MCPServer: content-block returns are unstructured, prompt messages take Image/Audio (#3320)

tests/docs_src/test_structured_output.py

@@ -2,7 +2,7 @@
 
 import pytest
 from inline_snapshot import snapshot
-from mcp_types import TextContent
+from mcp_types import EmbeddedResource, ImageContent, TextContent, TextResourceContents
 
 from docs_src.structured_output import (
     tutorial001,
@@ -17,6 +17,7 @@ from docs_src.structured_output import (
 )
 from mcp import Client
 from mcp.server import MCPServer
+from mcp.server.mcpserver import Image
 from mcp.server.mcpserver.exceptions import InvalidSignature
 
 # See test_index.py for why this is a per-module mark and not a conftest hook.
@@ -173,6 +174,36 @@ async def test_structured_output_false_opts_out() -> None:
         ]
 
 
+async def test_content_blocks_and_media_are_opted_out_of_structured_output() -> None:
+    """The "Content blocks and media" section: a content-block or `Image`/`Audio` return annotation, bare or
+    as list items, derives no output schema and no structured content; the blocks are the result."""
+    mcp = MCPServer("Reports")
+    document = EmbeddedResource(
+        type="resource", resource=TextResourceContents(uri="report://q3", mime_type="text/markdown", text="# Q3")
+    )
+
+    @mcp.tool()
+    def report() -> EmbeddedResource:
+        return document
+
+    @mcp.tool()
+    def chart() -> list[str | Image]:
+        return ["Sales by region:", Image(data=b"png", format="png")]
+
+    async with Client(mcp) as client:
+        tools = {tool.name: tool for tool in (await client.list_tools()).tools}
+        assert tools["report"].output_schema is None
+        assert tools["chart"].output_schema is None
+        report_result = await client.call_tool("report", {})
+        assert (report_result.content, report_result.structured_content) == ([document], None)
+        chart_result = await client.call_tool("chart", {})
+        assert chart_result.structured_content is None
+        assert chart_result.content == [
+            TextContent(type="text", text="Sales by region:"),
+            ImageContent(type="image", data="cG5n", mime_type="image/png"),
+        ]
+
+
 async def test_class_without_type_hints_is_silently_unstructured() -> None:
     """tutorial009: a class with no annotations on its body gets no schema, and the model gets a `repr`."""
     async with Client(tutorial009.mcp) as client: