test: cover branch where reconnection exhausts max attempts

Add test_sse_error_when_reconnection_exhausted to exercise the
_handle_sse_response path where SSE events are received (setting
last_event_id) but reconnection fails, ensuring the JSONRPCError
is sent to unblock the waiting request.

Author: heyhayes

src/mcp/client/streamable_http.py

@@ -211,10 +211,7 @@ async def handle_get_stream(self, client: httpx.AsyncClient, read_stream_writer:
 
                     # Only reset attempts if we actually received events;
                     # empty connections count toward MAX_RECONNECTION_ATTEMPTS
-                    if received_events:
-                        attempt = 0
-                    else:
-                        attempt += 1
+                    attempt = 0 if received_events else attempt + 1
 
             except Exception:  # pragma: lax no cover
                 logger.debug("GET stream error", exc_info=True)

tests/shared/test_streamable_http.py

@@ -2266,15 +2266,42 @@ async def test_sse_read_timeout_propagates_error(basic_server: None, basic_serve
             read_stream,
             write_stream,
         ):
-            async with ClientSession(read_stream, write_stream) as session:
+            async with ClientSession(read_stream, write_stream) as session:  # pragma: no branch
                 await session.initialize()
 
                 # Read a "slow" resource that takes 2s — longer than our 0.5s read timeout
-                with pytest.raises(MCPError):
-                    with anyio.fail_after(10):
+                with pytest.raises(MCPError):  # pragma: no branch
+                    with anyio.fail_after(10):  # pragma: no branch
                         await session.read_resource("slow://test")
 
 
+@pytest.mark.anyio
+async def test_sse_error_when_reconnection_exhausted(
+    event_server: tuple[SimpleEventStore, str],
+    monkeypatch: pytest.MonkeyPatch,
+):
+    """When SSE stream closes after events and reconnection fails, MCPError is raised."""
+    _, server_url = event_server
+
+    async def _always_fail_reconnection(
+        self: Any, ctx: Any, last_event_id: Any, retry_interval_ms: Any = None, attempt: int = 0
+    ) -> bool:
+        return False
+
+    monkeypatch.setattr(StreamableHTTPTransport, "_handle_reconnection", _always_fail_reconnection)
+
+    async with streamable_http_client(f"{server_url}/mcp") as (read_stream, write_stream):
+        async with ClientSession(read_stream, write_stream) as session:  # pragma: no branch
+            await session.initialize()
+
+            # tool_with_stream_close sends a priming event (setting last_event_id),
+            # then closes the SSE stream. With reconnection patched to fail,
+            # _handle_sse_response falls through to send the error.
+            with pytest.raises(MCPError):  # pragma: no branch
+                with anyio.fail_after(10):  # pragma: no branch
+                    await session.call_tool("tool_with_stream_close", {})
+
+
 @pytest.mark.anyio
 async def test_handle_reconnection_returns_false_on_max_attempts():
     """_handle_reconnection returns False when max attempts exceeded."""