fix: raise clear error for server-to-client requests in stateless mode (#1827)
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Max Isbey
GitHub
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d52937b39c
commit
bb6cb029f0
2 changed files with 295 additions and 0 deletions
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@ -93,6 +93,7 @@ class ServerSession(
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stateless: bool = False,
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) -> None:
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super().__init__(read_stream, write_stream, types.ClientRequest, types.ClientNotification)
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self._stateless = stateless
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self._initialization_state = (
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InitializationState.Initialized if stateless else InitializationState.NotInitialized
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)
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@ -156,6 +157,26 @@ class ServerSession(
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return True
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def _require_stateful_mode(self, feature_name: str) -> None:
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"""Raise an error if trying to use a feature that requires stateful mode.
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Server-to-client requests (sampling, elicitation, list_roots) are not
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supported in stateless HTTP mode because there is no persistent connection
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for bidirectional communication.
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Args:
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feature_name: Name of the feature being used (for error message)
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Raises:
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RuntimeError: If the session is in stateless mode
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"""
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if self._stateless:
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raise RuntimeError(
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f"Cannot use {feature_name} in stateless HTTP mode. "
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"Stateless mode does not support server-to-client requests. "
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"Use stateful mode (stateless_http=False) to enable this feature."
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)
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async def _receive_loop(self) -> None:
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async with self._incoming_message_stream_writer:
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await super()._receive_loop()
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@ -311,7 +332,9 @@ class ServerSession(
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Raises:
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McpError: If tools are provided but client doesn't support them.
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ValueError: If tool_use or tool_result message structure is invalid.
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RuntimeError: If called in stateless HTTP mode.
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"""
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self._require_stateful_mode("sampling")
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client_caps = self._client_params.capabilities if self._client_params else None
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validate_sampling_tools(client_caps, tools, tool_choice)
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validate_tool_use_result_messages(messages)
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@ -349,6 +372,7 @@ class ServerSession(
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async def list_roots(self) -> types.ListRootsResult:
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"""Send a roots/list request."""
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self._require_stateful_mode("list_roots")
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return await self.send_request(
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types.ServerRequest(types.ListRootsRequest()),
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types.ListRootsResult,
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@ -391,7 +415,11 @@ class ServerSession(
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Returns:
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The client's response with form data
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Raises:
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RuntimeError: If called in stateless HTTP mode.
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"""
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self._require_stateful_mode("elicitation")
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return await self.send_request(
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types.ServerRequest(
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types.ElicitRequest(
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@ -425,7 +453,11 @@ class ServerSession(
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Returns:
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The client's response indicating acceptance, decline, or cancellation
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Raises:
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RuntimeError: If called in stateless HTTP mode.
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"""
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self._require_stateful_mode("elicitation")
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return await self.send_request(
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types.ServerRequest(
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types.ElicitRequest(
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263
tests/server/test_stateless_mode.py
Normal file
263
tests/server/test_stateless_mode.py
Normal file
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@ -0,0 +1,263 @@
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"""Tests for stateless HTTP mode limitations.
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Stateless HTTP mode does not support server-to-client requests because there
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is no persistent connection for bidirectional communication. These tests verify
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that appropriate errors are raised when attempting to use unsupported features.
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See: https://github.com/modelcontextprotocol/python-sdk/issues/1097
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"""
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import anyio
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import pytest
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import mcp.types as types
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from mcp.server.models import InitializationOptions
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from mcp.server.session import ServerSession
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from mcp.shared.message import SessionMessage
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from mcp.types import ServerCapabilities
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def create_test_streams():
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"""Create memory streams for testing."""
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server_to_client_send, server_to_client_receive = anyio.create_memory_object_stream[SessionMessage](1)
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client_to_server_send, client_to_server_receive = anyio.create_memory_object_stream[SessionMessage | Exception](1)
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return (
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server_to_client_send,
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server_to_client_receive,
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client_to_server_send,
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client_to_server_receive,
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)
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def create_init_options():
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"""Create default initialization options for testing."""
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return InitializationOptions(
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server_name="test",
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server_version="0.1.0",
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capabilities=ServerCapabilities(),
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)
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@pytest.mark.anyio
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async def test_list_roots_fails_in_stateless_mode():
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"""Test that list_roots raises RuntimeError in stateless mode."""
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(
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server_to_client_send,
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server_to_client_receive,
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client_to_server_send,
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client_to_server_receive,
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) = create_test_streams()
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async with (
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client_to_server_send,
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client_to_server_receive,
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server_to_client_send,
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server_to_client_receive,
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):
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async with ServerSession(
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client_to_server_receive,
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server_to_client_send,
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create_init_options(),
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stateless=True,
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) as session:
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with pytest.raises(RuntimeError) as exc_info:
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await session.list_roots()
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assert "stateless HTTP mode" in str(exc_info.value)
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assert "list_roots" in str(exc_info.value)
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@pytest.mark.anyio
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async def test_create_message_fails_in_stateless_mode():
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"""Test that create_message raises RuntimeError in stateless mode."""
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(
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server_to_client_send,
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server_to_client_receive,
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client_to_server_send,
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client_to_server_receive,
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) = create_test_streams()
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async with (
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client_to_server_send,
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client_to_server_receive,
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server_to_client_send,
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server_to_client_receive,
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):
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async with ServerSession(
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client_to_server_receive,
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server_to_client_send,
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create_init_options(),
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stateless=True,
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) as session:
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with pytest.raises(RuntimeError) as exc_info:
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await session.create_message(
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messages=[
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types.SamplingMessage(
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role="user",
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content=types.TextContent(type="text", text="hello"),
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)
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],
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max_tokens=100,
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)
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assert "stateless HTTP mode" in str(exc_info.value)
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assert "sampling" in str(exc_info.value)
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@pytest.mark.anyio
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async def test_elicit_form_fails_in_stateless_mode():
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"""Test that elicit_form raises RuntimeError in stateless mode."""
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(
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server_to_client_send,
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server_to_client_receive,
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client_to_server_send,
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client_to_server_receive,
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) = create_test_streams()
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async with (
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client_to_server_send,
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client_to_server_receive,
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server_to_client_send,
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server_to_client_receive,
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):
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async with ServerSession(
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client_to_server_receive,
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server_to_client_send,
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create_init_options(),
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stateless=True,
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) as session:
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with pytest.raises(RuntimeError) as exc_info:
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await session.elicit_form(
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message="Please provide input",
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requestedSchema={"type": "object", "properties": {}},
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)
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assert "stateless HTTP mode" in str(exc_info.value)
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assert "elicitation" in str(exc_info.value)
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@pytest.mark.anyio
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async def test_elicit_url_fails_in_stateless_mode():
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"""Test that elicit_url raises RuntimeError in stateless mode."""
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(
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server_to_client_send,
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server_to_client_receive,
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client_to_server_send,
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client_to_server_receive,
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) = create_test_streams()
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async with (
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client_to_server_send,
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client_to_server_receive,
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server_to_client_send,
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server_to_client_receive,
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):
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async with ServerSession(
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client_to_server_receive,
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server_to_client_send,
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create_init_options(),
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stateless=True,
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) as session:
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with pytest.raises(RuntimeError) as exc_info:
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await session.elicit_url(
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message="Please authenticate",
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url="https://example.com/auth",
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elicitation_id="test-123",
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)
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assert "stateless HTTP mode" in str(exc_info.value)
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assert "elicitation" in str(exc_info.value)
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@pytest.mark.anyio
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async def test_elicit_deprecated_fails_in_stateless_mode():
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"""Test that the deprecated elicit method also fails in stateless mode."""
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(
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server_to_client_send,
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server_to_client_receive,
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client_to_server_send,
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client_to_server_receive,
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) = create_test_streams()
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async with (
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client_to_server_send,
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client_to_server_receive,
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server_to_client_send,
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server_to_client_receive,
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):
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async with ServerSession(
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client_to_server_receive,
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server_to_client_send,
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create_init_options(),
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stateless=True,
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) as session:
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with pytest.raises(RuntimeError) as exc_info:
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await session.elicit(
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message="Please provide input",
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requestedSchema={"type": "object", "properties": {}},
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)
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assert "stateless HTTP mode" in str(exc_info.value)
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assert "elicitation" in str(exc_info.value)
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@pytest.mark.anyio
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async def test_require_stateful_mode_does_not_raise_in_stateful_mode():
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"""Test that _require_stateful_mode does not raise in stateful mode."""
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(
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server_to_client_send,
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server_to_client_receive,
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client_to_server_send,
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client_to_server_receive,
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) = create_test_streams()
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async with (
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client_to_server_send,
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client_to_server_receive,
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server_to_client_send,
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server_to_client_receive,
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):
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async with ServerSession(
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client_to_server_receive,
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server_to_client_send,
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create_init_options(),
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stateless=False, # Stateful mode
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) as session:
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# These should not raise - the check passes in stateful mode
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session._require_stateful_mode("list_roots")
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session._require_stateful_mode("sampling")
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session._require_stateful_mode("elicitation")
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@pytest.mark.anyio
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async def test_stateless_error_message_is_actionable():
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"""Test that the error message provides actionable guidance."""
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(
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server_to_client_send,
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server_to_client_receive,
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client_to_server_send,
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client_to_server_receive,
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) = create_test_streams()
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async with (
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client_to_server_send,
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client_to_server_receive,
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server_to_client_send,
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server_to_client_receive,
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):
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async with ServerSession(
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client_to_server_receive,
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server_to_client_send,
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create_init_options(),
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stateless=True,
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) as session:
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with pytest.raises(RuntimeError) as exc_info:
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await session.list_roots()
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error_message = str(exc_info.value)
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# Should mention it's stateless mode
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assert "stateless HTTP mode" in error_message
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# Should explain why it doesn't work
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assert "server-to-client requests" in error_message
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# Should tell user how to fix it
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assert "stateless_http=False" in error_message
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