diff --git a/.gitignore b/.gitignore
index c101618..a06e3ca 100644
--- a/.gitignore
+++ b/.gitignore
@@ -21,6 +21,8 @@ checkpoints/
 *.pt
 *.pth
 *.onnx
+*.db
+*.db-journal
 
 # OS
 .DS_Store
diff --git a/labeling/backend/README.md b/labeling/backend/README.md
new file mode 100644
index 0000000..74a36fe
--- /dev/null
+++ b/labeling/backend/README.md
@@ -0,0 +1,58 @@
+# Labeling Tool — Backend
+
+API + data model for the bounding-box labeling tool. See CLAUDE.md §3.2 for the
+full requirements this is built against.
+
+**Status:** core data model + CRUD API implemented (sets, hierarchical classes,
+frames, labels, per-frame/per-set status). **Not yet implemented:** dataset
+versioning/promotion (`enemy-v1`, `enemy-v2`, ...) — deferred since it needs its
+own design pass (snapshot semantics: labels are editable at any time, but a
+promoted dataset version must stay reproducible). Also not yet implemented: any
+frontend, auth/login (see below), or the active-learning auto-label workflow.
+
+## Stack
+
+- **FastAPI** + **SQLAlchemy** (2.0), **SQLite** by default (`./labeling.db`),
+  swappable via the `DATABASE_URL` env var (e.g. to Postgres later without code
+  changes — one Postgres-compatible ORM).
+- **Multi-user, no auth yet:** labels/status changes take a plain `created_by` /
+  `updated_by` username string, resolved via get-or-create (`users.py`). There's no
+  login flow — attribution only, since there's no UI yet that would need real auth.
+
+## Data model
+
+- `LabelSet` — a label set (`Enemy`, `Items`, `Traps`, ...), one per detector model.
+- `MainClass` / `SubClass` — the per-set Main → Sub class hierarchy (e.g. `Enemy` →
+  `Bat`, `Snake`), created ad hoc via the API, no migration needed to add classes.
+- `Frame` — one labelable image, identified by `(session_name, frame_index)` —
+  matches the recording tool's frame-extraction output 1:1.
+- `Label` — one bounding box (`x, y, width, height` in pixel space), scoped to a
+  frame + set + sub-class.
+- `FrameSetStatus` — per-frame, per-set label state (`unlabeled` / `auto_labeled` /
+  `reviewed`).
+
+## Setup
+
+```
+python -m venv .venv
+source .venv/bin/activate
+pip install -e ".[dev]"
+```
+
+## Run
+
+```
+spelunkai-labeling-backend
+# or: uvicorn spelunkai_labeling_backend.main:create_app --factory --reload
+```
+
+Interactive API docs at `http://127.0.0.1:8000/docs` once running.
+
+## Testing
+
+```
+pytest
+```
+
+Each test gets a fully isolated app + SQLite file via `create_app(database_url=...)`
+(see `tests/conftest.py`) — no shared state between tests, no real server needed.
diff --git a/labeling/backend/pyproject.toml b/labeling/backend/pyproject.toml
index 6f16138..2e23f88 100644
--- a/labeling/backend/pyproject.toml
+++ b/labeling/backend/pyproject.toml
@@ -3,7 +3,17 @@ name = "spelunkai-labeling-backend"
 version = "0.0.0"
 description = "SpelunkAI Labeling Tool backend: API + data model for bounding-box labels"
 requires-python = ">=3.10"
-dependencies = []
+dependencies = [
+    "fastapi>=0.115",
+    "uvicorn[standard]>=0.32",
+    "sqlalchemy>=2.0",
+]
+
+[project.optional-dependencies]
+dev = ["pytest>=8", "httpx>=0.27"]
+
+[project.scripts]
+spelunkai-labeling-backend = "spelunkai_labeling_backend.cli:main"
 
 [build-system]
 requires = ["setuptools>=68"]
diff --git a/labeling/backend/src/spelunkai_labeling_backend/__main__.py b/labeling/backend/src/spelunkai_labeling_backend/__main__.py
new file mode 100644
index 0000000..9ae637f
--- /dev/null
+++ b/labeling/backend/src/spelunkai_labeling_backend/__main__.py
@@ -0,0 +1,4 @@
+from .cli import main
+
+if __name__ == "__main__":
+    main()
diff --git a/labeling/backend/src/spelunkai_labeling_backend/cli.py b/labeling/backend/src/spelunkai_labeling_backend/cli.py
new file mode 100644
index 0000000..3bc4713
--- /dev/null
+++ b/labeling/backend/src/spelunkai_labeling_backend/cli.py
@@ -0,0 +1,14 @@
+"""Convenience entry point to run the labeling backend with uvicorn."""
+from __future__ import annotations
+
+import uvicorn
+
+from .main import create_app
+
+
+def main() -> None:
+    uvicorn.run(create_app(), host="127.0.0.1", port=8000)
+
+
+if __name__ == "__main__":
+    main()
diff --git a/labeling/backend/src/spelunkai_labeling_backend/db.py b/labeling/backend/src/spelunkai_labeling_backend/db.py
new file mode 100644
index 0000000..62d2ac7
--- /dev/null
+++ b/labeling/backend/src/spelunkai_labeling_backend/db.py
@@ -0,0 +1,40 @@
+"""Database engine/session setup.
+
+Each `Database` instance owns its own engine and session factory, so a
+production run and a test can each get a fully isolated database instead of
+sharing process-global state (see `main.create_app`).
+"""
+from __future__ import annotations
+
+import os
+from typing import Iterator, Optional
+
+from sqlalchemy import create_engine
+from sqlalchemy.orm import Session, sessionmaker
+
+from .models import Base
+
+DEFAULT_DATABASE_URL = "sqlite:///./labeling.db"
+
+
+class Database:
+    def __init__(self, database_url: Optional[str] = None):
+        self.url = database_url or os.environ.get("DATABASE_URL", DEFAULT_DATABASE_URL)
+        connect_args = {"check_same_thread": False} if self.url.startswith("sqlite") else {}
+        self.engine = create_engine(self.url, connect_args=connect_args)
+        self.session_factory = sessionmaker(autocommit=False, autoflush=False, bind=self.engine)
+
+    def init_models(self) -> None:
+        Base.metadata.create_all(bind=self.engine)
+
+    def get_session(self) -> Iterator[Session]:
+        session = self.session_factory()
+        try:
+            yield session
+        finally:
+            session.close()
+
+
+def get_db() -> Iterator[Session]:
+    """Marker dependency; `create_app` overrides this with a real `Database.get_session`."""
+    raise NotImplementedError("get_db must be overridden via app.dependency_overrides")
diff --git a/labeling/backend/src/spelunkai_labeling_backend/main.py b/labeling/backend/src/spelunkai_labeling_backend/main.py
new file mode 100644
index 0000000..e6574f3
--- /dev/null
+++ b/labeling/backend/src/spelunkai_labeling_backend/main.py
@@ -0,0 +1,38 @@
+"""FastAPI app factory for the labeling backend.
+
+Run with `uvicorn spelunkai_labeling_backend.main:create_app --factory` (or
+`spelunkai-labeling-backend`) so the app — and its database — is constructed
+lazily instead of at import time; this also means importing this module has
+no side effects, and tests can call `create_app(database_url=...)` to get a
+fully isolated instance.
+"""
+from __future__ import annotations
+
+from typing import Optional
+
+from fastapi import FastAPI
+
+from .db import Database, get_db
+from .routers.frames import router as frames_router
+from .routers.labels import router as labels_router
+from .routers.sets import main_classes_router, sets_router
+
+
+def create_app(database_url: Optional[str] = None) -> FastAPI:
+    database = Database(database_url)
+    database.init_models()
+
+    app = FastAPI(title="SpelunkAI Labeling Backend")
+    app.state.database = database
+    app.dependency_overrides[get_db] = database.get_session
+
+    app.include_router(sets_router)
+    app.include_router(main_classes_router)
+    app.include_router(frames_router)
+    app.include_router(labels_router)
+
+    @app.get("/health")
+    def health() -> dict:
+        return {"status": "ok"}
+
+    return app
diff --git a/labeling/backend/src/spelunkai_labeling_backend/models.py b/labeling/backend/src/spelunkai_labeling_backend/models.py
new file mode 100644
index 0000000..692c86c
--- /dev/null
+++ b/labeling/backend/src/spelunkai_labeling_backend/models.py
@@ -0,0 +1,121 @@
+"""SQLAlchemy ORM models for the labeling data model.
+
+Main/sub classes are data rows (not enum columns), so users can add new
+classes on the fly per CLAUDE.md §3.2 without a schema migration.
+"""
+from __future__ import annotations
+
+import datetime
+import enum
+
+from sqlalchemy import Column, DateTime, Enum as SqlEnum, Float, ForeignKey, Integer, String, UniqueConstraint
+from sqlalchemy.orm import declarative_base, relationship
+
+Base = declarative_base()
+
+
+def _utcnow() -> datetime.datetime:
+    return datetime.datetime.now(datetime.timezone.utc)
+
+
+class LabelStatus(str, enum.Enum):
+    UNLABELED = "unlabeled"
+    AUTO_LABELED = "auto_labeled"
+    REVIEWED = "reviewed"
+
+
+class LabelSource(str, enum.Enum):
+    MANUAL = "manual"
+    AUTO = "auto"
+
+
+class User(Base):
+    __tablename__ = "users"
+
+    id = Column(Integer, primary_key=True)
+    username = Column(String, unique=True, nullable=False)
+    created_at = Column(DateTime, default=_utcnow)
+
+
+class LabelSet(Base):
+    __tablename__ = "label_sets"
+
+    id = Column(Integer, primary_key=True)
+    name = Column(String, unique=True, nullable=False)
+    description = Column(String, nullable=True)
+    created_at = Column(DateTime, default=_utcnow)
+
+    main_classes = relationship("MainClass", back_populates="label_set", cascade="all, delete-orphan")
+
+
+class MainClass(Base):
+    __tablename__ = "main_classes"
+    __table_args__ = (UniqueConstraint("set_id", "name", name="uq_main_class_set_name"),)
+
+    id = Column(Integer, primary_key=True)
+    set_id = Column(Integer, ForeignKey("label_sets.id"), nullable=False)
+    name = Column(String, nullable=False)
+
+    label_set = relationship("LabelSet", back_populates="main_classes")
+    sub_classes = relationship("SubClass", back_populates="main_class", cascade="all, delete-orphan")
+
+
+class SubClass(Base):
+    __tablename__ = "sub_classes"
+    __table_args__ = (UniqueConstraint("main_class_id", "name", name="uq_sub_class_main_name"),)
+
+    id = Column(Integer, primary_key=True)
+    main_class_id = Column(Integer, ForeignKey("main_classes.id"), nullable=False)
+    name = Column(String, nullable=False)
+
+    main_class = relationship("MainClass", back_populates="sub_classes")
+
+
+class Frame(Base):
+    """A single labelable image, produced by the recording tool's frame extraction."""
+
+    __tablename__ = "frames"
+    __table_args__ = (UniqueConstraint("session_name", "frame_index", name="uq_frame_session_index"),)
+
+    id = Column(Integer, primary_key=True)
+    session_name = Column(String, nullable=False)
+    frame_index = Column(Integer, nullable=False)
+    image_path = Column(String, nullable=False)
+    width = Column(Integer, nullable=False)
+    height = Column(Integer, nullable=False)
+    created_at = Column(DateTime, default=_utcnow)
+
+
+class FrameSetStatus(Base):
+    """Per-image, per-set label state (unlabeled / auto_labeled / reviewed)."""
+
+    __tablename__ = "frame_set_status"
+    __table_args__ = (UniqueConstraint("frame_id", "set_id", name="uq_frame_set_status"),)
+
+    id = Column(Integer, primary_key=True)
+    frame_id = Column(Integer, ForeignKey("frames.id"), nullable=False)
+    set_id = Column(Integer, ForeignKey("label_sets.id"), nullable=False)
+    status = Column(SqlEnum(LabelStatus), nullable=False, default=LabelStatus.UNLABELED)
+    updated_at = Column(DateTime, default=_utcnow, onupdate=_utcnow)
+    updated_by_id = Column(Integer, ForeignKey("users.id"), nullable=True)
+
+
+class Label(Base):
+    """A single bounding-box annotation, scoped to one frame within one label set."""
+
+    __tablename__ = "labels"
+
+    id = Column(Integer, primary_key=True)
+    frame_id = Column(Integer, ForeignKey("frames.id"), nullable=False)
+    set_id = Column(Integer, ForeignKey("label_sets.id"), nullable=False)
+    sub_class_id = Column(Integer, ForeignKey("sub_classes.id"), nullable=False)
+    x = Column(Float, nullable=False)
+    y = Column(Float, nullable=False)
+    width = Column(Float, nullable=False)
+    height = Column(Float, nullable=False)
+    source = Column(SqlEnum(LabelSource), nullable=False, default=LabelSource.MANUAL)
+    created_by_id = Column(Integer, ForeignKey("users.id"), nullable=True)
+    created_at = Column(DateTime, default=_utcnow)
+    updated_at = Column(DateTime, default=_utcnow, onupdate=_utcnow)
+
+    sub_class = relationship("SubClass")
diff --git a/labeling/backend/src/spelunkai_labeling_backend/routers/__init__.py b/labeling/backend/src/spelunkai_labeling_backend/routers/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/labeling/backend/src/spelunkai_labeling_backend/routers/frames.py b/labeling/backend/src/spelunkai_labeling_backend/routers/frames.py
new file mode 100644
index 0000000..83f290d
--- /dev/null
+++ b/labeling/backend/src/spelunkai_labeling_backend/routers/frames.py
@@ -0,0 +1,50 @@
+"""Frame ingestion and listing."""
+from __future__ import annotations
+
+from typing import Optional
+
+from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy.exc import IntegrityError
+from sqlalchemy.orm import Session
+
+from .. import models, schemas
+from ..db import get_db
+
+router = APIRouter(prefix="/frames", tags=["frames"])
+
+
+@router.post("", response_model=schemas.FrameRead, status_code=201)
+def create_frame(payload: schemas.FrameCreate, db: Session = Depends(get_db)):
+    existing = (
+        db.query(models.Frame)
+        .filter_by(session_name=payload.session_name, frame_index=payload.frame_index)
+        .first()
+    )
+    if existing is not None:
+        return existing
+
+    frame = models.Frame(**payload.model_dump())
+    db.add(frame)
+    try:
+        db.commit()
+    except IntegrityError:
+        db.rollback()
+        raise HTTPException(409, "frame already exists")
+    db.refresh(frame)
+    return frame
+
+
+@router.get("", response_model=list[schemas.FrameRead])
+def list_frames(session_name: Optional[str] = None, db: Session = Depends(get_db)):
+    query = db.query(models.Frame)
+    if session_name is not None:
+        query = query.filter_by(session_name=session_name)
+    return query.order_by(models.Frame.frame_index).all()
+
+
+@router.get("/{frame_id}", response_model=schemas.FrameRead)
+def get_frame(frame_id: int, db: Session = Depends(get_db)):
+    frame = db.get(models.Frame, frame_id)
+    if frame is None:
+        raise HTTPException(404, "frame not found")
+    return frame
diff --git a/labeling/backend/src/spelunkai_labeling_backend/routers/labels.py b/labeling/backend/src/spelunkai_labeling_backend/routers/labels.py
new file mode 100644
index 0000000..464a5be
--- /dev/null
+++ b/labeling/backend/src/spelunkai_labeling_backend/routers/labels.py
@@ -0,0 +1,97 @@
+"""Bounding-box labels and per-frame/per-set label status."""
+from __future__ import annotations
+
+from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy.orm import Session
+
+from .. import models, schemas
+from ..db import get_db
+from ..users import get_or_create_user
+
+router = APIRouter(tags=["labels"])
+
+
+def _require_frame_and_set(db: Session, frame_id: int, set_id: int) -> None:
+    if db.get(models.Frame, frame_id) is None:
+        raise HTTPException(404, "frame not found")
+    if db.get(models.LabelSet, set_id) is None:
+        raise HTTPException(404, "set not found")
+
+
+@router.post("/frames/{frame_id}/sets/{set_id}/labels", response_model=schemas.LabelRead, status_code=201)
+def create_label(frame_id: int, set_id: int, payload: schemas.LabelCreate, db: Session = Depends(get_db)):
+    _require_frame_and_set(db, frame_id, set_id)
+
+    sub_class = db.get(models.SubClass, payload.sub_class_id)
+    if sub_class is None or sub_class.main_class.set_id != set_id:
+        raise HTTPException(422, "sub_class_id does not belong to this set")
+
+    label = models.Label(
+        frame_id=frame_id,
+        set_id=set_id,
+        sub_class_id=payload.sub_class_id,
+        x=payload.x,
+        y=payload.y,
+        width=payload.width,
+        height=payload.height,
+        source=payload.source,
+        created_by_id=get_or_create_user(db, payload.created_by).id if payload.created_by else None,
+    )
+    db.add(label)
+    db.commit()
+    db.refresh(label)
+    return label
+
+
+@router.get("/frames/{frame_id}/sets/{set_id}/labels", response_model=list[schemas.LabelRead])
+def list_labels(frame_id: int, set_id: int, db: Session = Depends(get_db)):
+    _require_frame_and_set(db, frame_id, set_id)
+    return db.query(models.Label).filter_by(frame_id=frame_id, set_id=set_id).all()
+
+
+@router.patch("/labels/{label_id}", response_model=schemas.LabelRead)
+def update_label(label_id: int, payload: schemas.LabelUpdate, db: Session = Depends(get_db)):
+    label = db.get(models.Label, label_id)
+    if label is None:
+        raise HTTPException(404, "label not found")
+    for field, value in payload.model_dump(exclude_unset=True).items():
+        setattr(label, field, value)
+    db.commit()
+    db.refresh(label)
+    return label
+
+
+@router.delete("/labels/{label_id}", status_code=204)
+def delete_label(label_id: int, db: Session = Depends(get_db)):
+    label = db.get(models.Label, label_id)
+    if label is None:
+        raise HTTPException(404, "label not found")
+    db.delete(label)
+    db.commit()
+
+
+@router.get("/frames/{frame_id}/sets/{set_id}/status", response_model=schemas.FrameSetStatusRead)
+def get_status(frame_id: int, set_id: int, db: Session = Depends(get_db)):
+    _require_frame_and_set(db, frame_id, set_id)
+    status = db.query(models.FrameSetStatus).filter_by(frame_id=frame_id, set_id=set_id).first()
+    if status is None:
+        status = models.FrameSetStatus(frame_id=frame_id, set_id=set_id, status=models.LabelStatus.UNLABELED)
+        db.add(status)
+        db.commit()
+        db.refresh(status)
+    return status
+
+
+@router.put("/frames/{frame_id}/sets/{set_id}/status", response_model=schemas.FrameSetStatusRead)
+def set_status(frame_id: int, set_id: int, payload: schemas.FrameSetStatusUpdate, db: Session = Depends(get_db)):
+    _require_frame_and_set(db, frame_id, set_id)
+    status = db.query(models.FrameSetStatus).filter_by(frame_id=frame_id, set_id=set_id).first()
+    if status is None:
+        status = models.FrameSetStatus(frame_id=frame_id, set_id=set_id)
+        db.add(status)
+    status.status = payload.status
+    if payload.updated_by:
+        status.updated_by_id = get_or_create_user(db, payload.updated_by).id
+    db.commit()
+    db.refresh(status)
+    return status
diff --git a/labeling/backend/src/spelunkai_labeling_backend/routers/sets.py b/labeling/backend/src/spelunkai_labeling_backend/routers/sets.py
new file mode 100644
index 0000000..1978805
--- /dev/null
+++ b/labeling/backend/src/spelunkai_labeling_backend/routers/sets.py
@@ -0,0 +1,63 @@
+"""Label sets and their Main -> Sub class hierarchy."""
+from __future__ import annotations
+
+from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy.orm import Session
+
+from .. import models, schemas
+from ..db import get_db
+
+sets_router = APIRouter(prefix="/sets", tags=["sets"])
+main_classes_router = APIRouter(prefix="/main-classes", tags=["sets"])
+
+
+@sets_router.post("", response_model=schemas.LabelSetRead, status_code=201)
+def create_set(payload: schemas.LabelSetCreate, db: Session = Depends(get_db)):
+    if db.query(models.LabelSet).filter_by(name=payload.name).first():
+        raise HTTPException(409, f"set '{payload.name}' already exists")
+    label_set = models.LabelSet(name=payload.name, description=payload.description)
+    db.add(label_set)
+    db.commit()
+    db.refresh(label_set)
+    return label_set
+
+
+@sets_router.get("", response_model=list[schemas.LabelSetRead])
+def list_sets(db: Session = Depends(get_db)):
+    return db.query(models.LabelSet).all()
+
+
+@sets_router.get("/{set_id}", response_model=schemas.LabelSetRead)
+def get_set(set_id: int, db: Session = Depends(get_db)):
+    label_set = db.get(models.LabelSet, set_id)
+    if label_set is None:
+        raise HTTPException(404, "set not found")
+    return label_set
+
+
+@sets_router.post("/{set_id}/main-classes", response_model=schemas.MainClassRead, status_code=201)
+def create_main_class(set_id: int, payload: schemas.MainClassCreate, db: Session = Depends(get_db)):
+    label_set = db.get(models.LabelSet, set_id)
+    if label_set is None:
+        raise HTTPException(404, "set not found")
+    if db.query(models.MainClass).filter_by(set_id=set_id, name=payload.name).first():
+        raise HTTPException(409, f"main class '{payload.name}' already exists in this set")
+    main_class = models.MainClass(set_id=set_id, name=payload.name)
+    db.add(main_class)
+    db.commit()
+    db.refresh(main_class)
+    return main_class
+
+
+@main_classes_router.post("/{main_class_id}/sub-classes", response_model=schemas.SubClassRead, status_code=201)
+def create_sub_class(main_class_id: int, payload: schemas.SubClassCreate, db: Session = Depends(get_db)):
+    main_class = db.get(models.MainClass, main_class_id)
+    if main_class is None:
+        raise HTTPException(404, "main class not found")
+    if db.query(models.SubClass).filter_by(main_class_id=main_class_id, name=payload.name).first():
+        raise HTTPException(409, f"sub class '{payload.name}' already exists under this main class")
+    sub_class = models.SubClass(main_class_id=main_class_id, name=payload.name)
+    db.add(sub_class)
+    db.commit()
+    db.refresh(sub_class)
+    return sub_class
diff --git a/labeling/backend/src/spelunkai_labeling_backend/schemas.py b/labeling/backend/src/spelunkai_labeling_backend/schemas.py
new file mode 100644
index 0000000..5177137
--- /dev/null
+++ b/labeling/backend/src/spelunkai_labeling_backend/schemas.py
@@ -0,0 +1,109 @@
+"""Pydantic request/response schemas."""
+from __future__ import annotations
+
+from typing import Optional
+
+from pydantic import BaseModel, ConfigDict
+
+from .models import LabelSource, LabelStatus
+
+
+class SubClassCreate(BaseModel):
+    name: str
+
+
+class SubClassRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    name: str
+
+
+class MainClassCreate(BaseModel):
+    name: str
+
+
+class MainClassRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    name: str
+    sub_classes: list[SubClassRead] = []
+
+
+class LabelSetCreate(BaseModel):
+    name: str
+    description: Optional[str] = None
+
+
+class LabelSetRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    name: str
+    description: Optional[str] = None
+    main_classes: list[MainClassRead] = []
+
+
+class FrameCreate(BaseModel):
+    session_name: str
+    frame_index: int
+    image_path: str
+    width: int
+    height: int
+
+
+class FrameRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    session_name: str
+    frame_index: int
+    image_path: str
+    width: int
+    height: int
+
+
+class LabelCreate(BaseModel):
+    sub_class_id: int
+    x: float
+    y: float
+    width: float
+    height: float
+    source: LabelSource = LabelSource.MANUAL
+    created_by: Optional[str] = None
+
+
+class LabelUpdate(BaseModel):
+    sub_class_id: Optional[int] = None
+    x: Optional[float] = None
+    y: Optional[float] = None
+    width: Optional[float] = None
+    height: Optional[float] = None
+
+
+class LabelRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    frame_id: int
+    set_id: int
+    sub_class_id: int
+    x: float
+    y: float
+    width: float
+    height: float
+    source: LabelSource
+
+
+class FrameSetStatusRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    frame_id: int
+    set_id: int
+    status: LabelStatus
+
+
+class FrameSetStatusUpdate(BaseModel):
+    status: LabelStatus
+    updated_by: Optional[str] = None
diff --git a/labeling/backend/src/spelunkai_labeling_backend/users.py b/labeling/backend/src/spelunkai_labeling_backend/users.py
new file mode 100644
index 0000000..bbc266c
--- /dev/null
+++ b/labeling/backend/src/spelunkai_labeling_backend/users.py
@@ -0,0 +1,22 @@
+"""Lightweight user resolution.
+
+No login/auth flow yet — just get-or-create by username, so labels and
+status changes can be attributed to a person (CLAUDE.md §3.2 multi-user
+support) without building a full auth system before there's a UI to need it.
+"""
+from __future__ import annotations
+
+from sqlalchemy.orm import Session
+
+from . import models
+
+
+def get_or_create_user(db: Session, username: str) -> models.User:
+    user = db.query(models.User).filter_by(username=username).first()
+    if user is not None:
+        return user
+    user = models.User(username=username)
+    db.add(user)
+    db.commit()
+    db.refresh(user)
+    return user
diff --git a/labeling/backend/tests/conftest.py b/labeling/backend/tests/conftest.py
new file mode 100644
index 0000000..b78434f
--- /dev/null
+++ b/labeling/backend/tests/conftest.py
@@ -0,0 +1,12 @@
+import pytest
+from fastapi.testclient import TestClient
+
+from spelunkai_labeling_backend.main import create_app
+
+
+@pytest.fixture()
+def client(tmp_path):
+    db_path = tmp_path / "test.db"
+    app = create_app(database_url=f"sqlite:///{db_path}")
+    with TestClient(app) as test_client:
+        yield test_client
diff --git a/labeling/backend/tests/test_frames.py b/labeling/backend/tests/test_frames.py
new file mode 100644
index 0000000..b78578b
--- /dev/null
+++ b/labeling/backend/tests/test_frames.py
@@ -0,0 +1,32 @@
+def test_create_and_list_frames(client):
+    payload = {
+        "session_name": "run01",
+        "frame_index": 0,
+        "image_path": "run01_frames/frame_000000.png",
+        "width": 1280,
+        "height": 720,
+    }
+    resp = client.post("/frames", json=payload)
+    assert resp.status_code == 201
+    frame_id = resp.json()["id"]
+
+    resp = client.post("/frames", json=payload)
+    assert resp.status_code == 201
+    assert resp.json()["id"] == frame_id
+
+    resp = client.get("/frames", params={"session_name": "run01"})
+    assert resp.status_code == 200
+    assert len(resp.json()) == 1
+
+
+def test_list_frames_filters_by_session(client):
+    client.post("/frames", json={"session_name": "run01", "frame_index": 0, "image_path": "a.png", "width": 1, "height": 1})
+    client.post("/frames", json={"session_name": "run02", "frame_index": 0, "image_path": "b.png", "width": 1, "height": 1})
+
+    resp = client.get("/frames", params={"session_name": "run02"})
+    assert [f["image_path"] for f in resp.json()] == ["b.png"]
+
+
+def test_get_missing_frame(client):
+    resp = client.get("/frames/999")
+    assert resp.status_code == 404
diff --git a/labeling/backend/tests/test_labels.py b/labeling/backend/tests/test_labels.py
new file mode 100644
index 0000000..610ab2a
--- /dev/null
+++ b/labeling/backend/tests/test_labels.py
@@ -0,0 +1,75 @@
+def _make_set_with_subclass(client, name="Enemy", sub_name="Bat"):
+    set_id = client.post("/sets", json={"name": name}).json()["id"]
+    main_class_id = client.post(f"/sets/{set_id}/main-classes", json={"name": name}).json()["id"]
+    sub_class_id = client.post(f"/main-classes/{main_class_id}/sub-classes", json={"name": sub_name}).json()["id"]
+    return set_id, sub_class_id
+
+
+def _make_frame(client, session_name="run01", frame_index=0):
+    payload = {"session_name": session_name, "frame_index": frame_index, "image_path": "x.png", "width": 1280, "height": 720}
+    return client.post("/frames", json=payload).json()["id"]
+
+
+def test_create_list_update_delete_label(client):
+    set_id, sub_class_id = _make_set_with_subclass(client)
+    frame_id = _make_frame(client)
+
+    resp = client.post(
+        f"/frames/{frame_id}/sets/{set_id}/labels",
+        json={"sub_class_id": sub_class_id, "x": 10, "y": 20, "width": 30, "height": 40, "created_by": "jonas"},
+    )
+    assert resp.status_code == 201
+    label_id = resp.json()["id"]
+
+    resp = client.get(f"/frames/{frame_id}/sets/{set_id}/labels")
+    assert len(resp.json()) == 1
+
+    resp = client.patch(f"/labels/{label_id}", json={"x": 15})
+    assert resp.status_code == 200
+    assert resp.json()["x"] == 15
+
+    resp = client.delete(f"/labels/{label_id}")
+    assert resp.status_code == 204
+
+    resp = client.get(f"/frames/{frame_id}/sets/{set_id}/labels")
+    assert resp.json() == []
+
+
+def test_label_rejects_sub_class_from_another_set(client):
+    _, sub_class_id = _make_set_with_subclass(client, name="Enemy", sub_name="Bat")
+    other_set_id, _ = _make_set_with_subclass(client, name="Items", sub_name="Gold")
+    frame_id = _make_frame(client)
+
+    resp = client.post(
+        f"/frames/{frame_id}/sets/{other_set_id}/labels",
+        json={"sub_class_id": sub_class_id, "x": 0, "y": 0, "width": 1, "height": 1},
+    )
+    assert resp.status_code == 422
+
+
+def test_labels_require_existing_frame_and_set(client):
+    _, sub_class_id = _make_set_with_subclass(client)
+    resp = client.post(
+        "/frames/999/sets/999/labels",
+        json={"sub_class_id": sub_class_id, "x": 0, "y": 0, "width": 1, "height": 1},
+    )
+    assert resp.status_code == 404
+
+
+def test_frame_set_status_defaults_and_updates(client):
+    set_id, _ = _make_set_with_subclass(client)
+    frame_id = _make_frame(client)
+
+    resp = client.get(f"/frames/{frame_id}/sets/{set_id}/status")
+    assert resp.status_code == 200
+    assert resp.json()["status"] == "unlabeled"
+
+    resp = client.put(
+        f"/frames/{frame_id}/sets/{set_id}/status",
+        json={"status": "reviewed", "updated_by": "jonas"},
+    )
+    assert resp.status_code == 200
+    assert resp.json()["status"] == "reviewed"
+
+    resp = client.get(f"/frames/{frame_id}/sets/{set_id}/status")
+    assert resp.json()["status"] == "reviewed"
diff --git a/labeling/backend/tests/test_sets.py b/labeling/backend/tests/test_sets.py
new file mode 100644
index 0000000..2094f2a
--- /dev/null
+++ b/labeling/backend/tests/test_sets.py
@@ -0,0 +1,47 @@
+def test_create_and_list_sets(client):
+    resp = client.post("/sets", json={"name": "Enemy", "description": "Enemy sprites"})
+    assert resp.status_code == 201
+
+    resp = client.get("/sets")
+    assert resp.status_code == 200
+    assert [s["name"] for s in resp.json()] == ["Enemy"]
+
+
+def test_create_set_rejects_duplicate_name(client):
+    client.post("/sets", json={"name": "Enemy"})
+    resp = client.post("/sets", json={"name": "Enemy"})
+    assert resp.status_code == 409
+
+
+def test_get_missing_set(client):
+    resp = client.get("/sets/999")
+    assert resp.status_code == 404
+
+
+def test_create_main_and_sub_class(client):
+    set_id = client.post("/sets", json={"name": "Enemy"}).json()["id"]
+
+    resp = client.post(f"/sets/{set_id}/main-classes", json={"name": "Enemy"})
+    assert resp.status_code == 201
+    main_class_id = resp.json()["id"]
+
+    resp = client.post(f"/main-classes/{main_class_id}/sub-classes", json={"name": "Bat"})
+    assert resp.status_code == 201
+    assert resp.json()["name"] == "Bat"
+
+    resp = client.get(f"/sets/{set_id}")
+    assert resp.json()["main_classes"][0]["sub_classes"][0]["name"] == "Bat"
+
+
+def test_main_class_requires_existing_set(client):
+    resp = client.post("/sets/999/main-classes", json={"name": "Enemy"})
+    assert resp.status_code == 404
+
+
+def test_sub_class_rejects_duplicate_name_under_same_main_class(client):
+    set_id = client.post("/sets", json={"name": "Enemy"}).json()["id"]
+    main_class_id = client.post(f"/sets/{set_id}/main-classes", json={"name": "Enemy"}).json()["id"]
+    client.post(f"/main-classes/{main_class_id}/sub-classes", json={"name": "Bat"})
+
+    resp = client.post(f"/main-classes/{main_class_id}/sub-classes", json={"name": "Bat"})
+    assert resp.status_code == 409