diff --git a/app.py b/app.py
new file mode 100644
index 0000000..21fbb7e
--- /dev/null
+++ b/app.py
@@ -0,0 +1,114 @@
+"""FastAPI backend for garment photo cleaning."""
+
+import os
+from pathlib import Path
+from fastapi import FastAPI, File, Form, HTTPException, UploadFile, status
+from fastapi.middleware.cors import CORSMiddleware
+from fastapi.responses import FileResponse, JSONResponse
+from fastapi.staticfiles import StaticFiles
+
+from cleaner import (
+    DEFAULT_MODEL,
+    CleanerError,
+    _get_image_info,
+    build_prompt,
+    clean_garment,
+    encode_image_bytes,
+)
+
+DEFAULT_PROMPT = build_prompt("beige")
+STATIC_DIR = Path(__file__).resolve().parent / "static"
+STATIC_DIR.mkdir(parents=True, exist_ok=True)
+
+app = FastAPI(title="unwrap-clothes")
+
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=["*"],
+    allow_credentials=True,
+    allow_methods=["*"],
+    allow_headers=["*"],
+)
+
+
+@app.get("/api/health")
+def health_check():
+    return {"status": "ok"}
+
+
+@app.get("/")
+def read_root():
+    index_file = STATIC_DIR / "index.html"
+    if index_file.is_file():
+        return FileResponse(index_file)
+    return JSONResponse({"status": "ok", "message": "unwrap-clothes backend is running"})
+
+
+
+@app.post("/api/clean")
+async def clean_endpoint(
+    file: UploadFile = File(...),
+    prompt: str = Form(default=DEFAULT_PROMPT),
+    model: str = Form(default=DEFAULT_MODEL),
+    api_key: str | None = Form(default=None),
+    restore_res: bool = Form(default=True),
+):
+    effective_api_key = (api_key.strip() if api_key and api_key.strip() else None) or os.environ.get(
+        "OPENROUTER_API_KEY"
+    )
+    if not effective_api_key:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="API key is required. Provide it in the form or set OPENROUTER_API_KEY.",
+        )
+
+    file_bytes = await file.read()
+    if not file_bytes:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="Uploaded file is empty.",
+        )
+
+    orig_dims, _ = _get_image_info(file_bytes)
+    if orig_dims is None:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="Invalid image file. Could not decode image.",
+        )
+    try:
+        clean_result = clean_garment(
+            image_data=file_bytes,
+            prompt=prompt,
+            api_key=effective_api_key,
+            model=model,
+            restore_res=restore_res,
+        )
+    except CleanerError as exc:
+        raise HTTPException(
+            status_code=status.HTTP_502_BAD_GATEWAY,
+            detail=str(exc),
+        )
+
+    return {
+        "image": encode_image_bytes(clean_result.image_bytes, mime=clean_result.media_type),
+        "cost": clean_result.cost,
+        "width": clean_result.width,
+        "height": clean_result.height,
+        "original_dimensions": list(clean_result.original_dimensions)
+        if clean_result.original_dimensions
+        else None,
+        "model_dimensions": list(clean_result.model_dimensions)
+        if clean_result.model_dimensions
+        else None,
+        "was_rescaled": clean_result.was_rescaled,
+        "media_type": clean_result.media_type,
+    }
+
+if STATIC_DIR.is_dir():
+    app.mount("/static", StaticFiles(directory=STATIC_DIR), name="static")
+
+
+if __name__ == "__main__":
+    import uvicorn
+
+    uvicorn.run("app:app", host="127.0.0.1", port=8000, reload=True)
diff --git a/requirements.txt b/requirements.txt
index eb20200..13309ff 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,2 +1,5 @@
 requests>=2.28.0
-Pillow>=9.0.0
\ No newline at end of file
+Pillow>=9.0.0
+fastapi>=0.100.0
+uvicorn>=0.20.0
+python-multipart>=0.0.6
diff --git a/tests/test_app.py b/tests/test_app.py
new file mode 100644
index 0000000..8cba9cd
--- /dev/null
+++ b/tests/test_app.py
@@ -0,0 +1,217 @@
+"""Tests for FastAPI backend in app.py."""
+
+import base64
+import io
+from unittest.mock import patch
+
+from fastapi.testclient import TestClient
+from PIL import Image
+
+from cleaner import CleanerError, CleanResult
+
+
+def test_health_check():
+    """GET /api/health returns 200 OK with status ok."""
+    from app import app
+
+    client = TestClient(app)
+    response = client.get("/api/health")
+    assert response.status_code == 200
+    assert response.json() == {"status": "ok"}
+
+
+def test_root_fallback_when_no_index_html(monkeypatch, tmp_path):
+    """GET / returns a graceful fallback when static/index.html does not exist."""
+    import app as app_module
+
+    # Point STATIC_DIR to a temporary directory without index.html
+    monkeypatch.setattr(app_module, "STATIC_DIR", tmp_path)
+    client = TestClient(app_module.app)
+    response = client.get("/")
+    assert response.status_code == 200
+    assert response.json()["status"] == "ok"
+
+
+def test_root_serves_index_html_when_present(monkeypatch, tmp_path):
+    """GET / serves static/index.html when it exists."""
+    import app as app_module
+
+    index_file = tmp_path / "index.html"
+    index_file.write_text("<!DOCTYPE html><html><body>Unwrap Clothes</body></html>")
+    monkeypatch.setattr(app_module, "STATIC_DIR", tmp_path)
+
+    client = TestClient(app_module.app)
+    response = client.get("/")
+    assert response.status_code == 200
+    assert "Unwrap Clothes" in response.text
+    assert "text/html" in response.headers.get("content-type", "")
+
+
+def _create_test_image_bytes() -> bytes:
+    buf = io.BytesIO()
+    Image.new("RGB", (50, 50), color="blue").save(buf, format="JPEG")
+    return buf.getvalue()
+
+
+def test_clean_missing_api_key_returns_400(monkeypatch):
+    """POST /api/clean without API key when OPENROUTER_API_KEY is unset returns 400."""
+    from app import app
+
+    monkeypatch.delenv("OPENROUTER_API_KEY", raising=False)
+    client = TestClient(app)
+    image_bytes = _create_test_image_bytes()
+    files = {"file": ("test.jpg", image_bytes, "image/jpeg")}
+
+    response = client.post("/api/clean", files=files)
+    assert response.status_code == 400
+    assert "API key" in response.json()["detail"]
+
+
+def test_clean_uses_env_api_key_when_form_omitted(monkeypatch):
+    """POST /api/clean uses OPENROUTER_API_KEY from environment if api_key form field is omitted."""
+    from app import app
+
+    monkeypatch.setenv("OPENROUTER_API_KEY", "sk-or-v1-env-key")
+    client = TestClient(app)
+    image_bytes = _create_test_image_bytes()
+    files = {"file": ("test.jpg", image_bytes, "image/jpeg")}
+
+    mock_result = CleanResult(
+        image_bytes=image_bytes,
+        media_type="image/jpeg",
+        cost=0.005,
+        width=50,
+        height=50,
+        original_dimensions=(50, 50),
+        model_dimensions=(50, 50),
+    )
+    with patch("app.clean_garment", return_value=mock_result) as mock_clean:
+        response = client.post("/api/clean", files=files)
+        assert response.status_code == 200
+        mock_clean.assert_called_once()
+        _, kwargs = mock_clean.call_args
+        assert kwargs["api_key"] == "sk-or-v1-env-key"
+
+
+def test_clean_empty_file_returns_400(monkeypatch):
+    """POST /api/clean with empty file returns 400 Bad Request."""
+    from app import app
+
+    monkeypatch.setenv("OPENROUTER_API_KEY", "sk-or-v1-env-key")
+    client = TestClient(app)
+    files = {"file": ("empty.jpg", b"", "image/jpeg")}
+
+    response = client.post("/api/clean", files=files)
+    assert response.status_code == 400
+    assert "empty" in response.json()["detail"].lower()
+
+
+def test_clean_invalid_image_file_returns_400(monkeypatch):
+    """POST /api/clean with invalid image content returns 400 Bad Request."""
+    from app import app
+
+    monkeypatch.setenv("OPENROUTER_API_KEY", "sk-or-v1-env-key")
+    client = TestClient(app)
+    files = {"file": ("corrupt.jpg", b"not-a-valid-image-content", "image/jpeg")}
+
+    response = client.post("/api/clean", files=files)
+    assert response.status_code == 400
+    assert "invalid" in response.json()["detail"].lower()
+
+
+def test_clean_cleaner_error_returns_502(monkeypatch):
+    """POST /api/clean when clean_garment raises CleanerError returns 502 Bad Gateway."""
+    from app import app
+
+    monkeypatch.setenv("OPENROUTER_API_KEY", "sk-or-v1-env-key")
+    client = TestClient(app)
+    image_bytes = _create_test_image_bytes()
+    files = {"file": ("test.jpg", image_bytes, "image/jpeg")}
+
+    with patch("app.clean_garment", side_effect=CleanerError("OpenRouter upstream failure")):
+        response = client.post("/api/clean", files=files)
+        assert response.status_code == 502
+        assert "OpenRouter upstream failure" in response.json()["detail"]
+
+
+def test_clean_success_happy_path():
+    """POST /api/clean with valid inputs returns full JSON result with image data URI."""
+    from app import app
+
+    client = TestClient(app)
+    image_bytes = _create_test_image_bytes()
+    output_bytes = b"cleaned-image-data-bytes"
+    mock_result = CleanResult(
+        image_bytes=output_bytes,
+        media_type="image/png",
+        cost=0.0125,
+        width=1000,
+        height=1500,
+        original_dimensions=(1000, 1500),
+        model_dimensions=(800, 1200),
+    )
+
+    files = {"file": ("garment.jpg", image_bytes, "image/jpeg")}
+    data = {
+        "prompt": "Custom studio prompt with white background.",
+        "model": "custom/test-model",
+        "api_key": "sk-or-v1-custom-form-key",
+        "restore_res": "true",
+    }
+
+    with patch("app.clean_garment", return_value=mock_result) as mock_clean:
+        response = client.post("/api/clean", files=files, data=data)
+        assert response.status_code == 200
+        payload = response.json()
+
+        # Assert response keys and values
+        expected_data_uri = f"data:image/png;base64,{base64.b64encode(output_bytes).decode()}"
+        assert payload["image"] == expected_data_uri
+        assert payload["cost"] == 0.0125
+        assert payload["width"] == 1000
+        assert payload["height"] == 1500
+        assert payload["original_dimensions"] == [1000, 1500]
+        assert payload["model_dimensions"] == [800, 1200]
+        assert payload["was_rescaled"] is True
+        assert payload["media_type"] == "image/png"
+
+        # Assert arguments passed to clean_garment
+        mock_clean.assert_called_once_with(
+            image_data=image_bytes,
+            prompt="Custom studio prompt with white background.",
+            api_key="sk-or-v1-custom-form-key",
+            model="custom/test-model",
+            restore_res=True,
+        )
+
+
+def test_clean_default_parameters():
+    """POST /api/clean uses default prompt, model, and restore_res if omitted in form."""
+    from app import app, DEFAULT_PROMPT
+    from cleaner import DEFAULT_MODEL
+
+    client = TestClient(app)
+    image_bytes = _create_test_image_bytes()
+    mock_result = CleanResult(
+        image_bytes=image_bytes,
+        media_type="image/jpeg",
+        cost=None,
+        width=50,
+        height=50,
+        original_dimensions=(50, 50),
+        model_dimensions=(50, 50),
+    )
+
+    files = {"file": ("garment.jpg", image_bytes, "image/jpeg")}
+    data = {"api_key": "sk-or-v1-key"}
+
+    with patch("app.clean_garment", return_value=mock_result) as mock_clean:
+        response = client.post("/api/clean", files=files, data=data)
+        assert response.status_code == 200
+        mock_clean.assert_called_once_with(
+            image_data=image_bytes,
+            prompt=DEFAULT_PROMPT,
+            api_key="sk-or-v1-key",
+            model=DEFAULT_MODEL,
+            restore_res=True,
+        )