid]
-            s = _apply_sidecar_summary(s, fm)
+        fm, _ = all_sidecars[aid] if aid in all_sidecars else ({}, "")
+        s = _apply_sidecar_summary(s, fm)
         activities.append(s)
 
     activities.sort(key=lambda a: a.get("started_at", ""), reverse=True)
@@ -243,6 +259,17 @@ def _merge_all_locked(data_dir: Path) -> int:
 
     to_merge = set(sidecars) | set(image_lists)
 
+    # Also include activities whose title implies indoor (no sidecar required)
+    _index_path = data_dir / "index.json"
+    _cached_index: dict | None = None
+    if _index_path.exists():
+        from bincio.extract.writer import _infer_indoor_title
+        _cached_index = json.loads(_index_path.read_text(encoding="utf-8"))
+        for _s in _cached_index.get("activities", []):
+            _aid = _s.get("id", "")
+            if _aid and not _s.get("sub_sport") and _infer_indoor_title(_s.get("title") or ""):
+                to_merge.add(_aid)
+
     # Wipe and recreate _merged/activities/
     shutil.rmtree(merged_acts, ignore_errors=True)
     merged_acts.mkdir(parents=True, exist_ok=True)
@@ -259,6 +286,8 @@ def _merge_all_locked(data_dir: Path) -> int:
                 if activity_id in sidecars:
                     fm, body = sidecars[activity_id]
                     detail = apply_sidecar(detail, fm, body)
+                else:
+                    detail = apply_sidecar(detail, {}, "")
                 if activity_id in image_lists:
                     detail["custom"] = dict(detail.get("custom") or {})
                     detail["custom"]["images"] = image_lists[activity_id]
@@ -303,13 +332,12 @@ def _merge_all_locked(data_dir: Path) -> int:
     # Write merged index.json (private filtered, highlight sorted)
     index_path = data_dir / "index.json"
     if index_path.exists():
-        index = json.loads(index_path.read_text(encoding="utf-8"))
+        index = _cached_index or json.loads(index_path.read_text(encoding="utf-8"))
         activities = []
         for s in index.get("activities", []):
             aid = s.get("id", "")
-            if aid in sidecars:
-                fm, _ = sidecars[aid]
-                s = _apply_sidecar_summary(s, fm)
+            fm, _ = sidecars[aid] if aid in sidecars else ({}, "")
+            s = _apply_sidecar_summary(s, fm)
             activities.append(s)
 
         # "unlisted" (and legacy "private") activities are kept in the index so