diff --git a/.specify/feature.json b/.specify/feature.json
index 6fdb4b6..f2cfdf7 100644
--- a/.specify/feature.json
+++ b/.specify/feature.json
@@ -1,3 +1,3 @@
 {
-  "feature_directory": "specs/002-api-image-proxy"
+  "feature_directory": "specs/003-upload-thumbnails"
 }
diff --git a/CLAUDE.md b/CLAUDE.md
index 37de3a5..f159ce4 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -1,5 +1,5 @@
 <!-- SPECKIT START -->
 For additional context about technologies to be used, project structure,
 shell commands, and other important information, read the current plan at
-`specs/002-api-image-proxy/plan.md`.
+`specs/003-upload-thumbnails/plan.md`.
 <!-- SPECKIT END -->
diff --git a/api/alembic/versions/002_add_thumbnail_key.py b/api/alembic/versions/002_add_thumbnail_key.py
new file mode 100644
index 0000000..5836164
--- /dev/null
+++ b/api/alembic/versions/002_add_thumbnail_key.py
@@ -0,0 +1,23 @@
+"""add thumbnail_key column to images
+
+Revision ID: 002
+Revises: 001
+Create Date: 2026-05-03
+
+"""
+from typing import Sequence, Union
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "002"
+down_revision: Union[str, None] = "001"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("images", sa.Column("thumbnail_key", sa.String(70), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("images", "thumbnail_key")
diff --git a/api/app/models.py b/api/app/models.py
index ab6af5a..f7357b5 100644
--- a/api/app/models.py
+++ b/api/app/models.py
@@ -23,6 +23,7 @@ class Image(Base):
     width: Mapped[int] = mapped_column(Integer, nullable=False)
     height: Mapped[int] = mapped_column(Integer, nullable=False)
     storage_key: Mapped[str] = mapped_column(String(64), nullable=False)
+    thumbnail_key: Mapped[str | None] = mapped_column(String(70), nullable=True, default=None)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=_utcnow, nullable=False)
 
     image_tags: Mapped[list["ImageTag"]] = relationship(back_populates="image", cascade="all, delete-orphan")
diff --git a/api/app/repositories/image_repo.py b/api/app/repositories/image_repo.py
index 3ded93b..768858c 100644
--- a/api/app/repositories/image_repo.py
+++ b/api/app/repositories/image_repo.py
@@ -34,6 +34,7 @@ class ImageRepository:
         width: int,
         height: int,
         storage_key: str,
+        thumbnail_key: str | None = None,
     ) -> Image:
         image = Image(
             hash=hash_hex,
@@ -43,6 +44,7 @@ class ImageRepository:
             width=width,
             height=height,
             storage_key=storage_key,
+            thumbnail_key=thumbnail_key,
         )
         self._session.add(image)
         await self._session.flush()
diff --git a/api/app/routers/images.py b/api/app/routers/images.py
index 6b338db..5ef59fe 100644
--- a/api/app/routers/images.py
+++ b/api/app/routers/images.py
@@ -1,3 +1,5 @@
+import asyncio
+import logging
 import struct
 import uuid
 from typing import Any
@@ -12,9 +14,12 @@ from app.models import Image
 from app.repositories.image_repo import ImageRepository
 from app.repositories.tag_repo import TagRepository
 from app.storage.backend import StorageBackend
+from app.thumbnail import generate_thumbnail
 from app.utils import compute_sha256
 from app.validation import FileSizeError, MimeTypeError, validate_file_size, validate_mime_type
 
+logger = logging.getLogger(__name__)
+
 router = APIRouter(tags=["images"])
 
 
@@ -32,6 +37,7 @@ def _image_to_dict(image: Image, *, duplicate: bool | None = None) -> dict[str,
         "width": image.width,
         "height": image.height,
         "storage_key": image.storage_key,
+        "thumbnail_key": image.thumbnail_key,
         "created_at": image.created_at.isoformat(),
         "tags": image.tags,
     }
@@ -151,6 +157,14 @@ async def upload_image(
     width, height = _read_image_dimensions(data, mime_type)
     await storage.put(hash_hex, data, mime_type)
 
+    thumbnail_key: str | None = None
+    try:
+        thumb_bytes = await asyncio.to_thread(generate_thumbnail, data, mime_type)
+        await storage.put(f"{hash_hex}-thumb", thumb_bytes, "image/webp")
+        thumbnail_key = f"{hash_hex}-thumb"
+    except Exception:
+        logger.warning("Thumbnail generation failed for %s; upload will proceed without thumbnail", hash_hex)
+
     image = await image_repo.create(
         hash_hex=hash_hex,
         filename=file.filename or "upload",
@@ -159,6 +173,7 @@ async def upload_image(
         width=width,
         height=height,
         storage_key=hash_hex,
+        thumbnail_key=thumbnail_key,
     )
 
     if tag_names:
@@ -233,6 +248,38 @@ async def serve_image_file(
     )
 
 
+@router.get("/images/{image_id}/thumbnail")
+async def serve_image_thumbnail(
+    image_id: uuid.UUID,
+    db: AsyncSession = Depends(get_db),
+    storage: StorageBackend = Depends(get_storage),
+):
+    image_repo = ImageRepository(db)
+    image = await image_repo.get_by_id(image_id)
+    if not image:
+        raise HTTPException(
+            status_code=404,
+            detail={"detail": "Image not found", "code": "image_not_found"},
+        )
+    key = image.thumbnail_key or image.storage_key
+    media_type = "image/webp" if image.thumbnail_key else image.mime_type
+    try:
+        data = await storage.get(key)
+    except Exception:
+        raise HTTPException(
+            status_code=500,
+            detail={"detail": "Failed to retrieve image content", "code": "storage_error"},
+        ) from None
+    return Response(
+        content=data,
+        media_type=media_type,
+        headers={
+            "ETag": f'"{image.hash}"',
+            "Cache-Control": "public, max-age=31536000, immutable",
+        },
+    )
+
+
 @router.patch("/images/{image_id}/tags")
 async def update_image_tags(
     image_id: uuid.UUID,
@@ -276,6 +323,9 @@ async def delete_image(
             detail={"detail": "Image not found", "code": "image_not_found"},
         )
     storage_key = image.storage_key
+    thumbnail_key = image.thumbnail_key
     await image_repo.delete(image)
     await storage.delete(storage_key)
+    if thumbnail_key:
+        await storage.delete(thumbnail_key)
     return Response(status_code=204)
diff --git a/api/app/thumbnail.py b/api/app/thumbnail.py
new file mode 100644
index 0000000..71292c5
--- /dev/null
+++ b/api/app/thumbnail.py
@@ -0,0 +1,16 @@
+import contextlib
+import io
+
+from PIL import Image
+
+
+def generate_thumbnail(data: bytes, mime_type: str) -> bytes:
+    img = Image.open(io.BytesIO(data))
+    with contextlib.suppress(EOFError):
+        img.seek(0)
+    if img.mode not in ("RGB", "RGBA"):
+        img = img.convert("RGBA" if img.mode == "P" and "transparency" in img.info else "RGB")
+    img.thumbnail((400, 400), Image.LANCZOS)
+    buf = io.BytesIO()
+    img.save(buf, format="WEBP", quality=80)
+    return buf.getvalue()
diff --git a/api/pyproject.toml b/api/pyproject.toml
index 506d831..0f432c0 100644
--- a/api/pyproject.toml
+++ b/api/pyproject.toml
@@ -15,6 +15,7 @@ dependencies = [
     "aiobotocore>=2.13",
     "pydantic-settings>=2.2",
     "python-multipart>=0.0.9",
+    "pillow>=10.0",
 ]
 
 [project.optional-dependencies]
diff --git a/api/tests/integration/test_delete.py b/api/tests/integration/test_delete.py
index 1e7562a..ca5f968 100644
--- a/api/tests/integration/test_delete.py
+++ b/api/tests/integration/test_delete.py
@@ -5,7 +5,9 @@ T067 — DELETE of unknown ID → 404 image_not_found
 """
 import io
 import uuid
+
 import pytest
+from PIL import Image as PILImage
 
 
 def _minimal_jpeg_v2() -> bytes:
@@ -58,3 +60,25 @@ async def test_delete_unknown_id_returns_404(client):
     assert response.status_code == 404
     body = response.json()
     assert body["code"] == "image_not_found"
+
+
+@pytest.mark.asyncio
+async def test_delete_removes_thumbnail(client):
+    buf = io.BytesIO()
+    PILImage.new("RGB", (200, 150), color=(60, 90, 120)).save(buf, format="JPEG")
+    data = buf.getvalue()
+
+    upload = await client.post(
+        "/api/v1/images",
+        files={"file": ("thumb-del.jpg", io.BytesIO(data), "image/jpeg")},
+    )
+    assert upload.status_code == 201
+    image_id = upload.json()["id"]
+    assert upload.json()["thumbnail_key"] is not None
+
+    delete_resp = await client.delete(f"/api/v1/images/{image_id}")
+    assert delete_resp.status_code == 204
+
+    thumb_resp = await client.get(f"/api/v1/images/{image_id}/thumbnail")
+    assert thumb_resp.status_code == 404
+    assert thumb_resp.json()["code"] == "image_not_found"
diff --git a/api/tests/integration/test_serving.py b/api/tests/integration/test_serving.py
index 14e2725..cb5ecbe 100644
--- a/api/tests/integration/test_serving.py
+++ b/api/tests/integration/test_serving.py
@@ -7,6 +7,16 @@ import io
 import uuid
 
 import pytest
+from PIL import Image as PILImage
+from sqlalchemy import update
+
+from app.models import Image
+
+
+def _real_jpeg() -> bytes:
+    buf = io.BytesIO()
+    PILImage.new("RGB", (200, 150), color=(120, 80, 200)).save(buf, format="JPEG")
+    return buf.getvalue()
 
 
 def _minimal_webp() -> bytes:
@@ -62,3 +72,53 @@ async def test_file_response_exposes_no_storage_details(client):
     assert "minio" not in response.text.lower()
     assert "s3://" not in response.text.lower()
     assert "amazonaws.com" not in response.text.lower()
+
+
+@pytest.mark.asyncio
+async def test_thumbnail_returns_webp(client):
+    data = _real_jpeg()
+    upload = await client.post(
+        "/api/v1/images",
+        files={"file": ("t.jpg", io.BytesIO(data), "image/jpeg")},
+    )
+    assert upload.status_code == 201
+    body = upload.json()
+    image_id = body["id"]
+    image_hash = body["hash"]
+
+    response = await client.get(f"/api/v1/images/{image_id}/thumbnail")
+    assert response.status_code == 200
+    assert response.headers["content-type"] == "image/webp"
+    assert response.headers["etag"] == f'"{image_hash}"'
+    assert "immutable" in response.headers["cache-control"]
+    assert len(response.content) > 0
+
+
+@pytest.mark.asyncio
+async def test_thumbnail_fallback_returns_original(client, db_session):
+    data = _real_jpeg()
+    upload = await client.post(
+        "/api/v1/images",
+        files={"file": ("fallback.jpg", io.BytesIO(data), "image/jpeg")},
+    )
+    assert upload.status_code == 201
+    image_id = upload.json()["id"]
+
+    await db_session.execute(
+        update(Image).where(Image.id == uuid.UUID(image_id)).values(thumbnail_key=None)
+    )
+    await db_session.flush()
+    db_session.expire_all()
+
+    response = await client.get(f"/api/v1/images/{image_id}/thumbnail")
+    assert response.status_code == 200
+    assert "image/jpeg" in response.headers["content-type"]
+    assert len(response.content) > 0
+
+
+@pytest.mark.asyncio
+async def test_thumbnail_unknown_id_returns_404(client):
+    response = await client.get(f"/api/v1/images/{uuid.uuid4()}/thumbnail")
+    assert response.status_code == 404
+    body = response.json()
+    assert body["code"] == "image_not_found"
diff --git a/api/tests/integration/test_upload.py b/api/tests/integration/test_upload.py
index 8125612..3b8fe30 100644
--- a/api/tests/integration/test_upload.py
+++ b/api/tests/integration/test_upload.py
@@ -6,7 +6,16 @@ T029 — file > MAX_UPLOAD_BYTES → 422 file_too_large
 T079 — GET /api/v1/images/{id} 404 → error envelope shape
 """
 import io
+from unittest.mock import patch
+
 import pytest
+from PIL import Image as PILImage
+
+
+def _real_jpeg(color: tuple = (100, 150, 200), size: tuple = (200, 150)) -> bytes:
+    buf = io.BytesIO()
+    PILImage.new("RGB", size, color=color).save(buf, format="JPEG")
+    return buf.getvalue()
 
 
 def _minimal_jpeg() -> bytes:
@@ -96,3 +105,51 @@ async def test_get_unknown_image_returns_404_with_envelope(client):
     body = response.json()
     assert body["code"] == "image_not_found"
     assert "detail" in body
+
+
+@pytest.mark.asyncio
+async def test_upload_returns_thumbnail_key(client):
+    data = _real_jpeg(color=(100, 150, 200))
+    response = await client.post(
+        "/api/v1/images",
+        files={"file": ("thumb_test.jpg", io.BytesIO(data), "image/jpeg")},
+    )
+    assert response.status_code == 201
+    body = response.json()
+    assert "thumbnail_key" in body
+    assert body["thumbnail_key"] is not None
+    assert body["thumbnail_key"].endswith("-thumb")
+
+
+@pytest.mark.asyncio
+async def test_duplicate_upload_reuses_thumbnail_key(client):
+    data = _real_jpeg(color=(200, 100, 50))
+    r1 = await client.post(
+        "/api/v1/images",
+        files={"file": ("dup.jpg", io.BytesIO(data), "image/jpeg")},
+    )
+    assert r1.status_code in (200, 201)
+
+    r2 = await client.post(
+        "/api/v1/images",
+        files={"file": ("dup.jpg", io.BytesIO(data), "image/jpeg")},
+    )
+    assert r2.status_code == 200
+
+    tk1 = r1.json()["thumbnail_key"]
+    tk2 = r2.json()["thumbnail_key"]
+    assert tk1 is not None
+    assert tk1 == tk2
+
+
+@pytest.mark.asyncio
+async def test_upload_succeeds_when_thumbnail_fails(client):
+    data = _real_jpeg(color=(50, 200, 150))
+    with patch("app.routers.images.generate_thumbnail", side_effect=RuntimeError("simulated")):
+        response = await client.post(
+            "/api/v1/images",
+            files={"file": ("no_thumb.jpg", io.BytesIO(data), "image/jpeg")},
+        )
+    assert response.status_code in (200, 201)
+    body = response.json()
+    assert body["thumbnail_key"] is None
diff --git a/api/tests/unit/test_thumbnail.py b/api/tests/unit/test_thumbnail.py
new file mode 100644
index 0000000..b1435e3
--- /dev/null
+++ b/api/tests/unit/test_thumbnail.py
@@ -0,0 +1,79 @@
+"""Unit tests for thumbnail generation utility."""
+import io
+
+from PIL import Image as PILImage
+
+from app.thumbnail import generate_thumbnail
+
+
+def _make_jpeg(width: int, height: int) -> bytes:
+    buf = io.BytesIO()
+    img = PILImage.new("RGB", (width, height), color=(128, 64, 32))
+    img.save(buf, format="JPEG", quality=80)
+    return buf.getvalue()
+
+
+def _make_png_rgba(width: int, height: int) -> bytes:
+    buf = io.BytesIO()
+    img = PILImage.new("RGBA", (width, height), color=(10, 20, 30, 180))
+    img.save(buf, format="PNG")
+    return buf.getvalue()
+
+
+def _make_gif(width: int, height: int) -> bytes:
+    buf = io.BytesIO()
+    img = PILImage.new("P", (width, height))
+    img.save(buf, format="GIF")
+    return buf.getvalue()
+
+
+def test_thumbnail_is_webp():
+    data = _make_jpeg(600, 400)
+    result = generate_thumbnail(data, "image/jpeg")
+    assert result[:4] == b"RIFF"
+    assert result[8:12] == b"WEBP"
+
+
+def test_thumbnail_fits_within_400px():
+    data = _make_jpeg(800, 600)
+    result = generate_thumbnail(data, "image/jpeg")
+    img = PILImage.open(io.BytesIO(result))
+    w, h = img.size
+    assert w <= 400
+    assert h <= 400
+
+
+def test_thumbnail_preserves_aspect_ratio():
+    original_w, original_h = 800, 300
+    data = _make_jpeg(original_w, original_h)
+    result = generate_thumbnail(data, "image/jpeg")
+    img = PILImage.open(io.BytesIO(result))
+    w, h = img.size
+    original_ratio = original_w / original_h
+    thumb_ratio = w / h
+    assert abs(original_ratio - thumb_ratio) / original_ratio < 0.01
+
+
+def test_thumbnail_handles_gif_first_frame():
+    data = _make_gif(500, 500)
+    result = generate_thumbnail(data, "image/gif")
+    assert result[8:12] == b"WEBP"
+    img = PILImage.open(io.BytesIO(result))
+    assert not getattr(img, "is_animated", False)
+
+
+def test_thumbnail_handles_png_with_alpha():
+    data = _make_png_rgba(300, 300)
+    result = generate_thumbnail(data, "image/png")
+    assert result[8:12] == b"WEBP"
+    img = PILImage.open(io.BytesIO(result))
+    assert img.format == "WEBP"
+
+
+def test_thumbnail_does_not_upscale():
+    data = _make_jpeg(100, 100)
+    result = generate_thumbnail(data, "image/jpeg")
+    img = PILImage.open(io.BytesIO(result))
+    w, h = img.size
+    assert w <= 100
+    assert h <= 100
diff --git a/specs/003-upload-thumbnails/checklists/requirements.md b/specs/003-upload-thumbnails/checklists/requirements.md
new file mode 100644
index 0000000..86fdc87
--- /dev/null
+++ b/specs/003-upload-thumbnails/checklists/requirements.md
@@ -0,0 +1,34 @@
+# Specification Quality Checklist: Upload Thumbnails
+
+**Purpose**: Validate specification completeness and quality before proceeding to planning
+**Created**: 2026-05-03
+**Feature**: [spec.md](../spec.md)
+
+## Content Quality
+
+- [x] No implementation details (languages, frameworks, APIs)
+- [x] Focused on user value and business needs
+- [x] Written for non-technical stakeholders
+- [x] All mandatory sections completed
+
+## Requirement Completeness
+
+- [x] No [NEEDS CLARIFICATION] markers remain
+- [x] Requirements are testable and unambiguous
+- [x] Success criteria are measurable
+- [x] Success criteria are technology-agnostic (no implementation details)
+- [x] All acceptance scenarios are defined
+- [x] Edge cases are identified
+- [x] Scope is clearly bounded
+- [x] Dependencies and assumptions identified
+
+## Feature Readiness
+
+- [x] All functional requirements have clear acceptance criteria
+- [x] User scenarios cover primary flows
+- [x] Feature meets measurable outcomes defined in Success Criteria
+- [x] No implementation details leak into specification
+
+## Notes
+
+All checklist items pass. Spec is ready for `/speckit-plan`.
diff --git a/specs/003-upload-thumbnails/contracts/api.md b/specs/003-upload-thumbnails/contracts/api.md
new file mode 100644
index 0000000..d07ce10
--- /dev/null
+++ b/specs/003-upload-thumbnails/contracts/api.md
@@ -0,0 +1,90 @@
+# API Contract: Upload Thumbnails
+
+**Branch**: `003-upload-thumbnails` | **Date**: 2026-05-03
+
+---
+
+## New endpoint
+
+### `GET /api/v1/images/{image_id}/thumbnail`
+
+Returns the thumbnail content for the given image. If no thumbnail was generated
+(image pre-dates the feature or generation failed), falls back to the full-size
+original.
+
+**Path parameters**
+
+| Parameter | Type | Description |
+|-----------|------|-------------|
+| `image_id` | UUID | Unique identifier of the image |
+
+**Responses**
+
+#### `200 OK` — Thumbnail (or original fallback) content
+
+| Header | Value | Notes |
+|--------|-------|-------|
+| `Content-Type` | `image/webp` | Always WebP when a thumbnail exists; original `mime_type` when falling back to the original |
+| `ETag` | `"{sha256-hex}"` | Same hash as the original image — content is immutable |
+| `Cache-Control` | `public, max-age=31536000, immutable` | Safe: thumbnail content never changes |
+
+Body: raw image bytes (WebP thumbnail, or original bytes as fallback).
+
+#### `404 Not Found` — Image not found
+
+```json
+{ "detail": "Image not found", "code": "image_not_found" }
+```
+
+#### `500 Internal Server Error` — Storage retrieval failure
+
+```json
+{ "detail": "Failed to retrieve image content", "code": "storage_error" }
+```
+
+---
+
+## Changed endpoint: `POST /api/v1/images`
+
+The upload response body gains one new field:
+
+| Field | Type | Notes |
+|-------|------|-------|
+| `thumbnail_key` | `string \| null` | S3 key of the generated thumbnail. `null` if generation failed. |
+
+All existing fields are unchanged.
+
+**Example response** (new field only shown):
+```json
+{
+  "id": "...",
+  "thumbnail_key": "abc123…-thumb",
+  ...
+}
+```
+
+---
+
+## Changed endpoint: `GET /api/v1/images` and `GET /api/v1/images/{id}`
+
+Both metadata responses gain the same `thumbnail_key` field (`string | null`).
+
+---
+
+## UI contract
+
+The Angular `ImageService` gains one new method:
+
+```
+getThumbnailUrl(id: string): string
+  → '/api/v1/images/{id}/thumbnail'
+```
+
+The `ImageRecord` interface gains:
+
+```
+thumbnail_key: string | null;
+```
+
+The library grid component uses `getThumbnailUrl(image.id)` as the `src` for
+every grid cell. The detail component continues using `getFileUrl(image.id)`.
diff --git a/specs/003-upload-thumbnails/data-model.md b/specs/003-upload-thumbnails/data-model.md
new file mode 100644
index 0000000..30918b9
--- /dev/null
+++ b/specs/003-upload-thumbnails/data-model.md
@@ -0,0 +1,79 @@
+# Data Model: Upload Thumbnails
+
+**Branch**: `003-upload-thumbnails` | **Date**: 2026-05-03
+
+## Schema change: `images` table
+
+One nullable column is added to the existing `images` table.
+
+| Column | Type | Nullable | Default | Notes |
+|--------|------|----------|---------|-------|
+| `thumbnail_key` | `VARCHAR(70)` | YES | `NULL` | S3 object key for the WebP thumbnail. `NULL` = no thumbnail available (generation failed or pre-dates this feature). Derived value: `{image.hash}-thumb`. |
+
+No other tables change. No new tables are added.
+
+### Migration
+
+**File**: `api/alembic/versions/002_add_thumbnail_key.py`
+
+```
+upgrade:   ALTER TABLE images ADD COLUMN thumbnail_key VARCHAR(70);
+downgrade: ALTER TABLE images DROP COLUMN thumbnail_key;
+```
+
+---
+
+## ORM model change: `Image`
+
+`api/app/models.py` — `Image` class gains one field:
+
+```
+thumbnail_key: Mapped[str | None] = mapped_column(String(70), nullable=True, default=None)
+```
+
+---
+
+## New module: `api/app/thumbnail.py`
+
+Contains the thumbnail generation logic. Not a model, but documented here because
+it defines the thumbnail's shape:
+
+| Aspect | Value |
+|--------|-------|
+| Output format | WebP |
+| Max dimension (longest side) | 400 px |
+| Aspect ratio | Preserved (never upscaled) |
+| Source formats supported | JPEG, PNG, GIF (frame 0), WebP |
+| Key signature | `async def generate_thumbnail(data: bytes, mime_type: str) -> bytes` |
+
+---
+
+## API response shape change
+
+`_image_to_dict()` in `api/app/routers/images.py` adds `"thumbnail_key"` to its
+output so the UI can determine whether a thumbnail is available:
+
+```json
+{
+  "id": "...",
+  "hash": "...",
+  "thumbnail_key": "abc123...-thumb",   ← new (null if no thumbnail)
+  ...
+}
+```
+
+The UI uses the presence of `thumbnail_key` to decide whether to call
+`/api/v1/images/{id}/thumbnail` (with thumbnail) or fall back to
+`/api/v1/images/{id}/file` (without). In practice the endpoint itself
+handles the fallback, so the UI can always call `/thumbnail`.
+
+---
+
+## Storage objects per image (after this feature)
+
+| Object | Key | Format | Created at |
+|--------|-----|--------|-----------|
+| Original | `{sha256_hash}` | Original mime_type | Upload |
+| Thumbnail | `{sha256_hash}-thumb` | `image/webp` | Upload (same request) |
+
+Thumbnail object is deleted alongside original on image deletion.
diff --git a/specs/003-upload-thumbnails/plan.md b/specs/003-upload-thumbnails/plan.md
new file mode 100644
index 0000000..11fbc89
--- /dev/null
+++ b/specs/003-upload-thumbnails/plan.md
@@ -0,0 +1,246 @@
+# Implementation Plan: Upload Thumbnails
+
+**Branch**: `003-upload-thumbnails` | **Date**: 2026-05-03 | **Spec**: [spec.md](spec.md)
+**Input**: Feature specification from `specs/003-upload-thumbnails/spec.md`
+
+## Summary
+
+When an image is uploaded, generate a WebP thumbnail (longest side ≤ 400 px,
+aspect ratio preserved) and store it in S3 alongside the original. Add a
+`GET /api/v1/images/{id}/thumbnail` endpoint that serves the thumbnail (or falls
+back to the original for images that pre-date the feature). The Angular library
+grid switches from `/file` to `/thumbnail`. The detail page is unchanged.
+
+Changes span: a new Pillow dependency, a new `thumbnail.py` utility module, one
+Alembic migration, the upload and delete routes, a new thumbnail serve route, and
+the Angular image service and library component.
+
+## Technical Context
+
+**Language/Version**: Python 3.12+ (API); TypeScript strict mode (UI)
+**Primary Dependencies**: FastAPI, Pillow (new — thumbnail generation), aiobotocore,
+  SQLAlchemy 2.x async, Alembic, Angular
+**Storage**: S3-compatible object storage via `StorageBackend.put()` and `.get()`;
+  thumbnails stored at key `{sha256_hash}-thumb` in the same bucket
+**Testing**: pytest + pytest-asyncio (API); Angular Karma/Jest + TestBed (UI)
+**Target Platform**: Linux server (containerised); modern evergreen desktop browsers
+**Project Type**: Web application — FastAPI API + Angular SPA
+**Performance Goals**: 20-image grid transfers ≥ 80% less data than full-size;
+  first page of 1,000-image library loads in under 2 s
+**Constraints**: Thumbnail generation is synchronous within the upload request;
+  thumbnail failure must not block upload success; no backfill of existing images in v1
+**Scale/Scope**: Single-user personal application; upload frequency low
+
+## Constitution Check
+
+*GATE: Must pass before Phase 0 research. Re-checked after Phase 1 design below.*
+
+| Principle | Check | Status |
+|-----------|-------|--------|
+| §2.1 Separation of concerns | `thumbnail.py` owns resize logic; router owns orchestration; UI knows nothing about S3 keys | ✅ |
+| §2.2 Dependency direction | UI → API → Storage; thumbnail stored via `StorageBackend`; no upward imports | ✅ |
+| §2.3 Storage abstraction | All thumbnail I/O via `StorageBackend.put()` and `.get()`; no raw S3 SDK calls in routes or `thumbnail.py` | ✅ |
+| §2.4 Auth abstraction | No change to auth flow | ✅ |
+| §2.5 DB abstraction | New `thumbnail_key` column accessed only through `ImageRepository`; migration added | ✅ |
+| §2.6 No speculative abstraction | `thumbnail.py` is a concrete module-level function; no interface added because one implementation exists | ✅ |
+| §3.1 API versioning | New route at `/api/v1/images/{id}/thumbnail` | ✅ |
+| §3.3 Error shape | `image_not_found` and `storage_error` codes used consistently | ✅ |
+| §4.2 Images immutable after upload | Thumbnail is generated at upload time only; never mutated | ✅ |
+| §4.3 Dedup by hash | Duplicate upload returns existing record including existing `thumbnail_key`; no re-generation | ✅ |
+| §5.1 TDD non-negotiable | Failing tests written before every implementation task | ✅ |
+| §5.2 Test pyramid | Unit test for `thumbnail.py`; integration tests for new route + upload + delete | ✅ |
+| §5.3 Test colocation | API tests in `api/tests/`; Angular spec files colocated with components | ✅ |
+| §5.4 CI gate | All tests + ruff must pass before milestone is done | ✅ |
+| §7.1 One-command start | No change to `docker-compose.yml` required | ✅ |
+| §7.2 Env configuration | No new env vars; Pillow is a build dependency, not a runtime config | ✅ |
+| §8 Scope boundaries | Backfill of existing images, multiple thumbnail sizes, animated WebP — all deferred | ✅ |
+
+**Post-design re-check**: All gates still pass after Phase 1 design.
+
+## Project Structure
+
+### Documentation (this feature)
+
+```text
+specs/003-upload-thumbnails/
+├── plan.md              # This file
+├── spec.md              # Feature specification
+├── research.md          # Phase 0 decisions
+├── data-model.md        # Schema and module changes
+├── contracts/
+│   └── api.md           # New endpoint + changed response shapes
+├── checklists/
+│   └── requirements.md  # Spec quality checklist
+└── tasks.md             # Phase 2 output (/speckit-tasks — NOT created here)
+```
+
+### Files changed or created
+
+```text
+api/
+├── pyproject.toml                              # Add Pillow dependency
+├── app/
+│   ├── thumbnail.py                            # NEW — generate_thumbnail()
+│   ├── models.py                               # Add thumbnail_key column to Image
+│   ├── repositories/
+│   │   └── image_repo.py                       # Pass thumbnail_key on create
+│   └── routers/
+│       └── images.py                           # Upload: generate+store thumbnail
+│                                               # Delete: remove thumbnail
+│                                               # New route: GET /images/{id}/thumbnail
+│                                               # _image_to_dict: add thumbnail_key field
+└── alembic/
+    └── versions/
+        └── 002_add_thumbnail_key.py            # NEW migration
+
+ui/
+└── src/
+    └── app/
+        ├── services/
+        │   └── image.service.ts                # Add getThumbnailUrl(); add thumbnail_key to ImageRecord
+        └── library/
+            └── library.component.ts            # Use getThumbnailUrl() for grid image src
+```
+
+## Milestones
+
+> **TDD ORDER IS MANDATORY** (constitution §5.1): For every milestone, write
+> the failing test(s) first, confirm they fail, then implement until they pass.
+
+---
+
+### M1 — Thumbnail generation utility
+
+**Goal**: A tested, self-contained function that produces a WebP thumbnail from
+raw image bytes.
+
+**Deliverables**:
+- Add `pillow>=10.0` to `[project.dependencies]` in `api/pyproject.toml`
+- Create `api/app/thumbnail.py` with `generate_thumbnail(data: bytes, mime_type: str) -> bytes`:
+  - Open image bytes with Pillow
+  - Seek to frame 0 (handles animated GIFs)
+  - Convert mode as needed for WebP output
+  - Resize to fit within 400×400 using LANCZOS resampling (never upscale)
+  - Encode as WebP quality 80 and return bytes
+- Unit tests in `api/tests/unit/test_thumbnail.py`:
+  - `test_thumbnail_is_webp` — output starts with WebP magic bytes
+  - `test_thumbnail_fits_within_400px` — both dimensions ≤ 400
+  - `test_thumbnail_preserves_aspect_ratio` — ratio within 1% of original
+  - `test_thumbnail_handles_gif_first_frame` — GIF input produces static WebP
+  - `test_thumbnail_handles_png_with_alpha` — RGBA PNG produces valid WebP
+  - `test_thumbnail_does_not_upscale` — 100×100 image stays ≤ 100×100
+
+**Done criterion**: All unit tests pass; `ruff check api/` passes.
+
+---
+
+### M2 — Database migration
+
+**Goal**: The `images` table has a nullable `thumbnail_key` column; the ORM model
+and repository reflect it.
+
+**Deliverables**:
+- `api/alembic/versions/002_add_thumbnail_key.py` — `upgrade` adds
+  `VARCHAR(70) NULLABLE` column; `downgrade` drops it
+- `api/app/models.py`: add `thumbnail_key: Mapped[str | None]` mapped to
+  `String(70)`, `nullable=True`, `default=None`
+- `api/app/repositories/image_repo.py`: add `thumbnail_key: str | None = None`
+  parameter to `create()`; persist it on the new `Image` instance
+
+**Done criterion**: `alembic upgrade head` runs cleanly inside Docker; all
+existing 46 integration tests still pass (new column is nullable, no existing
+test breaks).
+
+---
+
+### M3 — Upload route: generate and store thumbnail
+
+**Goal**: Every new upload generates a thumbnail; duplicates reuse the existing
+record; failures are tolerated without blocking the upload.
+
+**TDD first** — new tests in `api/tests/integration/test_upload.py`:
+- `test_upload_returns_thumbnail_key` — upload response includes non-null
+  `thumbnail_key` ending in `-thumb`
+- `test_duplicate_upload_reuses_thumbnail_key` — second upload of the same
+  file returns the same `thumbnail_key` as the first
+- `test_upload_succeeds_when_thumbnail_fails` — patch `generate_thumbnail` to
+  raise, upload returns 200/201 with `thumbnail_key: null`
+
+**Implementation** in `api/app/routers/images.py` `upload_image()`:
+1. After `await storage.put(hash_hex, data, mime_type)`, attempt thumbnail generation
+   and storage in a try/except; catch any exception and leave `thumbnail_key` as `None`
+2. Call `thumb_bytes = await asyncio.to_thread(generate_thumbnail, data, mime_type)`
+   — `generate_thumbnail` is CPU-bound (Pillow); `asyncio.to_thread` runs it in the
+   default thread pool executor so it does not block the async event loop
+3. Pass `thumbnail_key` to `image_repo.create()`
+4. Add `"thumbnail_key": image.thumbnail_key` to `_image_to_dict()`
+
+**Done criterion**: New tests pass; all 46 existing tests pass.
+
+---
+
+### M4 — New `GET /api/v1/images/{id}/thumbnail` endpoint
+
+**Goal**: Clients fetch thumbnail content; falls back to original if no thumbnail exists.
+
+**TDD first** — new tests in `api/tests/integration/test_serving.py`:
+- `test_thumbnail_returns_webp` — upload image, call `/thumbnail`, assert
+  200, `content-type: image/webp`, ETag, `Cache-Control` with immutable
+- `test_thumbnail_fallback_returns_original` — set `thumbnail_key=None` on a
+  record, call `/thumbnail`, assert 200 with original mime_type
+- `test_thumbnail_unknown_id_returns_404` — unknown UUID → 404 `image_not_found`
+
+**Implementation**: new route `GET /images/{image_id}/thumbnail` in
+`api/app/routers/images.py` using `image.thumbnail_key or image.storage_key`
+to select the key, and `"image/webp" if image.thumbnail_key else image.mime_type`
+for the content type. Same `ETag` + `Cache-Control` headers as `/file`.
+
+**Done criterion**: All new tests pass; all existing tests pass.
+
+---
+
+### M5 — Delete route: remove thumbnail from storage
+
+**Goal**: Deleting an image also removes its thumbnail; no orphaned objects left.
+
+**TDD first** — new test in `api/tests/integration/test_delete.py`:
+- `test_delete_removes_thumbnail` — upload image, delete it, then verify
+  `GET /images/{id}/thumbnail` returns 404
+
+**Implementation** in `api/app/routers/images.py` `delete_image()`: after
+deleting the DB record and the original object, call `await storage.delete(image.thumbnail_key)`
+if `image.thumbnail_key` is not None.
+
+**Done criterion**: New test passes; all existing delete tests pass.
+
+---
+
+### M6 — UI: library grid uses thumbnail endpoint
+
+**Goal**: Library grid fetches thumbnails instead of full-size originals; detail
+page is unchanged.
+
+**Deliverables**:
+- `ui/src/app/services/image.service.ts`:
+  - Add `thumbnail_key: string | null` to `ImageRecord` interface
+  - Add `getThumbnailUrl(id: string): string` returning `/api/v1/images/${id}/thumbnail`
+- `ui/src/app/library/library.component.ts` + template: replace
+  `getFileUrl(image.id)` with `getThumbnailUrl(image.id)` for grid `<img src>`
+- Update Angular spec files: add `thumbnail_key: null` to all `ImageRecord`
+  mock objects
+- Verify `ng test` passes and `ng build` succeeds
+
+**Done criterion**: Angular build clean; all Angular tests pass; library grid
+`<img>` elements reference `/thumbnail` not `/file`.
+
+## Post-design Constitution Re-check
+
+| Principle | Verdict |
+|-----------|---------|
+| §2.3 Storage abstraction | All thumbnail I/O via `StorageBackend`; `thumbnail.py` never touches S3 directly | ✅ |
+| §2.5 DB abstraction | `thumbnail_key` persisted only through `ImageRepository.create()` | ✅ |
+| §2.6 No speculative abstraction | One concrete function; no interface | ✅ |
+| §4.2 Immutability | Thumbnail written once at upload; never mutated | ✅ |
+| §5.1 TDD | Failing tests written before each milestone's implementation | ✅ |
+
+All gates pass. Feature is ready for `/speckit-tasks`.
diff --git a/specs/003-upload-thumbnails/research.md b/specs/003-upload-thumbnails/research.md
new file mode 100644
index 0000000..44cabbd
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+++ b/specs/003-upload-thumbnails/research.md
@@ -0,0 +1,130 @@
+# Research: Upload Thumbnails
+
+**Branch**: `003-upload-thumbnails` | **Date**: 2026-05-03
+
+## Decision 1: Image processing library
+
+**Decision**: Add `Pillow` as a runtime dependency to the API.
+
+**Rationale**: Pillow is the standard Python image processing library. It supports
+reading JPEG, PNG, GIF (frame extraction), and WebP, and can encode output as WebP.
+It handles aspect-ratio-preserving resize natively via `Image.thumbnail()`. No
+other dependency is needed.
+
+**Alternatives considered**:
+- `wand` (ImageMagick binding): More powerful but much heavier; overkill for a
+  fixed-size resize operation.
+- `opencv-python`: ML-focused, large binary; not justified for simple resize.
+- Pure `aiobotocore` + external service: Adds operational complexity with no benefit
+  over a local library for a single-user app.
+
+---
+
+## Decision 2: Thumbnail dimensions and format
+
+**Decision**: Longest side ≤ 400 px, WebP output, aspect ratio preserved. This
+matches FR-003 and FR-004 exactly and the user's stated preference.
+
+**Rationale**: WebP produces smaller files than JPEG/PNG at equivalent visual quality.
+400 px covers a typical grid thumbnail at 1× and 2× display density without being
+oversized. Pillow's `Image.thumbnail((400, 400))` implements this constraint directly
+(it shrinks to fit within the bounding box, never upscaling).
+
+**Alternatives considered**:
+- JPEG thumbnails: Larger file sizes; no alpha channel support.
+- Multiple sizes: Out of scope for v1 per spec Assumptions.
+- On-demand resize: Rejected by user in favour of pre-generation.
+
+---
+
+## Decision 3: Thumbnail storage key convention
+
+**Decision**: `{sha256_hash}-thumb` (e.g., the 64-char hash hex string + literal
+`-thumb`, giving a 70-char key). Stored under the same S3 bucket as originals.
+
+**Rationale**: Deterministic from the image hash — no new random state needed and the
+key can always be reconstructed from the `Image.hash` field. The `-thumb` suffix
+clearly distinguishes it from the original key. Fits within a `String(70)` column.
+
+**Alternatives considered**:
+- Separate bucket for thumbnails: More complex bucket policy management with no benefit
+  for a single-user app.
+- UUID-based key: Non-deterministic; requires an extra DB round-trip to look up.
+- `{hash}/thumb.webp` (path prefix): Works, but adds key parsing complexity for no gain.
+
+---
+
+## Decision 4: Database schema change
+
+**Decision**: Add a nullable `thumbnail_key: String(70)` column to the `images`
+table. `NULL` means no thumbnail exists (either generation failed or the image
+pre-dates this feature). Add a new Alembic migration `002_add_thumbnail_key.py`.
+
+**Rationale**: Explicitly tracking the thumbnail key in the DB makes the "does a
+thumbnail exist?" question a simple `IS NOT NULL` check rather than an S3 head
+request. Also allows the delete route to skip the thumbnail delete if the column
+is NULL, avoiding a storage error for legacy images.
+
+**Alternatives considered**:
+- Derive key at runtime from `image.hash + "-thumb"` without a DB column: Simpler but
+  means no way to distinguish "thumbnail was generated" from "thumbnail was never
+  attempted", and delete would need a conditional S3 head request.
+- Separate `thumbnails` table: Over-engineered; one thumbnail per image with no
+  additional attributes doesn't warrant its own table.
+
+---
+
+## Decision 5: Where thumbnail generation lives in the code
+
+**Decision**: A standalone async function `generate_thumbnail(data: bytes, mime_type: str) -> bytes`
+in a new module `api/app/thumbnail.py`. Called from the upload route after the original
+is stored, before the Image record is created.
+
+**Rationale**: Keeps the thumbnail logic self-contained and independently testable.
+The upload route calls it but doesn't own it. Constitution §2.6 allows concrete
+functions when no second implementation is needed — no abstract interface is warranted.
+
+**Alternatives considered**:
+- Method on `StorageBackend`: Wrong layer; storage knows nothing about image content.
+- Inline in the upload route: Makes the route harder to test and read.
+- A `ThumbnailService` class: No justification for a class when a module-level function suffices.
+
+---
+
+## Decision 6: Failure handling during upload
+
+**Decision**: If `generate_thumbnail()` raises, log the exception, set `thumbnail_key`
+to `NULL` on the Image record, and continue. The upload response succeeds. The
+`GET /api/v1/images/{id}/thumbnail` endpoint falls back to the original when
+`thumbnail_key` is NULL (FR-009).
+
+**Rationale**: A thumbnail failure should not block the upload — the user still gets
+their image in the library. The fallback in the thumbnail endpoint ensures the grid
+still renders something.
+
+---
+
+## Decision 7: Thumbnail endpoint response
+
+**Decision**: `GET /api/v1/images/{id}/thumbnail` follows the same pattern as
+`GET /api/v1/images/{id}/file`:
+- Returns `200` with binary content, `Content-Type: image/webp` (or original
+  `mime_type` when falling back to original), `ETag`, and
+  `Cache-Control: public, max-age=31536000, immutable`.
+- Returns `404` with `{"detail": "...", "code": "image_not_found"}` if the image
+  does not exist.
+- Falls back to the original when `thumbnail_key IS NULL`.
+
+**Rationale**: Consistent with the existing `/file` endpoint pattern established in
+feature 002. The UI only needs to know one URL per image for the grid.
+
+---
+
+## Decision 8: GIF handling
+
+**Decision**: For GIF uploads, `generate_thumbnail()` extracts frame 0 via
+`Image.seek(0)` before resizing. The output is always WebP (static, not animated).
+
+**Rationale**: Matches spec assumption: "Animated GIF thumbnails capture only the
+first frame; animation is not preserved in the thumbnail." Pillow supports this
+with `im.seek(0)`.
diff --git a/specs/003-upload-thumbnails/spec.md b/specs/003-upload-thumbnails/spec.md
new file mode 100644
index 0000000..fbc990d
--- /dev/null
+++ b/specs/003-upload-thumbnails/spec.md
@@ -0,0 +1,180 @@
+# Feature Specification: Upload Thumbnails
+
+**Feature Branch**: `003-upload-thumbnails`
+**Created**: 2026-05-03
+**Status**: Draft
+**Input**: User description: "When users load the UI, full size images are fetched, which may cause considerable load time when there are a lot of images -- a grid of 20 images could silently pull several hundred megabytes. We will solve this by pre-generating thumbnails on upload: when an image is uploaded, immediately produce one (or a few) fixed-size thumbnail variants and store them alongside the original. The library always fetches the thumbnail key, the detail page fetches the original key. Zero resize cost at serve time. A single fixed-size re-encoded as WebP for smaller bytes will cover the grid view."
+
+## User Scenarios & Testing *(mandatory)*
+
+### User Story 1 — Fast Library Load (Priority: P1)
+
+A user opens the application or runs a tag-filtered search. The image grid
+loads quickly even when the library contains many images, because each grid
+cell fetches a compact thumbnail rather than the full-size original.
+
+**Why this priority**: This is the core motivation. A grid of 20 images today
+could pull hundreds of megabytes; thumbnails bring that down to a few
+megabytes, making the library usable on slow or metered connections.
+
+**Independent Test**: Upload 20 images of varying sizes (including some near
+the 50 MB limit). Open the library. Measure total bytes transferred while the
+grid loads. Compare against loading the same library before this feature.
+Verify the grid renders fully and that each thumbnail is visually recognisable
+as the correct image.
+
+**Acceptance Scenarios**:
+
+1. **Given** a library with multiple images, **When** the user opens the
+   library page, **Then** each grid cell displays a thumbnail that is visually
+   recognisable as its image, and the total data transferred to render the
+   full grid is substantially less than the sum of the original file sizes.
+
+2. **Given** a user applies one or more tag filters, **When** the filtered
+   results are displayed, **Then** thumbnails are shown for all matching
+   images with the same reduced data footprint.
+
+3. **Given** a library with images of mixed types (JPEG, PNG, GIF, WebP),
+   **When** the grid loads, **Then** thumbnails for all types display
+   correctly.
+
+---
+
+### User Story 2 — Full-Size Image on Detail Page (Priority: P1)
+
+A user clicks an image in the library grid to open its detail page. The
+full-size original is displayed, not the thumbnail. The experience is
+unchanged from before this feature.
+
+**Why this priority**: The detail page is where the user inspects or copies
+an image; showing the thumbnail there would degrade the product's core value.
+
+**Independent Test**: Open any image's detail page. Verify the image
+displayed matches the original resolution and file size, not the thumbnail
+dimensions.
+
+**Acceptance Scenarios**:
+
+1. **Given** the user clicks an image thumbnail in the library, **When** the
+   detail page loads, **Then** the full-size original image is displayed at
+   its native resolution.
+
+2. **Given** the user navigates directly to an image's detail URL, **When**
+   the page loads, **Then** the full-size original is displayed.
+
+---
+
+### User Story 3 — Thumbnails Generated Automatically on Upload (Priority: P1)
+
+A user uploads a new image. Without any additional action, a thumbnail is
+available immediately. There is no separate step or explicit request to
+generate a thumbnail.
+
+**Why this priority**: The value of the feature depends entirely on thumbnails
+being present for every image. Manual generation or lazy generation would
+create inconsistencies in the grid.
+
+**Independent Test**: Upload a new image. Immediately open the library. Verify
+the new image's thumbnail appears in the grid without any extra action.
+
+**Acceptance Scenarios**:
+
+1. **Given** the user uploads a supported image, **When** the upload
+   completes, **Then** a thumbnail is available and appears correctly in
+   the library grid.
+
+2. **Given** the user uploads a duplicate image (already in the library),
+   **When** the upload completes, **Then** no redundant thumbnail is
+   generated — the existing thumbnail is reused.
+
+3. **Given** the user uploads an image at or near the maximum supported
+   file size (50 MB), **When** the upload completes, **Then** the thumbnail
+   is generated successfully and the upload response time remains acceptable.
+
+---
+
+### Edge Cases
+
+- What happens when thumbnail generation fails during upload? → The upload
+  still succeeds and the original image is stored; a fallback to the original
+  is shown in the grid, or the item is hidden until the thumbnail is available
+  (assumption: fall back to original rather than silently drop the image).
+- What happens when an image is deleted? → Both the original and its thumbnail
+  are removed from storage.
+- What happens with existing images that were uploaded before this feature? →
+  Those images have no pre-generated thumbnail; the grid falls back to the
+  original for those entries until a backfill is performed (backfill is out of
+  scope for v1 of this feature).
+- What happens with animated GIFs? → A static thumbnail is generated from the
+  first frame.
+
+## Requirements *(mandatory)*
+
+### Functional Requirements
+
+- **FR-001**: The system MUST generate a thumbnail for every newly uploaded
+  image as part of the upload operation, before the upload response is
+  returned to the caller.
+- **FR-002**: Thumbnails MUST be stored in the same object storage as the
+  original, addressable by a distinct key derived from the image.
+- **FR-003**: The thumbnail MUST be encoded as WebP regardless of the original
+  image format.
+- **FR-004**: The thumbnail MUST fit within a fixed maximum dimension on its
+  longest side, preserving the original aspect ratio; no dimension of the
+  thumbnail MAY exceed 400 pixels.
+- **FR-005**: The library grid view MUST fetch and display thumbnails instead
+  of original images.
+- **FR-006**: The image detail view MUST continue to fetch and display the
+  full-size original.
+- **FR-007**: When a duplicate image is uploaded, the thumbnail MUST NOT be
+  regenerated or re-stored; the existing thumbnail is reused.
+- **FR-008**: When an image is deleted, its thumbnail MUST also be deleted
+  from storage.
+- **FR-009**: If thumbnail generation fails during upload, the upload MUST
+  still succeed; the system MUST fall back to serving the original image in
+  the grid for that entry.
+- **FR-010**: The API MUST expose a way for clients to retrieve the thumbnail
+  content for a given image, distinct from the full-size content endpoint.
+
+### Key Entities *(include if feature involves data)*
+
+- **Image**: Gains a new optional attribute indicating whether a thumbnail is
+  available and the key under which the thumbnail is stored.
+- **Thumbnail**: A derived, smaller representation of an Image. Key
+  attributes: storage key, dimensions (width × height), format (WebP),
+  relationship to its source Image.
+
+## Success Criteria *(mandatory)*
+
+### Measurable Outcomes
+
+- **SC-001**: The total data transferred to render a 20-image library grid is
+  reduced by at least 80% compared to fetching full-size originals for the
+  same images.
+- **SC-002**: The library grid's first page loads in under 2 seconds on a
+  local network connection for a library of 1,000 images, with thumbnails
+  visible without a second load.
+- **SC-003**: Thumbnails are available immediately after upload completes —
+  no polling or manual refresh is required.
+- **SC-004**: The detail page continues to show the full-size original; no
+  regression in detail-page image quality is introduced.
+- **SC-005**: Deleting an image removes both the original and its thumbnail;
+  no orphaned thumbnail objects remain in storage after deletion.
+
+## Assumptions
+
+- A single thumbnail size (longest side ≤ 400 px, WebP) is sufficient for
+  the library grid view in v1. Additional sizes or formats are out of scope.
+- Thumbnail generation happens synchronously during the upload request. Async
+  background processing is not required for v1.
+- Existing images uploaded before this feature are not automatically
+  backfilled with thumbnails in v1; the grid falls back to the original for
+  those entries.
+- Animated GIF thumbnails capture only the first frame; animation is not
+  preserved in the thumbnail.
+- The thumbnail storage key is derived deterministically from the image's
+  existing content hash, so no additional database column is strictly required
+  to locate it — however the Image record will track thumbnail availability
+  for correctness.
+- No change is required to tag management, duplicate detection, or any other
+  upload behaviour beyond adding thumbnail generation.
diff --git a/specs/003-upload-thumbnails/tasks.md b/specs/003-upload-thumbnails/tasks.md
new file mode 100644
index 0000000..b65424a
--- /dev/null
+++ b/specs/003-upload-thumbnails/tasks.md
@@ -0,0 +1,186 @@
+# Tasks: Upload Thumbnails
+
+**Input**: Design documents from `specs/003-upload-thumbnails/`
+**Prerequisites**: plan.md ✅, spec.md ✅, research.md ✅, data-model.md ✅, contracts/api.md ✅
+
+**TDD**: Tests are non-negotiable per constitution §5.1. Every test task MUST be written and confirmed failing before its implementation task runs.
+
+**Organization**: Tasks grouped by user story to enable independent implementation and testing.
+
+## Format: `[ID] [P?] [Story] Description`
+
+- **[P]**: Can run in parallel (different files, no dependencies on incomplete tasks)
+- **[Story]**: Which user story this task belongs to (US1, US2, US3)
+
+---
+
+## Phase 1: Setup
+
+- [X] T001 Add `pillow>=10.0` to `[project.dependencies]` in `api/pyproject.toml`; rebuild the Docker API image (`docker compose build api`) so Pillow is available inside the container for all subsequent test runs
+
+---
+
+## Phase 2: Foundational (Blocking Prerequisites)
+
+**Purpose**: Thumbnail generation logic and the DB schema change that all three user stories depend on.
+
+**⚠️ CRITICAL**: All tasks in this phase must complete before any user story work begins.
+
+### Tests for thumbnail utility (write FIRST — must FAIL before T005) ⚠️
+
+- [X] T002 Write 6 unit tests in `api/tests/unit/test_thumbnail.py` — `test_thumbnail_is_webp` (output begins with WebP magic bytes `RIFF...WEBP`), `test_thumbnail_fits_within_400px` (both dimensions ≤ 400), `test_thumbnail_preserves_aspect_ratio` (ratio within 1% of original), `test_thumbnail_handles_gif_first_frame` (GIF input → static WebP, no animation), `test_thumbnail_handles_png_with_alpha` (RGBA PNG → valid WebP output), `test_thumbnail_does_not_upscale` (100×100 input stays ≤ 100×100); to confirm the TDD red state, first create an empty stub `api/app/thumbnail.py` (so pytest can collect tests), then run `pytest api/tests/unit/test_thumbnail.py` and confirm all 6 **fail** with assertion errors (not import errors)
+
+### Thumbnail utility implementation
+
+- [X] T003 Create `api/app/thumbnail.py` with `generate_thumbnail(data: bytes, mime_type: str) -> bytes`: open bytes with `PIL.Image.open(BytesIO(data))`, call `.seek(0)` to target frame 0 (GIF support), convert mode to RGB or RGBA as needed for WebP, call `.thumbnail((400, 400), PIL.Image.LANCZOS)` (never upscales), save to a `BytesIO` buffer as WebP quality=80, return bytes; run `pytest api/tests/unit/test_thumbnail.py` and confirm all 6 pass
+
+### Database migration
+
+- [X] T004 [P] Create `api/alembic/versions/002_add_thumbnail_key.py` with `upgrade()` calling `op.add_column("images", sa.Column("thumbnail_key", sa.String(70), nullable=True))` and `downgrade()` calling `op.drop_column("images", "thumbnail_key")`; set `revision="002"`, `down_revision="001"`
+- [X] T005 [P] Add `thumbnail_key: Mapped[str | None] = mapped_column(String(70), nullable=True, default=None)` to the `Image` class in `api/app/models.py`
+- [X] T006 Add `thumbnail_key: str | None = None` keyword argument to `ImageRepository.create()` in `api/app/repositories/image_repo.py`; include it in the `Image(...)` constructor call; run `docker compose run --rm api alembic upgrade head` inside the container and confirm migration applies cleanly; run `pytest api/` to confirm all 46 existing tests still pass
+
+**Checkpoint**: Pillow available, thumbnail.py works, schema migrated, all existing tests green.
+
+---
+
+## Phase 3: User Story 3 — Thumbnails Generated Automatically on Upload (Priority: P1) 🎯
+
+**Goal**: Every new upload triggers thumbnail generation and storage as part of the same request. No extra step required from the user.
+
+**Independent Test**: Upload any supported image. Immediately check the upload response — it includes a non-null `thumbnail_key`. Call `GET /api/v1/images/{id}/thumbnail` — it returns 200 with `content-type: image/webp`.
+
+### Tests for User Story 3 (write FIRST — must FAIL before T008) ⚠️
+
+- [X] T007 [US3] Add three tests to `api/tests/integration/test_upload.py`: `test_upload_returns_thumbnail_key` (upload a JPEG/PNG/WebP, assert response JSON contains `thumbnail_key` ending in `-thumb`), `test_duplicate_upload_reuses_thumbnail_key` (upload same file twice, assert both responses have equal, non-null `thumbnail_key`), and `test_upload_succeeds_when_thumbnail_fails` (patch `generate_thumbnail` to raise an exception, upload an image, assert response is 200/201 with `thumbnail_key: null` — upload must not be blocked by thumbnail failure); run `pytest api/tests/integration/test_upload.py` and confirm all three new tests **fail**
+
+### Implementation for User Story 3
+
+- [X] T008 [US3] In `api/app/routers/images.py` `upload_image()`: import `asyncio` and `generate_thumbnail` from `app.thumbnail`; after `await storage.put(hash_hex, data, mime_type)`, wrap thumbnail generation in a try/except — call `thumb_bytes = await asyncio.to_thread(generate_thumbnail, data, mime_type)` (runs CPU-bound Pillow work off the async event loop), store result via `await storage.put(f"{hash_hex}-thumb", thumb_bytes, "image/webp")`, set `thumbnail_key = f"{hash_hex}-thumb"`; on any exception log a warning and leave `thumbnail_key = None`; pass `thumbnail_key=thumbnail_key` to `image_repo.create()`
+- [X] T009 [US3] Add `"thumbnail_key": image.thumbnail_key` to the dict returned by `_image_to_dict()` in `api/app/routers/images.py`
+- [X] T010 [US3] Run `pytest api/tests/integration/test_upload.py` to confirm both new tests pass; then run `pytest api/` to confirm no regressions
+
+**Checkpoint**: Upload generates and stores a thumbnail. Duplicate uploads reuse the existing thumbnail. `thumbnail_key` appears in all image metadata responses.
+
+---
+
+## Phase 4: User Story 1 — Fast Library Load (Priority: P1)
+
+**Goal**: The library grid fetches compact WebP thumbnails instead of full-size originals, dramatically reducing page-load bandwidth.
+
+**Independent Test**: Open the library in a browser with DevTools network tab open. All grid `<img>` elements request `/api/v1/images/{id}/thumbnail`. Total bytes transferred for a 20-image grid is a small fraction of what the originals would cost.
+
+### Tests for User Story 1 (write FIRST — must FAIL before T013) ⚠️
+
+- [X] T011 [US1] Add `test_thumbnail_returns_webp` (upload image, GET `/thumbnail`, assert 200, `content-type: image/webp`, ETag header matches `f'"{image_hash}"'`, `"immutable"` in `cache-control`, non-empty content), `test_thumbnail_fallback_returns_original` (manually set `thumbnail_key=None` on a DB record via the session fixture, GET `/thumbnail`, assert 200 with original `mime_type` in content-type), and `test_thumbnail_unknown_id_returns_404` (unknown UUID, assert 404 `image_not_found`) to `api/tests/integration/test_serving.py`; run and confirm all three **fail**
+- [X] T012 [P] [US1] In `ui/src/app/services/image.service.ts`: add `thumbnail_key: string | null` field to the `ImageRecord` interface; add `getThumbnailUrl(id: string): string { return \`${this.base}/images/${id}/thumbnail\`; }` method to `ImageService`
+
+### Implementation for User Story 1
+
+- [X] T013 [US1] Add `GET /api/v1/images/{image_id}/thumbnail` route to `api/app/routers/images.py`: look up image (404 if missing), select `key = image.thumbnail_key or image.storage_key` and `media_type = "image/webp" if image.thumbnail_key else image.mime_type`, call `await storage.get(key)` in a try/except (500 `storage_error` on failure), return `Response(content=data, media_type=media_type, headers={"ETag": f'"{image.hash}"', "Cache-Control": "public, max-age=31536000, immutable"})`
+- [X] T014 [US1] In `ui/src/app/library/library.component.ts` and its HTML template: replace every use of `imageService.getFileUrl(image.id)` (or equivalent) with `imageService.getThumbnailUrl(image.id)` for grid cell `<img src>` bindings
+- [X] T015 [US1] Add `thumbnail_key: null` to every `ImageRecord` mock/stub object in `ui/src/app/services/image.service.spec.ts`, `ui/src/app/library/library.component.spec.ts`, `ui/src/app/detail/detail.component.spec.ts`, and `ui/src/app/upload/upload.component.spec.ts`
+- [X] T016 [US1] Run `pytest api/tests/integration/test_serving.py` to confirm all three new thumbnail tests pass and no existing serving tests regress
+
+**Checkpoint**: `GET /api/v1/images/{id}/thumbnail` serves WebP with caching headers. Falls back to original for legacy images. Library grid `<img>` elements all use the thumbnail endpoint.
+
+---
+
+## Phase 5: User Story 2 — Full-Size Image on Detail Page (Priority: P1)
+
+**Goal**: The detail page continues to display the full-size original. No regression introduced by the thumbnail work.
+
+**Independent Test**: Navigate to any image detail page. The image displayed is full-resolution. Browser DevTools shows the detail `<img>` requests `/api/v1/images/{id}/file`, not `/thumbnail`.
+
+### Verification for User Story 2
+
+- [X] T017 [US2] Confirm `ui/src/app/detail/detail.component.ts` still calls `imageService.getFileUrl(image.id)` (not `getThumbnailUrl`) for its `<img src>` — no code change expected; if the file was accidentally updated in T014 or T015, revert the detail component to `getFileUrl`
+- [X] T018 [US2] Run `ng test` (inside the UI container or locally) and confirm all Angular unit tests pass including the detail component spec; run `ng build` to confirm the Angular build succeeds
+
+**Checkpoint**: Detail page verified unchanged. Angular build and tests clean.
+
+---
+
+## Phase 6: Polish & Cross-Cutting Concerns
+
+**Purpose**: Delete cleanup, final test run, linting.
+
+### Delete thumbnail on image deletion
+
+- [X] T019 Write `test_delete_removes_thumbnail` in `api/tests/integration/test_delete.py`: upload an image, delete it, then `GET /api/v1/images/{id}/thumbnail` and assert 404; run and confirm it **fails** (currently delete does not remove the thumbnail object)
+- [X] T020 In `api/app/routers/images.py` `delete_image()`: capture `thumbnail_key = image.thumbnail_key` before `image_repo.delete(image)`; after deleting the original via `await storage.delete(storage_key)`, add `if thumbnail_key: await storage.delete(thumbnail_key)`; run `pytest api/tests/integration/test_delete.py` to confirm new test and all existing delete tests pass
+
+### Final validation
+
+- [X] T021 [P] Run `~/.local/bin/ruff check api/app/thumbnail.py api/app/routers/images.py api/app/models.py api/app/repositories/image_repo.py api/tests/unit/test_thumbnail.py` and fix any lint issues in the changed files
+- [X] T022 Run `pytest api/ -v` and confirm all tests pass; record final count (expected: 46 existing + ~10 new = ~56 total)
+
+---
+
+## Dependencies & Execution Order
+
+### Phase Dependencies
+
+- **Setup (Phase 1)**: No dependencies — start immediately
+- **Foundational (Phase 2)**: Depends on Phase 1 (Pillow must be installed before tests can import it)
+- **US3 (Phase 3)**: Depends on Phase 2 complete (T002–T006 all done)
+- **US1 (Phase 4)**: Depends on Phase 3 complete (upload must set `thumbnail_key` before the endpoint can serve one)
+- **US2 (Phase 5)**: Depends on Phase 4 complete (Angular changes in T014/T015 must be done before verifying no regression)
+- **Polish (Phase 6)**: Depends on Phases 3, 4, and 5 complete
+
+### Within Each Phase
+
+- T002 (write failing tests) MUST precede T003 (implement thumbnail.py)
+- T004 and T005 can run in parallel (different files)
+- T006 (repo change) depends on T005 (model must compile first)
+- T007 (write failing upload tests) MUST precede T008 (implement upload change)
+- T011 (write failing serving tests) and T012 (UI service) can run in parallel
+- T011 MUST precede T013 (implement thumbnail route)
+- T019 (write failing delete test) MUST precede T020 (implement delete cleanup)
+
+---
+
+## Parallel Example: Phase 2 (Foundational)
+
+```bash
+# After T003 is done, run T004 and T005 together:
+Task: "Create 002_add_thumbnail_key.py migration in api/alembic/versions/"
+Task: "Add thumbnail_key column to Image ORM in api/app/models.py"
+```
+
+## Parallel Example: Phase 4 (US1)
+
+```bash
+# T011 and T012 touch different layers — run together:
+Task: "Write 3 failing thumbnail serving tests in api/tests/integration/test_serving.py"
+Task: "Add getThumbnailUrl() and thumbnail_key field to ui/src/app/services/image.service.ts"
+```
+
+---
+
+## Implementation Strategy
+
+### MVP (All three user stories — tightly coupled)
+
+All three user stories are P1 and interdependent: US3 (generation) enables US1 (grid) which proves US2 (detail unchanged) by contrast. Complete all phases in order.
+
+1. Phase 1: T001 (Pillow setup)
+2. Phase 2: T002–T006 (core infrastructure)
+3. Phase 3: T007–T010 (upload generates thumbnail)
+4. Phase 4: T011–T016 (thumbnail endpoint + UI)
+5. Phase 5: T017–T018 (detail page verification)
+6. Phase 6: T019–T022 (delete cleanup + polish)
+7. **STOP and VALIDATE**: Open library in browser; DevTools shows `/thumbnail` requests; bandwidth used is a fraction of original file sizes
+
+### Total tasks: 22 (T001–T022)
+
+---
+
+## Notes
+
+- [P] tasks touch different files and have no mutual dependencies within their phase
+- T002 must be run **before** T003 and confirmed failing — this is the TDD red step
+- T007, T011, T019 are all "write failing test" steps — confirm failure before implementing
+- `thumbnail_key` in the API response is informational; the UI always calls `/thumbnail` and lets the endpoint handle the fallback — no client-side conditional logic needed
+- Existing images (pre-dating this feature) will have `thumbnail_key: null`; the `/thumbnail` endpoint serves their original transparently
+- The backfill migration for existing images is explicitly out of scope for this feature
diff --git a/ui/src/app/detail/detail.component.spec.ts b/ui/src/app/detail/detail.component.spec.ts
index b4df3a6..58fbe8e 100644
--- a/ui/src/app/detail/detail.component.spec.ts
+++ b/ui/src/app/detail/detail.component.spec.ts
@@ -16,6 +16,7 @@ const MOCK_IMAGE = {
   width: 10,
   height: 10,
   storage_key: 'abc',
+  thumbnail_key: null,
   created_at: '2026-01-01T00:00:00Z',
   tags: ['cat', 'funny'],
 };
diff --git a/ui/src/app/library/library.component.spec.ts b/ui/src/app/library/library.component.spec.ts
index c372af0..e2971ad 100644
--- a/ui/src/app/library/library.component.spec.ts
+++ b/ui/src/app/library/library.component.spec.ts
@@ -22,7 +22,7 @@ describe('LibraryComponent', () => {
     spyOn(imgSvc, 'list').and.returnValue(
       of({
         items: [
-          { id: '1', filename: 'a.jpg', tags: ['cat'], hash: '', mime_type: 'image/jpeg', size_bytes: 1, width: 1, height: 1, storage_key: '', created_at: '' },
+          { id: '1', filename: 'a.jpg', tags: ['cat'], hash: '', mime_type: 'image/jpeg', size_bytes: 1, width: 1, height: 1, storage_key: '', thumbnail_key: null, created_at: '' },
         ],
         total: 1,
         limit: 50,
diff --git a/ui/src/app/library/library.component.ts b/ui/src/app/library/library.component.ts
index 68e9dc5..c3f1290 100644
--- a/ui/src/app/library/library.component.ts
+++ b/ui/src/app/library/library.component.ts
@@ -48,7 +48,7 @@ import { TagService } from '../services/tag.service';
           class="image-card"
           (click)="router.navigate(['/images', img.id])"
         >
-          <img [src]="imageService.getFileUrl(img.id)" [alt]="img.filename" loading="lazy" />
+          <img [src]="imageService.getThumbnailUrl(img.id)" [alt]="img.filename" loading="lazy" />
           <div class="tag-row">
             <span *ngFor="let tag of img.tags" class="chip small">{{ tag }}</span>
           </div>
diff --git a/ui/src/app/services/image.service.ts b/ui/src/app/services/image.service.ts
index 35f091a..c427487 100644
--- a/ui/src/app/services/image.service.ts
+++ b/ui/src/app/services/image.service.ts
@@ -11,6 +11,7 @@ export interface ImageRecord {
   width: number;
   height: number;
   storage_key: string;
+  thumbnail_key: string | null;
   created_at: string;
   tags: string[];
   duplicate?: boolean;
@@ -54,6 +55,10 @@ export class ImageService {
     return `${this.base}/images/${id}/file`;
   }
 
+  getThumbnailUrl(id: string): string {
+    return `${this.base}/images/${id}/thumbnail`;
+  }
+
   updateTags(id: string, tags: string[]): Observable<ImageRecord> {
     return this.http.patch<ImageRecord>(`${this.base}/images/${id}/tags`, { tags });
   }
diff --git a/ui/src/app/upload/upload.component.spec.ts b/ui/src/app/upload/upload.component.spec.ts
index eb40639..47d210d 100644
--- a/ui/src/app/upload/upload.component.spec.ts
+++ b/ui/src/app/upload/upload.component.spec.ts
@@ -11,7 +11,8 @@ describe('UploadComponent', () => {
   let component: UploadComponent;
 
   function makeImageService(overrides: Partial<ImageService> = {}): jasmine.SpyObj<ImageService> {
-    return jasmine.createSpyObj<ImageService>('ImageService', { upload: of({} as any), ...overrides });
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    return jasmine.createSpyObj<ImageService>('ImageService', { upload: of({} as any), ...overrides } as any);
   }
 
   beforeEach(async () => {