modelcontextprotocol · felixweinberger · Sep 29, 2025 · Sep 29, 2025
diff --git a/examples/fastmcp/icons_demo.py b/examples/fastmcp/icons_demo.py
@@ -14,7 +14,7 @@
 icon_data = base64.standard_b64encode(icon_path.read_bytes()).decode()
 icon_data_uri = f"data:image/png;base64,{icon_data}"

-icon_data = Icon(src=icon_data_uri, mimeType="image/png", sizes="64x64")
+icon_data = Icon(src=icon_data_uri, mimeType="image/png", sizes=["64x64"])

 # Create server with icons in implementation
 mcp = FastMCP("Icons Demo Server", website_url="https://github.com/modelcontextprotocol/python-sdk", icons=[icon_data])
@@ -40,9 +40,9 @@ def prompt_with_icon(text: str) -> str:

 @mcp.tool(
    icons=[
-        Icon(src=icon_data_uri, mimeType="image/png", sizes="16x16"),
-        Icon(src=icon_data_uri, mimeType="image/png", sizes="32x32"),
-        Icon(src=icon_data_uri, mimeType="image/png", sizes="64x64"),
+        Icon(src=icon_data_uri, mimeType="image/png", sizes=["16x16"]),
+        Icon(src=icon_data_uri, mimeType="image/png", sizes=["32x32"]),
+        Icon(src=icon_data_uri, mimeType="image/png", sizes=["64x64"]),
    ]
 )
 def multi_icon_tool(action: str) -> str:

diff --git a/src/mcp/types.py b/src/mcp/types.py
@@ -222,8 +222,8 @@ class Icon(BaseModel):
    mimeType: str | None = None
    """Optional MIME type for the icon."""

-    sizes: str | None = None
-    """Optional string specifying icon dimensions (e.g., "48x48 96x96")."""
+    sizes: list[str] | None = None
+    """Optional list of strings specifying icon dimensions (e.g., ["48x48", "96x96"])."""

    model_config = ConfigDict(extra="allow")


diff --git a/tests/issues/test_1338_icons_and_metadata.py b/tests/issues/test_1338_icons_and_metadata.py
@@ -15,7 +15,7 @@ async def test_icons_and_website_url():
    test_icon = Icon(
        src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNkYPhfDwAChwGA60e6kgAAAABJRU5ErkJggg==",
        mimeType="image/png",
-        sizes="1x1",
+        sizes=["1x1"],
    )

    # Create server with website URL and icon
@@ -80,9 +80,9 @@ async def test_multiple_icons():
    """Test that multiple icons can be added to tools, resources, and prompts."""

    # Create multiple test icons
-    icon1 = Icon(src="data:image/png;base64,icon1", mimeType="image/png", sizes="16x16")
-    icon2 = Icon(src="data:image/png;base64,icon2", mimeType="image/png", sizes="32x32")
-    icon3 = Icon(src="data:image/png;base64,icon3", mimeType="image/png", sizes="64x64")
+    icon1 = Icon(src="data:image/png;base64,icon1", mimeType="image/png", sizes=["16x16"])
+    icon2 = Icon(src="data:image/png;base64,icon2", mimeType="image/png", sizes=["32x32"])
+    icon3 = Icon(src="data:image/png;base64,icon3", mimeType="image/png", sizes=["64x64"])

    mcp = FastMCP("MultiIconServer")

@@ -98,9 +98,9 @@ def multi_icon_tool() -> str:
    tool = tools[0]
    assert tool.icons is not None
    assert len(tool.icons) == 3
-    assert tool.icons[0].sizes == "16x16"
-    assert tool.icons[1].sizes == "32x32"
-    assert tool.icons[2].sizes == "64x64"
+    assert tool.icons[0].sizes == ["16x16"]
+    assert tool.icons[1].sizes == ["32x32"]
+    assert tool.icons[2].sizes == ["64x64"]


 async def test_no_icons_or_website():