Add a pluggable server extension API with MCP Apps (#3003)

Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

tests/docs_src/test_extensions.py

@@ -0,0 +1,97 @@
+"""`docs/advanced/extensions.md`: every claim the page makes, proved against the real SDK."""
+
+import logging
+from typing import cast
+
+import mcp_types as types
+import pytest
+from inline_snapshot import snapshot
+from mcp_types import METHOD_NOT_FOUND, MISSING_REQUIRED_CLIENT_CAPABILITY, TextContent
+
+from docs_src.extensions import tutorial001, tutorial002, tutorial003, tutorial004, tutorial005
+from mcp import Client, MCPError
+from mcp.server.extension import Extension
+
+# See test_index.py for why this is a per-module mark and not a conftest hook.
+pytestmark = [pytest.mark.anyio, pytest.mark.filterwarnings("error::mcp.MCPDeprecationWarning")]
+
+
+async def test_using_an_extension_advertises_its_capability() -> None:
+    """tutorial001: `extensions=[Apps()]` is all it takes for the server to advertise
+    the extension under `capabilities.extensions`."""
+    async with Client(tutorial001.mcp) as client:
+        assert client.server_capabilities.extensions == {"io.modelcontextprotocol/ui": {}}
+
+
+def test_a_prefixless_identifier_fails_at_class_definition() -> None:
+    """tutorial002 + the page's TypeError block: the identifier is validated when the
+    subclass is defined, with the exact message the page shows."""
+    assert tutorial002.Stamps.identifier == "com.example/stamps"
+    with pytest.raises(TypeError) as exc_info:
+        type("Stamps", (Extension,), {"identifier": "stamps"})
+    assert str(exc_info.value) == snapshot(
+        "Stamps.identifier must be a `vendor-prefix/name` string (reverse-DNS prefix required), got 'stamps'"
+    )
+
+
+async def test_extension_settings_advertised_under_capabilities() -> None:
+    """tutorial003: `settings()` becomes the entry at `capabilities.extensions[identifier]`."""
+    async with Client(tutorial003.mcp) as client:
+        assert client.server_capabilities.extensions == {"com.example/stamps": {"sealed": True}}
+
+
+async def test_contributed_tool_is_listed_and_callable() -> None:
+    """tutorial003: a `ToolBinding` registers like any `add_tool` call: listed and callable."""
+    async with Client(tutorial003.mcp) as client:
+        listed = await client.list_tools()
+        assert [tool.name for tool in listed.tools] == ["stamp"]
+        result = await client.call_tool("stamp", {"text": "hello"})
+    assert result.content == [TextContent(type="text", text="[stamped] hello")]
+
+
+async def test_the_stamps_client_program_runs_as_shown(capsys: pytest.CaptureFixture[str]) -> None:
+    """tutorial003: `main()` is the literal client program on the page; both printed
+    lines match the page's comments."""
+    await tutorial003.main()
+    out = capsys.readouterr().out
+    assert "{'com.example/stamps': {'sealed': True}}" in out
+    assert "[stamped] hello" in out
+
+
+async def test_the_search_client_program_runs_as_shown(capsys: pytest.CaptureFixture[str]) -> None:
+    """tutorial004: `main()` declares the extension and gets the vendor method's result."""
+    await tutorial004.main()
+    assert "['mcp-0', 'mcp-1', 'mcp-2']" in capsys.readouterr().out
+
+
+async def test_vendor_method_rejects_a_non_declaring_client_with_32021() -> None:
+    """tutorial004: `require_client_extension` answers a non-declaring client with `-32021`
+    and the machine-readable `requiredCapabilities` payload."""
+    async with Client(tutorial004.mcp) as client:
+        request = tutorial004.SearchRequest(params=tutorial004.SearchParams(query="mcp"))
+        with pytest.raises(MCPError) as exc_info:
+            await client.session.send_request(cast("types.ClientRequest", request), tutorial004.SearchResult)
+    assert exc_info.value.code == MISSING_REQUIRED_CLIENT_CAPABILITY
+    assert exc_info.value.error.data == {"requiredCapabilities": {"extensions": {"com.example/search": {}}}}
+
+
+async def test_version_pinned_method_is_not_found_on_a_legacy_connection() -> None:
+    """tutorial004: `protocol_versions={"2026-07-28"}` makes the method METHOD_NOT_FOUND
+    at any other wire version; for a legacy client it doesn't exist."""
+    async with Client(tutorial004.mcp, mode="legacy", extensions={tutorial004.EXTENSION_ID: {}}) as client:
+        request = tutorial004.SearchRequest(params=tutorial004.SearchParams(query="mcp"))
+        with pytest.raises(MCPError) as exc_info:
+            await client.session.send_request(cast("types.ClientRequest", request), tutorial004.SearchResult)
+    assert exc_info.value.code == METHOD_NOT_FOUND
+
+
+async def test_interceptor_observes_the_call_and_passes_the_result_through(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """tutorial005: the interceptor logs the tool name and returns `call_next`'s result unchanged."""
+    with caplog.at_level(logging.INFO, logger=tutorial005.logger.name):
+        async with Client(tutorial005.mcp) as client:
+            result = await client.call_tool("add", {"a": 2, "b": 3})
+    assert result.structured_content == {"result": 5}
+    messages = [record.getMessage() for record in caplog.records if record.name == tutorial005.logger.name]
+    assert messages == ["tool 'add' called"]