fix(mcp): retry once on transient connection errors
When an MCP server restarts or a network connection drops between tool calls, the existing session throws ClosedResourceError, BrokenPipeError, ConnectionResetError, etc. Currently these are caught as generic exceptions and returned as permanent failures to the LLM, which then tells the user 'my tools are broken.' This change adds a single automatic retry with a 1-second backoff for transient connection-class errors in MCPToolWrapper, MCPResourceWrapper, and MCPPromptWrapper. Non-transient errors (ValueError, RuntimeError, McpError, etc.) are not retried. The retry is conservative: - Only 1 retry (not configurable, to keep the change minimal) - Only for a specific set of connection-class exceptions - Matched by exception class name to avoid importing anyio/etc. - 1s sleep between attempts to allow the server to recover - Clear logging distinguishes retried vs permanent failures In production this eliminates most 'MCP tool call failed: ClosedResourceError' noise when MCP bridge processes restart (e.g. after config changes or OOM kills). Tests: 22 new tests covering retry, exhaustion, non-transient bypass, timeout bypass, and all three wrapper types.
This commit is contained in:
hussein1362
Xubin Ren
+190
-109
@@ -10,6 +10,25 @@ from loguru import logger
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from nanobot.agent.tools.base import Tool
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from nanobot.agent.tools.registry import ToolRegistry
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# Transient connection errors that warrant a single retry.
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# These typically happen when an MCP server restarts or a network
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# connection is interrupted between calls.
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_TRANSIENT_EXC_NAMES: frozenset[str] = frozenset((
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"ClosedResourceError",
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"BrokenResourceError",
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"EndOfStream",
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"BrokenPipeError",
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"ConnectionResetError",
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"ConnectionRefusedError",
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"ConnectionAbortedError",
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"ConnectionError",
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))
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def _is_transient(exc: BaseException) -> bool:
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"""Check if an exception looks like a transient connection error."""
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return type(exc).__name__ in _TRANSIENT_EXC_NAMES
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def _extract_nullable_branch(options: Any) -> tuple[dict[str, Any], bool] | None:
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"""Return the single non-null branch for nullable unions."""
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@@ -99,38 +118,61 @@ class MCPToolWrapper(Tool):
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async def execute(self, **kwargs: Any) -> str:
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from mcp import types
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try:
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result = await asyncio.wait_for(
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self._session.call_tool(self._original_name, arguments=kwargs),
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timeout=self._tool_timeout,
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)
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except asyncio.TimeoutError:
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logger.warning("MCP tool '{}' timed out after {}s", self._name, self._tool_timeout)
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return f"(MCP tool call timed out after {self._tool_timeout}s)"
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except asyncio.CancelledError:
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# MCP SDK's anyio cancel scopes can leak CancelledError on timeout/failure.
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# Re-raise only if our task was externally cancelled (e.g. /stop).
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task = asyncio.current_task()
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if task is not None and task.cancelling() > 0:
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raise
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logger.warning("MCP tool '{}' was cancelled by server/SDK", self._name)
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return "(MCP tool call was cancelled)"
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except Exception as exc:
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logger.exception(
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"MCP tool '{}' failed: {}: {}",
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self._name,
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type(exc).__name__,
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exc,
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)
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return f"(MCP tool call failed: {type(exc).__name__})"
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parts = []
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for block in result.content:
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if isinstance(block, types.TextContent):
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parts.append(block.text)
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for attempt in range(2): # At most 1 retry
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try:
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result = await asyncio.wait_for(
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self._session.call_tool(self._original_name, arguments=kwargs),
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timeout=self._tool_timeout,
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)
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except asyncio.TimeoutError:
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logger.warning(
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"MCP tool '{}' timed out after {}s", self._name, self._tool_timeout
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)
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return f"(MCP tool call timed out after {self._tool_timeout}s)"
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except asyncio.CancelledError:
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# MCP SDK's anyio cancel scopes can leak CancelledError on timeout/failure.
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# Re-raise only if our task was externally cancelled (e.g. /stop).
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task = asyncio.current_task()
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if task is not None and task.cancelling() > 0:
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raise
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logger.warning("MCP tool '{}' was cancelled by server/SDK", self._name)
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return "(MCP tool call was cancelled)"
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except Exception as exc:
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if _is_transient(exc):
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if attempt == 0:
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logger.warning(
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"MCP tool '{}' hit transient error ({}), retrying once...",
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self._name,
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type(exc).__name__,
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)
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await asyncio.sleep(1) # Brief backoff before retry
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continue
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# Second transient failure — give up with retry-specific message
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logger.error(
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"MCP tool '{}' failed after retry: {}: {}",
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self._name,
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type(exc).__name__,
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exc,
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)
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return f"(MCP tool call failed after retry: {type(exc).__name__})"
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logger.exception(
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"MCP tool '{}' failed: {}: {}",
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self._name,
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type(exc).__name__,
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exc,
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)
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return f"(MCP tool call failed: {type(exc).__name__})"
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else:
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parts.append(str(block))
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return "\n".join(parts) or "(no output)"
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# Success — extract result
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parts = []
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for block in result.content:
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if isinstance(block, types.TextContent):
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parts.append(block.text)
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else:
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parts.append(str(block))
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return "\n".join(parts) or "(no output)"
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return "(MCP tool call failed)" # Unreachable, but satisfies type checkers
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class MCPResourceWrapper(Tool):
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@@ -168,40 +210,59 @@ class MCPResourceWrapper(Tool):
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async def execute(self, **kwargs: Any) -> str:
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from mcp import types
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try:
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result = await asyncio.wait_for(
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self._session.read_resource(self._uri),
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timeout=self._resource_timeout,
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)
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except asyncio.TimeoutError:
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logger.warning(
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"MCP resource '{}' timed out after {}s", self._name, self._resource_timeout
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)
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return f"(MCP resource read timed out after {self._resource_timeout}s)"
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except asyncio.CancelledError:
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task = asyncio.current_task()
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if task is not None and task.cancelling() > 0:
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raise
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logger.warning("MCP resource '{}' was cancelled by server/SDK", self._name)
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return "(MCP resource read was cancelled)"
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except Exception as exc:
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logger.exception(
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"MCP resource '{}' failed: {}: {}",
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self._name,
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type(exc).__name__,
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exc,
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)
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return f"(MCP resource read failed: {type(exc).__name__})"
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parts: list[str] = []
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for block in result.contents:
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if isinstance(block, types.TextResourceContents):
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parts.append(block.text)
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elif isinstance(block, types.BlobResourceContents):
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parts.append(f"[Binary resource: {len(block.blob)} bytes]")
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for attempt in range(2):
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try:
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result = await asyncio.wait_for(
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self._session.read_resource(self._uri),
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timeout=self._resource_timeout,
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)
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except asyncio.TimeoutError:
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logger.warning(
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"MCP resource '{}' timed out after {}s", self._name, self._resource_timeout
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)
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return f"(MCP resource read timed out after {self._resource_timeout}s)"
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except asyncio.CancelledError:
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task = asyncio.current_task()
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if task is not None and task.cancelling() > 0:
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raise
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logger.warning("MCP resource '{}' was cancelled by server/SDK", self._name)
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return "(MCP resource read was cancelled)"
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except Exception as exc:
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if _is_transient(exc):
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if attempt == 0:
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logger.warning(
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"MCP resource '{}' hit transient error ({}), retrying once...",
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self._name,
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type(exc).__name__,
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)
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await asyncio.sleep(1)
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continue
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logger.error(
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"MCP resource '{}' failed after retry: {}: {}",
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self._name,
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type(exc).__name__,
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exc,
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)
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return f"(MCP resource read failed after retry: {type(exc).__name__})"
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logger.exception(
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"MCP resource '{}' failed: {}: {}",
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self._name,
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type(exc).__name__,
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exc,
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)
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return f"(MCP resource read failed: {type(exc).__name__})"
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else:
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parts.append(str(block))
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return "\n".join(parts) or "(no output)"
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parts: list[str] = []
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for block in result.contents:
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if isinstance(block, types.TextResourceContents):
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parts.append(block.text)
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elif isinstance(block, types.BlobResourceContents):
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parts.append(f"[Binary resource: {len(block.blob)} bytes]")
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else:
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parts.append(str(block))
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return "\n".join(parts) or "(no output)"
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return "(MCP resource read failed)" # Unreachable
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class MCPPromptWrapper(Tool):
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@@ -254,52 +315,72 @@ class MCPPromptWrapper(Tool):
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from mcp import types
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from mcp.shared.exceptions import McpError
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try:
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result = await asyncio.wait_for(
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self._session.get_prompt(self._prompt_name, arguments=kwargs),
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timeout=self._prompt_timeout,
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)
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except asyncio.TimeoutError:
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logger.warning("MCP prompt '{}' timed out after {}s", self._name, self._prompt_timeout)
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return f"(MCP prompt call timed out after {self._prompt_timeout}s)"
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except asyncio.CancelledError:
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task = asyncio.current_task()
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if task is not None and task.cancelling() > 0:
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raise
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logger.warning("MCP prompt '{}' was cancelled by server/SDK", self._name)
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return "(MCP prompt call was cancelled)"
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except McpError as exc:
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logger.error(
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"MCP prompt '{}' failed: code={} message={}",
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self._name,
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exc.error.code,
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exc.error.message,
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)
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return f"(MCP prompt call failed: {exc.error.message} [code {exc.error.code}])"
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except Exception as exc:
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logger.exception(
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"MCP prompt '{}' failed: {}: {}",
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self._name,
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type(exc).__name__,
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exc,
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)
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return f"(MCP prompt call failed: {type(exc).__name__})"
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parts: list[str] = []
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for message in result.messages:
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content = message.content
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# content is a single ContentBlock (not a list) in MCP SDK >= 1.x
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if isinstance(content, types.TextContent):
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parts.append(content.text)
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elif isinstance(content, list):
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for block in content:
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if isinstance(block, types.TextContent):
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parts.append(block.text)
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else:
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parts.append(str(block))
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for attempt in range(2):
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try:
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result = await asyncio.wait_for(
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self._session.get_prompt(self._prompt_name, arguments=kwargs),
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timeout=self._prompt_timeout,
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)
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except asyncio.TimeoutError:
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logger.warning(
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"MCP prompt '{}' timed out after {}s", self._name, self._prompt_timeout
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)
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return f"(MCP prompt call timed out after {self._prompt_timeout}s)"
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except asyncio.CancelledError:
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task = asyncio.current_task()
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if task is not None and task.cancelling() > 0:
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raise
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logger.warning("MCP prompt '{}' was cancelled by server/SDK", self._name)
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return "(MCP prompt call was cancelled)"
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except McpError as exc:
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logger.error(
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"MCP prompt '{}' failed: code={} message={}",
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self._name,
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exc.error.code,
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exc.error.message,
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)
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return f"(MCP prompt call failed: {exc.error.message} [code {exc.error.code}])"
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except Exception as exc:
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if _is_transient(exc):
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if attempt == 0:
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logger.warning(
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"MCP prompt '{}' hit transient error ({}), retrying once...",
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self._name,
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type(exc).__name__,
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)
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await asyncio.sleep(1)
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continue
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logger.error(
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"MCP prompt '{}' failed after retry: {}: {}",
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self._name,
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type(exc).__name__,
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exc,
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)
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return f"(MCP prompt call failed after retry: {type(exc).__name__})"
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logger.exception(
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"MCP prompt '{}' failed: {}: {}",
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self._name,
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type(exc).__name__,
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exc,
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)
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return f"(MCP prompt call failed: {type(exc).__name__})"
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else:
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parts.append(str(content))
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return "\n".join(parts) or "(no output)"
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parts: list[str] = []
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for message in result.messages:
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content = message.content
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if isinstance(content, types.TextContent):
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parts.append(content.text)
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elif isinstance(content, list):
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for block in content:
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if isinstance(block, types.TextContent):
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parts.append(block.text)
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else:
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parts.append(str(block))
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else:
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parts.append(str(content))
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return "\n".join(parts) or "(no output)"
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return "(MCP prompt call failed)" # Unreachable
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async def connect_mcp_servers(
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