Fail fast on server-to-client requests in JSON-response mode instead of hanging (#3195)

tests/interaction/_requirements.py

@@ -2608,6 +2608,16 @@ REQUIREMENTS: dict[str, Requirement] = {
             "overtake anything already queued for the request."
         ),
     ),
+    "transport:streamable-http:json-response-restrictions": Requirement(
+        source="sdk",
+        behavior=(
+            "In JSON-response mode a handler's request-scoped server-initiated request fails fast with an "
+            "INVALID_REQUEST protocol error and request-scoped notifications are not delivered, because the "
+            "single JSON body carries only the response; the connection's standalone stream is unaffected."
+        ),
+        transports=("streamable-http",),
+        note="Only observable over streamable HTTP: JSON-response mode is an HTTP framing option.",
+    ),
     "transport:streamable-http:stateless": Requirement(
         source=f"{SPEC_BASE_URL}/basic/transports#streamable-http",
         behavior=(

tests/interaction/transports/test_streamable_http.py

@@ -64,7 +64,8 @@ def _smoke_server() -> MCPServer:
     async def ask(ctx: Context) -> str:
         """Elicit a confirmation from the client and report the outcome."""
         answer = await ctx.elicit("Proceed?", Confirmation)
-        # In stateless mode the elicit raises before this point: there is no session to call back through.
+        # In stateless and JSON-response modes the elicit raises before this point: there is no
+        # request-scoped channel to call back through.
         assert isinstance(answer, AcceptedElicitation)
         return f"confirmed={answer.data.confirmed}"
 
@@ -120,6 +121,48 @@ async def test_stateless_streamable_http_rejects_server_initiated_requests() ->
     assert exc_info.value.error.code == INVALID_REQUEST
 
 
+@requirement("transport:streamable-http:json-response-restrictions")
+async def test_json_response_streamable_http_rejects_request_scoped_server_requests() -> None:
+    """A handler that calls back to the client mid-request fails fast when the server answers with
+    JSON: the one response body cannot carry the nested `elicitation/create`, so the request-scoped
+    channel raises `NoBackChannelError` (a top-level `MCPError`) instead of parking a waiter no reply
+    could ever reach. Bounded, because before the fix this call hung until it timed out."""
+    async with connect_over_streamable_http(_smoke_server(), json_response=True) as client:
+        with anyio.fail_after(5), pytest.raises(MCPError) as exc_info:
+            await client.call_tool("ask", {})
+
+    assert exc_info.value.error.code == INVALID_REQUEST
+
+
+@requirement("transport:streamable-http:json-response-restrictions")
+@requirement("transport:streamable-http:unrelated-messages")
+@requirement("hosting:http:standalone-sse")
+async def test_json_response_streamable_http_delivers_only_unrelated_notifications() -> None:
+    """In JSON-response mode the call's own log notification has no stream to ride and never
+    reaches the client, while the tool result comes back as the JSON body and the unrelated
+    resource-updated notification arrives on the standalone stream. The handler writes both
+    notifications before returning, so once the result and the unrelated message are in, no
+    request-scoped message can still be in flight."""
+    received: list[IncomingMessage] = []
+    server_message_seen = anyio.Event()
+
+    async def collect(message: IncomingMessage) -> None:
+        received.append(message)
+        server_message_seen.set()
+
+    async with connect_over_streamable_http(_smoke_server(), json_response=True, message_handler=collect) as client:
+        with anyio.fail_after(5):
+            result = await client.call_tool("announce", {})
+            await server_message_seen.wait()
+
+    assert result == snapshot(
+        CallToolResult(content=[TextContent(text="announced")], structured_content={"result": "announced"})
+    )
+    assert received == snapshot(
+        [ResourceUpdatedNotification(params=ResourceUpdatedNotificationParams(uri="file:///watched.txt"))]
+    )
+
+
 @requirement("transport:streamable-http:notifications")
 @requirement("transport:streamable-http:unrelated-messages")
 @requirement("hosting:http:standalone-sse")