diff --git a/scripts/dispatcher.py b/scripts/dispatcher.py
index 0770513..c073485 100644
--- a/scripts/dispatcher.py
+++ b/scripts/dispatcher.py
@@ -25,6 +25,7 @@ import shlex
 import sqlite3
 import subprocess
 import sys
+import threading
 import time
 from contextlib import closing
 from datetime import datetime, timezone
@@ -59,6 +60,9 @@ DEFAULT_WORKERS = [
 ]
 WORKERS = json.loads(os.environ.get("COSMA_QC_WORKERS", json.dumps(DEFAULT_WORKERS)))
 
+_worker_lock = threading.Lock()
+_reserved_vram = {w["host"]: 0 for w in WORKERS}
+
 
 def db() -> sqlite3.Connection:
     conn = sqlite3.connect(DB_PATH, isolation_level=None)
@@ -84,12 +88,24 @@ def worker_free_vram_mib(worker: dict) -> int:
 
 
 def pick_worker(estimated_vram_mib: int) -> dict | None:
-    best = None
-    for w in WORKERS:
-        free = worker_free_vram_mib(w)
-        if free >= estimated_vram_mib and (best is None or free > best[0]):
-            best = (free, w)
-    return best[1] if best else None
+    """Pick worker with most effective free VRAM (actual free minus local reservations)
+    and reserve the estimated VRAM. Returns None if none fit."""
+    with _worker_lock:
+        best = None
+        for w in WORKERS:
+            free = worker_free_vram_mib(w) - _reserved_vram.get(w["host"], 0)
+            if free >= estimated_vram_mib and (best is None or free > best[0]):
+                best = (free, w)
+        if best:
+            _reserved_vram[best[1]["host"]] = _reserved_vram.get(best[1]["host"], 0) + estimated_vram_mib
+            return best[1]
+    return None
+
+
+def release_worker(worker: dict, estimated_vram_mib: int):
+    with _worker_lock:
+        h = worker["host"]
+        _reserved_vram[h] = max(0, _reserved_vram.get(h, 0) - estimated_vram_mib)
 
 
 def estimate_vram_mib(frame_count: int) -> int:
@@ -399,6 +415,8 @@ def run_one(job: sqlite3.Row) -> bool:
     estimated = estimate_vram_mib(job["frame_count"] or 150)
     worker = pick_worker(estimated)
     if not worker:
+        # release claim so the job can be re-tried by main loop
+        set_status(job_id, status="queued")
         return False
     set_status(job_id, status="extracting", worker_host=worker["host"], started_at=_now_iso())
     try:
@@ -412,21 +430,42 @@ def run_one(job: sqlite3.Row) -> bool:
         _maybe_create_per_auv_stitch(job_id)
     except Exception as e:
         set_status(job_id, status="error", error=str(e)[:2000], finished_at=_now_iso())
+    finally:
+        release_worker(worker, estimated)
     return True
 
 
 def pop_queued() -> sqlite3.Row | None:
+    """Atomic claim: grab a queued job and mark it 'claimed' to prevent double-dispatch."""
     with closing(db()) as conn:
-        return conn.execute(
-            "SELECT * FROM jobs WHERE status='queued' ORDER BY created_at LIMIT 1"
-        ).fetchone()
+        try:
+            conn.execute("BEGIN IMMEDIATE")
+            row = conn.execute(
+                "SELECT * FROM jobs WHERE status='queued' ORDER BY created_at LIMIT 1"
+            ).fetchone()
+            if row:
+                conn.execute("UPDATE jobs SET status='claimed' WHERE id=?", (row["id"],))
+            conn.execute("COMMIT")
+        except Exception:
+            conn.execute("ROLLBACK")
+            return None
+    return row
 
 
 def pop_queued_stitch() -> sqlite3.Row | None:
     with closing(db()) as conn:
-        return conn.execute(
-            "SELECT * FROM stitches WHERE status='queued' ORDER BY created_at LIMIT 1"
-        ).fetchone()
+        try:
+            conn.execute("BEGIN IMMEDIATE")
+            row = conn.execute(
+                "SELECT * FROM stitches WHERE status='queued' ORDER BY created_at LIMIT 1"
+            ).fetchone()
+            if row:
+                conn.execute("UPDATE stitches SET status='claimed' WHERE id=?", (row["id"],))
+            conn.execute("COMMIT")
+        except Exception:
+            conn.execute("ROLLBACK")
+            return None
+    return row
 
 
 def write_heartbeat():
@@ -437,23 +476,48 @@ def write_heartbeat():
         pass
 
 
+_heartbeat_stop = threading.Event()
+
+
+def _heartbeat_loop():
+    while not _heartbeat_stop.is_set():
+        write_heartbeat()
+        _heartbeat_stop.wait(5)
+
+
+def _run_one_thread(job: sqlite3.Row):
+    try:
+        if not run_one(job):
+            print(f"  ↳ job #{job['id']}: pas de worker dispo, remis en queued")
+    except Exception as e:
+        set_status(job["id"], status="error", error=f"run_one thread crashed: {str(e)[:1500]}",
+                   finished_at=_now_iso())
+
+
 def main():
     print(f"cosma-qc dispatcher · DB={DB_PATH} · workers={[w['host'] for w in WORKERS]}")
+    threading.Thread(target=_heartbeat_loop, daemon=True).start()
+    active: set[threading.Thread] = set()
+    max_parallel = max(1, len(WORKERS))
     while True:
-        write_heartbeat()
-        job = pop_queued()
-        if job:
-            print(f"→ job #{job['id']} ({job['auv']}/{job['gopro_serial']}/{job['segment_label']})")
-            if not run_one(job):
-                print("  ↳ pas de worker dispo, retry dans 30s")
-                time.sleep(30)
-            continue
-        stitch = pop_queued_stitch()
-        if stitch:
-            label = f"{stitch['level']} {stitch['auv'] or ''} acq#{stitch['acquisition_id']}"
-            print(f"→ stitch #{stitch['id']} ({label})")
-            run_one_stitch(stitch)
-            continue
+        # drain finished threads
+        active = {t for t in active if t.is_alive()}
+        if len(active) < max_parallel:
+            job = pop_queued()
+            if job:
+                print(f"→ job #{job['id']} ({job['auv']}/{job['gopro_serial']}/{job['segment_label']}) [active={len(active) + 1}]")
+                t = threading.Thread(target=_run_one_thread, args=(job,), daemon=False)
+                t.start()
+                active.add(t)
+                # brief pause so the thread can reserve its worker before we pop another
+                time.sleep(0.5)
+                continue
+            stitch = pop_queued_stitch()
+            if stitch:
+                label = f"{stitch['level']} {stitch['auv'] or ''} acq#{stitch['acquisition_id']}"
+                print(f"→ stitch #{stitch['id']} ({label})")
+                run_one_stitch(stitch)
+                continue
         time.sleep(POLL_S)