Graphs are languages.
Edit them like code.

#!/usr/bin/env python3
# A language server: warns about states that cannot be left.
import json, sys

def read():
    headers = {}
    while line := sys.stdin.buffer.readline().strip():
        name, _, value = line.partition(b":")
        headers[name.strip().lower()] = value.strip()
    length = int(headers.get(b"content-length", 0))
    return json.loads(sys.stdin.buffer.read(length)) if length else None

def send(message):
    body = json.dumps({"jsonrpc": "2.0", **message}).encode()
    sys.stdout.buffer.write(b"Content-Length: %d\r\n\r\n" % len(body))
    sys.stdout.buffer.write(body)
    sys.stdout.buffer.flush()

def diagnose(buffer):
    graph = buffer["graph"]
    sources = {edge["source"] for edge in graph["edges"].values()}
    return [{"lintIdentifier": "DL-001", "severity": "warning",
             "title": "Deadlock",
             "message": "There is no way out of this state",
             "affectedElements": [vertex]}
            for vertex in graph["vertices"] if vertex not in sources]

while message := read():
    method, params = message.get("method"), message.get("params")
    if method == "initialize":
        send({"id": message["id"], "result": {
            "serverInfo": {"name": "deadlock-ls", "version": "1.0"},
            "language": "LTS",
            "capabilities": {"sync": "full", "diagnostics": True}}})
    elif method in ("graph/didOpen", "graph/didChange"):
        buffer = params["buffer"]
        send({"method": "graph/publishDiagnostics", "params": {
            "key": buffer["key"], "diagnostics": diagnose(buffer)}})
    elif method == "shutdown":
        send({"id": message["id"], "result": None})
    elif method == "exit":
        break

// A plugin with a syntax: flowcharts of steps and decisions.
class flowchart_plugin : public QObject, public graphedit::plugin {
    Q_OBJECT
    Q_PLUGIN_METADATA(IID GRAPHEDIT_PLUGIN_IID FILE "flowchart_plugin.json")
    Q_INTERFACES(graphedit::plugin)
public:
    auto name() const -> QString override {
        return QStringLiteral("Flowchart");
    }
    auto syntax_factories()
            -> std::vector<std::shared_ptr<graphedit::syntax_factory>> override {
        return {std::make_shared<flowchart_syntax>()};
    }
};

// The element types of the syntax, with their properties.
auto flowchart_syntax::vertex_types() const
        -> std::vector<graphedit::element_type_t> {
    return {{.name = QStringLiteral("decision"),
             .display_name = QStringLiteral("Decision"),
             .description = QStringLiteral("A question with two outcomes"),
             .icon = QStringLiteral("diamond"),
             .properties = {label_property(QStringLiteral("Decision"))}}};
}

cmake_minimum_required(VERSION 3.21)
project(flowchart LANGUAGES CXX)
set(CMAKE_CXX_STANDARD 20)

find_package(Qt6 6.5 REQUIRED COMPONENTS Core)
find_package(graphedit 2 REQUIRED)
qt_standard_project_setup()

qt_add_plugin(flowchart SHARED
    flowchart.cpp flowchart.h flowchart_plugin.cpp)
target_link_libraries(flowchart PRIVATE graphedit::core)
install(TARGETS flowchart
    LIBRARY DESTINATION "${GRAPHEDIT_PLUGIN_DIR}/flowchart")