diff --git a/src/main.rs b/src/main.rs
index 05959ae..4644a6d 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -2126,6 +2126,17 @@ fn process_new_files(
     // disabled (no Apollo integration configured) — Phase 3 wires this
     // up; the watcher remains usable on legacy deploys.
     if face_client.is_enabled() {
+        // Opportunistic content_hash backfill: photos indexed before
+        // content-hashing landed (or where the hash compute failed
+        // silently on insert) end up in image_exif with NULL
+        // content_hash. build_face_candidates keys on content_hash, so
+        // those files would never become candidates without backfill.
+        // Idempotent — subsequent scans see the populated hashes and
+        // no-op. The dedicated `backfill_hashes` binary is still the
+        // right tool for very large legacy libraries; this branch
+        // ensures small/medium deploys self-heal without operator
+        // action.
+        backfill_missing_content_hashes(&context, &files, library, &exif_dao);
         let candidates = build_face_candidates(&context, &files, &exif_dao, &face_dao);
         debug!(
             "face_watch: scan tick — {} image file(s) walked, {} candidate(s) (library '{}', modified_since={})",
@@ -2270,6 +2281,95 @@ fn process_new_files(
     }
 }
 
+/// Compute and persist content_hash for image_exif rows where it's NULL.
+///
+/// Bounded per call by `FACE_HASH_BACKFILL_MAX_PER_TICK` (default 500) so
+/// a watcher tick on a large legacy library doesn't block for hours
+/// blake3-ing every photo at once. Subsequent scans pick up the rest.
+/// For 50k+ libraries the dedicated `cargo run --bin backfill_hashes`
+/// is still faster (it doesn't fight a watcher loop for the DAO mutex).
+fn backfill_missing_content_hashes(
+    context: &opentelemetry::Context,
+    files: &[(PathBuf, String)],
+    library: &libraries::Library,
+    exif_dao: &Arc<Mutex<Box<dyn ExifDao>>>,
+) {
+    let image_paths: Vec<String> = files
+        .iter()
+        .filter(|(p, _)| !is_video_file(p))
+        .map(|(_, rel)| rel.clone())
+        .collect();
+    if image_paths.is_empty() {
+        return;
+    }
+
+    let exif_records = {
+        let mut dao = exif_dao.lock().expect("Unable to lock ExifDao");
+        dao.get_exif_batch(context, &image_paths)
+            .unwrap_or_default()
+    };
+    // Cheap lookup back from rel_path → absolute file_path so
+    // content_hash::compute can read the bytes.
+    let path_by_rel: HashMap<String, &PathBuf> =
+        files.iter().map(|(p, rel)| (rel.clone(), p)).collect();
+
+    let cap: usize = dotenv::var("FACE_HASH_BACKFILL_MAX_PER_TICK")
+        .ok()
+        .and_then(|s| s.parse().ok())
+        .filter(|n: &usize| *n > 0)
+        .unwrap_or(500);
+
+    let mut backfilled = 0usize;
+    let mut errors = 0usize;
+    for record in &exif_records {
+        if backfilled + errors >= cap {
+            break;
+        }
+        if record.content_hash.is_some() {
+            continue;
+        }
+        let Some(file_path) = path_by_rel.get(&record.file_path) else {
+            // Walked file went missing between the directory scan and now;
+            // next tick will retry naturally.
+            continue;
+        };
+        match content_hash::compute(file_path) {
+            Ok(id) => {
+                let mut dao = exif_dao.lock().expect("Unable to lock ExifDao");
+                if let Err(e) = dao.backfill_content_hash(
+                    context,
+                    library.id,
+                    &record.file_path,
+                    &id.content_hash,
+                    id.size_bytes,
+                ) {
+                    warn!(
+                        "face_watch: backfill_content_hash failed for {}: {:?}",
+                        record.file_path, e
+                    );
+                    errors += 1;
+                } else {
+                    backfilled += 1;
+                }
+            }
+            Err(e) => {
+                debug!(
+                    "face_watch: hash compute failed for {} ({:?})",
+                    file_path.display(),
+                    e
+                );
+                errors += 1;
+            }
+        }
+    }
+    if backfilled > 0 || errors > 0 {
+        info!(
+            "face_watch: backfilled content_hash for {} file(s) in library '{}' ({} error(s); cap={})",
+            backfilled, library.name, errors, cap
+        );
+    }
+}
+
 /// Build the face-detection candidate list for a scan tick.
 ///
 /// We need `(rel_path, content_hash)` for every image file that has a