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Commit c8ce49c

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dsp-antdsp-ant
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Fix #177: Returning multiple tool results (#222)
* feat: allow lowlevel servers to return a list of resources The resource/read message in MCP allows of multiple resources to be returned. However, in the SDK we do not allow this. This change is such that we allow returning multiple resource in the lowlevel API if needed. However in FastMCP we stick to one, since a FastMCP resource defines the mime_type in the decorator and hence a resource cannot dynamically return different mime_typed resources. It also is just the better default to only return one resource. However in the lowlevel API we will allow this. Strictly speaking this is not a BC break since the new return value is additive, but if people subclassed server, it will break them. * feat: lower the type requriements for call_tool to Iterable
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‎src/mcp/server/fastmcp/server.py

Copy file name to clipboardExpand all lines: src/mcp/server/fastmcp/server.py
+4-4Lines changed: 4 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -3,7 +3,7 @@
33
import inspect
44
import json
55
import re
6-
from collections.abc import AsyncIterator
6+
from collections.abc import AsyncIterator, Iterable
77
from contextlib import (
88
AbstractAsyncContextManager,
99
asynccontextmanager,
@@ -236,7 +236,7 @@ async def list_resource_templates(self) -> list[MCPResourceTemplate]:
236236
for template in templates
237237
]
238238

239-
async def read_resource(self, uri: AnyUrl | str) -> ReadResourceContents:
239+
async def read_resource(self, uri: AnyUrl | str) -> Iterable[ReadResourceContents]:
240240
"""Read a resource by URI."""
241241

242242
resource = await self._resource_manager.get_resource(uri)
@@ -245,7 +245,7 @@ async def read_resource(self, uri: AnyUrl | str) -> ReadResourceContents:
245245

246246
try:
247247
content = await resource.read()
248-
return ReadResourceContents(content=content, mime_type=resource.mime_type)
248+
return [ReadResourceContents(content=content, mime_type=resource.mime_type)]
249249
except Exception as e:
250250
logger.error(f"Error reading resource {uri}: {e}")
251251
raise ResourceError(str(e))
@@ -649,7 +649,7 @@ async def report_progress(
649649
progress_token=progress_token, progress=progress, total=total
650650
)
651651

652-
async def read_resource(self, uri: str | AnyUrl) -> ReadResourceContents:
652+
async def read_resource(self, uri: str | AnyUrl) -> Iterable[ReadResourceContents]:
653653
"""Read a resource by URI.
654654
655655
Args:

‎src/mcp/server/lowlevel/server.py

Copy file name to clipboardExpand all lines: src/mcp/server/lowlevel/server.py
+18-7Lines changed: 18 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -67,9 +67,9 @@ async def main():
6767
import contextvars
6868
import logging
6969
import warnings
70-
from collections.abc import Awaitable, Callable
70+
from collections.abc import Awaitable, Callable, Iterable
7171
from contextlib import AbstractAsyncContextManager, AsyncExitStack, asynccontextmanager
72-
from typing import Any, AsyncIterator, Generic, Sequence, TypeVar
72+
from typing import Any, AsyncIterator, Generic, TypeVar
7373

7474
import anyio
7575
from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
@@ -279,7 +279,9 @@ async def handler(_: Any):
279279

280280
def read_resource(self):
281281
def decorator(
282-
func: Callable[[AnyUrl], Awaitable[str | bytes | ReadResourceContents]],
282+
func: Callable[
283+
[AnyUrl], Awaitable[str | bytes | Iterable[ReadResourceContents]]
284+
],
283285
):
284286
logger.debug("Registering handler for ReadResourceRequest")
285287

@@ -307,13 +309,22 @@ def create_content(data: str | bytes, mime_type: str | None):
307309
case str() | bytes() as data:
308310
warnings.warn(
309311
"Returning str or bytes from read_resource is deprecated. "
310-
"Use ReadResourceContents instead.",
312+
"Use Iterable[ReadResourceContents] instead.",
311313
DeprecationWarning,
312314
stacklevel=2,
313315
)
314316
content = create_content(data, None)
315-
case ReadResourceContents() as contents:
316-
content = create_content(contents.content, contents.mime_type)
317+
case Iterable() as contents:
318+
contents_list = [
319+
create_content(content_item.content, content_item.mime_type)
320+
for content_item in contents
321+
if isinstance(content_item, ReadResourceContents)
322+
]
323+
return types.ServerResult(
324+
types.ReadResourceResult(
325+
contents=contents_list,
326+
)
327+
)
317328
case _:
318329
raise ValueError(
319330
f"Unexpected return type from read_resource: {type(result)}"
@@ -387,7 +398,7 @@ def decorator(
387398
func: Callable[
388399
...,
389400
Awaitable[
390-
Sequence[
401+
Iterable[
391402
types.TextContent | types.ImageContent | types.EmbeddedResource
392403
]
393404
],

‎tests/issues/test_141_resource_templates.py

Copy file name to clipboardExpand all lines: tests/issues/test_141_resource_templates.py
+4-2Lines changed: 4 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -51,8 +51,10 @@ def get_user_profile_missing(user_id: str) -> str:
5151

5252
# Verify valid template works
5353
result = await mcp.read_resource("resource://users/123/posts/456")
54-
assert result.content == "Post 456 by user 123"
55-
assert result.mime_type == "text/plain"
54+
result_list = list(result)
55+
assert len(result_list) == 1
56+
assert result_list[0].content == "Post 456 by user 123"
57+
assert result_list[0].mime_type == "text/plain"
5658

5759
# Verify invalid parameters raise error
5860
with pytest.raises(ValueError, match="Unknown resource"):

‎tests/issues/test_152_resource_mime_type.py

Copy file name to clipboardExpand all lines: tests/issues/test_152_resource_mime_type.py
+4-4Lines changed: 4 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -99,11 +99,11 @@ async def handle_list_resources():
9999
@server.read_resource()
100100
async def handle_read_resource(uri: AnyUrl):
101101
if str(uri) == "test://image":
102-
return ReadResourceContents(content=base64_string, mime_type="image/png")
102+
return [ReadResourceContents(content=base64_string, mime_type="image/png")]
103103
elif str(uri) == "test://image_bytes":
104-
return ReadResourceContents(
105-
content=bytes(image_bytes), mime_type="image/png"
106-
)
104+
return [
105+
ReadResourceContents(content=bytes(image_bytes), mime_type="image/png")
106+
]
107107
raise Exception(f"Resource not found: {uri}")
108108

109109
# Test that resources are listed with correct mime type

‎tests/server/fastmcp/servers/test_file_server.py

Copy file name to clipboardExpand all lines: tests/server/fastmcp/servers/test_file_server.py
+12-3Lines changed: 12 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -88,7 +88,10 @@ async def test_list_resources(mcp: FastMCP):
8888

8989
@pytest.mark.anyio
9090
async def test_read_resource_dir(mcp: FastMCP):
91-
res = await mcp.read_resource("dir://test_dir")
91+
res_iter = await mcp.read_resource("dir://test_dir")
92+
res_list = list(res_iter)
93+
assert len(res_list) == 1
94+
res = res_list[0]
9295
assert res.mime_type == "text/plain"
9396

9497
files = json.loads(res.content)
@@ -102,7 +105,10 @@ async def test_read_resource_dir(mcp: FastMCP):
102105

103106
@pytest.mark.anyio
104107
async def test_read_resource_file(mcp: FastMCP):
105-
res = await mcp.read_resource("file://test_dir/example.py")
108+
res_iter = await mcp.read_resource("file://test_dir/example.py")
109+
res_list = list(res_iter)
110+
assert len(res_list) == 1
111+
res = res_list[0]
106112
assert res.content == "print('hello world')"
107113

108114

@@ -119,5 +125,8 @@ async def test_delete_file_and_check_resources(mcp: FastMCP, test_dir: Path):
119125
await mcp.call_tool(
120126
"delete_file", arguments=dict(path=str(test_dir / "example.py"))
121127
)
122-
res = await mcp.read_resource("file://test_dir/example.py")
128+
res_iter = await mcp.read_resource("file://test_dir/example.py")
129+
res_list = list(res_iter)
130+
assert len(res_list) == 1
131+
res = res_list[0]
123132
assert res.content == "File not found"

‎tests/server/fastmcp/test_server.py

Copy file name to clipboardExpand all lines: tests/server/fastmcp/test_server.py
+4-1Lines changed: 4 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -581,7 +581,10 @@ def test_resource() -> str:
581581

582582
@mcp.tool()
583583
async def tool_with_resource(ctx: Context) -> str:
584-
r = await ctx.read_resource("test://data")
584+
r_iter = await ctx.read_resource("test://data")
585+
r_list = list(r_iter)
586+
assert len(r_list) == 1
587+
r = r_list[0]
585588
return f"Read resource: {r.content} with mime type {r.mime_type}"
586589

587590
async with client_session(mcp._mcp_server) as client:

‎tests/server/test_read_resource.py

Copy file name to clipboardExpand all lines: tests/server/test_read_resource.py
+16-11Lines changed: 16 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -1,3 +1,4 @@
1+
from collections.abc import Iterable
12
from pathlib import Path
23
from tempfile import NamedTemporaryFile
34

@@ -26,8 +27,8 @@ async def test_read_resource_text(temp_file: Path):
2627
server = Server("test")
2728

2829
@server.read_resource()
29-
async def read_resource(uri: AnyUrl) -> ReadResourceContents:
30-
return ReadResourceContents(content="Hello World", mime_type="text/plain")
30+
async def read_resource(uri: AnyUrl) -> Iterable[ReadResourceContents]:
31+
return [ReadResourceContents(content="Hello World", mime_type="text/plain")]
3132

3233
# Get the handler directly from the server
3334
handler = server.request_handlers[types.ReadResourceRequest]
@@ -54,10 +55,12 @@ async def test_read_resource_binary(temp_file: Path):
5455
server = Server("test")
5556

5657
@server.read_resource()
57-
async def read_resource(uri: AnyUrl) -> ReadResourceContents:
58-
return ReadResourceContents(
59-
content=b"Hello World", mime_type="application/octet-stream"
60-
)
58+
async def read_resource(uri: AnyUrl) -> Iterable[ReadResourceContents]:
59+
return [
60+
ReadResourceContents(
61+
content=b"Hello World", mime_type="application/octet-stream"
62+
)
63+
]
6164

6265
# Get the handler directly from the server
6366
handler = server.request_handlers[types.ReadResourceRequest]
@@ -83,11 +86,13 @@ async def test_read_resource_default_mime(temp_file: Path):
8386
server = Server("test")
8487

8588
@server.read_resource()
86-
async def read_resource(uri: AnyUrl) -> ReadResourceContents:
87-
return ReadResourceContents(
88-
content="Hello World",
89-
# No mime_type specified, should default to text/plain
90-
)
89+
async def read_resource(uri: AnyUrl) -> Iterable[ReadResourceContents]:
90+
return [
91+
ReadResourceContents(
92+
content="Hello World",
93+
# No mime_type specified, should default to text/plain
94+
)
95+
]
9196

9297
# Get the handler directly from the server
9398
handler = server.request_handlers[types.ReadResourceRequest]

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