diff --git a/alembic/versions/0080_tag_head_train_fingerprint.py b/alembic/versions/0080_tag_head_train_fingerprint.py
new file mode 100644
index 0000000..b4bd224
--- /dev/null
+++ b/alembic/versions/0080_tag_head_train_fingerprint.py
@@ -0,0 +1,31 @@
+"""tag_head.train_fingerprint (#1317 phase 2) — incremental head retraining
+
+A per-head training-data fingerprint (positive + rejection count/latest-timestamp)
+so a manual Retrain refits only the tags whose data changed; the nightly run
+ignores it (full reconcile). Nullable — a NULL fingerprint (existing heads) forces
+a refit on the first incremental run, then it's stamped.
+
+Revision ID: 0080
+Revises: 0079
+Create Date: 2026-07-06
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0080"
+down_revision: Union[str, None] = "0079"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "tag_head",
+        sa.Column("train_fingerprint", sa.String(128), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("tag_head", "train_fingerprint")
diff --git a/backend/app/models/tag_head.py b/backend/app/models/tag_head.py
index 34a5bcb..0390810 100644
--- a/backend/app/models/tag_head.py
+++ b/backend/app/models/tag_head.py
@@ -73,5 +73,12 @@ class TagHead(Base):
     trained_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now()
     )
+    # Training-data fingerprint (positives + rejections) at last fit — the
+    # incremental-retrain change detector (#1317 p2). A manual Retrain refits only
+    # heads whose fingerprint moved; the nightly run ignores it (full reconcile).
+    # NULL forces a refit (pre-fingerprint heads).
+    train_fingerprint: Mapped[str | None] = mapped_column(
+        String(128), nullable=True
+    )
     # Extra detail (auto-apply operating point, F1, etc.) — non-load-bearing.
     metrics: Mapped[dict[str, Any] | None] = mapped_column(JSONB, nullable=True)
diff --git a/backend/app/services/ml/heads.py b/backend/app/services/ml/heads.py
index bb18c25..41059cd 100644
--- a/backend/app/services/ml/heads.py
+++ b/backend/app/services/ml/heads.py
@@ -150,24 +150,103 @@ def _eligible_tag_ids(session: Session, min_pos: int) -> list[int]:
     return [r[0] for r in rows]
 
 
+def _head_fingerprints(session: Session, tag_ids: list[int]) -> dict[int, str]:
+    """Per-tag training-data fingerprint: (positive count, latest positive
+    created_at) + (rejection count, latest rejected_at). It moves whenever a tag
+    gains/loses a positive or a rejection — the incremental-retrain change
+    detector (#1317 p2). A newly-added positive/rejection always has the latest
+    timestamp, so even a remove-one-add-one (unchanged count) is caught. The
+    sampled-unlabeled negative pool + the hygiene set drift GLOBALLY and are
+    reconciled by the nightly full run, not captured here."""
+    if not tag_ids:
+        return {}
+    pos = session.execute(
+        select(
+            image_tag.c.tag_id,
+            func.count(image_tag.c.image_record_id),
+            func.max(image_tag.c.created_at),
+        )
+        .where(image_tag.c.tag_id.in_(tag_ids))
+        .group_by(image_tag.c.tag_id)
+    ).all()
+    pos_map = {t: (c, m) for t, c, m in pos}
+    rej = session.execute(
+        select(
+            TagSuggestionRejection.tag_id,
+            func.count(),
+            func.max(TagSuggestionRejection.rejected_at),
+        )
+        .where(TagSuggestionRejection.tag_id.in_(tag_ids))
+        .group_by(TagSuggestionRejection.tag_id)
+    ).all()
+    rej_map = {t: (c, m) for t, c, m in rej}
+    out = {}
+    for t in tag_ids:
+        pc, pm = pos_map.get(t, (0, None))
+        rc, rm = rej_map.get(t, (0, None))
+        out[t] = f"{pc}:{pm}:{rc}:{rm}"
+    return out
+
+
+def _heads_needing_retrain(
+    session: Session, eligible: list[int], embedding_version: str,
+    fps: dict[int, str], full: bool,
+) -> list[int]:
+    """The eligible tag_ids to (re)fit: no head yet, a head trained in a DIFFERENT
+    embedding space (a model swap), or a changed training-data fingerprint.
+    full=True forces every eligible tag. sklearn-free (train_head itself needs
+    scikit-learn) so the incremental decision is unit-testable on its own."""
+    if full:
+        return list(eligible)
+    existing = {
+        tag_id: (fp, ev)
+        for tag_id, fp, ev in session.execute(
+            select(
+                TagHead.tag_id, TagHead.train_fingerprint,
+                TagHead.embedding_version,
+            )
+        ).all()
+    }
+    out = []
+    for tag_id in eligible:
+        prev = existing.get(tag_id)
+        if (
+            prev is None
+            or prev[1] != embedding_version
+            or prev[0] != fps.get(tag_id)
+        ):
+            out.append(tag_id)
+    return out
+
+
 def train_all_heads(
     session: Session, params: dict[str, Any], run: HeadTrainingRun | None = None
 ) -> dict[str, int]:
-    """(Re)train a head for every eligible concept; prune heads whose tag is no
-    longer eligible. Commits per head so a SIGKILL leaves trained heads durable
-    (training is idempotent). Returns {n_trained, n_skipped}."""
+    """(Re)train eligible concept heads, INCREMENTALLY by default (#1317 p2):
+    refit only the tags whose training data changed since last fit, so a manual
+    Retrain click is fast. `params["full"]=True` (the nightly run) refits every
+    head to reconcile sampled-negative + hygiene drift. Prunes heads whose tag is
+    no longer eligible. Commits per head so a SIGKILL leaves trained heads durable.
+    Returns {n_trained, n_skipped} (n_skipped = unchanged + too-few-examples)."""
     import numpy as np
 
     cfg = _normalize_params(session, params)
     embedding_version = _embedder_version(session)
+    full = bool((params or {}).get("full"))
     eligible = _eligible_tag_ids(session, cfg["min_positives"])
     eligible_set = set(eligible)
     # Computed once per run, not per head — the hygiene set is identical for
     # every non-system concept.
     hygiene = _hygiene_excluded_ids(session)
+    fps = _head_fingerprints(session, eligible)
+    to_train = set(
+        _heads_needing_retrain(session, eligible, embedding_version, fps, full)
+    )
     trained = 0
-    skipped = 0
+    failed = 0
     for i, tag_id in enumerate(eligible):
+        if tag_id not in to_train:
+            continue
         try:
             ok = train_head(
                 session, tag_id, embedding_version, cfg, np, hygiene=hygiene
@@ -175,9 +254,15 @@ def train_all_heads(
         except Exception:
             log.exception("train_head failed for tag %d", tag_id)
             ok = False
+        if ok:
+            # Stamp the fingerprint we trained against so an unchanged tag is
+            # skipped on the next incremental run.
+            head = session.get(TagHead, tag_id)
+            if head is not None:
+                head.train_fingerprint = fps.get(tag_id)
         session.commit()
         trained += int(ok)
-        skipped += int(not ok)
+        failed += int(not ok)
         if run is not None and i % 10 == 0:
             run.last_progress_at = datetime.now(UTC)
             session.commit()
@@ -188,7 +273,11 @@ def train_all_heads(
     else:
         session.execute(delete(TagHead))
     session.commit()
-    return {"n_trained": trained, "n_skipped": skipped}
+    # n_skipped = unchanged (not attempted) + failed-to-fit (too few examples).
+    return {
+        "n_trained": trained,
+        "n_skipped": (len(eligible) - len(to_train)) + failed,
+    }
 
 
 def head_training_ids(
diff --git a/backend/app/tasks/ml.py b/backend/app/tasks/ml.py
index bfb8d0c..4959099 100644
--- a/backend/app/tasks/ml.py
+++ b/backend/app/tasks/ml.py
@@ -356,7 +356,10 @@ def scheduled_train_heads() -> str:
         if running is not None:
             return "already running"
         run = HeadTrainingRun(
-            params={"source": "scheduled"}, status="running",
+            # Nightly = FULL reconcile (refit every head) so sampled-negative +
+            # hygiene drift is folded in; the manual Retrain button stays
+            # incremental (#1317 p2).
+            params={"source": "scheduled", "full": True}, status="running",
             last_progress_at=datetime.now(UTC),
         )
         session.add(run)
diff --git a/tests/test_head_incremental.py b/tests/test_head_incremental.py
new file mode 100644
index 0000000..b16b9dd
--- /dev/null
+++ b/tests/test_head_incremental.py
@@ -0,0 +1,136 @@
+"""Incremental head retraining (#1317 phase 2). The refit-decision + fingerprint
+are split out sklearn-free (train_head itself needs scikit-learn), so they're
+tested directly via db_sync."""
+import pytest
+from sqlalchemy import select
+
+from backend.app.models import (
+    ImageRecord,
+    MLSettings,
+    Tag,
+    TagHead,
+    TagKind,
+    TagSuggestionRejection,
+)
+from backend.app.models.tag import image_tag
+from backend.app.services.ml.heads import (
+    _head_fingerprints,
+    _heads_needing_retrain,
+)
+
+pytestmark = pytest.mark.integration
+
+
+def _img(db, sha: str) -> ImageRecord:
+    img = ImageRecord(
+        path=f"/images/{sha}.jpg", sha256=sha, size_bytes=1, mime="image/jpeg",
+        width=1, height=1, origin="imported_filesystem",
+        integrity_status="unknown",
+    )
+    db.add(img)
+    db.flush()
+    return img
+
+
+def _tag(db, name: str) -> Tag:
+    t = Tag(name=name, kind=TagKind.general)
+    db.add(t)
+    db.flush()
+    return t
+
+
+def _apply(db, image_id: int, tag_id: int) -> None:
+    db.execute(image_tag.insert().values(
+        image_record_id=image_id, tag_id=tag_id, source="manual",
+    ))
+
+
+def _reject(db, image_id: int, tag_id: int) -> None:
+    db.add(TagSuggestionRejection(image_record_id=image_id, tag_id=tag_id))
+    db.flush()
+
+
+def _version(db) -> str:
+    return db.execute(
+        select(MLSettings.embedder_model_version).where(MLSettings.id == 1)
+    ).scalar_one()
+
+
+def _head(db, tag_id: int, fp: str | None, version: str) -> None:
+    db.add(TagHead(
+        tag_id=tag_id, embedding_version=version,
+        weights=[0.0] * 1152, bias=0.0, suggest_threshold=0.5,
+        auto_apply_threshold=None, n_pos=10, n_neg=30,
+        ap=0.8, precision_cv=0.9, recall=0.6, train_fingerprint=fp,
+    ))
+    db.flush()
+
+
+def test_fingerprint_changes_on_new_positive(db_sync):
+    tag = _tag(db_sync, "glasses")
+    i1 = _img(db_sync, "a" * 64)
+    _apply(db_sync, i1.id, tag.id)
+    db_sync.commit()
+    fp1 = _head_fingerprints(db_sync, [tag.id])[tag.id]
+
+    i2 = _img(db_sync, "b" * 64)
+    _apply(db_sync, i2.id, tag.id)
+    db_sync.commit()
+    assert _head_fingerprints(db_sync, [tag.id])[tag.id] != fp1
+
+
+def test_fingerprint_changes_on_new_rejection(db_sync):
+    tag = _tag(db_sync, "glasses")
+    i1 = _img(db_sync, "c" * 64)
+    _apply(db_sync, i1.id, tag.id)
+    db_sync.commit()
+    fp1 = _head_fingerprints(db_sync, [tag.id])[tag.id]
+
+    _reject(db_sync, i1.id, tag.id)
+    db_sync.commit()
+    assert _head_fingerprints(db_sync, [tag.id])[tag.id] != fp1
+
+
+def test_needing_retrain_selects_only_changed(db_sync):
+    ver = _version(db_sync)
+    a = _tag(db_sync, "A")
+    _apply(db_sync, _img(db_sync, "d" * 64).id, a.id)
+    b = _tag(db_sync, "B")
+    _apply(db_sync, _img(db_sync, "e" * 64).id, b.id)
+    c = _tag(db_sync, "C")
+    _apply(db_sync, _img(db_sync, "f" * 64).id, c.id)
+    db_sync.commit()
+
+    ids = [a.id, b.id, c.id]
+    fps = _head_fingerprints(db_sync, ids)
+    _head(db_sync, a.id, fps[a.id], ver)          # A: head with CURRENT fp → skip
+    _head(db_sync, b.id, "stale", ver)            # B: head with STALE fp → retrain
+    db_sync.commit()                              # C: no head → retrain
+
+    need = set(_heads_needing_retrain(db_sync, ids, ver, fps, full=False))
+    assert a.id not in need
+    assert {b.id, c.id} <= need
+
+
+def test_stale_embedding_version_forces_retrain(db_sync):
+    ver = _version(db_sync)
+    a = _tag(db_sync, "A")
+    _apply(db_sync, _img(db_sync, "g" * 64).id, a.id)
+    db_sync.commit()
+    fps = _head_fingerprints(db_sync, [a.id])
+    # Matching fingerprint but a DIFFERENT embedding space → must refit.
+    _head(db_sync, a.id, fps[a.id], "old-model-v0")
+    db_sync.commit()
+    assert a.id in set(_heads_needing_retrain(db_sync, [a.id], ver, fps, full=False))
+
+
+def test_full_forces_all(db_sync):
+    ver = _version(db_sync)
+    a = _tag(db_sync, "A")
+    _apply(db_sync, _img(db_sync, "h" * 64).id, a.id)
+    db_sync.commit()
+    fps = _head_fingerprints(db_sync, [a.id])
+    _head(db_sync, a.id, fps[a.id], ver)          # current fp → would be skipped
+    db_sync.commit()
+    # full=True ignores the fingerprint (nightly reconcile).
+    assert a.id in set(_heads_needing_retrain(db_sync, [a.id], ver, fps, full=True))