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fix: support List[<MCP types>] return values for mcp_tool #337

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fix: support List[<MCP types>] return values for mcp_tool #337

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453bc88
fix List[] return values
hallvictoria May 1, 2026
1ebfee6
lint
hallvictoria May 4, 2026
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74 changes: 64 additions & 10 deletions azure/functions/decorators/function_app.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1663,16 +1663,44 @@ def decorator(fb: FunctionBuilder) -> FunctionBuilder:
args = typing.get_args(return_annotation)

# Check for official MCP SDK types in lists
try:
if isinstance(args[0], type):
# Check if the type is from the mcp.types module
if hasattr(args[0], '__module__'):
module = args[0].__module__
if module and (module.startswith('mcp.types')
or module == 'mcp.types'):
is_mcp_sdk_type = True
except (ImportError, TypeError, AttributeError):
pass
if origin in (list, List):
# For List[T], check if T is an MCP type
try:
if len(args) > 0:
list_item_type = args[0]
# Check if it's a direct MCP type
if isinstance(list_item_type, type):
if hasattr(list_item_type, '__module__'):
module = list_item_type.__module__
if (module
and (module.startswith('mcp.types')
or module == 'mcp.types')):
is_mcp_sdk_type = True
# Check if it's a Union of MCP types
elif hasattr(list_item_type, '__origin__'):
union_origin = typing.get_origin(
list_item_type)
if union_origin is Union:
union_args = typing.get_args(
list_item_type)
for union_arg in union_args:
if (isinstance(union_arg, type)
and union_arg is not
type(None)):
if hasattr(union_arg,
'__module__'):
module = (
union_arg.__module__)
if (module
and (module.startswith(
'mcp.types')
or module
== 'mcp.types')):
is_mcp_sdk_type = True
break
except (ImportError, TypeError, AttributeError,
IndexError):
pass

# Check for Optional[T] where T is an MCP type
if origin is Union:
Expand Down Expand Up @@ -1801,6 +1829,32 @@ async def wrapper(context: str, *args, **kwargs):
"structuredContent": structured_content_json
}))

# Handle lists of MCP SDK content blocks
# Wrap them in a CallToolResult structure
elif isinstance(result, list) and len(result) > 0:
first_item = result[0]
if _is_mcp_sdk_type(first_item):
# Serialize all blocks in the list
from ..mcp import _serialize_content_block
blocks_list = [_serialize_content_block(block)
for block in result]

# Create a CallToolResult-like structure
# containing the blocks
call_tool_result = {
"content": blocks_list
}
full_result_json = json.dumps(call_tool_result)

# Return in CallToolResult format
# (list of blocks doesn't have separate
# structuredContent)
return str(json.dumps({
"type": "call_tool_result",
"content": full_result_json,
"structuredContent": None
}))

# Handle all other MCP SDK types
# (TextContent, ImageContent, ResourceLink, etc.)
elif _is_mcp_sdk_type(result):
Expand Down
40 changes: 40 additions & 0 deletions tests/decorators/test_mcp.py
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Original file line number Diff line number Diff line change
Expand Up @@ -675,6 +675,18 @@ def get_texts() -> List[TextContent]:
trigger = get_texts._function._bindings[0]
self.assertTrue(trigger.use_result_schema)

def test_auto_detect_list_union_mcp_types(self):
"""Test auto-detection of List[Union[MCP types]] return type"""
from typing import Union

@self.app.mcp_tool()
def get_mixed_content() -> List[Union[TextContent, ImageContent]]:
"""Returns mixed content blocks"""
return [TextContent(type="text", text="test")]

trigger = get_mixed_content._function._bindings[0]
self.assertTrue(trigger.use_result_schema)

def test_auto_detect_optional_mcp_image_content(self):
"""Test auto-detection of Optional[ImageContent] return type"""
@self.app.mcp_tool()
Expand Down Expand Up @@ -907,6 +919,34 @@ def test_func() -> str:
self.assertIn("content", result_obj)
self.assertIn("structuredContent", result_obj)

def test_structured_content_in_list_of_mcp_types(self):
"""Test that List[MCP SDK types] includes structuredContent"""
@self.app.mcp_tool()
def test_func() -> List[TextContent]:
"""Test function"""
return [
TextContent(type="text", text="First item"),
TextContent(type="text", text="Second item")
]

wrapper = test_func._function._func
context = json.dumps({"arguments": {}})
result = asyncio.run(wrapper(context))
result_obj = json.loads(result)

# List of content blocks is wrapped as CallToolResult
self.assertIn("type", result_obj)
self.assertEqual(result_obj["type"], "call_tool_result")
self.assertIn("content", result_obj)
self.assertIn("structuredContent", result_obj)

# Content contains the CallToolResult structure with the blocks
content_obj = json.loads(result_obj["content"])
self.assertIn("content", content_obj)
self.assertEqual(len(content_obj["content"]), 2)
self.assertEqual(content_obj["content"][0]["text"], "First item")
self.assertEqual(content_obj["content"][1]["text"], "Second item")


class TestMCPPackageNotInstalled(unittest.TestCase):
"""Tests for graceful degradation when mcp package is not installed"""
Expand Down
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