diff --git a/gui_v6/processing_runner.py b/gui_v6/processing_runner.py
index 7a5c6b8..1c34f85 100644
--- a/gui_v6/processing_runner.py
+++ b/gui_v6/processing_runner.py
@@ -99,6 +99,10 @@ def discover_documents(input_path, extensions: Optional[Sequence[str]] = None) -
     return []
 
 
+class OutputNotWritableError(RuntimeError):
+    """Le dossier de sortie n'est pas inscriptible (échec amont, message clair)."""
+
+
 @dataclass
 class DocResult:
     """Détail anonymisé d'un document traité (pour la télémétrie d'usage).
@@ -195,6 +199,19 @@ class ProcessingRunner:
             log("Aucun document supporté détecté.")
             return summary
 
+        # Sonde amont : on vérifie une seule fois que le dossier de sortie est
+        # inscriptible AVANT la boucle, pour un échec clair et unique (P1-6)
+        # plutôt qu'une erreur cryptique répétée à chaque document.
+        try:
+            out_root.mkdir(parents=True, exist_ok=True)
+            probe = out_root / ".anon_write_test"
+            probe.write_text("", encoding="utf-8")
+            probe.unlink()
+        except Exception as exc:
+            raise OutputNotWritableError(
+                f"Dossier de sortie non inscriptible : {out_root} ({exc})"
+            ) from exc
+
         for index, doc in enumerate(docs, start=1):
             if stop_event is not None and stop_event.is_set():
                 summary.stopped = True
diff --git a/tests/unit/test_gui_v6_processing_runner.py b/tests/unit/test_gui_v6_processing_runner.py
index e78820d..1a3565a 100644
--- a/tests/unit/test_gui_v6_processing_runner.py
+++ b/tests/unit/test_gui_v6_processing_runner.py
@@ -197,6 +197,23 @@ def test_progress_callbacks(tmp_path):
     assert (2, 2) in events  # progression finale atteinte
 
 
+def test_run_fails_fast_when_output_not_writable(tmp_path, monkeypatch):
+    from gui_v6.processing_runner import ProcessingRunner, OutputNotWritableError
+    src = tmp_path / "in"
+    src.mkdir()
+    (src / "a.txt").write_text("x", encoding="utf-8")
+    out = tmp_path / "ro"
+    out.mkdir()
+
+    def boom(*a, **k):
+        raise PermissionError("read-only")
+
+    monkeypatch.setattr("gui_v6.processing_runner.Path.mkdir", boom)
+    runner = ProcessingRunner(process_fn=lambda d, o: {})
+    with pytest.raises(OutputNotWritableError):
+        runner.run(src, out)
+
+
 def test_no_double_run(tmp_path):
     _touch(tmp_path / "a.pdf")
     started = threading.Event()