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Suhas Aggarwal (suhasagg) wants to merge 3 commits into from
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Python: fix(python): prevent duplicate MCP tool execution after ambiguous connection loss #9247

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c10b7ef
fix(python): prevent duplicate MCP tool execution after connection loss
suhasagg Oct 8, 2026
ef53ec3
test(python): cover MCP no-replay behavior and advisory tool hints
suhasagg Oct 10, 2026
ced5913
docs(python): clarify explicit MCP recovery in integration test
suhasagg Oct 10, 2026
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100 changes: 46 additions & 54 deletions python/packages/core/agent_framework/_mcp.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2852,80 +2852,72 @@ async def _call_tool(
OtelAttr.OPERATION: OtelAttr.TOOL_EXECUTION_OPERATION,
})
with create_mcp_client_span("tools/call", target=tool_name, attributes=mcp_span_attrs) as span:
return await self._call_tool_with_retries(
return await self._call_tool_once(
tool_name,
filtered_kwargs,
meta,
parser,
span,
)

async def _call_tool_with_retries(
async def _call_tool_once(
self,
tool_name: str,
filtered_kwargs: dict[str, Any],
meta: dict[str, Any] | None,
parser: Callable[..., str | list[Content]],
span: otel_trace.Span,
) -> str | list[Content]:
"""Execute the MCP tools/call RPC with retry logic."""
"""Execute an MCP tool call without replaying uncertain outcomes."""
from anyio import ClosedResourceError
from mcp.shared.exceptions import McpError

for attempt in range(2):
try:
result = await self.session.call_tool(tool_name, arguments=filtered_kwargs, meta=meta) # type: ignore
_capture_mcp_tool_result(result)
if result.isError:
parsed = parser(result)
text = (
"\n".join(c.text for c in parsed if c.type == "text" and c.text)
if isinstance(parsed, list)
else str(parsed)
)
# Per OTel MCP semconv: set error.type="tool_error" for isError results
if span.is_recording():
set_mcp_span_error(span, "tool_error", text or str(parsed))
raise ToolExecutionException(text or str(parsed))
return parser(result)
except ToolExecutionException:
raise
except (ClosedResourceError, McpError) as call_ex:
is_session_terminated = (
isinstance(call_ex, McpError) and "session terminated" in call_ex.error.message.lower()
try:
result = await self.session.call_tool(tool_name, arguments=filtered_kwargs, meta=meta) # type: ignore
_capture_mcp_tool_result(result)
if result.isError:
parsed = parser(result)
text = (
"\n".join(c.text for c in parsed if c.type == "text" and c.text)
if isinstance(parsed, list)
else str(parsed)
)
is_connection_lost = isinstance(call_ex, ClosedResourceError) or is_session_terminated
if not is_connection_lost:
error_message = call_ex.error.message if isinstance(call_ex, McpError) else str(call_ex)
if span.is_recording():
set_mcp_span_error(span, type(call_ex).__name__, error_message)
raise ToolExecutionException(error_message, inner_exception=call_ex) from call_ex

if attempt == 0:
# First attempt failed, try reconnecting.
logger.info("MCP connection closed or terminated unexpectedly. Reconnecting...")
try:
await self.connect(reset=True)
continue
except Exception as reconn_ex:
raise ToolExecutionException(
"Failed to reconnect to MCP server.",
inner_exception=reconn_ex,
) from reconn_ex

# Second attempt also failed, give up.
logger.error("MCP connection closed unexpectedly after reconnection: %s", call_ex)
# Per OTel MCP semconv: set error.type="tool_error" for isError results
if span.is_recording():
set_mcp_span_error(span, type(call_ex).__name__, str(call_ex))
raise ToolExecutionException(
f"Failed to call tool '{tool_name}' - connection lost.",
inner_exception=call_ex,
) from call_ex
except Exception as ex:
set_mcp_span_error(span, "tool_error", text or str(parsed))
raise ToolExecutionException(text or str(parsed))
return parser(result)
except ToolExecutionException:
raise
except (ClosedResourceError, McpError) as call_ex:
is_session_terminated = (
isinstance(call_ex, McpError) and "session terminated" in call_ex.error.message.lower()
)
is_connection_lost = isinstance(call_ex, ClosedResourceError) or is_session_terminated
if not is_connection_lost:
error_message = call_ex.error.message if isinstance(call_ex, McpError) else str(call_ex)
if span.is_recording():
set_mcp_span_error(span, type(ex).__name__, str(ex))
raise ToolExecutionException(f"Failed to call tool '{tool_name}'.", inner_exception=ex) from ex
raise ToolExecutionException(f"Failed to call tool '{tool_name}' after retries.")
set_mcp_span_error(span, type(call_ex).__name__, error_message)
raise ToolExecutionException(error_message, inner_exception=call_ex) from call_ex

# The server may have executed this operation before the
# connection failed. Reissuing tools/call could duplicate
# non-idempotent side effects.
logger.warning(
"MCP connection lost during tool '%s'; execution outcome is unknown. "
"Not retrying to avoid duplicate side effects.",
tool_name,
)
if span.is_recording():
set_mcp_span_error(span, type(call_ex).__name__, str(call_ex))
raise ToolExecutionException(
f"Failed to call tool '{tool_name}' - connection lost; remote execution outcome is unknown.",
inner_exception=call_ex,
) from call_ex
except Exception as ex:
if span.is_recording():
set_mcp_span_error(span, type(ex).__name__, str(ex))
raise ToolExecutionException(f"Failed to call tool '{tool_name}'.", inner_exception=ex) from ex

def _resolved_extra_args(self, tool_name: str) -> set[str]:
"""Return the user-configured extra argument names allowed for a tool."""
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