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"""Tests for the streamable-HTTP client transport.
The full client<->server round trip is also pinned by the interaction suite under
tests/interaction/transports/. Tests here cover SSE event sizes and HTTP-seam details
that the public client never exposes.
"""
import base64
import json
from collections.abc import AsyncIterator, Callable, Mapping
from typing import Any
import anyio
import httpx2
import pytest
from inline_snapshot import snapshot
from mcp_types import (
CLIENT_CAPABILITIES_META_KEY,
CLIENT_INFO_META_KEY,
CONNECTION_CLOSED,
INVALID_REQUEST,
METHOD_NOT_FOUND,
PROTOCOL_VERSION_META_KEY,
CallToolRequestParams,
CallToolResult,
JSONRPCError,
JSONRPCNotification,
JSONRPCRequest,
JSONRPCResponse,
ListToolsResult,
PaginatedRequestParams,
TextContent,
Tool,
)
from mcp_types.version import LATEST_MODERN_VERSION
from starlette.applications import Starlette
from starlette.routing import Mount
from starlette.types import Receive, Scope, Send
from mcp import Client, MCPError
from mcp.client.streamable_http import (
MAX_RECONNECTION_ATTEMPTS,
RequestContext,
StreamableHTTPTransport,
streamable_http_client,
)
from mcp.server import Server, ServerRequestContext
from mcp.server._streamable_http_modern import handle_modern_request
from mcp.server.streamable_http_manager import StreamableHTTPSessionManager
from mcp.server.subscriptions import InMemorySubscriptionBus, ListenHandler, ServerEvent
from mcp.shared._context_streams import ContextSendStream, create_context_streams
from mcp.shared.dispatcher import CallOptions, DispatchContext
from mcp.shared.inbound import MCP_METHOD_HEADER, MCP_PROTOCOL_VERSION_HEADER, encode_header_value
from mcp.shared.jsonrpc_dispatcher import JSONRPCDispatcher
from mcp.shared.message import ClientMessageMetadata, ServerMessageMetadata, SessionMessage
from mcp.shared.transport_context import TransportContext
from tests.interaction.transports import StreamingASGITransport
from tests.shared.test_dispatcher import Recorder, echo_handlers
@pytest.mark.anyio
async def test_configured_sse_limit_accepts_a_tool_result_larger_than_one_mebibyte() -> None:
"""SDK-defined: a larger SSE limit accepts a large tool result sent as one POST SSE event."""
payload = "x" * (2 * 1024 * 1024)
async def list_tools(ctx: ServerRequestContext, params: PaginatedRequestParams | None) -> ListToolsResult:
return ListToolsResult(tools=[Tool(name="large", input_schema={"type": "object"})])
async def call_tool(ctx: ServerRequestContext, params: CallToolRequestParams) -> CallToolResult:
assert params.name == "large"
return CallToolResult(content=[TextContent(text=payload)])
manager = StreamableHTTPSessionManager(app=Server("large-result", on_list_tools=list_tools, on_call_tool=call_tool))
app = Starlette(routes=[Mount("/mcp", app=manager.handle_request)])
with anyio.fail_after(5):
async with (
manager.run(),
httpx2.AsyncClient(transport=StreamingASGITransport(app)) as http,
Client(
streamable_http_client("http://localhost/mcp", http_client=http, max_sse_event_size=4 * 1024 * 1024),
mode="legacy",
) as client,
):
result = await client.call_tool("large", {})
assert result.content == [TextContent(text=payload)]
@pytest.mark.anyio
async def test_default_sse_limit_fails_one_large_tool_call_and_keeps_the_session_usable() -> None:
"""SDK-defined: the 1 MiB default rejects one oversized tool result without poisoning the session."""
calls: list[str] = []
async def list_tools(ctx: ServerRequestContext, params: PaginatedRequestParams | None) -> ListToolsResult:
return ListToolsResult(tools=[Tool(name=name, input_schema={"type": "object"}) for name in ("large", "small")])
async def call_tool(ctx: ServerRequestContext, params: CallToolRequestParams) -> CallToolResult:
calls.append(params.name)
return CallToolResult(content=[TextContent(text="x" * (2 * 1024 * 1024) if params.name == "large" else "ok")])
manager = StreamableHTTPSessionManager(
app=Server("bounded-result", on_list_tools=list_tools, on_call_tool=call_tool)
)
app = Starlette(routes=[Mount("/mcp", app=manager.handle_request)])
with anyio.fail_after(5):
async with (
manager.run(),
httpx2.AsyncClient(transport=StreamingASGITransport(app)) as http,
Client(streamable_http_client("http://localhost/mcp", http_client=http), mode="legacy") as client,
):
with pytest.raises(MCPError) as exc_info:
await client.call_tool("large", {})
result = await client.call_tool("small", {})
assert exc_info.value.error.code == CONNECTION_CLOSED
# httpx2 owns the error detail after this prefix.
assert exc_info.value.error.message.startswith("SSE stream failed: ")
assert result.content == [TextContent(text="ok")]
assert calls == ["large", "small"]
@pytest.mark.anyio
async def test_configured_sse_limit_accepts_a_large_get_notification() -> None:
"""SDK-defined: a configured limit accepts a large standalone GET event.
The raw peer forces the notification onto GET, a transport detail hidden by the typed client.
"""
payload = "x" * (2 * 1024 * 1024)
notification = json.dumps({"jsonrpc": "2.0", "method": "notifications/message", "params": {"data": payload}})
def handler(request: httpx2.Request) -> httpx2.Response:
if request.method == "GET":
return httpx2.Response(
200, content=f"data: {notification}\n\n".encode(), headers={"content-type": "text/event-stream"}
)
body = json.loads(request.content)
if body.get("method") == "initialize":
return httpx2.Response(
200, json={"jsonrpc": "2.0", "id": body["id"], "result": {}}, headers={"mcp-session-id": "s1"}
)
return httpx2.Response(202)
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client(
"http://test/mcp", http_client=http, terminate_on_close=False, max_sse_event_size=4 * 1024 * 1024
) as (
read,
write,
),
):
await write.send(SessionMessage(JSONRPCRequest(jsonrpc="2.0", id=1, method="initialize", params={})))
await read.receive()
await write.send(SessionMessage(JSONRPCNotification(jsonrpc="2.0", method="notifications/initialized")))
received = await read.receive()
assert isinstance(received, SessionMessage)
assert isinstance(received.message, JSONRPCNotification)
assert received.message.method == "notifications/message"
assert received.message.params == {"data": payload}
class _ParkedSSEStream(httpx2.AsyncByteStream):
"""An SSE response body that emits one frame, then parks until closed.
`opened` fires when the body is consumed; `closed` fires when httpx2 releases it.
"""
def __init__(self, content: bytes = b": parked\n\n") -> None:
self.opened = anyio.Event()
self.closed = anyio.Event()
self._release = anyio.Event()
self._content = content
async def __aiter__(self) -> AsyncIterator[bytes]:
self.opened.set()
yield self._content
await self._release.wait()
async def aclose(self) -> None:
self.closed.set()
self._release.set()
@pytest.mark.anyio
async def test_oversized_get_event_reconnects_and_delivers_later_messages() -> None:
"""SDK-defined: a bad GET event does not prevent later server messages or POSTs.
The raw peer forces a bad GET followed by a fresh GET stream, which the typed client cannot schedule.
"""
gets: list[httpx2.Request] = []
def handler(request: httpx2.Request) -> httpx2.Response:
if request.method == "GET":
gets.append(request)
if len(gets) == 1:
content = b'retry: 0\n\ndata: {"jsonrpc":"2.0","method":"notifications/message","params":{"data":"'
content += b"x" * 2048 + b'"}}\n\n'
return httpx2.Response(200, content=content, headers={"content-type": "text/event-stream"})
return httpx2.Response(
200,
stream=_ParkedSSEStream(
b'data: {"jsonrpc":"2.0","method":"notifications/message","params":{"data":"ok"}}\n\n'
),
headers={"content-type": "text/event-stream"},
)
body = json.loads(request.content)
if body.get("method") == "initialize":
return httpx2.Response(
200, json={"jsonrpc": "2.0", "id": body["id"], "result": {}}, headers={"mcp-session-id": "s1"}
)
if "id" in body:
return httpx2.Response(200, json={"jsonrpc": "2.0", "id": body["id"], "result": {}})
return httpx2.Response(202)
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client(
"http://test/mcp", http_client=http, terminate_on_close=False, max_sse_event_size=1024
) as (read, write),
):
await write.send(SessionMessage(JSONRPCRequest(jsonrpc="2.0", id=1, method="initialize", params={})))
await read.receive()
await write.send(SessionMessage(JSONRPCNotification(jsonrpc="2.0", method="notifications/initialized")))
notification = await read.receive()
await write.send(SessionMessage(JSONRPCRequest(jsonrpc="2.0", id=2, method="tools/list", params={})))
reply = await read.receive()
assert isinstance(notification, SessionMessage)
assert isinstance(notification.message, JSONRPCNotification)
assert notification.message.params == {"data": "ok"}
assert isinstance(reply, SessionMessage)
assert isinstance(reply.message, JSONRPCResponse)
assert reply.message.id == 2
assert len(gets) == 2
@pytest.mark.anyio
@pytest.mark.parametrize("limit", [0, -1])
async def test_streamable_http_rejects_nonpositive_sse_event_limits(limit: int) -> None:
"""SDK-defined: a nonpositive per-event byte limit is rejected before any HTTP request is sent."""
with pytest.raises(ValueError) as exc_info:
async with streamable_http_client("http://test/mcp", max_sse_event_size=limit):
raise NotImplementedError
assert str(exc_info.value) == snapshot("max_sse_event_size must be positive or None")
@pytest.mark.anyio
async def test_none_disables_the_sse_event_limit() -> None:
"""SDK-defined: opting out accepts a POST SSE response larger than the 1 MiB default.
The raw peer forces SSE for the request; the typed server can choose a JSON response instead.
"""
payload = "x" * (2 * 1024 * 1024)
def handler(request: httpx2.Request) -> httpx2.Response:
body = json.loads(request.content)
return httpx2.Response(
200,
content=f'data: {{"jsonrpc":"2.0","id":{body["id"]},"result":{{"text":"{payload}"}}}}\n\n'.encode(),
headers={"content-type": "text/event-stream"},
)
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client("http://test/mcp", http_client=http, max_sse_event_size=None) as (read, write),
):
await write.send(SessionMessage(JSONRPCRequest(jsonrpc="2.0", id=1, method="tools/list", params={})))
reply = await read.receive()
assert isinstance(reply, SessionMessage)
assert isinstance(reply.message, JSONRPCResponse)
assert reply.message.result == {"text": payload}
@pytest.mark.anyio
async def test_configured_sse_limit_fails_an_oversized_resumption_event() -> None:
"""SDK-defined: an oversized resumption event fails its request.
The raw peer sends an oversized replay regardless of token state, which a typed server cannot force.
"""
seen: list[str] = []
def handler(request: httpx2.Request) -> httpx2.Response:
seen.append(request.method)
return httpx2.Response(
200,
content=b'data: {"jsonrpc":"2.0","id":1,"result":{"text":"' + b"x" * 2048 + b'"}}\n\n',
headers={"content-type": "text/event-stream"},
)
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client("http://test/mcp", http_client=http, max_sse_event_size=1024) as (read, write),
):
await write.send(
SessionMessage(
JSONRPCRequest(jsonrpc="2.0", id=1, method="tools/call", params={}),
metadata=ClientMessageMetadata(resumption_token="evt-1"),
)
)
reply = await read.receive()
assert isinstance(reply, SessionMessage)
assert isinstance(reply.message, JSONRPCError)
assert reply.message.error.code == CONNECTION_CLOSED
# httpx2 owns the error detail after this prefix.
assert reply.message.error.message.startswith("SSE stream failed: ")
assert seen == ["GET"]
@pytest.mark.anyio
async def test_configured_sse_limit_does_not_replay_an_oversized_reconnected_event() -> None:
"""SDK-defined: a reconnect that receives an oversized event fails once; replay cannot make it smaller.
The raw peer forces a priming event followed by an oversized replay, which a typed server cannot schedule exactly.
"""
seen: list[str] = []
def handler(request: httpx2.Request) -> httpx2.Response:
seen.append(request.method)
if request.method == "POST":
return httpx2.Response(
200, content=b"id: evt-1\nretry: 0\n\n", headers={"content-type": "text/event-stream"}
)
assert request.headers["last-event-id"] == "evt-1"
return httpx2.Response(
200,
content=b'data: {"jsonrpc":"2.0","id":1,"result":{"text":"' + b"x" * 2048 + b'"}}\n\n',
headers={"content-type": "text/event-stream"},
)
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client("http://test/mcp", http_client=http, max_sse_event_size=1024) as (read, write),
):
await write.send(SessionMessage(JSONRPCRequest(jsonrpc="2.0", id=1, method="tools/call", params={})))
reply = await read.receive()
assert isinstance(reply, SessionMessage)
assert isinstance(reply.message, JSONRPCError)
assert reply.message.error.code == CONNECTION_CLOSED
# httpx2 owns the error detail after this prefix.
assert reply.message.error.message.startswith("SSE stream failed: ")
assert seen == ["POST", "GET"]
@pytest.mark.anyio
async def test_non_sse_reconnection_response_retries_before_failing_the_request() -> None:
"""SDK-defined: a temporary non-SSE reconnect response does not discard the request.
The raw peer forces a non-SSE GET followed by a valid replay, which the typed server cannot produce.
"""
seen: list[str] = []
def handler(request: httpx2.Request) -> httpx2.Response:
seen.append(request.method)
if request.method == "POST":
return httpx2.Response(
200, content=b"id: evt-1\nretry: 0\n\n", headers={"content-type": "text/event-stream"}
)
assert request.headers["last-event-id"] == "evt-1"
if len(seen) == 2:
return httpx2.Response(200, content=b"temporary proxy response", headers={"content-type": "text/plain"})
return httpx2.Response(
200,
content=b'data: {"jsonrpc":"2.0","id":1,"result":{}}\n\n',
headers={"content-type": "text/event-stream"},
)
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client("http://test/mcp", http_client=http, max_sse_event_size=1024) as (read, write),
):
await write.send(SessionMessage(JSONRPCRequest(jsonrpc="2.0", id=1, method="tools/call", params={})))
reply = await read.receive()
assert isinstance(reply, SessionMessage)
assert isinstance(reply.message, JSONRPCResponse)
assert reply.message.id == 1
assert seen == ["POST", "GET", "GET"]
@pytest.mark.parametrize(
("raw", "expected", "wrapped"),
[
("add", snapshot("add"), False),
("", snapshot(""), False),
("tool with spaces", snapshot("tool with spaces"), False),
(" add", snapshot("=?base64?IGFkZA==?="), True),
("add ", snapshot("=?base64?YWRkIA==?="), True),
("résumé", snapshot("=?base64?csOpc3Vtw6k=?="), True),
("a\r\nb", snapshot("=?base64?YQ0KYg==?="), True),
("=?base64?Zm9v?=", snapshot("=?base64?PT9iYXNlNjQ/Wm05dj89?="), True),
],
)
def test_mcp_name_header_values_are_base64_wrapped_when_unsafe_for_an_http_field(
raw: str, expected: str, wrapped: bool
) -> None:
"""Printable-ASCII names pass verbatim; CR/LF, non-ASCII, edge-whitespace, and sentinel-shaped names are wrapped.
The ``=?base64?...?=`` sentinel is the spec's RFC 7230 safety gate for the ``Mcp-Name`` header.
Wrapped values round-trip through base64 so the server can recover the original name. A leading
or trailing space is wrapped because RFC 7230 forbids it in field-values (h11 rejects on real
transports); an empty value is allowed and passes verbatim.
"""
encoded = encode_header_value(raw)
assert encoded == expected
if wrapped:
assert encoded.startswith("=?base64?") and encoded.endswith("?=")
assert base64.b64decode(encoded.removeprefix("=?base64?").removesuffix("?=")).decode() == raw
else:
assert encoded == raw
@pytest.mark.anyio
async def test_post_request_merges_per_message_metadata_headers() -> None:
"""`ClientMessageMetadata.headers` on a `SessionMessage` are merged into the outgoing POST headers
(SDK-defined: the headers sidecar is the path the session uses to reach the transport)."""
recorded: list[httpx2.Request] = []
def handler(request: httpx2.Request) -> httpx2.Response:
recorded.append(request)
body = json.loads(request.content)
return httpx2.Response(200, json={"jsonrpc": "2.0", "id": body["id"], "result": {}})
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client("http://test/mcp", http_client=http) as (read, write),
):
await write.send(
SessionMessage(
message=JSONRPCRequest(jsonrpc="2.0", id=1, method="tools/list", params={}),
metadata=ClientMessageMetadata(headers={"x-test": "v"}),
)
)
reply = await read.receive()
assert isinstance(reply, SessionMessage)
assert [r.method for r in recorded] == ["POST"]
assert recorded[0].headers["x-test"] == "v"
@pytest.mark.anyio
async def test_pre_session_bare_404_maps_to_method_not_found() -> None:
"""A bare HTTP 404 (no JSON-RPC body) before any session-id is held maps to METHOD_NOT_FOUND.
Gateways and legacy servers 404 at the HTTP layer for unknown methods; with no session yet,
"Session terminated" is meaningless, and the discover→initialize fallback ladder keys on -32601.
"""
def handler(request: httpx2.Request) -> httpx2.Response:
return httpx2.Response(404)
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client("http://test/mcp", http_client=http) as (read, write),
):
await write.send(SessionMessage(JSONRPCRequest(jsonrpc="2.0", id=1, method="server/discover", params={})))
reply = await read.receive()
assert isinstance(reply, SessionMessage)
assert isinstance(reply.message, JSONRPCError)
assert reply.message.error.code == METHOD_NOT_FOUND
@pytest.mark.anyio
async def test_initialize_post_clears_cached_pv_header_and_unstamped_posts_read_it() -> None:
"""``initialize`` discards the cached protocol-version header; every other POST reads it.
Steps:
1. A stamped probe POST caches its ``MCP-Protocol-Version`` header.
2. An ``initialize`` POST clears that cache before building headers, so the fallback
handshake never carries a probe-stamped value.
3. A subsequent stamped POST re-seeds the cache with the negotiated version.
4. An unstamped POST (a JSON-RPC response written by the dispatcher, which never
passes through the session's stamp) then reads the cache and carries the
negotiated version — the spec MUST for all post-initialization HTTP requests.
"""
recorded: list[httpx2.Request] = []
def handler(request: httpx2.Request) -> httpx2.Response:
recorded.append(request)
body = json.loads(request.content)
if "id" not in body or "result" in body:
return httpx2.Response(202)
return httpx2.Response(200, json={"jsonrpc": "2.0", "id": body["id"], "result": {}})
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client("http://test/mcp", http_client=http) as (read, write),
):
await write.send(
SessionMessage(
message=JSONRPCRequest(jsonrpc="2.0", id=1, method="server/discover", params={}),
metadata=ClientMessageMetadata(headers={MCP_PROTOCOL_VERSION_HEADER: "2026-07-28"}),
)
)
await read.receive()
await write.send(SessionMessage(JSONRPCRequest(jsonrpc="2.0", id=2, method="initialize", params={})))
await read.receive()
await write.send(
SessionMessage(
message=JSONRPCNotification(jsonrpc="2.0", method="notifications/initialized"),
metadata=ClientMessageMetadata(headers={MCP_PROTOCOL_VERSION_HEADER: "2025-11-25"}),
)
)
# An unstamped JSON-RPC response — what the dispatcher writes when answering
# a server-initiated request (sampling/elicitation/roots).
await write.send(SessionMessage(JSONRPCResponse(jsonrpc="2.0", id=99, result={})))
assert [r.method for r in recorded] == ["POST", "POST", "POST", "POST"]
assert recorded[0].headers[MCP_PROTOCOL_VERSION_HEADER] == "2026-07-28"
assert MCP_PROTOCOL_VERSION_HEADER not in recorded[1].headers
assert recorded[2].headers[MCP_PROTOCOL_VERSION_HEADER] == "2025-11-25"
assert recorded[3].headers[MCP_PROTOCOL_VERSION_HEADER] == "2025-11-25"
def _sse_or_ack_handler(
parked: _ParkedSSEStream, posted: list[dict[str, Any]], frame_posted: anyio.Event
) -> Callable[[httpx2.Request], httpx2.Response]:
"""Requests get the parked SSE body; notifications get 202 and set `frame_posted`."""
def handler(request: httpx2.Request) -> httpx2.Response:
body = json.loads(request.content)
posted.append(body)
if "id" in body:
return httpx2.Response(200, headers={"content-type": "text/event-stream"}, stream=parked)
frame_posted.set()
return httpx2.Response(202)
return handler
@pytest.mark.anyio
async def test_modern_cancelled_frame_aborts_the_matching_in_flight_post() -> None:
"""At 2026 an outbound `notifications/cancelled` never POSTs — closing the named
request's response stream IS the wire's cancellation signal — so the transport
aborts the in-flight POST and swallows the frame."""
parked = _ParkedSSEStream()
posted: list[dict[str, Any]] = []
def handler(request: httpx2.Request) -> httpx2.Response:
posted.append(json.loads(request.content))
return httpx2.Response(200, headers={"content-type": "text/event-stream"}, stream=parked)
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client("http://test/mcp", http_client=http) as (_read, write),
):
await write.send(
SessionMessage(
message=JSONRPCRequest(jsonrpc="2.0", id="listen-1", method="subscriptions/listen", params={}),
metadata=ClientMessageMetadata(headers={MCP_PROTOCOL_VERSION_HEADER: LATEST_MODERN_VERSION}),
)
)
await parked.opened.wait()
await write.send(
SessionMessage(
JSONRPCNotification(
jsonrpc="2.0", method="notifications/cancelled", params={"requestId": "listen-1"}
)
)
)
await parked.closed.wait()
assert [body["method"] for body in posted] == ["subscriptions/listen"]
@pytest.mark.anyio
@pytest.mark.parametrize("stamped_version", [None, "2025-11-25"], ids=["no-version-yet", "2025-11-25"])
async def test_legacy_cancelled_frame_posts_and_leaves_the_stream_open(stamped_version: str | None) -> None:
"""Below 2026 — or before any stamped POST has revealed the version — the frame is
the spec's cancellation signal: it POSTs, and the request's stream stays open
(a 2025 disconnect is explicitly not a cancel)."""
parked = _ParkedSSEStream()
posted: list[dict[str, Any]] = []
frame_posted = anyio.Event()
handler = _sse_or_ack_handler(parked, posted, frame_posted)
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client("http://test/mcp", http_client=http) as (_read, write),
):
metadata = (
ClientMessageMetadata(headers={MCP_PROTOCOL_VERSION_HEADER: stamped_version})
if stamped_version is not None
else None
)
await write.send(
SessionMessage(
message=JSONRPCRequest(jsonrpc="2.0", id=1, method="tools/call", params={}),
metadata=metadata,
)
)
await parked.opened.wait()
await write.send(
SessionMessage(
JSONRPCNotification(jsonrpc="2.0", method="notifications/cancelled", params={"requestId": 1})
)
)
await frame_posted.wait()
# Checked before teardown: exiting the transport cancels the parked POST.
assert not parked.closed.is_set()
assert [body["method"] for body in posted] == ["tools/call", "notifications/cancelled"]
@pytest.mark.anyio
@pytest.mark.parametrize(
"params",
[
pytest.param({"requestId": 999}, id="unknown-id"),
pytest.param({"requestId": True}, id="bool-must-not-alias-request-id-1"),
pytest.param({"requestId": "1"}, id="string-1-must-not-match-int-1"),
pytest.param({}, id="no-request-id"),
pytest.param(None, id="no-params"),
],
)
async def test_modern_cancelled_frames_matching_no_post_are_swallowed(params: dict[str, Any] | None) -> None:
"""At 2026 the frame is swallowed even when it aborts nothing — the wire defines no
client-to-server notifications, so a late cancel racing the response must not leak
a POST — and a mismatched id must not abort someone else's stream."""
parked = _ParkedSSEStream()
posted: list[dict[str, Any]] = []
def handler(request: httpx2.Request) -> httpx2.Response:
body = json.loads(request.content)
posted.append(body)
if body.get("id") == 1:
return httpx2.Response(200, headers={"content-type": "text/event-stream"}, stream=parked)
return httpx2.Response(200, json={"jsonrpc": "2.0", "id": body["id"], "result": {}})
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client("http://test/mcp", http_client=http) as (read, write),
):
await write.send(
SessionMessage(
message=JSONRPCRequest(jsonrpc="2.0", id=1, method="subscriptions/listen", params={}),
metadata=ClientMessageMetadata(headers={MCP_PROTOCOL_VERSION_HEADER: LATEST_MODERN_VERSION}),
)
)
await parked.opened.wait()
await write.send(
SessionMessage(JSONRPCNotification(jsonrpc="2.0", method="notifications/cancelled", params=params))
)
# A follow-up request completing proves the loop moved past the swallowed frame.
await write.send(SessionMessage(JSONRPCRequest(jsonrpc="2.0", id=2, method="ping", params={})))
reply = await read.receive()
# Checked before teardown: exiting the transport cancels the parked POST.
assert not parked.closed.is_set()
assert isinstance(reply, SessionMessage)
assert isinstance(reply.message, JSONRPCResponse)
assert reply.message.id == 2
assert [body["method"] for body in posted] == ["subscriptions/listen", "ping"]
@pytest.mark.anyio
async def test_handler_scoped_cancelled_frames_are_translated_at_modern_too() -> None:
"""A cancel carrying `ServerMessageMetadata` (a handler abandoning its own
back-channel request) still names one of OUR outbound ids — every spec-legal
cancel names a request its sender issued — so at 2026 it aborts that POST and
stays off the wire like any other."""
parked = _ParkedSSEStream()
posted: list[dict[str, Any]] = []
frame_posted = anyio.Event()
handler = _sse_or_ack_handler(parked, posted, frame_posted)
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client("http://test/mcp", http_client=http) as (_read, write),
):
await write.send(
SessionMessage(
message=JSONRPCRequest(jsonrpc="2.0", id=1, method="tools/call", params={}),
metadata=ClientMessageMetadata(headers={MCP_PROTOCOL_VERSION_HEADER: LATEST_MODERN_VERSION}),
)
)
await parked.opened.wait()
await write.send(
SessionMessage(
message=JSONRPCNotification(
jsonrpc="2.0", method="notifications/cancelled", params={"requestId": 1}
),
metadata=ServerMessageMetadata(related_request_id=99),
)
)
await parked.closed.wait()
assert [body["method"] for body in posted] == ["tools/call"]
assert not frame_posted.is_set()
@pytest.mark.anyio
async def test_cancel_for_a_request_sent_under_2025_still_posts_after_modern_adoption() -> None:
"""The translation follows the era the NAMED request was sent under, not the
cache at cancel time: a request POSTed under 2025 keeps 2025 cancellation
semantics (frame on the wire, stream left open) even after a later message
flips the negotiated version to 2026."""
parked = _ParkedSSEStream()
posted: list[dict[str, Any]] = []
frame_posted = anyio.Event()
def handler(request: httpx2.Request) -> httpx2.Response:
body = json.loads(request.content)
posted.append(body)
if body.get("id") == 1:
return httpx2.Response(200, headers={"content-type": "text/event-stream"}, stream=parked)
if "id" in body:
return httpx2.Response(200, json={"jsonrpc": "2.0", "id": body["id"], "result": {}})
frame_posted.set()
return httpx2.Response(202)
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client("http://test/mcp", http_client=http) as (read, write),
):
await write.send(
SessionMessage(
message=JSONRPCRequest(jsonrpc="2.0", id=1, method="tools/call", params={}),
metadata=ClientMessageMetadata(headers={MCP_PROTOCOL_VERSION_HEADER: "2025-11-25"}),
)
)
await parked.opened.wait()
# A modern-stamped request flips the cached negotiated version.
await write.send(
SessionMessage(
message=JSONRPCRequest(jsonrpc="2.0", id=2, method="ping", params={}),
metadata=ClientMessageMetadata(headers={MCP_PROTOCOL_VERSION_HEADER: LATEST_MODERN_VERSION}),
)
)
reply = await read.receive()
assert isinstance(reply, SessionMessage)
assert isinstance(reply.message, JSONRPCResponse)
await write.send(
SessionMessage(
JSONRPCNotification(jsonrpc="2.0", method="notifications/cancelled", params={"requestId": 1})
)
)
await frame_posted.wait()
# Checked before teardown: exiting the transport cancels the parked POST.
assert not parked.closed.is_set()
assert [body["method"] for body in posted] == ["tools/call", "ping", "notifications/cancelled"]
class _SignalingBus(InMemorySubscriptionBus):
"""Signals subscribe/unsubscribe so a test observes the stream lifecycle through
the bus Protocol (the public seam) instead of polling handler internals."""
def __init__(self) -> None:
super().__init__()
self.subscribed = anyio.Event()
self.unsubscribed = anyio.Event()
def subscribe(self, listener: Callable[[ServerEvent], None]) -> Callable[[], None]:
unsubscribe = super().subscribe(listener)
self.subscribed.set()
def unsubscribe_and_signal() -> None:
unsubscribe()
self.unsubscribed.set()
return unsubscribe_and_signal
@pytest.mark.anyio
async def test_scope_cancel_aborts_a_modern_listen_post_end_to_end() -> None:
"""Over a real ASGI bridge: cancelling the caller of a parked `subscriptions/listen`
closes the POST's response stream — the server treats the disconnect as the cancel
and releases the subscription — and no `notifications/cancelled` crosses the wire."""
bus = _SignalingBus()
server = Server("test", on_subscriptions_listen=ListenHandler(bus))
async def app(scope: Scope, receive: Receive, send: Send) -> None:
async with server.lifespan(server) as lifespan_state:
await handle_modern_request(server, None, False, lifespan_state, scope, receive, send)
posted_methods: list[str] = []
async def record_request(request: httpx2.Request) -> None:
posted_methods.append(json.loads(request.content)["method"])
acked = anyio.Event()
async def on_notify(dctx: DispatchContext[TransportContext], method: str, params: Mapping[str, Any] | None) -> None:
assert method == "notifications/subscriptions/acknowledged"
acked.set()
on_request, _ = echo_handlers(Recorder())
with anyio.fail_after(15):
async with (
httpx2.AsyncClient(
transport=StreamingASGITransport(app),
base_url="http://testserver",
event_hooks={"request": [record_request]},
) as http,
streamable_http_client("http://testserver/mcp", http_client=http) as (read, write),
):
dispatcher: JSONRPCDispatcher[TransportContext] = JSONRPCDispatcher(read, write)
async with anyio.create_task_group() as tg: # pragma: no branch
await tg.start(dispatcher.run, on_request, on_notify)
listen_scope = anyio.CancelScope()
async def send_listen() -> None:
params: dict[str, Any] = {
"_meta": {
PROTOCOL_VERSION_META_KEY: LATEST_MODERN_VERSION,
CLIENT_INFO_META_KEY: {"name": "test-client", "version": "0"},
CLIENT_CAPABILITIES_META_KEY: {},
},
"notifications": {"toolsListChanged": True},
}
opts: CallOptions = {
"request_id": "listen-1",
"headers": {
MCP_PROTOCOL_VERSION_HEADER: LATEST_MODERN_VERSION,
MCP_METHOD_HEADER: "subscriptions/listen",
},
}
with listen_scope:
await dispatcher.send_raw_request("subscriptions/listen", params, opts)
tg.start_soon(send_listen)
await acked.wait()
assert bus.subscribed.is_set()
assert not bus.unsubscribed.is_set()
listen_scope.cancel()
await bus.unsubscribed.wait()
tg.cancel_scope.cancel()
assert posted_methods == ["subscriptions/listen"]
class _CompletingSSEStream(httpx2.AsyncByteStream):
"""An SSE body that delivers one JSON-RPC response, then parks in `aclose`.
Holding `aclose` keeps the finished POST task alive past its response, so a
test can re-register the same request id underneath it before releasing.
"""
def __init__(self, response_body: dict[str, Any]) -> None:
self._event = f"data: {json.dumps(response_body)}\n\n".encode()
self.release = anyio.Event()
async def __aiter__(self) -> AsyncIterator[bytes]:
yield self._event
async def aclose(self) -> None:
await self.release.wait()
@pytest.mark.anyio
async def test_a_finished_post_task_does_not_evict_a_reused_ids_new_registration() -> None:
"""Request ids are reusable once resolved; a finished POST task unwinding late
must not pop the successor's registration, or a cancel for the reused id would
find nothing to abort and the live POST would leak past the cancellation."""
completing = _CompletingSSEStream({"jsonrpc": "2.0", "id": "dup-1", "result": {}})
parked = _ParkedSSEStream()
posted: list[dict[str, Any]] = []
streams = [completing, parked]
def handler(request: httpx2.Request) -> httpx2.Response:
posted.append(json.loads(request.content))
return httpx2.Response(200, headers={"content-type": "text/event-stream"}, stream=streams.pop(0))
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client("http://test/mcp", http_client=http) as (read, write),
):
modern = ClientMessageMetadata(headers={MCP_PROTOCOL_VERSION_HEADER: LATEST_MODERN_VERSION})
await write.send(
SessionMessage(
message=JSONRPCRequest(jsonrpc="2.0", id="dup-1", method="tools/call", params={}),
metadata=modern,
)
)
reply = await read.receive()
assert isinstance(reply, SessionMessage)
assert isinstance(reply.message, JSONRPCResponse)
# The first task is now parked in `aclose`; reuse its id underneath it.
await write.send(
SessionMessage(
message=JSONRPCRequest(jsonrpc="2.0", id="dup-1", method="subscriptions/listen", params={}),
metadata=modern,
)
)
await parked.opened.wait()
completing.release.set()
await anyio.wait_all_tasks_blocked()
# The successor's registration survived: a cancel still aborts it.
await write.send(
SessionMessage(
JSONRPCNotification(jsonrpc="2.0", method="notifications/cancelled", params={"requestId": "dup-1"})
)
)
await parked.closed.wait()
assert [body["method"] for body in posted] == ["tools/call", "subscriptions/listen"]
class _DyingSSEStream(httpx2.AsyncByteStream):
"""Emits one id-less comment then breaks - a non-resumable stream dropping."""
def __init__(self) -> None:
self.opened = anyio.Event()
async def __aiter__(self) -> AsyncIterator[bytes]:
self.opened.set()
yield b": hello\n\n"
raise httpx2.ReadError("connection reset")
async def aclose(self) -> None:
pass
@pytest.mark.anyio
async def test_a_non_resumable_sse_drop_resolves_the_request_with_an_error() -> None:
"""A per-request SSE stream that dies having carried no event ids can never deliver its
response; the transport resolves the waiter with CONNECTION_CLOSED instead of hanging forever."""
dying = _DyingSSEStream()
def handler(request: httpx2.Request) -> httpx2.Response:
return httpx2.Response(200, headers={"content-type": "text/event-stream"}, stream=dying)
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client("http://test/mcp", http_client=http) as (read, write),
):
await write.send(
SessionMessage(JSONRPCRequest(jsonrpc="2.0", id="listen-1", method="subscriptions/listen", params={}))
)
reply = await read.receive()
assert isinstance(reply, SessionMessage)
assert isinstance(reply.message, JSONRPCError)
assert reply.message.id == "listen-1"
assert reply.message.error.code == CONNECTION_CLOSED
class _DeliverOnCommandSSEStream(httpx2.AsyncByteStream):
"""Parks after opening, then delivers one JSON-RPC response when told."""
def __init__(self, response_body: dict[str, Any]) -> None:
self._event = f"data: {json.dumps(response_body)}\n\n".encode()
self.opened = anyio.Event()
self.deliver = anyio.Event()
async def __aiter__(self) -> AsyncIterator[bytes]:
self.opened.set()
await self.deliver.wait()
yield self._event
async def aclose(self) -> None:
pass
@pytest.mark.anyio
async def test_a_superseded_posts_late_real_response_cannot_answer_the_successor() -> None:
"""SDK-defined: re-issuing an id severs the superseded POST, so nothing from its
stream (a late real response, or a synthesized error for its death) can resolve
the reused id's waiter; only the successor's own response arrives."""
stale = _DeliverOnCommandSSEStream({"jsonrpc": "2.0", "id": "dup-1", "result": {"origin": "stale"}})
succeeding = _DeliverOnCommandSSEStream({"jsonrpc": "2.0", "id": "dup-1", "result": {"origin": "fresh"}})
streams: list[httpx2.AsyncByteStream] = [stale, succeeding]
def handler(request: httpx2.Request) -> httpx2.Response:
return httpx2.Response(200, headers={"content-type": "text/event-stream"}, stream=streams.pop(0))
with anyio.fail_after(5):
async with (
httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http,
streamable_http_client("http://test/mcp", http_client=http) as (read, write),
):
await write.send(SessionMessage(JSONRPCRequest(jsonrpc="2.0", id="dup-1", method="tools/call", params={})))
await stale.opened.wait()
await write.send(SessionMessage(JSONRPCRequest(jsonrpc="2.0", id="dup-1", method="tools/call", params={})))
await succeeding.opened.wait()
stale.deliver.set()
await anyio.wait_all_tasks_blocked()
succeeding.deliver.set()
reply = await read.receive()
assert isinstance(reply, SessionMessage)
assert isinstance(reply.message, JSONRPCResponse), reply.message
assert reply.message.result == {"origin": "fresh"}
@pytest.mark.anyio
async def test_a_202_to_a_request_resolves_the_waiter_with_an_error() -> None:
You can’t perform that action at this time.