diff --git a/frontend/js/db.js b/frontend/js/db.js
index 6dbed57..364f7a0 100644
--- a/frontend/js/db.js
+++ b/frontend/js/db.js
@@ -1001,6 +1001,65 @@ async function splitGpsSession(sessionId, part2Ids, part1EndTime, part2StartTime
   return String(newId);
 }
 
+// Undo a merge: split a merged session back into its original sessions by orig_session_id
+// (merge preserved it on every point). The current session keeps its own original group; each
+// other orig_session_id gets its session row recreated (metadata copied). Points are retagged
+// (orig_session_id cleared, seq cleared → timestamp order). Returns the list of resulting
+// session id_strs (incl. the current one), or null if this session isn't a merge. The caller
+// recomputes stats + times for each (computeStats + finishGpsSession).
+async function unmergeGpsSession(sessionId) {
+  const s = await getGpsSession(sessionId);
+  if (!s) return null;
+  const rows = await select(
+    "SELECT DISTINCT CAST(orig_session_id AS TEXT) AS o FROM gps_points WHERE session_id = CAST(?1 AS INTEGER) AND orig_session_id IS NOT NULL",
+    [s.id_str]
+  );
+  const origs = rows.map(r => r.o).filter(Boolean);
+  if (origs.length < 2) return null; // not a merge — nothing to split
+  const resultIds = [];
+  for (const oid of origs) {
+    if (oid !== s.id_str) {
+      await execute(
+        `INSERT INTO gps_sessions (id, date, start_time, end_time, activity_type, source, bike_id)
+         VALUES (CAST(?1 AS INTEGER), ?2, ?3, ?4, ?5, ?6, ?7)`,
+        [oid, s.date, s.start_time, s.end_time, s.activity_type, s.source ?? "recorded", s.bike_id ?? null]
+      );
+      await execute(
+        "UPDATE gps_points SET session_id = CAST(?1 AS INTEGER), orig_session_id = NULL, seq = NULL WHERE session_id = CAST(?2 AS INTEGER) AND orig_session_id = CAST(?1 AS TEXT)",
+        [oid, s.id_str]
+      );
+    } else {
+      await execute(
+        "UPDATE gps_points SET orig_session_id = NULL, seq = NULL WHERE session_id = CAST(?1 AS INTEGER) AND orig_session_id = CAST(?1 AS TEXT)",
+        [s.id_str]
+      );
+    }
+    resultIds.push(oid);
+  }
+  // Fix each piece's date/times from its own points (merge had stretched them to the whole span).
+  for (const id of resultIds) {
+    const ts = await select("SELECT MIN(ts) AS a, MAX(ts) AS b FROM gps_points WHERE session_id = CAST(?1 AS INTEGER)", [id]);
+    const a = ts[0] && ts[0].a;
+    if (a) {
+      const hm = (t) => new Date(t).toTimeString().slice(0, 8);
+      await execute("UPDATE gps_sessions SET date = ?1, start_time = ?2, end_time = ?3 WHERE id = CAST(?4 AS INTEGER)",
+        [new Date(a).toISOString().slice(0, 10), hm(a), hm(ts[0].b), id]);
+    }
+  }
+  return resultIds;
+}
+
+// How many distinct original sessions are glued into this one (>1 ⇒ it's a merge that can be split).
+async function countMergedOrigins(sessionId) {
+  const s = await getGpsSession(sessionId);
+  if (!s) return 0;
+  const r = await select(
+    "SELECT COUNT(DISTINCT orig_session_id) AS c FROM gps_points WHERE session_id = CAST(?1 AS INTEGER) AND orig_session_id IS NOT NULL",
+    [s.id_str]
+  );
+  return (r[0] && r[0].c) || 0;
+}
+
 async function importGpxSession(session, points) {
   await execute(
     `INSERT INTO gps_sessions (date, start_time, end_time, activity_type,
@@ -1389,6 +1448,8 @@ window.DB.getGpsSession          = getGpsSession;
 window.DB.getGpsPoints           = getGpsPoints;
 window.DB.deleteGpsSession       = deleteGpsSession;
 window.DB.mergeGpsSessions       = mergeGpsSessions;
+window.DB.unmergeGpsSession      = unmergeGpsSession;
+window.DB.countMergedOrigins     = countMergedOrigins;
 window.DB.splitGpsSession        = splitGpsSession;
 window.DB.importGpxSession       = importGpxSession;
 window.DB.saveGapFill            = saveGapFill;