diff --git a/core/export.py b/core/export.py
new file mode 100644
index 0000000..2873014
--- /dev/null
+++ b/core/export.py
@@ -0,0 +1,123 @@
+import os
+import subprocess
+import threading
+from concurrent.futures import ThreadPoolExecutor, as_completed
+from typing import Callable
+
+from .ffmpeg import build_ffmpeg_command, build_audio_extract_command
+from .paths import _log
+
+
+class ExportRunner:
+    """Run ffmpeg export jobs in a background thread pool.
+
+    Callbacks:
+        on_clip_done(path: str)
+        on_all_done()
+        on_error(msg: str)
+    """
+
+    def __init__(
+        self,
+        input_path: str,
+        jobs: list[tuple[float, str, str | None, float]],
+        short_side: int | None = None,
+        image_sequence: bool = False,
+        max_workers: int | None = None,
+        encoder: str = "libx264",
+        on_clip_done: Callable[[str], None] | None = None,
+        on_all_done: Callable[[], None] | None = None,
+        on_error: Callable[[str], None] | None = None,
+    ):
+        self._input = input_path
+        self._jobs = jobs
+        self._short_side = short_side
+        self._image_sequence = image_sequence
+        self._max_workers = max_workers
+        self._encoder = encoder
+        self._on_clip_done = on_clip_done
+        self._on_all_done = on_all_done
+        self._on_error = on_error
+        self._cancel = False
+        self._procs: list[subprocess.Popen] = []
+        self._procs_lock = threading.Lock()
+        self._thread: threading.Thread | None = None
+
+    def start(self):
+        self._thread = threading.Thread(target=self._run, daemon=True)
+        self._thread.start()
+
+    def cancel(self):
+        self._cancel = True
+        with self._procs_lock:
+            for proc in self._procs:
+                try:
+                    proc.kill()
+                except OSError:
+                    pass
+
+    def is_running(self) -> bool:
+        return self._thread is not None and self._thread.is_alive()
+
+    def _run_one(self, start: float, output: str,
+                 portrait_ratio: str | None, crop_center: float) -> str:
+        if self._cancel:
+            raise RuntimeError("cancelled")
+        if self._image_sequence:
+            os.makedirs(output, exist_ok=True)
+        cmd = build_ffmpeg_command(
+            self._input, start, output,
+            short_side=self._short_side,
+            portrait_ratio=portrait_ratio,
+            crop_center=crop_center,
+            image_sequence=self._image_sequence,
+            encoder=self._encoder,
+        )
+        proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+        with self._procs_lock:
+            self._procs.append(proc)
+        try:
+            _, stderr = proc.communicate(timeout=120)
+        except subprocess.TimeoutExpired:
+            proc.kill()
+            raise RuntimeError("ffmpeg timed out")
+        finally:
+            with self._procs_lock:
+                self._procs.remove(proc)
+        if self._cancel:
+            raise RuntimeError("cancelled")
+        if proc.returncode != 0:
+            msg = stderr.decode(errors='replace')[-500:] if stderr else "ffmpeg failed"
+            raise RuntimeError(msg)
+        if self._image_sequence:
+            audio_cmd = build_audio_extract_command(self._input, start, output)
+            subprocess.run(audio_cmd, capture_output=True, text=True, timeout=60)
+        return output
+
+    def _run(self):
+        cap = self._max_workers or (os.cpu_count() or 2)
+        workers = min(len(self._jobs), cap)
+        try:
+            with ThreadPoolExecutor(max_workers=workers) as pool:
+                futures = {
+                    pool.submit(self._run_one, s, o, pr, cc): o
+                    for s, o, pr, cc in self._jobs
+                }
+                for fut in as_completed(futures):
+                    if self._cancel:
+                        break
+                    try:
+                        path = fut.result()
+                        if self._on_clip_done:
+                            self._on_clip_done(path)
+                    except Exception as e:
+                        if "cancelled" not in str(e) and self._on_error:
+                            self._on_error(str(e))
+        except Exception as e:
+            if self._on_error:
+                self._on_error(str(e))
+            return
+        if self._cancel:
+            return
+        if self._on_all_done:
+            self._on_all_done()