diff --git a/studio/backend/tests/test_diffusion_dataset_api.py b/studio/backend/tests/test_diffusion_dataset_api.py
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+++ b/studio/backend/tests/test_diffusion_dataset_api.py
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+# SPDX-License-Identifier: AGPL-3.0-only
+# Copyright 2026-present the Unsloth AI Inc. team. All rights reserved. See /studio/LICENSE.AGPL-3.0
+
+"""Tests for the diffusion dataset labeling + example-import routes.
+
+The routes are hit with the FastAPI TestClient; the datasets root is redirected to a
+tmp_path so nothing touches a real Studio home. The example importer is exercised with a
+mocked datasets.load_dataset so no network / GPU is needed.
+"""
+
+from __future__ import annotations
+
+import io
+import json
+
+import pytest
+from fastapi import FastAPI, HTTPException
+from fastapi.testclient import TestClient
+from PIL import Image
+
+from auth.authentication import get_current_subject
+from routes.training import router as training_router
+
+
+def _png_bytes(color = (200, 100, 50), size = (8, 8)) -> bytes:
+    buf = io.BytesIO()
+    Image.new("RGB", size, color).save(buf, format = "PNG")
+    return buf.getvalue()
+
+
+def _write_png(path, color = (200, 100, 50), size = (8, 8)) -> None:
+    Image.new("RGB", size, color).save(path, format = "PNG")
+
+
+@pytest.fixture
+def client():
+    app = FastAPI()
+    app.include_router(training_router, prefix = "/api/train")
+    app.dependency_overrides[get_current_subject] = lambda: "test-user"
+    return TestClient(app)
+
+
+@pytest.fixture
+def ds_root(monkeypatch, tmp_path):
+    import utils.paths as up
+
+    root = tmp_path / "assets" / "datasets"
+    root.mkdir(parents = True)
+    monkeypatch.setattr(up, "datasets_root", lambda: root)
+    return root
+
+
+# ── listing + caption precedence ─────────────────────────────────────────────
+def test_list_images_caption_precedence(client, ds_root):
+    folder = ds_root / "styleset"
+    folder.mkdir()
+    _write_png(folder / "a.png")
+    _write_png(folder / "b.png")
+    _write_png(folder / "c.png")
+    # a.png -> metadata (beats a stray sidecar), b.png -> sidecar, c.png -> none.
+    (folder / "metadata.jsonl").write_text(
+        json.dumps({"file_name": "a.png", "text": "from metadata"}) + "\n", encoding = "utf-8"
+    )
+    (folder / "a.txt").write_text("stray sidecar", encoding = "utf-8")
+    (folder / "b.txt").write_text("from sidecar", encoding = "utf-8")
+
+    r = client.get("/api/train/diffusion/dataset/styleset/images")
+    assert r.status_code == 200, r.text
+    body = r.json()
+    assert body["name"] == "styleset"
+    recs = {rec["filename"]: rec for rec in body["images"]}
+    assert set(recs) == {"a.png", "b.png", "c.png"}
+    assert recs["a.png"]["caption"] == "from metadata"
+    assert recs["a.png"]["caption_source"] == "metadata"
+    assert recs["b.png"]["caption"] == "from sidecar"
+    assert recs["b.png"]["caption_source"] == "sidecar"
+    assert recs["c.png"]["caption"] is None
+    assert recs["c.png"]["caption_source"] == "none"
+    assert recs["a.png"]["width"] == 8 and recs["a.png"]["height"] == 8
+
+
+def test_list_images_missing_dataset_404(client, ds_root):
+    assert client.get("/api/train/diffusion/dataset/nope/images").status_code == 404
+
+
+# ── image serving + thumbnails ───────────────────────────────────────────────
+def test_get_image_and_thumbnail_excluded_from_listing(client, ds_root):
+    folder = ds_root / "pics"
+    folder.mkdir()
+    _write_png(folder / "one.png", size = (64, 48))
+
+    full = client.get("/api/train/diffusion/dataset/pics/image/one.png")
+    assert full.status_code == 200, full.text
+
+    thumb = client.get("/api/train/diffusion/dataset/pics/image/one.png?thumb=32")
+    assert thumb.status_code == 200
+    assert thumb.headers["content-type"] == "image/jpeg"
+    assert (folder / ".thumbs").is_dir()
+
+    # The .thumbs cache dir must not surface as a dataset image.
+    listing = client.get("/api/train/diffusion/dataset/pics/images").json()
+    assert [rec["filename"] for rec in listing["images"]] == ["one.png"]
+
+
+def test_get_image_missing_404(client, ds_root):
+    (ds_root / "pics").mkdir()
+    assert (
+        client.get("/api/train/diffusion/dataset/pics/image/ghost.png").status_code == 404
+    )
+
+
+# ── caption write / clear ────────────────────────────────────────────────────
+def test_put_caption_roundtrip_and_clear(client, ds_root):
+    folder = ds_root / "cap"
+    folder.mkdir()
+    _write_png(folder / "x.png")
+
+    r = client.put(
+        "/api/train/diffusion/dataset/cap/caption/x.png", json = {"caption": "a red apple"}
+    )
+    assert r.status_code == 200, r.text
+    assert r.json()["caption"] == "a red apple"
+    assert r.json()["caption_source"] == "sidecar"
+    assert (folder / "x.txt").read_text(encoding = "utf-8") == "a red apple"
+
+    # Blank clears the sidecar.
+    r = client.put("/api/train/diffusion/dataset/cap/caption/x.png", json = {"caption": "  "})
+    assert r.status_code == 200
+    assert r.json()["caption"] is None
+    assert r.json()["caption_source"] == "none"
+    assert not (folder / "x.txt").exists()
+
+
+def test_put_caption_missing_image_404(client, ds_root):
+    (ds_root / "cap").mkdir()
+    r = client.put(
+        "/api/train/diffusion/dataset/cap/caption/ghost.png", json = {"caption": "hi"}
+    )
+    assert r.status_code == 404
+
+
+def test_put_caption_too_long_400(client, ds_root):
+    folder = ds_root / "cap"
+    folder.mkdir()
+    _write_png(folder / "x.png")
+    r = client.put(
+        "/api/train/diffusion/dataset/cap/caption/x.png", json = {"caption": "z" * 2001}
+    )
+    assert r.status_code == 400
+
+
+# ── delete ───────────────────────────────────────────────────────────────────
+def test_delete_image_cleans_sidecar_and_thumb(client, ds_root):
+    folder = ds_root / "d"
+    folder.mkdir()
+    _write_png(folder / "x.png")
+    (folder / "x.txt").write_text("cap", encoding = "utf-8")
+    # Generate a thumbnail so we can assert it is cleaned up too.
+    client.get("/api/train/diffusion/dataset/d/image/x.png?thumb=32")
+    assert list((folder / ".thumbs").glob("x_*.jpg"))
+
+    r = client.delete("/api/train/diffusion/dataset/d/image/x.png")
+    assert r.status_code == 200, r.text
+    assert not (folder / "x.png").exists()
+    assert not (folder / "x.txt").exists()
+    assert not list((folder / ".thumbs").glob("x_*.jpg"))
+
+
+# ── traversal / validation ───────────────────────────────────────────────────
+def test_dataset_name_traversal_rejected_over_http(client, ds_root):
+    # A name that fails the folder-name validator returns 400, never touches disk.
+    assert client.get("/api/train/diffusion/dataset/bad name!/images").status_code == 400
+
+
+def test_image_filename_validation_rejects_traversal():
+    from pathlib import Path
+
+    from routes.training import _safe_dataset_image_path
+
+    folder = Path("/tmp/some-dataset")
+    for bad in ("../../etc/passwd", "/etc/passwd", "..", "sub/dir.png", "notimage.txt"):
+        with pytest.raises(HTTPException) as exc:
+            _safe_dataset_image_path(folder, bad)
+        assert exc.value.status_code == 400
+    # A plain image name resolves inside the folder.
+    assert _safe_dataset_image_path(folder, "ok.png") == folder / "ok.png"
+
+
+def test_clean_dataset_name_rejects_dotdot():
+    from routes.training import _clean_diffusion_dataset_name
+
+    for bad in ("../x", "a/b", "..", " "):
+        with pytest.raises(HTTPException) as exc:
+            _clean_diffusion_dataset_name(bad)
+        assert exc.value.status_code == 400
+
+
+# ── examples registry + import ───────────────────────────────────────────────
+def test_list_dataset_examples(client, ds_root):
+    r = client.get("/api/train/diffusion/dataset-examples")
+    assert r.status_code == 200, r.text
+    ids = {e["id"] for e in r.json()["examples"]}
+    assert {"dreambooth-dog", "tuxemon", "tarot-1920"} <= ids
+    dog = next(e for e in r.json()["examples"] if e["id"] == "dreambooth-dog")
+    assert dog["suggested_trigger"] == "a photo of sks dog"
+    assert dog["license"]
+
+
+class _FakeImageFeature:
+    # Mimics datasets.Image so _detect_image_column matches by class name.
+    pass
+
+
+_FakeImageFeature.__name__ = "Image"
+
+
+class _FakeDS:
+    def __init__(self, rows, features):
+        self._rows = rows
+        self.features = features
+
+    def __iter__(self):
+        return iter(self._rows)
+
+
+def _install_fake_load_dataset(monkeypatch, n_rows):
+    calls = {"count": 0}
+    rows = [
+        {"image": Image.new("RGB", (8, 8), (i * 30 % 255, 60, 90)), "prompt": f"caption {i}"}
+        for i in range(n_rows)
+    ]
+    features = {"image": _FakeImageFeature(), "prompt": object()}
+
+    def fake_load(repo, **kwargs):
+        calls["count"] += 1
+        assert kwargs.get("split") == "train"
+        return _FakeDS(rows, features)
+
+    import datasets
+
+    monkeypatch.setattr(datasets, "load_dataset", fake_load)
+    return calls
+
+
+def test_import_example_writes_images_and_captions(client, ds_root, monkeypatch):
+    calls = _install_fake_load_dataset(monkeypatch, n_rows = 3)
+    r = client.post(
+        "/api/train/diffusion/dataset/import-example",
+        json = {"id": "tuxemon", "name": "my-tux"},
+    )
+    assert r.status_code == 200, r.text
+    body = r.json()
+    assert body["name"] == "my-tux"
+    assert body["imported"] == 3
+    assert body["image_count"] == 3
+    assert body["caption_count"] == 3
+    assert body["license"] == "cc-by-sa-3.0"
+    assert body["source_repo"] == "linoyts/Tuxemon"
+    folder = ds_root / "my-tux"
+    assert sorted(p.name for p in folder.glob("*.png")) == [f"img_{i:04d}.png" for i in range(3)]
+    assert (folder / "img_0000.txt").read_text(encoding = "utf-8") == "caption 0"
+
+    # Idempotent: a second call does not reload or duplicate.
+    r2 = client.post(
+        "/api/train/diffusion/dataset/import-example",
+        json = {"id": "tuxemon", "name": "my-tux"},
+    )
+    assert r2.status_code == 200
+    assert r2.json()["imported"] == 0
+    assert r2.json()["image_count"] == 3
+    assert calls["count"] == 1
+
+
+def test_import_example_respects_cap(client, ds_root, monkeypatch):
+    _install_fake_load_dataset(monkeypatch, n_rows = 5)
+    entry = next(e for e in __import__("routes.training", fromlist = ["_DATASET_EXAMPLES"])._DATASET_EXAMPLES if e["id"] == "tuxemon")
+    monkeypatch.setitem(entry, "image_cap", 2)
+    r = client.post(
+        "/api/train/diffusion/dataset/import-example", json = {"id": "tuxemon"}
+    )
+    assert r.status_code == 200, r.text
+    assert r.json()["imported"] == 2
+    assert r.json()["image_count"] == 2
+
+
+def test_import_example_unknown_id_404(client, ds_root):
+    r = client.post(
+        "/api/train/diffusion/dataset/import-example", json = {"id": "does-not-exist"}
+    )
+    assert r.status_code == 404
+
+
+def test_import_example_load_failure_maps_to_502(client, ds_root, monkeypatch):
+    import datasets
+
+    def boom(repo, **kwargs):
+        raise RuntimeError("network down")
+
+    monkeypatch.setattr(datasets, "load_dataset", boom)
+    r = client.post(
+        "/api/train/diffusion/dataset/import-example", json = {"id": "tuxemon"}
+    )
+    assert r.status_code == 502
+    assert "Could not import" in r.json()["detail"]