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fix(client): propagate HTTP transport exceptions to caller by gspeter-max · Pull Request #2124 · modelcontextprotocol/python-sdk · GitHub

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fix(client): propagate HTTP transport exceptions to caller #2124

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gspeter-max wants to merge 3 commits into from
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fix(client): propagate HTTP transport exceptions to caller #2124

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80d1386
fix(client): propagate HTTP transport exceptions to caller
peter-luminova Feb 22, 2026
5584cc6
fix(tests): use condition-based waiting for child process tests
peter-luminova Feb 22, 2026
c339047
test(tests): add coverage for _wait_for_file_to_exist error paths
peter-luminova Feb 22, 2026
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3 changes: 2 additions & 1 deletion src/mcp/client/sse.py
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Original file line number Diff line number Diff line change
Expand Up @@ -144,8 +144,9 @@ async def post_writer(endpoint_url: str):
)
response.raise_for_status()
logger.debug(f"Client message sent successfully: {response.status_code}")
except Exception: # pragma: lax no cover
except Exception as exc: # pragma: lax no cover
logger.exception("Error in post_writer")
await read_stream_writer.send(exc)
finally:
await write_stream.aclose()

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3 changes: 2 additions & 1 deletion src/mcp/client/streamable_http.py
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Expand Up @@ -478,8 +478,9 @@ async def handle_request_async():
else:
await handle_request_async()

except Exception: # pragma: lax no cover
except Exception as exc: # pragma: lax no cover
logger.exception("Error in post_writer")
await read_stream_writer.send(exc)
finally:
await read_stream_writer.aclose()
await write_stream.aclose()
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106 changes: 77 additions & 29 deletions tests/client/test_stdio.py
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Expand Up @@ -30,6 +30,64 @@
tee = shutil.which("tee")


async def _wait_for_file_to_exist(file_path: str, timeout: float = 5.0) -> None:
"""Wait for a file to exist and be non-empty.

Uses condition-based waiting instead of arbitrary sleep to eliminate
race conditions in tests that verify child processes have started
writing to marker files.

Args:
file_path: Path to the file to wait for
timeout: Maximum time to wait in seconds

Raises:
TimeoutError: If file doesn't exist or remains empty after timeout
"""
start_time = time.time()
poll_interval = 0.01 # Poll every 10ms

while time.time() - start_time < timeout:
if os.path.exists(file_path):
size = os.path.getsize(file_path)
if size > 0:
return
await anyio.sleep(poll_interval)

# Check one more time for better error message
if os.path.exists(file_path):
size = os.path.getsize(file_path)
raise TimeoutError(f"File {file_path} exists but is empty after {timeout}s (size={size})")
else:
raise TimeoutError(f"File {file_path} does not exist after {timeout}s")


@pytest.mark.anyio
async def test_wait_for_file_to_exist_timeout():
"""Test _wait_for_file_to_exist raises TimeoutError when file doesn't exist."""
nonexistent_file = "/tmp/nonexistent_file_test_1234567890"

with pytest.raises(TimeoutError, match="does not exist"):
with anyio.fail_after(1.0):
await _wait_for_file_to_exist(nonexistent_file, timeout=0.1)


@pytest.mark.anyio
async def test_wait_for_file_to_exist_empty_file():
"""Test _wait_for_file_to_exist raises TimeoutError when file exists but is empty."""
import tempfile

with tempfile.NamedTemporaryFile(mode="w", delete=False) as f:
empty_file = f.name

try:
with pytest.raises(TimeoutError, match="exists but is empty"):
with anyio.fail_after(1.0):
await _wait_for_file_to_exist(empty_file, timeout=0.1)
finally:
os.unlink(empty_file)


@pytest.mark.anyio
@pytest.mark.skipif(tee is None, reason="could not find tee command")
async def test_stdio_context_manager_exiting():
Expand Down Expand Up @@ -296,19 +354,15 @@ async def test_basic_child_process_cleanup(self):
# Start the parent process
proc = await _create_platform_compatible_process(sys.executable, ["-c", parent_script])

# Wait for processes to start
await anyio.sleep(0.5)

# Verify parent started
# Wait for parent to start (condition-based)
with anyio.fail_after(5.0):
await _wait_for_file_to_exist(parent_marker)
assert os.path.exists(parent_marker), "Parent process didn't start"

# Verify child is writing
if os.path.exists(marker_file): # pragma: no branch
initial_size = os.path.getsize(marker_file)
await anyio.sleep(0.3)
size_after_wait = os.path.getsize(marker_file)
assert size_after_wait > initial_size, "Child process should be writing"
print(f"Child is writing (file grew from {initial_size} to {size_after_wait} bytes)")
# Wait for child to start writing (condition-based instead of arbitrary 0.5s + 0.3s)
with anyio.fail_after(5.0):
await _wait_for_file_to_exist(marker_file)
print(f"Child is writing (file size: {os.path.getsize(marker_file)} bytes)")

# Terminate using our function
print("Terminating process and children...")
Expand Down Expand Up @@ -398,16 +452,15 @@ async def test_nested_process_tree(self):
# Start the parent process
proc = await _create_platform_compatible_process(sys.executable, ["-c", parent_script])

# Let all processes start
await anyio.sleep(1.0)

# Verify all are writing
# Wait for all processes to start (condition-based)
for file_path, name in [(parent_file, "parent"), (child_file, "child"), (grandchild_file, "grandchild")]:
if os.path.exists(file_path): # pragma: no branch
initial_size = os.path.getsize(file_path)
await anyio.sleep(0.3)
new_size = os.path.getsize(file_path)
assert new_size > initial_size, f"{name} process should be writing"
with anyio.fail_after(5.0):
await _wait_for_file_to_exist(file_path)

parent_size = os.path.getsize(parent_file)
child_size = os.path.getsize(child_file)
grandchild_size = os.path.getsize(grandchild_file)
print(f"parent={parent_size}, child={child_size}, grandchild={grandchild_size}")

# Terminate the whole tree
await _terminate_process_tree(proc)
Expand Down Expand Up @@ -477,15 +530,10 @@ def handle_term(sig, frame):
# Start the parent process
proc = await _create_platform_compatible_process(sys.executable, ["-c", parent_script])

# Let child start writing
await anyio.sleep(0.5)

# Verify child is writing
if os.path.exists(marker_file): # pragma: no branch
size1 = os.path.getsize(marker_file)
await anyio.sleep(0.3)
size2 = os.path.getsize(marker_file)
assert size2 > size1, "Child should be writing"
# Wait for child to start writing (condition-based)
with anyio.fail_after(5.0):
await _wait_for_file_to_exist(marker_file)
print(f"Child is writing (file size: {os.path.getsize(marker_file)} bytes)")

# Terminate - this will kill the process group even if parent exits first
await _terminate_process_tree(proc)
Expand Down
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