feat(b3): ml-worker becomes optional — embed-only role, decoupled GPU coordination, cpu-embed switch

The ml-worker's ONLY processing role is now the CPU whole-image embed fallback
(tag_and_embed renamed embed_image — Camie tagging was retired #1189 and the
name kept implying otherwise; videos were already handled agent-style: frame
sampling + mean-pool). Detection/cropping/CCIP stay GPU-agent-only, and their
completion is judged per-pipeline: ccip by gpu_job rows, siglip by concept
regions at the current model version — never by image_record.siglip_embedding.
A CPU embed therefore can NEVER close crop work for the agent (regression test
pins this; only the whole-image 'embed' job, the same artifact, is satisfied).

Making removal actually safe (operator will drop the container):
- GPU-queue coordination (enqueue_gpu_backfill, recover_orphaned_gpu_jobs,
  reprocess_gpu_jobs) moved verbatim to tasks/gpu_queue.py on the maintenance
  quick lane — it lived on the 'ml' queue only by module colocation, which made
  the ml-worker a hard dependency of the whole agent pipeline.
- New ml_settings.cpu_embed_enabled (migration 0074, default ON so agent-less
  installs keep working): OFF stops the four import hooks queueing embed work
  nothing will consume and no-ops the manual backfill; switch lives on the
  renamed 'CPU embedding backfill' card.
- NB heads training / auto-apply still run on the ml image (sklearn) — a stack
  that removes the container gives those up too.

Deploy note: in-flight messages under the old task names are dropped by the
new workers; the 60s orphan sweep + hourly backfill re-fire under the new
names immediately.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

alembic/versions/0074_ml_settings_cpu_embed_enabled.py

@@ -0,0 +1,35 @@
+"""ml_settings.cpu_embed_enabled — the CPU embed fallback becomes a switch
+
+B3 (operator 2026-07-02): the ml-worker's only processing role is the CPU
+whole-image embed for stacks without a GPU agent. ON by default (a fresh
+install works agent-less); agent-equipped stacks that drop the ml-worker
+container turn it off so import hooks stop queueing embed work into a queue
+nothing consumes — the daily GPU 'embed' backfill covers those images.
+
+Revision ID: 0074
+Revises: 0073
+Create Date: 2026-07-02
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0074"
+down_revision: Union[str, None] = "0073"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "ml_settings",
+        sa.Column(
+            "cpu_embed_enabled", sa.Boolean(), nullable=False,
+            server_default=sa.true(),
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("ml_settings", "cpu_embed_enabled")

backend/app/api/gpu.py

@@ -96,7 +96,7 @@ async def backfill():
     """Enqueue a job for every image that doesn't already have one for `task`."""
     body = await request.get_json(silent=True) or {}
     task = str(body.get("task") or "ccip")
-    from ..tasks.ml import enqueue_gpu_backfill
+    from ..tasks.gpu_queue import enqueue_gpu_backfill
 
     r = enqueue_gpu_backfill.delay(task)
     return jsonify({"celery_task_id": r.id, "task": task}), 202
@@ -109,7 +109,7 @@ async def reprocess():
     detectors). Heavy — the back-catalogue is otherwise skipped by the backfills."""
     body = await request.get_json(silent=True) or {}
     task = str(body.get("task") or "ccip")
-    from ..tasks.ml import reprocess_gpu_jobs
+    from ..tasks.gpu_queue import reprocess_gpu_jobs
 
     r = reprocess_gpu_jobs.delay(task)
     return jsonify({"celery_task_id": r.id, "task": task}), 202

backend/app/api/ml_admin.py

@@ -9,6 +9,7 @@ ml_admin_bp = Blueprint("ml_admin", __name__, url_prefix="/api/ml")
 
 
 _EDITABLE = (
+    "cpu_embed_enabled",
     "video_frame_interval_seconds",
     "video_max_frames",
     "head_min_positives",
@@ -63,6 +64,7 @@ async def get_settings():
         ).scalar_one()
     return jsonify(
         {
+            "cpu_embed_enabled": s.cpu_embed_enabled,
             "video_frame_interval_seconds": s.video_frame_interval_seconds,
             "video_max_frames": s.video_max_frames,
             "embedder_model_version": s.embedder_model_version,

backend/app/celery_app.py

@@ -29,6 +29,7 @@ def make_celery() -> Celery:
             "backend.app.tasks.thumbnail",
             "backend.app.tasks.maintenance",
             "backend.app.tasks.ml",
+            "backend.app.tasks.gpu_queue",
             "backend.app.tasks.download",
             "backend.app.tasks.external",
             "backend.app.tasks.backup",
@@ -41,6 +42,11 @@ def make_celery() -> Celery:
         task_routes={
             "backend.app.tasks.import_file.*": {"queue": "import"},
             "backend.app.tasks.ml.*": {"queue": "ml"},
+            # GPU-queue coordination (backfill enqueues, orphan recovery,
+            # reprocess) is pure DB work — it rides the maintenance quick lane
+            # so the GPU agent pipeline works even on stacks that drop the
+            # (now-optional, B3) ml-worker container entirely.
+            "backend.app.tasks.gpu_queue.*": {"queue": "maintenance"},
             "backend.app.tasks.thumbnail.*": {"queue": "thumbnail"},
             "backend.app.tasks.download.*": {"queue": "download"},
             # External file-host fetches are downloads — same lane (they can run
@@ -106,7 +112,7 @@ def make_celery() -> Celery:
                 "schedule": 86400.0,  # no-op unless head_auto_apply_enabled
             },
             "recover-orphaned-gpu-jobs": {
-                "task": "backend.app.tasks.ml.recover_orphaned_gpu_jobs",
+                "task": "backend.app.tasks.gpu_queue.recover_orphaned_gpu_jobs",
                 "schedule": 60.0,  # quick pickup of work a dead agent orphaned
             },
             "triage-gpu-errors": {
@@ -114,17 +120,17 @@ def make_celery() -> Celery:
                 "schedule": 900.0,  # probe errored jobs' files → defect/file_ok
             },
             "enqueue-ccip-backfill-hourly": {
-                "task": "backend.app.tasks.ml.enqueue_gpu_backfill",
+                "task": "backend.app.tasks.gpu_queue.enqueue_gpu_backfill",
                 "schedule": 3600.0,  # auto-feed NEW images; errored are
                 "args": ("ccip",),   # tombstoned — retry is the button only
             },
             "enqueue-siglip-backfill-daily": {
-                "task": "backend.app.tasks.ml.enqueue_gpu_backfill",
+                "task": "backend.app.tasks.gpu_queue.enqueue_gpu_backfill",
                 "schedule": 86400.0,  # drain the concept-crop back-catalogue
                 "args": ("siglip",),  # (errored are tombstoned, not retried)
             },
             "enqueue-embed-backfill-daily": {
-                "task": "backend.app.tasks.ml.enqueue_gpu_backfill",
+                "task": "backend.app.tasks.gpu_queue.enqueue_gpu_backfill",
                 "schedule": 86400.0,  # whole-image re-embed under the current
                 "args": ("embed",),   # model (an operator swap) drains via agent
             },

backend/app/models/ml_settings.py

@@ -23,6 +23,15 @@ class MLSettings(Base):
     __table_args__ = (CheckConstraint("id = 1", name="singleton"),)
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    # CPU whole-image embedding (B3, operator 2026-07-02). The ml-worker's ONLY
+    # processing role is the embed fallback for stacks WITHOUT a GPU agent — ON
+    # by default so a fresh install works with no agent. Stacks that run the
+    # agent and drop the ml-worker container turn this OFF so import hooks stop
+    # queueing embed work nothing will consume (the daily GPU 'embed' backfill
+    # covers those images instead).
+    cpu_embed_enabled: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=True
+    )
     # Video embedding (#747). Sample one frame every N seconds (fixed CADENCE, not
     # a fixed count) so coverage reflects real screen time regardless of length;
     # cap the total so a long video can't explode into hundreds of embeds. The

backend/app/services/cleanup_service.py

@@ -1008,7 +1008,7 @@ def reextract_archive_attachments(
     still an archive on disk, so the cursor is what guarantees forward progress.
     """
     from ..models import ImportSettings, Post, PostAttachment, Source
-    from ..tasks.ml import tag_and_embed
+    from ..tasks.ml import cpu_embed_enabled, embed_image
     from ..tasks.thumbnail import generate_thumbnail
     from .archive_extractor import is_archive
     from .importer import Importer
@@ -1089,10 +1089,12 @@ def reextract_archive_attachments(
 
     # Thumbnails + ML for the newly-imported members (best-effort; off the
     # critical path — a Redis hiccup must not fail the whole re-extract).
+    do_embed = cpu_embed_enabled()
     for img_id in enqueue_ids:
         try:
             generate_thumbnail.delay(img_id)
-            tag_and_embed.delay(img_id)
+            if do_embed:
+                embed_image.delay(img_id)
         except Exception as exc:
             log.warning("re-extract enqueue failed for image %s: %s", img_id, exc)
     return summary

backend/app/services/download_service.py

@@ -326,14 +326,16 @@ class DownloadService:
                 # for hours after a download landed. Lazy import to avoid
                 # circular-import risk between this service and the
                 # tasks/* modules that import it.
-                from ..tasks.ml import tag_and_embed
+                from ..tasks.ml import cpu_embed_enabled, embed_image
                 from ..tasks.thumbnail import generate_thumbnail
+                do_embed = cpu_embed_enabled()
                 ids = list(result.member_image_ids)
                 if result.image_id is not None and result.image_id not in ids:
                     ids.append(result.image_id)
                 for img_id in ids:
                     generate_thumbnail.delay(img_id)
-                    tag_and_embed.delay(img_id)
+                    if do_embed:
+                        embed_image.delay(img_id)
             elif result.status == "attached":
                 # Non-media or extracted archive captured as PostAttachment
                 # (FC-2d-iii). The canonical copy lives in the attachments

backend/app/tasks/external.py

@@ -216,11 +216,13 @@ def fetch_external_link(self, link_id: int, _serialize_waits: int = 0) -> dict:
         # Thumbnails + ML for any newly-attached images (mirrors the download
         # path). Lazy import to dodge a task-module import cycle.
         if image_ids:
-            from .ml import tag_and_embed
+            from .ml import cpu_embed_enabled, embed_image
             from .thumbnail import generate_thumbnail
+            do_embed = cpu_embed_enabled()
             for img_id in image_ids:
                 generate_thumbnail.delay(img_id)
-                tag_and_embed.delay(img_id)
+                if do_embed:
+                    embed_image.delay(img_id)
         return {"link_id": link_id, "files": len(result.files), "images": len(image_ids)}
     except Exception as exc:  # never leave a link stuck in 'downloading'
         log.exception("external fetch task failed for link %s", link_id)

backend/app/tasks/gpu_queue.py

@@ -0,0 +1,171 @@
+"""GPU-job queue coordination: backfill enqueues, orphan recovery, reprocess.
+
+These are pure-DB sweeps (INSERT…SELECT / UPDATE) — no torch, no sklearn —
+that keep the desktop GPU agent's work queue fed and self-healing. They lived
+in tasks/ml.py (routed to the 'ml' queue) purely by colocation, which made the
+ml-worker container a hard dependency of the GPU pipeline; under B3 the
+ml-worker is OPTIONAL (its only processing role is the CPU embed fallback), so
+these moved here and route to the 'maintenance' quick lane with the other
+recovery sweeps. A stack with no ml-worker keeps a fully-working GPU pipeline.
+"""
+
+import logging
+
+from sqlalchemy import select
+
+from ..celery_app import celery
+from ._sync_engine import sync_session_factory as _sync_session_factory
+
+log = logging.getLogger(__name__)
+
+
+@celery.task(name="backend.app.tasks.gpu_queue.enqueue_gpu_backfill")
+def enqueue_gpu_backfill(task_name: str) -> int:
+    """Enqueue a gpu_job for every image that still needs `task_name` (one
+    INSERT…SELECT, so it scales to a full library). The desktop agent drains the
+    queue over HTTP. Returns the number enqueued.
+
+    Completion is judged PER PIPELINE, never across them (B3, operator
+    2026-07-02): 'ccip' by prior gpu_job rows, 'siglip' by concept regions at
+    the current model version, and only 'embed' by image_record's whole-image
+    embedding — the one artifact the CPU fallback also produces. A CPU embed
+    therefore never closes crop/detect work for the agent.
+
+    An ERRORED job is a tombstone for its (image, task): no variant re-enqueues
+    it. Retry is deliberate-only (/retry_errors), which also means an errored
+    back-catalogue needs one "Retry errored jobs" press after a model swap.
+    Before the tombstone rule, this loop re-minted a fresh doomed job for every
+    permanently-bad file each run — ~24 duplicate error rows/day per file (the
+    2026-07-02 "unprocessable" flood)."""
+    from sqlalchemy import exists, insert, literal, or_
+    from sqlalchemy import select as sa_select
+
+    from ..models import GpuJob, ImageRecord, ImageRegion, MLSettings
+    from ..services.ml.gpu_jobs import error_dedupe_statements
+
+    SessionLocal = _sync_session_factory()
+    with SessionLocal() as session:
+        # Prune stale tombstones first (loop-era duplicates + rows made moot by
+        # a later success), so 'error' reads as one row per distinct failing
+        # file and the skip-guards below see a clean picture.
+        pruned = sum(
+            session.execute(s).rowcount or 0 for s in error_dedupe_statements()
+        )
+        if pruned:
+            log.info("gpu backfill: pruned %d stale/duplicate error rows", pruned)
+        cur_version = session.execute(
+            select(MLSettings.embedder_model_version).where(MLSettings.id == 1)
+        ).scalar_one()
+        if task_name == "embed":
+            # Whole-image GPU re-embed (#1190): images with no embedding, or one
+            # stamped under a DIFFERENT model version (an operator model swap).
+            stale = or_(
+                ImageRecord.siglip_embedding.is_(None),
+                ImageRecord.siglip_model_version.is_(None),
+                ImageRecord.siglip_model_version != cur_version,
+            )
+            # 'error' blocks too — tombstone rule, see docstring.
+            blocked = exists().where(
+                GpuJob.image_record_id == ImageRecord.id,
+                GpuJob.task == "embed",
+                GpuJob.status.in_(["pending", "leased", "error"]),
+            )
+            sel = sa_select(
+                ImageRecord.id, literal("embed"), literal("pending")
+            ).where(stale).where(~blocked)
+        elif task_name == "siglip":
+            # Concept-crop re-embed: enqueue when there's no concept region AT THE
+            # CURRENT model version — so a model swap re-triggers crops too, not
+            # only the never-embedded back-catalogue.
+            has_current_concept = exists().where(
+                ImageRegion.image_record_id == ImageRecord.id,
+                ImageRegion.kind == "concept",
+                ImageRegion.embedding_version == cur_version,
+            )
+            # 'error' blocks too — tombstone rule, see docstring.
+            blocked = exists().where(
+                GpuJob.image_record_id == ImageRecord.id,
+                GpuJob.task == "siglip",
+                GpuJob.status.in_(["pending", "leased", "error"]),
+            )
+            sel = sa_select(
+                ImageRecord.id, literal("siglip"), literal("pending")
+            ).where(~has_current_concept).where(~blocked)
+        else:
+            # ANY prior row blocks — including 'error' (tombstone rule, see
+            # docstring): pre-fix this branch ran HOURLY and was the loop.
+            already = exists().where(
+                GpuJob.image_record_id == ImageRecord.id,
+                GpuJob.task == task_name,
+                GpuJob.status.in_(["pending", "leased", "done", "error"]),
+            )
+            sel = sa_select(
+                ImageRecord.id, literal(task_name), literal("pending")
+            ).where(~already)
+        # RETURNING + count: result.rowcount is unreliable for INSERT…SELECT.
+        rows = session.execute(
+            insert(GpuJob)
+            .from_select(["image_record_id", "task", "status"], sel)
+            .returning(GpuJob.id)
+        ).fetchall()
+        session.commit()
+        return len(rows)
+
+
+@celery.task(name="backend.app.tasks.gpu_queue.recover_orphaned_gpu_jobs")
+def recover_orphaned_gpu_jobs() -> int:
+    """Reset expired GPU-job leases back to pending — recovers work orphaned by an
+    agent that died mid-job (no graceful release) — and convert poison-loopers
+    (release/expiry cycles that never reach fail()'s attempt cap) to 'error'.
+    Statements are shared with GpuJobService.recover_orphaned so the sweep and
+    the service can't drift. Short beat cadence so orphans get picked back up
+    quickly + the queue counts read honestly. Returns the number recovered."""
+    from datetime import UTC, datetime
+
+    from ..services.ml.gpu_jobs import recover_statements
+
+    SessionLocal = _sync_session_factory()
+    with SessionLocal() as session:
+        counts = {
+            name: session.execute(stmt).rowcount or 0
+            for name, stmt in recover_statements(datetime.now(UTC)).items()
+        }
+        session.commit()
+        if counts["poison_expired"] or counts["poison_pending"]:
+            log.warning(
+                "gpu jobs poisoned -> error: %d crash-loop (expired lease), "
+                "%d never-complete (pending)",
+                counts["poison_expired"], counts["poison_pending"],
+            )
+        return counts["recovered"]
+
+
+@celery.task(name="backend.app.tasks.gpu_queue.reprocess_gpu_jobs")
+def reprocess_gpu_jobs(task_name: str = "ccip") -> int:
+    """Reset every done/error job of `task_name` back to pending so the agent
+    re-runs the WHOLE library under the CURRENT pipeline — e.g. after adding crop
+    detectors (#1202), re-cropping existing images. Heavy + operator-triggered;
+    the back-catalogue won't otherwise re-process (the backfills skip images that
+    already have current-version regions). Returns the number reset."""
+    from datetime import UTC, datetime
+
+    from sqlalchemy import update
+
+    from ..models import GpuJob
+
+    SessionLocal = _sync_session_factory()
+    with SessionLocal() as session:
+        now = datetime.now(UTC)
+        res = session.execute(
+            update(GpuJob)
+            .where(
+                GpuJob.task == task_name,
+                GpuJob.status.in_(["done", "error"]),
+            )
+            .values(
+                status="pending", attempts=0, lease_token=None, leased_at=None,
+                lease_expires_at=None, updated_at=now,
+            )
+        )
+        session.commit()
+        return res.rowcount or 0

backend/app/tasks/import_file.py

@@ -228,15 +228,17 @@ def _do_import(session, task, import_task_id: int) -> dict:
     # Enqueue thumbnail + ML for newly imported AND superseded images
     # (a superseded row has cleared ML + no thumbnail).
     if result.status in ("imported", "superseded"):
-        from .ml import tag_and_embed
+        from .ml import cpu_embed_enabled, embed_image
         from .thumbnail import generate_thumbnail
 
+        do_embed = cpu_embed_enabled()
         ids = list(result.member_image_ids)
         if result.image_id is not None and result.image_id not in ids:
             ids.append(result.image_id)
         for img_id in ids:
             generate_thumbnail.delay(img_id)
-            tag_and_embed.delay(img_id)
+            if do_embed:
+                embed_image.delay(img_id)
 
     # If this was the last task in the batch, mark the batch complete.
     remaining = session.execute(

backend/app/tasks/maintenance.py

@@ -121,7 +121,7 @@ IMPORT_BATCH_KEEP_DAYS = 30
 # task.time_limit + a small buffer. task_name overrides take precedence
 # over queue overrides.
 #
-# ml queue: tag_and_embed video branch (≈20 GPU ops); time_limit=1200.
+# ml queue: embed_image video branch (≈20 GPU ops); time_limit=1200.
 # import_archive_file: shares the 'import' queue with the fast
 #   single-file import_media_file, so it needs a task-name override
 #   (the import queue itself stays at the 5-min default for single

backend/app/tasks/ml.py

@@ -1,8 +1,15 @@
 """ML Celery tasks: per-image embedding, backfill discovery, head training,
 model self-heal.
 
-All run on the ml-worker (queue 'ml'). Sync sessions (Celery workers are sync
-processes), same pattern as FC-2a tasks.
+All run on the ml-worker (queue 'ml'), which under B3 (2026-07-02) is an
+OPTIONAL container: its only processing role is the CPU whole-image embed
+fallback (gated by ml_settings.cpu_embed_enabled) for stacks without a GPU
+agent — plus head training / auto-apply, which need sklearn/numpy and so
+live on this image. GPU-queue coordination (backfill enqueues, orphan
+recovery, reprocess) deliberately does NOT live here — see tasks/gpu_queue.py
+(maintenance lane), so the agent pipeline works with no ml-worker at all.
+Sync sessions (Celery workers are sync processes), same pattern as FC-2a
+tasks.
 """
 
 import logging
@@ -26,8 +33,24 @@ def _is_video(path: Path) -> bool:
     return path.suffix.lower() in VIDEO_EXTS
 
 
+def cpu_embed_enabled() -> bool:
+    """Dispatch gate for the CPU embed fallback (B3, operator 2026-07-02):
+    stacks that run a GPU agent and DROP the (optional) ml-worker container
+    turn ml_settings.cpu_embed_enabled off, so the import hooks stop queueing
+    embed work into a queue nothing consumes — the daily GPU 'embed' backfill
+    covers those images instead. Opens its own short session because the four
+    dispatch sites sit in different session scopes; defaults ON when the
+    settings row is missing (a fresh install must work agent-less)."""
+    SessionLocal = _sync_session_factory()
+    with SessionLocal() as session:
+        val = session.execute(
+            select(MLSettings.cpu_embed_enabled).where(MLSettings.id == 1)
+        ).scalar_one_or_none()
+    return True if val is None else bool(val)
+
+
 @celery.task(
-    name="backend.app.tasks.ml.tag_and_embed",
+    name="backend.app.tasks.ml.embed_image",
     bind=True,
     autoretry_for=(OperationalError, DBAPIError, OSError),
     retry_backoff=5,
@@ -44,13 +67,21 @@ def _is_video(path: Path) -> bool:
     soft_time_limit=900,   # 15 min
     time_limit=1200,       # 20 min hard
 )
-def tag_and_embed(self, image_id: int) -> dict:
-    """Compute + store one image's SigLIP embedding.
+def embed_image(self, image_id: int) -> dict:
+    """Compute + store one image's whole-image SigLIP embedding — the CPU
+    fallback path (B3, operator 2026-07-02): this is the ml-worker's ONLY
+    processing role, keeping search/similarity/head-suggestions alive on
+    deployments without a GPU agent. Detection, cropping and CCIP are
+    deliberately agent-only, and their backfill predicates read image_region /
+    gpu_job state — never image_record.siglip_embedding — so a CPU whole-image
+    embed can NEVER mark crop work as done. (Renamed from tag_and_embed —
+    Camie tagging was retired #1189; the old name kept implying a tagging step
+    that no longer exists.)
 
     Video (#747): sample frames at a fixed cadence (ml_settings
     video_frame_interval_seconds, capped at video_max_frames) and mean-pool the
-    per-frame SigLIP embeddings. On no-frames returns status='no_frames' (not an
-    error). (Camie tagging was retired #1189 — heads + CCIP are the tag source.)
+    per-frame SigLIP embeddings — the same shape as the GPU agent's video
+    handling. On no-frames returns status='no_frames' (not an error).
     """
     import time
 
@@ -84,9 +115,9 @@ def tag_and_embed(self, image_id: int) -> dict:
                 f"image_id={image_id} path={record.path} mime={record.mime} "
                 f"bytes={record.size_bytes} video={is_vid}"
             )
-            log.info("tag_and_embed start: %s", ctx)
+            log.info("embed_image start: %s", ctx)
             if not src.is_file():
-                log.warning("tag_and_embed file missing on disk: %s", ctx)
+                log.warning("embed_image file missing on disk: %s", ctx)
                 return {"status": "file_missing", "image_id": image_id}
 
             phase = "load_models"
@@ -102,7 +133,7 @@ def tag_and_embed(self, image_id: int) -> dict:
                 vprobe = safe_probe.probe_video(src)
                 if not vprobe.ok:
                     log.warning(
-                        "tag_and_embed bad video (%s): %s", vprobe.reason, ctx
+                        "embed_image bad video (%s): %s", vprobe.reason, ctx
                     )
                     return {
                         "status": "bad_video", "image_id": image_id,
@@ -130,7 +161,7 @@ def tag_and_embed(self, image_id: int) -> dict:
                 t0 = time.monotonic()
                 embedding = embedder.infer(src)
                 log.info(
-                    "tag_and_embed embedded in %.1fs: %s",
+                    "embed_image embedded in %.1fs: %s",
                     time.monotonic() - t0, ctx,
                 )
 
@@ -141,7 +172,7 @@ def tag_and_embed(self, image_id: int) -> dict:
             session.commit()
     except SoftTimeLimitExceeded:
         log.error(
-            "tag_and_embed TIMED OUT after %.0fs in phase=%s: %s",
+            "embed_image TIMED OUT after %.0fs in phase=%s: %s",
             _elapsed(), phase, ctx,
         )
         # Re-raise as SoftTimeLimitExceeded (preserves the 'timeout' status in
@@ -155,12 +186,12 @@ def tag_and_embed(self, image_id: int) -> dict:
         # ORIGINAL so the type is preserved; just make sure it's logged with
         # context first.
         log.exception(
-            "tag_and_embed FAILED in phase=%s after %.0fs: %s",
+            "embed_image FAILED in phase=%s after %.0fs: %s",
             phase, _elapsed(), ctx,
         )
         raise
 
-    log.info("tag_and_embed ok in %.1fs: %s", _elapsed(), ctx)
+    log.info("embed_image ok in %.1fs: %s", _elapsed(), ctx)
     return {"status": "ok", "image_id": image_id}
 
 
@@ -222,13 +253,17 @@ def _sample_video_frames(
 
 @celery.task(name="backend.app.tasks.ml.backfill", bind=True)
 def backfill(self) -> int:
-    """Enqueue tag_and_embed (embed-only) for images with no SigLIP embedding.
+    """Enqueue embed_image (embed-only) for images with no SigLIP embedding.
     Keyset pagination by id ASC (restart-safe).
 
     NB: a siglip MODEL-VERSION mismatch (an operator model swap, #1190) is NOT
     re-embedded here — the CPU ml-worker can't churn the library at 384/512px;
     the GPU agent owns version re-embeds via the 'embed' job.
     """
+    if not cpu_embed_enabled():
+        log.info("cpu backfill skipped: cpu_embed_enabled is off (B3 — the "
+                 "GPU 'embed' backfill owns whole-image embeds on this stack)")
+        return 0
     SessionLocal = _sync_session_factory()
     enqueued = 0
     last_id = 0
@@ -244,7 +279,7 @@ def backfill(self) -> int:
             if not rows:
                 break
             for image_id in rows:
-                tag_and_embed.delay(image_id)
+                embed_image.delay(image_id)
                 enqueued += 1
             last_id = rows[-1]
     return enqueued
@@ -405,156 +440,6 @@ def scheduled_apply_head_tags() -> str:
     return "dispatched"
 
 
-@celery.task(name="backend.app.tasks.ml.enqueue_gpu_backfill")
-def enqueue_gpu_backfill(task_name: str) -> int:
-    """Enqueue a gpu_job for every image that still needs `task_name` (one
-    INSERT…SELECT, so it scales to a full library). The desktop agent drains the
-    queue over HTTP. Returns the number enqueued.
-
-    'siglip' gates on the RESULT (no concept region yet) rather than on a prior
-    job, so it picks up the back-catalogue of images that were CCIP-embedded
-    before concept crops existed — without re-touching their figure/CCIP regions.
-
-    An ERRORED job is a tombstone for its (image, task): no variant re-enqueues
-    it. Retry is deliberate-only (/retry_errors), which also means an errored
-    back-catalogue needs one "Retry errored jobs" press after a model swap.
-    Before the tombstone rule, this loop re-minted a fresh doomed job for every
-    permanently-bad file each run — ~24 duplicate error rows/day per file (the
-    2026-07-02 "unprocessable" flood)."""
-    from sqlalchemy import exists, insert, literal, or_
-    from sqlalchemy import select as sa_select
-
-    from ..models import GpuJob, ImageRecord, ImageRegion, MLSettings
-    from ..services.ml.gpu_jobs import error_dedupe_statements
-
-    SessionLocal = _sync_session_factory()
-    with SessionLocal() as session:
-        # Prune stale tombstones first (loop-era duplicates + rows made moot by
-        # a later success), so 'error' reads as one row per distinct failing
-        # file and the skip-guards below see a clean picture.
-        pruned = sum(
-            session.execute(s).rowcount or 0 for s in error_dedupe_statements()
-        )
-        if pruned:
-            log.info("gpu backfill: pruned %d stale/duplicate error rows", pruned)
-        cur_version = session.execute(
-            select(MLSettings.embedder_model_version).where(MLSettings.id == 1)
-        ).scalar_one()
-        if task_name == "embed":
-            # Whole-image GPU re-embed (#1190): images with no embedding, or one
-            # stamped under a DIFFERENT model version (an operator model swap).
-            stale = or_(
-                ImageRecord.siglip_embedding.is_(None),
-                ImageRecord.siglip_model_version.is_(None),
-                ImageRecord.siglip_model_version != cur_version,
-            )
-            # 'error' blocks too — tombstone rule, see docstring.
-            blocked = exists().where(
-                GpuJob.image_record_id == ImageRecord.id,
-                GpuJob.task == "embed",
-                GpuJob.status.in_(["pending", "leased", "error"]),
-            )
-            sel = sa_select(
-                ImageRecord.id, literal("embed"), literal("pending")
-            ).where(stale).where(~blocked)
-        elif task_name == "siglip":
-            # Concept-crop re-embed: enqueue when there's no concept region AT THE
-            # CURRENT model version — so a model swap re-triggers crops too, not
-            # only the never-embedded back-catalogue.
-            has_current_concept = exists().where(
-                ImageRegion.image_record_id == ImageRecord.id,
-                ImageRegion.kind == "concept",
-                ImageRegion.embedding_version == cur_version,
-            )
-            # 'error' blocks too — tombstone rule, see docstring.
-            blocked = exists().where(
-                GpuJob.image_record_id == ImageRecord.id,
-                GpuJob.task == "siglip",
-                GpuJob.status.in_(["pending", "leased", "error"]),
-            )
-            sel = sa_select(
-                ImageRecord.id, literal("siglip"), literal("pending")
-            ).where(~has_current_concept).where(~blocked)
-        else:
-            # ANY prior row blocks — including 'error' (tombstone rule, see
-            # docstring): pre-fix this branch ran HOURLY and was the loop.
-            already = exists().where(
-                GpuJob.image_record_id == ImageRecord.id,
-                GpuJob.task == task_name,
-                GpuJob.status.in_(["pending", "leased", "done", "error"]),
-            )
-            sel = sa_select(
-                ImageRecord.id, literal(task_name), literal("pending")
-            ).where(~already)
-        # RETURNING + count: result.rowcount is unreliable for INSERT…SELECT.
-        rows = session.execute(
-            insert(GpuJob)
-            .from_select(["image_record_id", "task", "status"], sel)
-            .returning(GpuJob.id)
-        ).fetchall()
-        session.commit()
-        return len(rows)
-
-
-@celery.task(name="backend.app.tasks.ml.recover_orphaned_gpu_jobs")
-def recover_orphaned_gpu_jobs() -> int:
-    """Reset expired GPU-job leases back to pending — recovers work orphaned by an
-    agent that died mid-job (no graceful release) — and convert poison-loopers
-    (release/expiry cycles that never reach fail()'s attempt cap) to 'error'.
-    Statements are shared with GpuJobService.recover_orphaned so the sweep and
-    the service can't drift. Short beat cadence so orphans get picked back up
-    quickly + the queue counts read honestly. Returns the number recovered."""
-    from datetime import UTC, datetime
-
-    from ..services.ml.gpu_jobs import recover_statements
-
-    SessionLocal = _sync_session_factory()
-    with SessionLocal() as session:
-        counts = {
-            name: session.execute(stmt).rowcount or 0
-            for name, stmt in recover_statements(datetime.now(UTC)).items()
-        }
-        session.commit()
-        if counts["poison_expired"] or counts["poison_pending"]:
-            log.warning(
-                "gpu jobs poisoned -> error: %d crash-loop (expired lease), "
-                "%d never-complete (pending)",
-                counts["poison_expired"], counts["poison_pending"],
-            )
-        return counts["recovered"]
-
-
-@celery.task(name="backend.app.tasks.ml.reprocess_gpu_jobs")
-def reprocess_gpu_jobs(task_name: str = "ccip") -> int:
-    """Reset every done/error job of `task_name` back to pending so the agent
-    re-runs the WHOLE library under the CURRENT pipeline — e.g. after adding crop
-    detectors (#1202), re-cropping existing images. Heavy + operator-triggered;
-    the back-catalogue won't otherwise re-process (the backfills skip images that
-    already have current-version regions). Returns the number reset."""
-    from datetime import UTC, datetime
-
-    from sqlalchemy import update
-
-    from ..models import GpuJob
-
-    SessionLocal = _sync_session_factory()
-    with SessionLocal() as session:
-        now = datetime.now(UTC)
-        res = session.execute(
-            update(GpuJob)
-            .where(
-                GpuJob.task == task_name,
-                GpuJob.status.in_(["done", "error"]),
-            )
-            .values(
-                status="pending", attempts=0, lease_token=None, leased_at=None,
-                lease_expires_at=None, updated_at=now,
-            )
-        )
-        session.commit()
-        return res.rowcount or 0
-
-
 @celery.task(
     name="backend.app.tasks.ml.scheduled_ccip_auto_apply",
     soft_time_limit=1800, time_limit=2100,

frontend/src/components/settings/MLBackfillCard.vue

@@ -1,16 +1,33 @@
 <template>
   <MaintenanceTile
     icon="mdi-refresh"
-    title="ML backfill"
-    blurb="Compute SigLIP embeddings on images missing them."
+    title="CPU embedding backfill"
+    blurb="Whole-image embeddings without a GPU agent — the built-in fallback."
     :open="busy"
   >
       <p class="text-body-2 mb-3">
-        Compute the SigLIP embedding for any image that doesn't have one yet
-        (CPU). Safe to re-run. To re-embed under a NEW model, use the GPU
-        agent's "Re-embed library" instead.
+        Computes the whole-image SigLIP embedding for anything missing one —
+        images directly, videos by sampling frames (the same approach as the
+        GPU agent). Runs on the ml-worker's CPU, so search, similarity and
+        head suggestions work <strong>without</strong> a GPU agent; new imports
+        are embedded this way automatically. Detection, cropping and character
+        (CCIP) embeddings are GPU-agent-only. Safe to re-run. To re-embed under
+        a NEW model, use the GPU agent's "Re-embed library" instead.
       </p>
-      <v-btn color="primary" rounded="pill" :loading="busy" @click="run">
+      <v-switch
+        v-model="enabled" color="accent" hide-details density="compact"
+        :loading="saving" label="CPU embedding enabled"
+        class="mb-1" @update:model-value="onToggle"
+      />
+      <p class="fc-muted text-caption mb-3">
+        Turn OFF if you run the GPU agent and removed the ml-worker container —
+        imports then stop queueing CPU embed work nothing will consume (the
+        daily GPU embed backfill covers those images instead).
+      </p>
+      <v-btn
+        color="primary" rounded="pill" :loading="busy" :disabled="!enabled"
+        @click="run"
+      >
         <v-icon start>mdi-refresh</v-icon> Run backfill now
       </v-btn>
       <span v-if="done" class="ml-3 text-caption">Enqueued.</span>
@@ -20,13 +37,40 @@
 
 <script setup>
 import { toast } from '../../utils/toast.js'
-import { ref } from 'vue'
+import { onMounted, ref } from 'vue'
 import { useMLStore } from '../../stores/ml.js'
 import MaintenanceTile from '../common/MaintenanceTile.vue'
 import QueueStatusBar from './QueueStatusBar.vue'
 const store = useMLStore()
 const busy = ref(false)
 const done = ref(false)
+const enabled = ref(true)
+const saving = ref(false)
+onMounted(async () => {
+  try {
+    await store.loadSettings()
+    if (store.settings?.cpu_embed_enabled != null) {
+      enabled.value = store.settings.cpu_embed_enabled
+    }
+  } catch { /* non-fatal */ }
+})
+async function onToggle() {
+  saving.value = true
+  try {
+    await store.patchSettings({ cpu_embed_enabled: enabled.value })
+    toast({
+      text: enabled.value
+        ? 'CPU embedding on — imports queue embeds for the ml-worker'
+        : 'CPU embedding off — the GPU embed backfill owns whole-image embeds',
+      type: 'success',
+    })
+  } catch (e) {
+    toast({ text: `Could not save: ${e.message}`, type: 'error' })
+    enabled.value = !enabled.value
+  } finally {
+    saving.value = false
+  }
+}
 async function run() {
   busy.value = true
   try { await store.triggerBackfill(); done.value = true }
@@ -34,3 +78,7 @@ async function run() {
   finally { busy.value = false }
 }
 </script>
+
+<style scoped>
+.fc-muted { color: rgb(var(--v-theme-on-surface-variant)); }
+</style>

tests/test_api_gpu.py

@@ -127,7 +127,7 @@ async def test_backfill_enqueues_then_is_idempotent(db):
     await _img(db, "c" * 64)
     await _img(db, "d" * 64)
     await db.commit()
-    from backend.app.tasks.ml import enqueue_gpu_backfill
+    from backend.app.tasks.gpu_queue import enqueue_gpu_backfill
 
     n = enqueue_gpu_backfill("ccip")     # sync task, own session
     assert n >= 2
@@ -260,3 +260,33 @@ async def test_errors_endpoint_reports_triage_view(client, db):
     assert item["reason_class"] == "truncated_or_corrupt"
     assert item["triage_status"] is None
     assert item["image_url"].startswith("/images/")
+
+
+@pytest.mark.asyncio
+async def test_cpu_embed_never_blocks_gpu_crop_backfills(db):
+    """B3 invariant (operator 2026-07-02): ccip (detect + character) and
+    siglip (concept crops) completion is judged per-pipeline — gpu_job rows and
+    image_region state — never inferred from image_record.siglip_embedding. So
+    an image the CPU fallback already embedded still gets both crop jobs; only
+    the whole-image 'embed' job (the SAME artifact the CPU path produces) is
+    satisfied by it."""
+    from backend.app.models import MLSettings
+    from backend.app.tasks.gpu_queue import enqueue_gpu_backfill
+
+    img = await _img(db, "7" * 64)
+    cur = (await db.execute(
+        select(MLSettings.embedder_model_version).where(MLSettings.id == 1)
+    )).scalar_one()
+    # As if the CPU fallback already embedded it under the current model.
+    img.siglip_embedding = [0.1] * 1152
+    img.siglip_model_version = cur
+    await db.commit()
+
+    assert enqueue_gpu_backfill("ccip") == 1      # crops still open
+    assert enqueue_gpu_backfill("siglip") == 1    # concept crops still open
+    assert enqueue_gpu_backfill("embed") == 0     # same artifact — already done
+
+    tasks = set((await db.execute(
+        select(GpuJob.task).where(GpuJob.image_record_id == img.id)
+    )).scalars().all())
+    assert tasks == {"ccip", "siglip"}

tests/test_download_service.py

@@ -922,7 +922,7 @@ async def test_download_enqueues_thumbnail_and_ml_per_attached_image(
         lambda image_id: thumb_calls.append(image_id),
     )
     monkeypatch.setattr(
-        ml_mod.tag_and_embed, "delay",
+        ml_mod.embed_image, "delay",
         lambda image_id: ml_calls.append(image_id),
     )
 

tests/test_external_worker.py

@@ -125,7 +125,7 @@ def test_refetch_same_link_keeps_canonical_file(db_sync, tmp_path, monkeypatch):
 
     import backend.app.tasks.ml as ml_mod
     import backend.app.tasks.thumbnail as thumb_mod
-    monkeypatch.setattr(ml_mod.tag_and_embed, "delay", lambda i: None)
+    monkeypatch.setattr(ml_mod.embed_image, "delay", lambda i: None)
     monkeypatch.setattr(thumb_mod.generate_thumbnail, "delay", lambda i: None)
 
     out = ext.fetch_external_link(link_id)
@@ -234,7 +234,7 @@ def test_downloaded_archive_gets_provenance_and_tagging(db_sync, tmp_path, monke
     tagged, thumbed = [], []
     import backend.app.tasks.ml as ml_mod
     import backend.app.tasks.thumbnail as thumb_mod
-    monkeypatch.setattr(ml_mod.tag_and_embed, "delay", lambda i: tagged.append(i))
+    monkeypatch.setattr(ml_mod.embed_image, "delay", lambda i: tagged.append(i))
     monkeypatch.setattr(thumb_mod.generate_thumbnail, "delay", lambda i: thumbed.append(i))
 
     out = ext.fetch_external_link(link_id)

tests/test_gpu_jobs.py

@@ -30,7 +30,7 @@ async def test_enqueue_siglip_backfill_gates_on_concept_region(db):
     # back-catalogue) and skips ones that already have one — and never double-
     # enqueues an image that already has a pending siglip job.
     from backend.app.models import MLSettings
-    from backend.app.tasks.ml import enqueue_gpu_backfill
+    from backend.app.tasks.gpu_queue import enqueue_gpu_backfill
 
     cur = (await db.execute(
         select(MLSettings.embedder_model_version).where(MLSettings.id == 1)
@@ -71,7 +71,7 @@ async def test_enqueue_embed_backfill_selects_stale_and_unembedded(db):
     # stamped under a DIFFERENT model version (an operator swap); skip ones
     # already at the current version.
     from backend.app.models import MLSettings
-    from backend.app.tasks.ml import enqueue_gpu_backfill
+    from backend.app.tasks.gpu_queue import enqueue_gpu_backfill
 
     cur = (await db.execute(
         select(MLSettings.embedder_model_version).where(MLSettings.id == 1)
@@ -99,7 +99,7 @@ async def test_enqueue_embed_backfill_selects_stale_and_unembedded(db):
 async def test_reprocess_resets_done_jobs_to_pending(db):
     # Re-process (#1202): done/error jobs of a task go back to pending so the
     # agent re-runs the whole library under the current pipeline.
-    from backend.app.tasks.ml import reprocess_gpu_jobs
+    from backend.app.tasks.gpu_queue import reprocess_gpu_jobs
 
     img = await _img(db, "r1" * 32)
     job = await GpuJobService(db).enqueue(img.id, "ccip")
@@ -274,7 +274,7 @@ async def test_backfill_skips_errored_images(db):
     # An errored job is a TOMBSTONE for its (image, task): no backfill variant
     # re-enqueues it — retry is deliberate-only (/retry_errors). Pre-fix, the
     # hourly ccip run minted a fresh doomed job per bad file forever.
-    from backend.app.tasks.ml import enqueue_gpu_backfill
+    from backend.app.tasks.gpu_queue import enqueue_gpu_backfill
 
     img = await _img(db, "f1" * 32)
     svc = GpuJobService(db)
@@ -294,7 +294,7 @@ async def test_backfill_prunes_moot_error_tombstones(db):
     # Loop-era duplicates: several error rows for one (image, task), all made
     # moot by a later done row. The backfill's dedupe pass removes them, and
     # the done row still blocks re-enqueue.
-    from backend.app.tasks.ml import enqueue_gpu_backfill
+    from backend.app.tasks.gpu_queue import enqueue_gpu_backfill
 
     img = await _img(db, "f2" * 32)
     for i in range(3):

tests/test_maintenance.py

@@ -310,7 +310,7 @@ def test_recover_stalled_task_runs_skips_fresh_running(db_sync):
 
 
 def test_recover_stalled_task_runs_ml_queue_uses_longer_threshold(db_sync):
-    """ml-queue tasks (tag_and_embed video branch) legitimately run
+    """ml-queue tasks (embed_image video branch) legitimately run
     past the default 5-min threshold. The sweep must NOT flag an
     ml-queue task that's only been running 10 min — the override
     threshold (25 min via QUEUE_STUCK_THRESHOLD_MINUTES) protects

tests/test_reextract_archives.py

@@ -46,7 +46,7 @@ def test_reextract_links_archive_members_to_post(db_sync, tmp_path, monkeypatch)
 
     # No broker in this path — the post-import enqueue is best-effort anyway.
     monkeypatch.setattr(thumb_mod.generate_thumbnail, "delay", lambda *a, **k: None)
-    monkeypatch.setattr(ml_mod.tag_and_embed, "delay", lambda *a, **k: None)
+    monkeypatch.setattr(ml_mod.embed_image, "delay", lambda *a, **k: None)
 
     images_root = tmp_path / "images"
     images_root.mkdir()
@@ -116,7 +116,7 @@ def test_reextract_timebox_resumes_from_cursor(db_sync, tmp_path, monkeypatch):
     from backend.app.tasks import thumbnail as thumb_mod
 
     monkeypatch.setattr(thumb_mod.generate_thumbnail, "delay", lambda *a, **k: None)
-    monkeypatch.setattr(ml_mod.tag_and_embed, "delay", lambda *a, **k: None)
+    monkeypatch.setattr(ml_mod.embed_image, "delay", lambda *a, **k: None)
 
     images_root = tmp_path / "images"
     images_root.mkdir()

tests/test_tasks_ml.py

@@ -1,4 +1,4 @@
-"""tag_and_embed (embed-only) / backfill task tests. The pure _is_video helper
+"""embed_image (embed-only) / backfill task tests. The pure _is_video helper
 is a unit test; the DB-touching backfill query is an integration test with
 monkeypatched dispatch."""
 
@@ -23,7 +23,7 @@ async def test_backfill_enqueues_missing(db, monkeypatch):
 
     calls = []
     monkeypatch.setattr(
-        ml_tasks.tag_and_embed, "delay", lambda image_id: calls.append(image_id)
+        ml_tasks.embed_image, "delay", lambda image_id: calls.append(image_id)
     )
 
     img = ImageRecord(
@@ -38,3 +38,35 @@ async def test_backfill_enqueues_missing(db, monkeypatch):
     count = ml_tasks.backfill()
     assert count >= 1
     assert img.id in calls
+
+
+@pytest.mark.integration
+@pytest.mark.asyncio
+async def test_backfill_respects_cpu_embed_toggle(db, monkeypatch):
+    """B3: with cpu_embed_enabled off (agent-equipped stack, no ml-worker),
+    the CPU backfill is a no-op — the GPU 'embed' backfill owns whole-image
+    embeds there. Same gate the import hooks consult before dispatching."""
+    from sqlalchemy import update
+
+    from backend.app.models import ImageRecord, MLSettings
+    from backend.app.tasks import ml as ml_tasks
+
+    calls = []
+    monkeypatch.setattr(
+        ml_tasks.embed_image, "delay", lambda image_id: calls.append(image_id)
+    )
+    db.add(ImageRecord(
+        path="/images/o.jpg", sha256="o" * 64, size_bytes=1,
+        mime="image/jpeg", width=1, height=1,
+        origin="imported_filesystem", integrity_status="unknown",
+        siglip_embedding=None,
+    ))
+    await db.execute(
+        update(MLSettings).where(MLSettings.id == 1)
+        .values(cpu_embed_enabled=False)
+    )
+    await db.commit()
+
+    assert ml_tasks.cpu_embed_enabled() is False
+    assert ml_tasks.backfill() == 0
+    assert calls == []