docs: document ValidationError from typed request result validation

A peer result that fails model validation raises pydantic.ValidationError
out of send_request and the peer convenience methods; list it in the
Raises sections and pin the behavior with a test. ServerSession.send_request
was also missing MCPError in its Raises section.

Author: maxisbey

src/mcp/server/_typed_request.py

@@ -79,6 +79,7 @@ async def send_request(
         Raises:
             MCPError: The peer responded with an error.
             NoBackChannelError: No back-channel for server-initiated requests.
+            pydantic.ValidationError: The peer's result does not match the expected result type.
             KeyError: `result_type` omitted for a non-spec request type.
         """
         raw = await self.send_raw_request(req.method, dump_params(req.params), opts)

src/mcp/server/session.py

@@ -65,9 +65,11 @@ async def send_request(
         streamable HTTP; it is the only metadata field honored here.
 
         Raises:
+            MCPError: The peer responded with an error.
             NoBackChannelError: If there is no related request to ride on and
                 the connection has no standalone channel (stateless HTTP), so
                 a response could never arrive.
+            pydantic.ValidationError: The peer's result does not match `result_type`.
         """
         data = request.model_dump(by_alias=True, mode="json", exclude_none=True)
         opts: CallOptions = {}

src/mcp/shared/peer.py

@@ -125,6 +125,7 @@ async def sample(
             MCPError: The peer responded with an error.
             NoBackChannelError: The host's transport context has no
                 back-channel for server-initiated requests.
+            pydantic.ValidationError: The peer's result does not match the expected result type.
         """
         params = CreateMessageRequestParams(
             messages=messages,
@@ -156,6 +157,7 @@ async def elicit_form(
         Raises:
             MCPError: The peer responded with an error.
             NoBackChannelError: No back-channel for server-initiated requests.
+            pydantic.ValidationError: The peer's result does not match the expected result type.
         """
         params = ElicitRequestFormParams(message=message, requested_schema=requested_schema)
         result = await self.send_raw_request("elicitation/create", dump_params(params, meta), opts)
@@ -175,6 +177,7 @@ async def elicit_url(
         Raises:
             MCPError: The peer responded with an error.
             NoBackChannelError: No back-channel for server-initiated requests.
+            pydantic.ValidationError: The peer's result does not match the expected result type.
         """
         params = ElicitRequestURLParams(message=message, url=url, elicitation_id=elicitation_id)
         result = await self.send_raw_request("elicitation/create", dump_params(params, meta), opts)
@@ -188,6 +191,7 @@ async def list_roots(
         Raises:
             MCPError: The peer responded with an error.
             NoBackChannelError: No back-channel for server-initiated requests.
+            pydantic.ValidationError: The peer's result does not match the expected result type.
         """
         result = await self.send_raw_request("roots/list", dump_params(None, meta), opts)
         return ListRootsResult.model_validate(result, by_name=False)

tests/server/test_connection.py

@@ -12,6 +12,7 @@
 
 import anyio
 import pytest
+from pydantic import ValidationError
 
 from mcp.server.connection import Connection
 from mcp.shared.dispatcher import CallOptions
@@ -136,6 +137,13 @@ async def test_connection_send_request_with_result_type_kwarg_validates_custom_t
     assert isinstance(result, EmptyResult)
 
 
+@pytest.mark.anyio
+async def test_connection_send_request_nonconforming_result_raises_validation_error():
+    conn = Connection(StubOutbound(result={"bogus": 1}), has_standalone_channel=True)
+    with pytest.raises(ValidationError):
+        await conn.send_request(ListRootsRequest())
+
+
 @pytest.mark.anyio
 async def test_connection_ping_sends_ping_on_standalone():
     out = StubOutbound()