mcp.shared.exceptions.McpError: Connection closed
(One of the three shapes, as a Python traceback’s last line would print it; assembled by us from the class’s module path and the message quoted as source below. The other two are Timed out while waiting for response to … and whatever message the server sent.)
In the MCP Python SDK, ClientSession’s initialize,
list_tools and call_tool each send their request with
send_request, and that method raises McpError in two places: when no response arrives
before the read timeout, and when what arrives is a JSON-RPC error. That error is either the server’s own,
or one the SDK hands every request still waiting when the connection’s read stream ends (our reading).
Every claim about the library below is read off the Python source of
modelcontextprotocol/
What it is: class McpError(Exception):, in
src/
Where: BaseSession.send_request in
src/
What to do: for a timeout, give the call more time with read_timeout_seconds, per call or per session; for Connection closed, find out why the server or transport went away; for anything else, read e.error.code and e.error.message (our reading).
In src/
except TimeoutError: raise McpError( ErrorData( code=httpx.codes.REQUEST_TIMEOUT, message=( f"Timed out while waiting for response to " f"{request.__class__.__name__}. Waited " f"{timeout} seconds." ), ) )
ClientSession’s methods wrap their request in types.ClientRequest(
before passing it (for example lines 184, 402 and 555 of client/
Timed out while waiting for response to ClientRequest. Waited <seconds> seconds.
(Assembled by us from the f-string above; <seconds> is our placeholder for the timeout in seconds.) The code is httpx.codes.REQUEST_TIMEOUT, an httpx constant (we did not fetch httpx); the SDK leaves # REQUEST_TIMEOUT = -32001 # the typescript sdk uses this commented out at line 183 of types.py.
Lines 283–288 of the same file pick it: timeout = None, then the per-request value if if request_read_timeout_seconds is not None:, otherwise the session’s if elif self._session_read_timeout_seconds is not None:, each converted with .total_seconds(). The session value is the read_timeout_seconds parameter of BaseSession.__init__, commented # If none, reading will never time out (lines 191–192, stored at line 200).
In src/
Right after the wait, lines 305–306:
if isinstance(response_or_error, JSONRPCError): raise McpError(response_or_error.error)
A JSON-RPC error from the server reaches this point through _handle_response (line 481), which first offers it to any response routers (lines 501–505) and otherwise sends it to the waiting request’s stream (lines 514–516). Whatever code, message and data the server put in its error are what you get (our reading).
The session’s _receive_loop (line 351) reads async for message in self._read_stream: (line 357). Its finally block (line 445) is commented # after the read stream is closed, we need to send errors (line 446), and lines 450–453 are:
for id, stream in list(self._response_streams.items()): error = ErrorData(code=CONNECTION_CLOSED, message="Connection closed") try: await stream.send(JSONRPCError(jsonrpc="2.0", id=id, error=error))
Each request still waiting receives that JSONRPCError and raises it at line 306, so
Connection closed means the read stream ended — or the loop stopped on an exception
(lines 436 and 441) — while your call was outstanding (our reading). Which transport closed the stream
and why is not in these files: we did not fetch the transports in src/
In src/
In src/
def __init__(self, error: ErrorData): """Initialize McpError.""" super().__init__(error.message) self.error = error
The same file defines class UrlElicitationRequiredError(McpError): (line 21), which an
except McpError also catches (our reading). A grep of the four v1.30.0 files we fetched —
shared/
from datetime import timedelta import httpx from mcp.client.session import ClientSession from mcp.shared.exceptions import McpError from mcp.types import CONNECTION_CLOSED async def call(session: ClientSession, name: str, arguments: dict, seconds: float = 120): # Our sketch: give the call a deadline and tell the three cases apart. try: return await session.call_tool(name, arguments, read_timeout_seconds=timedelta(seconds=seconds)) except McpError as e: if e.error.code == httpx.codes.REQUEST_TIMEOUT: print("timed out:", e.error.message) elif e.error.code == CONNECTION_CLOSED and e.error.message == "Connection closed": print("connection closed: check the server's stderr and logs") else: print("server error:", e.error.code, e.error.message, e.error.data) raise
(Our sketch, not library code, and not run against your version. It re-raises in every case; what to do next is yours.)
There is a written guide: the step-and-turn arithmetic as a formula you can run against a brief before you launch it, why raising a cap does not finish the job, and batch.py, one standard-library file that collapses a per-item loop into a single pass — fewer turns, which means fewer final calls made from an unfinished transcript when a run hits its cap. It is $19, on a storefront that delivers the files automatically and carries a 30-day money-back guarantee (checked 7 October 2026). One working way to pay today is 19 USDC on Base, and delivery is manual: you email the transaction hash and the files come back as a reply. This page is free, ungated, and sells nothing on its own.
The short version: McpError, from mcp.shared.exceptions, is what send_request in MCP Python SDK v1.30.0 raises when a request gets no response before its read timeout (Timed out while waiting for response to …) or gets a JSON-RPC error back — the server’s, or the Connection closed error the SDK sends to requests still waiting when the read stream ends. Read e.error.code and e.error.message; for a timeout, raise read_timeout_seconds; for a closed connection, look at why the server or transport went away.
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