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Fix bug with tools that return lists

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This commit is contained in:
Jeremiah Lowin 2025-04-11 10:51:07 -04:00
commit 5b32ffb77f
2 changed files with 44 additions and 55 deletions
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27
src/fastmcp/server/server.py
View file

@ -8,7 +8,6 @@ from contextlib import (
AbstractAsyncContextManager,
asynccontextmanager,
)
from itertools import chain
from typing import TYPE_CHECKING, Any, Generic, Literal
import anyio
@ -618,7 +617,8 @@ class FastMCP(Generic[LifespanResultT]):
def _convert_to_content(
result: Any,
) -> Sequence[TextContent | ImageContent | EmbeddedResource]:
_process_as_single_item: bool = False,
) -> list[TextContent | ImageContent | EmbeddedResource]:
"""Convert a result to a sequence of content objects."""
if result is None:
return []
@ -629,8 +629,27 @@ def _convert_to_content(
if isinstance(result, Image):
return [result.to_image_content()]
if isinstance(result, list | tuple):
return list(chain.from_iterable(_convert_to_content(item) for item in result)) # type: ignore[reportUnknownVariableType]
if isinstance(result, list | tuple) and not _process_as_single_item:
# if the result is a list, then it could either be a list of MCP types,
# or a "regular" list that the tool is returning, or a mix of both.
#
# so we extract all the MCP types / images and convert them as individual content elements,
# and aggregate the rest as a single content element
mcp_types = []
other_content = []
for item in result:
if isinstance(item, (TextContent, ImageContent, EmbeddedResource, Image)):
mcp_types.append(_convert_to_content(item)[0])
else:
other_content.append(item)
if other_content:
other_content = _convert_to_content(
other_content, _process_as_single_item=True
)
return other_content + mcp_types
if not isinstance(result, str):
try:

72
tests/server/test_server.py
View file

@ -1,4 +1,5 @@
import base64
import json
from pathlib import Path
from typing import TYPE_CHECKING
@ -25,13 +26,11 @@ if TYPE_CHECKING:
class TestServer:
@pytest.mark.anyio
async def test_create_server(self):
mcp = FastMCP(instructions="Server instructions")
assert mcp.name == "FastMCP"
assert mcp.instructions == "Server instructions"
@pytest.mark.anyio
async def test_non_ascii_description(self):
"""Test that FastMCP handles non-ASCII characters in descriptions correctly"""
mcp = FastMCP()
@ -59,7 +58,6 @@ class TestServer:
assert isinstance(content, TextContent)
assert "¡Hola, 世界! 👋" == content.text
@pytest.mark.anyio
async def test_add_tool_decorator(self):
mcp = FastMCP()
@ -69,7 +67,6 @@ class TestServer:
assert len(mcp._tool_manager.list_tools()) == 1
@pytest.mark.anyio
async def test_add_tool_decorator_incorrect_usage(self):
mcp = FastMCP()
@ -79,7 +76,6 @@ class TestServer:
def add(x: int, y: int) -> int:
return x + y
@pytest.mark.anyio
async def test_add_resource_decorator(self):
mcp = FastMCP()
@ -89,7 +85,6 @@ class TestServer:
assert len(mcp._resource_manager._templates) == 1
@pytest.mark.anyio
async def test_add_resource_decorator_incorrect_usage(self):
mcp = FastMCP()
@ -106,6 +101,10 @@ def tool_fn(x: int, y: int) -> int:
return x + y
def tool_fn_list() -> list[str | int]:
return ["x", 2]
def error_tool_fn() -> None:
raise ValueError("Test error")
@ -122,14 +121,12 @@ def mixed_content_tool_fn() -> list[TextContent | ImageContent]:
class TestServerTools:
@pytest.mark.anyio
async def test_add_tool(self):
mcp = FastMCP()
mcp.add_tool(tool_fn)
mcp.add_tool(tool_fn)
assert len(mcp._tool_manager.list_tools()) == 1
@pytest.mark.anyio
async def test_list_tools(self):
mcp = FastMCP()
mcp.add_tool(tool_fn)
@ -137,7 +134,6 @@ class TestServerTools:
tools = await client.list_tools()
assert len(tools.tools) == 1
@pytest.mark.anyio
async def test_call_tool(self):
mcp = FastMCP()
mcp.add_tool(tool_fn)
@ -146,7 +142,6 @@ class TestServerTools:
assert not hasattr(result, "error")
assert len(result.content) > 0
@pytest.mark.anyio
async def test_tool_exception_handling(self):
mcp = FastMCP()
mcp.add_tool(error_tool_fn)
@ -158,7 +153,6 @@ class TestServerTools:
assert "Test error" in content.text
assert result.isError is True
@pytest.mark.anyio
async def test_tool_error_handling(self):
mcp = FastMCP()
mcp.add_tool(error_tool_fn)
@ -170,7 +164,6 @@ class TestServerTools:
assert "Test error" in content.text
assert result.isError is True
@pytest.mark.anyio
async def test_tool_error_details(self):
"""Test that exception details are properly formatted in the response"""
mcp = FastMCP()
@ -183,7 +176,6 @@ class TestServerTools:
assert "Test error" in content.text
assert result.isError is True
@pytest.mark.anyio
async def test_tool_return_value_conversion(self):
mcp = FastMCP()
mcp.add_tool(tool_fn)
@ -194,7 +186,16 @@ class TestServerTools:
assert isinstance(content, TextContent)
assert content.text == "3"
@pytest.mark.anyio
async def test_tool_returns_list(self):
mcp = FastMCP()
mcp.add_tool(tool_fn_list)
async with client_session(mcp._mcp_server) as client:
result = await client.call_tool("tool_fn_list", {})
assert len(result.content) == 1
content = result.content[0]
assert isinstance(content, TextContent)
assert json.loads(content.text) == ["x", 2]
async def test_tool_image_helper(self, tmp_path: Path):
# Create a test image
image_path = tmp_path / "test.png"
@ -213,12 +214,12 @@ class TestServerTools:
decoded = base64.b64decode(content.data)
assert decoded == b"fake png data"
@pytest.mark.anyio
async def test_tool_mixed_content(self):
mcp = FastMCP()
mcp.add_tool(mixed_content_tool_fn)
async with client_session(mcp._mcp_server) as client:
result = await client.call_tool("mixed_content_tool_fn", {})
assert len(result.content) == 2
content1 = result.content[0]
content2 = result.content[1]
@ -228,10 +229,9 @@ class TestServerTools:
assert content2.mimeType == "image/png"
assert content2.data == "abc"
@pytest.mark.anyio
async def test_tool_mixed_list_with_image(self, tmp_path: Path):
"""Test that lists containing Image objects and other types are handled
correctly"""
correctly. Note that the non-MCP content will be grouped together."""
# Create a test image
image_path = tmp_path / "test.png"
image_path.write_bytes(b"test image data")
@ -248,24 +248,20 @@ class TestServerTools:
mcp.add_tool(mixed_list_fn)
async with client_session(mcp._mcp_server) as client:
result = await client.call_tool("mixed_list_fn", {})
assert len(result.content) == 4
assert len(result.content) == 3
# Check text conversion
content1 = result.content[0]
assert isinstance(content1, TextContent)
assert content1.text == "text message"
assert json.loads(content1.text) == ["text message", {"key": "value"}]
# Check image conversion
content2 = result.content[1]
assert isinstance(content2, ImageContent)
assert content2.mimeType == "image/png"
assert base64.b64decode(content2.data) == b"test image data"
# Check dict conversion
# Check direct TextContent
content3 = result.content[2]
assert isinstance(content3, TextContent)
assert '"key": "value"' in content3.text
# Check direct TextContent
content4 = result.content[3]
assert isinstance(content4, TextContent)
assert content4.text == "direct content"
assert content3.text == "direct content"
async def test_parameter_descriptions(self):
mcp = FastMCP("Test Server")
@ -291,7 +287,6 @@ class TestServerTools:
class TestServerResources:
@pytest.mark.anyio
async def test_text_resource(self):
mcp = FastMCP()
@ -308,7 +303,6 @@ class TestServerResources:
assert isinstance(result.contents[0], TextResourceContents)
assert result.contents[0].text == "Hello, world!"
@pytest.mark.anyio
async def test_binary_resource(self):
mcp = FastMCP()
@ -328,7 +322,6 @@ class TestServerResources:
assert isinstance(result.contents[0], BlobResourceContents)
assert result.contents[0].blob == base64.b64encode(b"Binary data").decode()
@pytest.mark.anyio
async def test_file_resource_text(self, tmp_path: Path):
mcp = FastMCP()
@ -346,7 +339,6 @@ class TestServerResources:
assert isinstance(result.contents[0], TextResourceContents)
assert result.contents[0].text == "Hello from file!"
@pytest.mark.anyio
async def test_file_resource_binary(self, tmp_path: Path):
mcp = FastMCP()
@ -372,7 +364,6 @@ class TestServerResources:
class TestServerResourceTemplates:
@pytest.mark.anyio
async def test_resource_with_params(self):
"""Test that a resource with function parameters raises an error if the URI
parameters don't match"""
@ -384,7 +375,6 @@ class TestServerResourceTemplates:
def get_data_fn(param: str) -> str:
return f"Data: {param}"
@pytest.mark.anyio
async def test_resource_with_uri_params(self):
"""Test that a resource with URI parameters is automatically a template"""
mcp = FastMCP()
@ -395,7 +385,6 @@ class TestServerResourceTemplates:
def get_data() -> str:
return "Data"
@pytest.mark.anyio
async def test_resource_with_untyped_params(self):
"""Test that a resource with untyped parameters raises an error"""
mcp = FastMCP()
@ -404,7 +393,6 @@ class TestServerResourceTemplates:
def get_data(param) -> str:
return "Data"
@pytest.mark.anyio
async def test_resource_matching_params(self):
"""Test that a resource with matching URI and function parameters works"""
mcp = FastMCP()
@ -418,7 +406,6 @@ class TestServerResourceTemplates:
assert isinstance(result.contents[0], TextResourceContents)
assert result.contents[0].text == "Data for test"
@pytest.mark.anyio
async def test_resource_mismatched_params(self):
"""Test that mismatched parameters raise an error"""
mcp = FastMCP()
@ -429,7 +416,6 @@ class TestServerResourceTemplates:
def get_data(user: str) -> str:
return f"Data for {user}"
@pytest.mark.anyio
async def test_resource_multiple_params(self):
"""Test that multiple parameters work correctly"""
mcp = FastMCP()
@ -445,7 +431,6 @@ class TestServerResourceTemplates:
assert isinstance(result.contents[0], TextResourceContents)
assert result.contents[0].text == "Data for cursor/fastmcp"
@pytest.mark.anyio
async def test_resource_multiple_mismatched_params(self):
"""Test that mismatched parameters raise an error"""
mcp = FastMCP()
@ -468,7 +453,6 @@ class TestServerResourceTemplates:
assert isinstance(result.contents[0], TextResourceContents)
assert result.contents[0].text == "Static data"
@pytest.mark.anyio
async def test_template_to_resource_conversion(self):
"""Test that templates are properly converted to resources when accessed"""
mcp = FastMCP()
@ -491,7 +475,6 @@ class TestServerResourceTemplates:
class TestContextInjection:
"""Test context injection in tools."""
@pytest.mark.anyio
async def test_context_detection(self):
"""Test that context parameters are properly detected."""
mcp = FastMCP()
@ -502,7 +485,6 @@ class TestContextInjection:
tool = mcp._tool_manager.add_tool(tool_with_context)
assert tool.context_kwarg == "ctx"
@pytest.mark.anyio
async def test_context_injection(self):
"""Test that context is properly injected into tool calls."""
mcp = FastMCP()
@ -520,7 +502,6 @@ class TestContextInjection:
assert "Request" in content.text
assert "42" in content.text
@pytest.mark.anyio
async def test_async_context(self):
"""Test that context works in async functions."""
mcp = FastMCP()
@ -538,7 +519,6 @@ class TestContextInjection:
assert "Async request" in content.text
assert "42" in content.text
@pytest.mark.anyio
async def test_context_logging(self):
from unittest.mock import patch
@ -576,7 +556,6 @@ class TestContextInjection:
level="error", data="Error message", logger=None
)
@pytest.mark.anyio
async def test_optional_context(self):
"""Test that context is optional."""
mcp = FastMCP()
@ -592,7 +571,6 @@ class TestContextInjection:
assert isinstance(content, TextContent)
assert content.text == "42"
@pytest.mark.anyio
async def test_context_resource_access(self):
"""Test that context can access resources."""
mcp = FastMCP()
@ -620,7 +598,6 @@ class TestContextInjection:
class TestServerPrompts:
"""Test prompt functionality in FastMCP server."""
@pytest.mark.anyio
async def test_prompt_decorator(self):
"""Test that the prompt decorator registers prompts correctly."""
mcp = FastMCP()
@ -637,7 +614,6 @@ class TestServerPrompts:
assert isinstance(content[0].content, TextContent)
assert content[0].content.text == "Hello, world!"
@pytest.mark.anyio
async def test_prompt_decorator_with_name(self):
"""Test prompt decorator with custom name."""
mcp = FastMCP()
@ -653,7 +629,6 @@ class TestServerPrompts:
assert isinstance(content[0].content, TextContent)
assert content[0].content.text == "Hello, world!"
@pytest.mark.anyio
async def test_prompt_decorator_with_description(self):
"""Test prompt decorator with custom description."""
mcp = FastMCP()
@ -678,7 +653,6 @@ class TestServerPrompts:
def fn() -> str:
return "Hello, world!"
@pytest.mark.anyio
async def test_list_prompts(self):
"""Test listing prompts through MCP protocol."""
mcp = FastMCP()
@ -700,7 +674,6 @@ class TestServerPrompts:
assert prompt.arguments[1].name == "optional"
assert prompt.arguments[1].required is False
@pytest.mark.anyio
async def test_get_prompt(self):
"""Test getting a prompt through MCP protocol."""
mcp = FastMCP()
@ -718,7 +691,6 @@ class TestServerPrompts:
assert isinstance(content, TextContent)
assert content.text == "Hello, World!"
@pytest.mark.anyio
async def test_get_prompt_with_resource(self):
"""Test getting a prompt that returns resource content."""
mcp = FastMCP()
@ -748,7 +720,6 @@ class TestServerPrompts:
assert resource.text == "File contents"
assert resource.mimeType == "text/plain"
@pytest.mark.anyio
async def test_get_unknown_prompt(self):
"""Test error when getting unknown prompt."""
mcp = FastMCP()
@ -756,7 +727,6 @@ class TestServerPrompts:
with pytest.raises(McpError, match="Unknown prompt"):
await client.get_prompt("unknown")
@pytest.mark.anyio
async def test_get_prompt_missing_args(self):
"""Test error when required arguments are missing."""
mcp = FastMCP()