diff --git a/bincio/render/merge.py b/bincio/render/merge.py
index d21a495..4bdefda 100644
--- a/bincio/render/merge.py
+++ b/bincio/render/merge.py
@@ -21,6 +21,31 @@ import yaml
 
 # Per-user-directory lock so concurrent upload requests and the dev file-watcher
 # cannot run merge_all simultaneously on the same directory.
+
+
+def _fix_surrogates(obj: object) -> object:
+    """Recursively replace surrogate pairs in strings with proper Unicode code points.
+
+    Surrogate pairs (U+D800–U+DFFF) are valid in Python str but not in UTF-8.
+    They typically arise when emoji from UTF-16-encoded sources (Strava, some FIT
+    devices) are decoded incorrectly. encode/decode via utf-16 with surrogatepass
+    reconstructs the intended characters.
+    """
+    if isinstance(obj, str):
+        try:
+            obj.encode("utf-8")
+            return obj
+        except UnicodeEncodeError:
+            return obj.encode("utf-16", "surrogatepass").decode("utf-16")
+    if isinstance(obj, dict):
+        return {k: _fix_surrogates(v) for k, v in obj.items()}
+    if isinstance(obj, list):
+        return [_fix_surrogates(v) for v in obj]
+    return obj
+
+
+def _dumps(obj: object) -> str:
+    return json.dumps(_fix_surrogates(obj), indent=2, ensure_ascii=False)
 _merge_locks: dict[str, threading.Lock] = {}
 _merge_locks_mu = threading.Lock()
 
@@ -152,7 +177,7 @@ def _merge_one_locked(data_dir: Path, activity_id: str) -> None:
         if image_files:
             detail["custom"] = dict(detail.get("custom") or {})
             detail["custom"]["images"] = image_files
-        dest.write_text(json.dumps(detail, indent=2, ensure_ascii=False))
+        dest.write_text(_dumps(detail))
     else:
         dest.symlink_to(src.resolve())
 
@@ -237,7 +262,7 @@ def _merge_all_locked(data_dir: Path) -> int:
                 if activity_id in image_lists:
                     detail["custom"] = dict(detail.get("custom") or {})
                     detail["custom"]["images"] = image_lists[activity_id]
-                dest.write_text(json.dumps(detail, indent=2, ensure_ascii=False))
+                dest.write_text(_dumps(detail))
             else:
                 if not dest.exists() and not dest.is_symlink():
                     dest.symlink_to(src.resolve())
@@ -271,7 +296,7 @@ def _merge_all_locked(data_dir: Path) -> int:
         if edits:
             athlete_data = json.loads(athlete_src.read_text(encoding="utf-8"))
             athlete_data.update({k: v for k, v in edits.items() if k in _ATHLETE_EDITABLE})
-            athlete_dest.write_text(json.dumps(athlete_data, indent=2, ensure_ascii=False))
+            athlete_dest.write_text(_dumps(athlete_data))
         else:
             athlete_dest.symlink_to(athlete_src.resolve())
 
@@ -338,7 +363,7 @@ def _write_year_shards(merged_dir: Path, activities: list[dict], index_meta: dic
             "activities": by_year[year],
         }
         fname = f"index-{year}.json"
-        (merged_dir / fname).write_text(json.dumps(shard_doc, indent=2, ensure_ascii=False))
+        (merged_dir / fname).write_text(_dumps(shard_doc))
         shards.append({"url": fname, "year": int(year) if year.isdigit() else 0,
                         "count": len(by_year[year])})
 
@@ -347,7 +372,7 @@ def _write_year_shards(merged_dir: Path, activities: list[dict], index_meta: dic
         "shards": shards,
         "activities": [],
     }
-    (merged_dir / "index.json").write_text(json.dumps(root_doc, indent=2, ensure_ascii=False))
+    (merged_dir / "index.json").write_text(_dumps(root_doc))
 
 
 FEED_PAGE_SIZE = 50
@@ -423,6 +448,6 @@ def write_combined_feed(data_dir: Path) -> int:
             "total_activities": len(all_activities),
             "activities": slim,
         }
-        (data_dir / fname).write_text(json.dumps(doc, indent=2, ensure_ascii=False))
+        (data_dir / fname).write_text(_dumps(doc))
 
     return len(all_activities)