diff --git a/frontend/src/components/subscriptions/SourceCard.vue b/frontend/src/components/subscriptions/SourceCard.vue
index 03464f8..7b8f9bc 100644
--- a/frontend/src/components/subscriptions/SourceCard.vue
+++ b/frontend/src/components/subscriptions/SourceCard.vue
@@ -25,9 +25,17 @@
         size="x-small" color="error" variant="tonal" label
       >{{ source.consecutive_failures }} err</v-chip>
       <v-chip
-        v-else-if="(source.backfill_runs_remaining || 0) > 0"
+        v-else-if="source.backfill_state === 'running'"
         size="x-small" color="info" variant="tonal" label
-      >backfill ({{ source.backfill_runs_remaining }}×)</v-chip>
+      >Backfilling{{ source.backfill_chunks ? ` (${source.backfill_chunks})` : '' }}</v-chip>
+      <v-chip
+        v-else-if="source.backfill_state === 'complete'"
+        size="x-small" color="success" variant="tonal" label
+      >Backfilled</v-chip>
+      <v-chip
+        v-else-if="source.backfill_state === 'stalled'"
+        size="x-small" color="warning" variant="tonal" label
+      >Stalled</v-chip>
     </div>
 
     <div class="fc-source-card__actions">
@@ -40,11 +48,13 @@
       </v-btn>
       <v-btn
         size="x-small" variant="text"
-        :disabled="(source.backfill_runs_remaining || 0) > 0"
+        :color="source.backfill_state === 'running' ? 'warning' : undefined"
         @click.stop="$emit('backfill', source)"
       >
-        <v-icon>mdi-magnify-scan</v-icon>
-        <v-tooltip activator="parent" location="top">Deep scan</v-tooltip>
+        <v-icon>{{ source.backfill_state === 'running' ? 'mdi-stop' : 'mdi-magnify-scan' }}</v-icon>
+        <v-tooltip activator="parent" location="top">
+          {{ source.backfill_state === 'running' ? 'Stop backfill' : 'Backfill full history' }}
+        </v-tooltip>
       </v-btn>
       <v-btn size="x-small" variant="text" @click.stop="$emit('edit', source)">
         <v-icon>mdi-pencil</v-icon>
diff --git a/frontend/src/components/subscriptions/SourceRow.vue b/frontend/src/components/subscriptions/SourceRow.vue
index 011645f..9fe412e 100644
--- a/frontend/src/components/subscriptions/SourceRow.vue
+++ b/frontend/src/components/subscriptions/SourceRow.vue
@@ -32,11 +32,17 @@
         size="x-small" color="error" variant="tonal" label
       >{{ source.consecutive_failures }}</v-chip>
       <v-chip
-        v-else-if="(source.backfill_runs_remaining || 0) > 0"
+        v-else-if="source.backfill_state === 'running'"
         size="x-small" color="info" variant="tonal" label
-      >
-        backfill ({{ source.backfill_runs_remaining }}×)
-      </v-chip>
+      >Backfilling{{ source.backfill_chunks ? ` (${source.backfill_chunks})` : '' }}</v-chip>
+      <v-chip
+        v-else-if="source.backfill_state === 'complete'"
+        size="x-small" color="success" variant="tonal" label
+      >Backfilled</v-chip>
+      <v-chip
+        v-else-if="source.backfill_state === 'stalled'"
+        size="x-small" color="warning" variant="tonal" label
+      >Stalled</v-chip>
       <span v-else class="fc-source-row__zero">0</span>
     </td>
     <td class="fc-source-row__actions">
@@ -49,13 +55,15 @@
         <v-tooltip activator="parent" location="top">Check now</v-tooltip>
       </v-btn>
       <v-btn
-        icon="mdi-magnify-scan" size="x-small" variant="text"
-        :disabled="(source.backfill_runs_remaining || 0) > 0"
+        size="x-small" variant="text"
+        :color="source.backfill_state === 'running' ? 'warning' : undefined"
         @click.stop="$emit('backfill', source)"
       >
-        <v-icon>mdi-magnify-scan</v-icon>
+        <v-icon>{{ source.backfill_state === 'running' ? 'mdi-stop' : 'mdi-magnify-scan' }}</v-icon>
         <v-tooltip activator="parent" location="top">
-          Deep scan — walk full history for next few runs
+          {{ source.backfill_state === 'running'
+            ? 'Stop backfill'
+            : 'Backfill — walk the full post history until complete' }}
         </v-tooltip>
       </v-btn>
       <v-btn
diff --git a/frontend/src/components/subscriptions/SubscriptionsTab.vue b/frontend/src/components/subscriptions/SubscriptionsTab.vue
index d6a73da..5d479c7 100644
--- a/frontend/src/components/subscriptions/SubscriptionsTab.vue
+++ b/frontend/src/components/subscriptions/SubscriptionsTab.vue
@@ -532,31 +532,23 @@ async function onCheck(source) {
   }
 }
 
-// Plan #544: arm a source for backfill mode (gallery-dl walks the full
-// post history) for the next N download runs. Default 3 — enough budget
-// to finish a deep creator without re-prompting the operator across
-// timeout boundaries. The chip on the row reflects the remaining count.
+// Plan #693: toggle a run-until-done backfill. Starting walks the full post
+// history in time-boxed chunks across ticks until it reaches the bottom (the
+// row badge tracks progress / completion); stopping cancels back to tick mode.
 async function onBackfill(source) {
-  const raw = globalThis.window?.prompt(
-    `Deep scan "${source.artist_name} (${source.platform})" — walk full history for the next how many download runs? (1–10, default 3)`,
-    '3',
-  )
-  if (raw == null) return
-  const runs = parseInt(raw, 10)
-  if (!Number.isFinite(runs) || runs < 1 || runs > 10) {
-    toast({ text: 'Deep scan: runs must be 1–10', type: 'error' })
-    return
-  }
+  const running = source.backfill_state === 'running'
   try {
-    await store.setBackfill(source.id, runs, source.artist_id)
-    toast({
-      text: `Deep scan armed for ${runs} run${runs === 1 ? '' : 's'}`,
-      type: 'success',
-    })
+    if (running) {
+      await store.stopBackfill(source.id, source.artist_id)
+      toast({ text: `Backfill stopped for ${source.artist_name}`, type: 'success' })
+    } else {
+      await store.startBackfill(source.id, source.artist_id)
+      toast({ text: `Backfill started for ${source.artist_name}`, type: 'success' })
+    }
     await store.loadAll()
   } catch (e) {
     toast({
-      text: `Deep scan failed: ${e?.detail || e?.message || e}`,
+      text: `Backfill ${running ? 'stop' : 'start'} failed: ${e?.detail || e?.message || e}`,
       type: 'error',
     })
   }
diff --git a/frontend/src/stores/sources.js b/frontend/src/stores/sources.js
index 2e9406e..5eb90fe 100644
--- a/frontend/src/stores/sources.js
+++ b/frontend/src/stores/sources.js
@@ -85,11 +85,16 @@ export const useSourcesStore = defineStore('sources', () => {
     }
   }
 
-  // Plan #544: arm a source for backfill mode. The next `runs` download
-  // runs (default 3) walk gallery-dl's full post history instead of
-  // exiting early at the first contiguous archived block.
-  async function setBackfill(id, runs = 3, artistIdHint = null) {
-    const body = await api.post(`/api/sources/${id}/backfill`, { body: { runs } })
+  // Plan #693: start/stop a run-until-done backfill. 'start' walks the full
+  // post history in time-boxed chunks until it reaches the bottom (then the
+  // source shows backfill_state 'complete'); 'stop' cancels back to tick mode.
+  async function startBackfill(id, artistIdHint = null) {
+    const body = await api.post(`/api/sources/${id}/backfill`, { body: { action: 'start' } })
+    _invalidate(artistIdHint ?? body.artist_id)
+    return body
+  }
+  async function stopBackfill(id, artistIdHint = null) {
+    const body = await api.post(`/api/sources/${id}/backfill`, { body: { action: 'stop' } })
     _invalidate(artistIdHint ?? body.artist_id)
     return body
   }
@@ -120,7 +125,8 @@ export const useSourcesStore = defineStore('sources', () => {
     loadAll, loadForArtist,
     create, update, remove,
     checkNow,
-    setBackfill,
+    startBackfill,
+    stopBackfill,
     findOrCreateArtist, autocompleteArtist,
     loadScheduleStatus,
     sourcesByArtistGrouped,