diff --git a/CLAUDE.md b/CLAUDE.md
index d4a7dce..0e21dcd 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -186,6 +186,15 @@ See [SPEC.md](SPEC.md) for full tool specifications and DSM call details.
   maps error 400 to a clear "requires Btrfs-formatted volume" message.
 - **`SYNO.FileStation.BackgroundTask`:** Only the `list` method is available (v1–v3).
   No stop/cancel/clear methods exist on this firmware.
+- **`SYNO.FileStation.Thumb` — `quality` parameter ignored:** DSM accepts the `quality`
+  field in the POST body but always returns the same JPEG regardless of the value.
+  No server-side quality control is available.
+- **`SYNO.FileStation.Thumb` — size limits:** `small` thumbnails range from ~5 KB to
+  ~548 KB raw depending on source image resolution. `medium`/`large` can exceed 380 KB
+  raw. `original` reflects the full stored image and may be several MB.
+  The `get_thumbnail` tool enforces: abort >1.5 MB raw (≈2 MB base64), warning >375 KB
+  raw (≈500 KB base64). Default changed from `large` to `small` to avoid MCP buffer
+  overflows.
 
 ## Roadmap
 
diff --git a/SPEC.md b/SPEC.md
index f2e8b7c..60573ff 100644
--- a/SPEC.md
+++ b/SPEC.md
@@ -460,12 +460,26 @@ pagination hint when more results are available.
 | Name | Type | Required | Default | Description |
 |------|------|----------|---------|-------------|
 | `path` | str | yes | — | Share-relative path to the image or video file |
-| `size` | str | no | `large` | `small`, `medium`, `large`, or `original` |
+| `size` | str | no | `small` | `small`, `medium`, `large`, or `original` |
 
 **Returns:** JSON `{filename, size_bytes, content_base64}` (JPEG bytes encoded as base64).
+If the thumbnail exceeds the soft limit (≈500 KB base64 / 375 KB raw), the JSON includes
+an additional `"warning"` field advising to use `size='small'`.
+If the thumbnail exceeds the hard limit (≈2 MB base64 / 1.5 MB raw), the tool returns
+`"Error: Thumbnail too large (… KB base64) — use size='small' to get a smaller version."`
+instead of the JSON payload.
 
 **DSM call:** `SYNO.FileStation.Thumb::get` (POST, v2)
 
+> **`quality` parameter is ignored:** DSM accepts `quality` in the request but always
+> returns the same JPEG regardless of the value. No server-side quality control is available.
+>
+> **Size limits confirmed against this NAS:**
+> - `small` → 5 KB–548 KB raw depending on source image resolution.
+> - `medium` / `large` → can exceed 380 KB raw even for modest photos.
+> - `original` → reflects the actual stored image; may be several MB.
+> Default was changed from `large` to `small` to avoid hitting MCP buffer limits.
+>
 > DSM returns image bytes directly when the file has a thumbnail. Non-image content-type
 > indicates a DSM error envelope — the tool parses and surfaces the error code.
 
diff --git a/pyproject.toml b/pyproject.toml
index d8e8203..b2d47da 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "mcp-synology-filestation"
-version = "0.3.4"
+version = "0.3.5"
 description = "MCP server for Synology FileStation"
 requires-python = ">=3.12"
 dependencies = [
diff --git a/src/mcp_synology_filestation/__init__.py b/src/mcp_synology_filestation/__init__.py
index cabd3c2..a026195 100644
--- a/src/mcp_synology_filestation/__init__.py
+++ b/src/mcp_synology_filestation/__init__.py
@@ -1,3 +1,3 @@
 """MCP server for Synology FileStation."""
 
-__version__ = "0.3.4"
+__version__ = "0.3.5"
diff --git a/src/mcp_synology_filestation/tools/filestation.py b/src/mcp_synology_filestation/tools/filestation.py
index 311a283..d972028 100644
--- a/src/mcp_synology_filestation/tools/filestation.py
+++ b/src/mcp_synology_filestation/tools/filestation.py
@@ -25,6 +25,11 @@ _VALID_SORT_DIR = frozenset({"asc", "desc"})
 # Cap on items returned by list_dir — DSM hard limit is 10000, we enforce lower.
 _MAX_LIMIT = 500
 
+# get_thumbnail: abort if raw bytes exceed this (≈2 MB base64 after encoding).
+_THUMB_ABORT_BYTES = 1_500_000
+# get_thumbnail: include a warning field if raw bytes exceed this (≈500 KB base64).
+_THUMB_WARN_BYTES = 375_000
+
 
 def _fmt_size(size: int | None) -> str:
     """Format a byte count as a human-readable string."""
@@ -1126,8 +1131,8 @@ def register_filestation(
     # ── thumbnail + favorites tools ───────────────────────────────────────
 
     @mcp.tool()
-    async def get_thumbnail(path: str, size: str = "large"):
-        """Fetch a thumbnail for an image. Returns JSON: filename, size_bytes, content_base64."""
+    async def get_thumbnail(path: str, size: str = "small"):
+        """Fetch a thumbnail for an image/video. Returns JSON with filename and base64 content."""
         import base64
         import json as _json
 
@@ -1142,14 +1147,32 @@ def register_filestation(
         except SynologyError as e:
             return f"Error: {e}"
 
+        raw_len = len(img_bytes)
+
+        # Hard limit: refuse to return a payload that would exceed ~2 MB base64.
+        if raw_len > _THUMB_ABORT_BYTES:
+            b64_kb = raw_len * 4 // 3 // 1024
+            return (
+                f"Error: Thumbnail too large ({b64_kb} KB base64) — "
+                f"use size='small' to get a smaller version."
+            )
+
         filename = path.rsplit("/", 1)[-1]
-        return _json.dumps(
-            {
-                "filename": filename,
-                "size_bytes": len(img_bytes),
-                "content_base64": base64.b64encode(img_bytes).decode(),
-            }
-        )
+        payload: dict = {
+            "filename": filename,
+            "size_bytes": raw_len,
+            "content_base64": base64.b64encode(img_bytes).decode(),
+        }
+
+        # Soft warning: note large payload without refusing it.
+        if raw_len > _THUMB_WARN_BYTES:
+            b64_kb = raw_len * 4 // 3 // 1024
+            payload["warning"] = (
+                f"Thumbnail is large ({b64_kb} KB base64). "
+                "Consider using size='small' for faster responses."
+            )
+
+        return _json.dumps(payload)
 
     @mcp.tool()
     async def list_favorites():
diff --git a/tests/test_tools_filestation.py b/tests/test_tools_filestation.py
index a7f793f..2439234 100644
--- a/tests/test_tools_filestation.py
+++ b/tests/test_tools_filestation.py
@@ -1768,6 +1768,105 @@ async def test_get_md5_missing_hash_in_response(config: AppConfig) -> None:
     assert "md5" in result.lower() or "hash" in result.lower()
 
 
+# ──────────────────────────────────────────────────────────────────────────
+# get_thumbnail
+# ──────────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.asyncio
+async def test_get_thumbnail_success(config: AppConfig) -> None:
+    """get_thumbnail returns JSON with filename, size_bytes, content_base64 for small images."""
+    import base64
+
+    client = MagicMock()
+    img_bytes = b"\xff\xd8\xff" + b"\x00" * 1000  # fake JPEG, well under limits
+    client.get_thumbnail_bytes = AsyncMock(return_value=img_bytes)
+    tools = _make_mcp_and_tools(config, client)
+
+    result = await tools["get_thumbnail"](path="/photo/test.jpg", size="small")
+
+    payload = json.loads(result)
+    assert payload["filename"] == "test.jpg"
+    assert payload["size_bytes"] == len(img_bytes)
+    assert base64.b64decode(payload["content_base64"]) == img_bytes
+    assert "warning" not in payload
+
+
+@pytest.mark.asyncio
+async def test_get_thumbnail_invalid_size(config: AppConfig) -> None:
+    """get_thumbnail returns Error: for an unrecognised size value."""
+    client = MagicMock()
+    tools = _make_mcp_and_tools(config, client)
+
+    result = await tools["get_thumbnail"](path="/photo/test.jpg", size="tiny")
+
+    assert result.startswith("Error:")
+    assert "size" in result.lower()
+
+
+@pytest.mark.asyncio
+async def test_get_thumbnail_dsm_error(config: AppConfig) -> None:
+    """get_thumbnail returns Error: when the client raises SynologyError."""
+    from mcp_synology_filestation.client import SynologyError
+
+    client = MagicMock()
+    client.get_thumbnail_bytes = AsyncMock(
+        side_effect=SynologyError("File not found: /photo/missing.jpg", code=404)
+    )
+    tools = _make_mcp_and_tools(config, client)
+
+    result = await tools["get_thumbnail"](path="/photo/missing.jpg")
+
+    assert result.startswith("Error:")
+    assert "not found" in result.lower()
+
+
+@pytest.mark.asyncio
+async def test_get_thumbnail_large_warning(config: AppConfig) -> None:
+    """get_thumbnail includes a warning field when image exceeds the soft limit (~375 KB raw)."""
+    client = MagicMock()
+    # 400 000 bytes raw > 375 000 threshold but < 1 500 000 hard limit
+    img_bytes = b"\x00" * 400_000
+    client.get_thumbnail_bytes = AsyncMock(return_value=img_bytes)
+    tools = _make_mcp_and_tools(config, client)
+
+    result = await tools["get_thumbnail"](path="/photo/big.jpg", size="medium")
+
+    payload = json.loads(result)
+    assert "content_base64" in payload
+    assert "warning" in payload
+    assert "small" in payload["warning"].lower()
+
+
+@pytest.mark.asyncio
+async def test_get_thumbnail_too_large_aborts(config: AppConfig) -> None:
+    """get_thumbnail returns Error: (no base64) when image exceeds the hard limit (~1.5 MB raw)."""
+    client = MagicMock()
+    # 2 000 000 bytes raw > 1 500 000 hard limit
+    img_bytes = b"\x00" * 2_000_000
+    client.get_thumbnail_bytes = AsyncMock(return_value=img_bytes)
+    tools = _make_mcp_and_tools(config, client)
+
+    result = await tools["get_thumbnail"](path="/photo/huge.jpg", size="original")
+
+    assert result.startswith("Error:")
+    assert "large" in result.lower() or "too large" in result.lower()
+    # Must NOT contain base64 payload
+    assert "content_base64" not in result
+
+
+@pytest.mark.asyncio
+async def test_get_thumbnail_default_size_is_small(config: AppConfig) -> None:
+    """get_thumbnail defaults to size='small'."""
+    client = MagicMock()
+    client.get_thumbnail_bytes = AsyncMock(return_value=b"\xff\xd8" + b"\x00" * 100)
+    tools = _make_mcp_and_tools(config, client)
+
+    await tools["get_thumbnail"](path="/photo/test.jpg")
+
+    client.get_thumbnail_bytes.assert_called_once_with("/photo/test.jpg", "small")
+
+
 # ──────────────────────────────────────────────────────────────────────────
 # background_tasks
 # ──────────────────────────────────────────────────────────────────────────