diff --git a/backend/app/api/sources.py b/backend/app/api/sources.py
index 56c5cd6..19d47ce 100644
--- a/backend/app/api/sources.py
+++ b/backend/app/api/sources.py
@@ -130,6 +130,15 @@ async def set_backfill(source_id: int):
     current pHash threshold; 'stop' cancels either back to tick mode. Returns the
     updated source dict (incl. backfill_state / backfill_chunks /
     backfill_bypass_seen)."""
+    from pathlib import Path
+
+    from ..services.credential_service import CredentialService
+    from ..services.download_backends import (
+        uses_native_ingester,
+        verify_source_credential,
+    )
+    from .credentials import _get_crypto
+
     payload = await request.get_json(silent=True) or {}
     action = payload.get("action", "start")
     if action not in ("start", "stop", "recover"):
@@ -137,6 +146,37 @@ async def set_backfill(source_id: int):
             "invalid_action",
             detail="action must be 'start', 'stop', or 'recover'",
         )
+
+    # Pre-flight (plan #703 #2): before arming a deep walk on a native-ingester
+    # platform (where verify is one cheap API page), refuse if the credential is
+    # DEFINITIVELY rejected — don't burn chunks against expired cookies. Proceed
+    # on valid OR inconclusive (a network blip shouldn't block). Gated to native
+    # platforms: gallery-dl verify is a slow --simulate subprocess, too heavy for
+    # an arm action. The credential read happens in a session that's CLOSED
+    # before the verify network call (don't hold a DB conn across the request).
+    if action in ("start", "recover"):
+        async with get_session() as session:
+            rec = await SourceService(session).get(source_id)
+            if rec is None:
+                return _bad("not_found", status=404)
+            native = uses_native_ingester(rec.platform)
+            if native:
+                cred = CredentialService(session, _get_crypto())
+                cookies_path = await cred.get_cookies_path(rec.platform)
+                auth_token = await cred.get_token(rec.platform)
+        if native:
+            ok, message = await verify_source_credential(
+                platform=rec.platform,
+                url=rec.url,
+                artist_slug=rec.artist_slug,
+                config_overrides=rec.config_overrides or {},
+                cookies_path=str(cookies_path) if cookies_path else None,
+                auth_token=auth_token,
+                images_root=Path("/images"),
+            )
+            if ok is False:
+                return _bad("credential_rejected", detail=message, status=409)
+
     async with get_session() as session:
         try:
             svc = SourceService(session)
diff --git a/frontend/src/components/subscriptions/SubscriptionsTab.vue b/frontend/src/components/subscriptions/SubscriptionsTab.vue
index cafd618..d804c7a 100644
--- a/frontend/src/components/subscriptions/SubscriptionsTab.vue
+++ b/frontend/src/components/subscriptions/SubscriptionsTab.vue
@@ -550,7 +550,7 @@ async function onBackfill(source) {
     await store.loadAll()
   } catch (e) {
     toast({
-      text: `Backfill ${running ? 'stop' : 'start'} failed: ${e?.detail || e?.message || e}`,
+      text: `Backfill ${running ? 'stop' : 'start'} failed: ${e?.body?.detail || e?.message || e}`,
       type: 'error',
     })
   }
@@ -566,7 +566,7 @@ async function onRecover(source) {
     await store.loadAll()
   } catch (e) {
     toast({
-      text: `Recovery start failed: ${e?.detail || e?.message || e}`,
+      text: `Recovery start failed: ${e?.body?.detail || e?.message || e}`,
       type: 'error',
     })
   }
diff --git a/tests/test_api_sources.py b/tests/test_api_sources.py
index 438b305..71f4dde 100644
--- a/tests/test_api_sources.py
+++ b/tests/test_api_sources.py
@@ -5,6 +5,20 @@ from backend.app.models import Artist, Source
 pytestmark = pytest.mark.integration
 
 
+@pytest.fixture(autouse=True)
+def _stub_preflight_verify(monkeypatch):
+    """Backfill/recover arms run a pre-flight credential verify (plan #703 #2)
+    which for Patreon would hit the network. Default it to 'valid' so the
+    endpoint tests stay network-free; the dedicated pre-flight tests override
+    this with a rejection/inconclusive verdict."""
+    from backend.app.services import download_backends as db_mod
+
+    async def _ok(**kwargs):
+        return (True, "Credentials valid")
+
+    monkeypatch.setattr(db_mod, "verify_source_credential", _ok)
+
+
 @pytest.fixture
 async def artist(db):
     a = Artist(name="Alice", slug="alice")
@@ -275,3 +289,92 @@ async def test_backfill_endpoint_recover_arms_bypass(client, artist, db):
     stopped_body = await stopped.get_json()
     assert stopped_body["backfill_state"] is None
     assert stopped_body["backfill_bypass_seen"] is False
+
+
+# --- Plan #703 #2: pre-flight credential verify on backfill/recover arm -----
+
+
+@pytest.mark.asyncio
+async def test_arm_blocked_when_credential_rejected(client, artist, db, monkeypatch):
+    """A definitively-rejected credential refuses the arm (409 + reason) instead
+    of starting a doomed walk; the source is NOT armed."""
+    from backend.app.services import download_backends as db_mod
+
+    async def _reject(**kwargs):
+        return (False, "Patreon rejected the credential — cookies expired")
+    monkeypatch.setattr(db_mod, "verify_source_credential", _reject)
+
+    src = Source(
+        artist_id=artist.id, platform="patreon",
+        url="https://patreon.com/alice-preflight", enabled=True,
+    )
+    db.add(src)
+    await db.commit()
+    sid = src.id
+
+    resp = await client.post(f"/api/sources/{sid}/backfill", json={"action": "recover"})
+    assert resp.status_code == 409
+    assert "expired" in ((await resp.get_json()).get("detail") or "")
+
+    # Not armed.
+    one = await (await client.get(f"/api/sources/{sid}")).get_json()
+    assert one["backfill_state"] is None
+
+
+@pytest.mark.asyncio
+async def test_arm_proceeds_when_credential_inconclusive(client, artist, db, monkeypatch):
+    """An inconclusive verify (network blip / drift) must NOT block the arm."""
+    from backend.app.services import download_backends as db_mod
+
+    async def _inconclusive(**kwargs):
+        return (None, "couldn't verify (network)")
+    monkeypatch.setattr(db_mod, "verify_source_credential", _inconclusive)
+
+    src = Source(
+        artist_id=artist.id, platform="patreon",
+        url="https://patreon.com/alice-incon", enabled=True,
+    )
+    db.add(src)
+    await db.commit()
+
+    resp = await client.post(f"/api/sources/{src.id}/backfill", json={"action": "start"})
+    assert resp.status_code == 200
+    assert (await resp.get_json())["backfill_state"] == "running"
+
+
+@pytest.mark.asyncio
+async def test_stop_never_pre_flights(client, artist, db, monkeypatch):
+    from backend.app.services import download_backends as db_mod
+
+    async def _boom(**kwargs):
+        raise AssertionError("stop must not pre-flight verify")
+    monkeypatch.setattr(db_mod, "verify_source_credential", _boom)
+
+    src = Source(
+        artist_id=artist.id, platform="patreon",
+        url="https://patreon.com/alice-stop", enabled=True,
+    )
+    db.add(src)
+    await db.commit()
+    resp = await client.post(f"/api/sources/{src.id}/backfill", json={"action": "stop"})
+    assert resp.status_code == 200
+
+
+@pytest.mark.asyncio
+async def test_gallery_dl_platform_arm_skips_pre_flight(client, artist, db, monkeypatch):
+    """Pre-flight is gated to native-ingester platforms (cheap verify); a
+    gallery-dl source arms without the slow --simulate probe."""
+    from backend.app.services import download_backends as db_mod
+
+    async def _boom(**kwargs):
+        raise AssertionError("gallery-dl arm must not pre-flight verify")
+    monkeypatch.setattr(db_mod, "verify_source_credential", _boom)
+
+    src = Source(
+        artist_id=artist.id, platform="subscribestar",
+        url="https://www.subscribestar.com/x", enabled=True,
+    )
+    db.add(src)
+    await db.commit()
+    resp = await client.post(f"/api/sources/{src.id}/backfill", json={"action": "start"})
+    assert resp.status_code == 200