gammaray-mcp/PLAN.md

QML SceneGraph MCP Bridge — Plan

Goal

Build an MCP server that exposes QML SceneGraph introspection data (from GammaRay's probe) to LLMs for assisted debugging. The bridge is a GammaRay client: it connects to a probe injected into the target Qt/QML app, reads the same models the GammaRay GUI uses, and translates them into MCP JSON-RPC tool calls.

Architecture

Target Qt/QML App
  │  GammaRay probe injected (preload/attach), loads quickinspector plugin
  │
  │  GammaRay binary protocol over TCP/local socket
  │  (common/protocol.h — QDataStream-based, NOT JSON-RPC)
  ▼
QML SceneGraph MCP Bridge (new project, GPL-2.0-or-later)
  │  Links gammaray_client + gammaray_common (installed from source)
  │  Uses ClientConnectionManager to connect, ObjectBroker to get models
  │  Connection is lazy: bridge starts without a probe, MCP client calls
  │  connectProbe(host, port) to establish; auto-reconnects on probe restart.
  │
  │  MCP JSON-RPC over stdio (or HTTP+SSE)
  ▼
LLM / AI Agent

The bridge is a GammaRay client peer to gammaray-client (GUI client) and gammaray (launcher+client GUI). It does NOT touch the probe side; it reuses GammaRay's existing probe injection + protocol + model sync.

License

• GammaRay libraries are GPL-2.0-or-later. Linking gammaray_client makes the bridge a derivative work → bridge must be GPL-compatible.
• Accepted by user. No commercial license needed.

GammaRay source location

The GammaRay source used for investigation: /home/blumia/Sources/GammaRay (version 3.4.0 per version.txt).

Step 1: Build and install GammaRay from source [DONE]

System packages are too old (system has 2.11.3); protocol version must match between probe and client (common/protocol.h:141 Protocol::version()). Built both probe and client libs from the 3.4.0 source.

Verified environment: Qt 6.8.0 (system), qt6-base-private-dev / qt6-declarative-private-dev installed, CMake 3.31, Ninja 1.12, GCC 12.3.

# Reused existing build/ dir; only reconfigured install prefix.
cmake -S /home/blumia/Sources/GammaRay -B /home/blumia/Sources/GammaRay/build \
  -DCMAKE_INSTALL_PREFIX=$(pwd)/install-prefix
cmake --build /home/blumia/Sources/GammaRay/build     # already built
cmake --install /home/blumia/Sources/GammaRay/build

Installed to install-prefix/ in this project dir (NOT $HOME/.local as originally drafted — keeps the toolchain self-contained). Contents:

• install-prefix/lib/libgammaray_{client,common,core,ui,...}-qt6_8-x86_64.so.3.4.0
• install-prefix/lib/cmake/GammaRay/ (GammaRayConfig.cmake + GammaRayTarget.cmake + GammaRayMacros.cmake)
• install-prefix/bin/gammaray (launcher+client)
• install-prefix/include/gammaray/{client,common,core,launcher,ui}/

Verified at runtime: Protocol::version() = 38, Endpoint::defaultPort() = 11732. gammaray_client.so runtime-links libgammaray_ui-qt6_8-x86_64.so (PRIVATE link, as the plan predicted) → install-prefix/lib must be on LD_LIBRARY_PATH at runtime.

VERIFIED — exported target namespace (gotcha #8): CMakeLists.txt:931-932 has NAMESPACE GammaRay:: commented out. Confirmed in installed GammaRayTarget.cmake:22: targets are unnamespaced — gammaray_client, gammaray_common, gammaray_core, gammaray_ui, gammaray_kitemmodels, gammaray_launcher, gammaray_launcher_ui, gammaray_kuserfeedback, gammaray_probe. Use gammaray_client / gammaray_common directly (no GammaRay:: prefix).

Step 2: Create the bridge project

New working directory, separate from GammaRay source tree.

Dependencies

1. GammaRay — installed from source (Step 1), provides gammaray_client, gammaray_common
2. qtmcp (https://github.com/signal-slot/qtmcp) — Qt-based MCP protocol implementation, eliminates hand-writing JSON-RPC/stdio/handshake

Why qtmcp

qtmcp provides a complete MCP server framework (QMcpServer) with:

• Built-in stdio and sse backends — no manual stdin/stdout handling
• Protocol handshake + version negotiation (supports 2024-11-05 and 2025-03-26)
• registerToolSet() + Q_INVOKABLE auto-exposes QObject methods as MCP tools
• Supports QFuture<Result> for async tool results (matches GammaRay's async APIs like getShader())
• Supports QImage return values (matches GammaRay texture grab / screenshot)
• License: LGPL-3.0 OR GPL-2.0 OR GPL-3.0 — choose GPL-2.0 to be compatible with GammaRay's GPL-2.0-or-later

qtmcp — consumed via FetchContent (no separate clone/install needed)

VERIFIED: qtmcp works with FetchContent_MakeAvailable() despite using Qt BuildInternals (qt_internal_project_setup() + qt_build_repo()). Tested empirically: configure + build + link all succeed. The resulting targets are Qt6::McpServer, Qt6::McpCommon, Qt6::McpClient (with Qt:: alias). Headers are generated into the consumer's build tree (${CMAKE_BINARY_DIR}/include/QtMcpServer/).

Skip examples/tests with the Qt-prefixed vars (not BUILD_EXAMPLES): -DQT_BUILD_EXAMPLES=OFF -DQT_BUILD_TESTS=OFF — verified to suppress the examples/ build dir.

Caveats of the FetchContent approach:

• qtmcp builds shared libs (libQt6McpServer.so.6.10.2) into the consumer's build tree. The bridge exe needs RPATH or LD_LIBRARY_PATH pointing at ${CMAKE_BINARY_DIR}/lib/x86_64-linux-gnu/ at runtime. For a distributable bridge, either install qtmcp or switch to static (-DBUILD_SHARED_LIBS=OFF, verify).
• MCP backend plugins (stdio, sse) are built as Qt plugins under ${CMAKE_BINARY_DIR}/lib/x86_64-linux-gnu/qt6/plugins/mcpserverbackend/. The bridge process must find them via the Qt plugin path / QT_PLUGIN_PATH at runtime, otherwise QMcpServer("stdio") can't load the stdio backend.
• qtmcp version at time of verification: 6.10.2 (alpha1 prerelease, from .cmake.conf). Pin GIT_TAG to a commit/tag for reproducibility rather than tracking main.

qtmcp requirements

• Qt 6.8+ (check your Qt version; GammaRay needs 6.5+, qtmcp needs 6.8+)
• C++20 (GammaRay is C++17; bridge project compiled as C++20 can link C++17 libs fine)
• Uses Qt BuildInternals CMake style (qt_build_repo())

CMakeLists.txt skeleton

cmake_minimum_required(VERSION 3.16)
project(QmlSceneGraphMcpBridge LANGUAGES CXX)
set(CMAKE_CXX_STANDARD 20)              # qtmcp requires C++20
set(CMAKE_AUTOMOC ON)

# GammaRay installed from Step 1; pass -DCMAKE_PREFIX_PATH=$(pwd)/install-prefix
find_package(GammaRay REQUIRED)
find_package(Qt6 COMPONENTS Core Gui Widgets Network REQUIRED)

# qtmcp via FetchContent (no separate install)
include(FetchContent)
FetchContent_Declare(
  qtmcp
  GIT_REPOSITORY https://github.com/signal-slot/qtmcp.git
  GIT_TAG main        # pin to a tag/commit for reproducibility
  GIT_SHALLOW TRUE
)
set(QT_BUILD_EXAMPLES OFF CACHE BOOL "" FORCE)
set(QT_BUILD_TESTS OFF CACHE BOOL "" FORCE)
FetchContent_MakeAvailable(qtmcp)

add_executable(qml-sg-mcp-bridge
    src/main.cpp
    src/gammaray_session.cpp           # wraps ClientConnectionManager + ObjectBroker
    src/gammaray_session.h
    src/scenegraph_tools.cpp            # Q_INVOKABLE methods exposed as MCP tools
    src/scenegraph_tools.h
)

target_link_libraries(qml-sg-mcp-bridge PRIVATE
    gammaray_client                    # VERIFIED: no GammaRay:: namespace
    gammaray_common
    Qt6::Core
    Qt6::Gui
    Qt6::Widgets                       # REQUIRED: see QApplication note below
    Qt6::Network
    Qt6::McpServer                     # qtmcp, provides Qt::McpServer alias
    Qt6::McpCommon
)

# qtmcp shared libs + stdio plugin live in build tree; make runtime find them:
set_target_properties(qml-sg-mcp-bridge PROPERTIES
  BUILD_RPATH "$<TARGET_FILE_DIR:Qt6::McpServer>;$<TARGET_FILE_DIR:Qt6::McpCommon>"
)

Build

cmake -S . -B build -DCMAKE_PREFIX_PATH=$(pwd)/install-prefix -G Ninja
cmake --build build
# Run with probe libs + qtmcp runtime libs on the path:
LD_LIBRARY_PATH=$(pwd)/install-prefix/lib QT_QPA_PLATFORM=offscreen \
  QT_PLUGIN_PATH=$PWD/build/lib/x86_64-linux-gnu/qt6/plugins \
  ./build/qml-sg-mcp-bridge --connect tcp://127.0.0.1:11732

❗ MUST use QApplication, not QCoreApplication

VERIFIED the hard way: ObjectBroker::model() on the client side goes through ClientConnectionManager's model factory (client/clientconnectionmanager.cpp:44) which constructs a RemoteModel. RemoteModel's constructor (client/remotemodel.cpp:97) calls QApplication::style()->sizeFromContents(...). With a plain QCoreApplication there is no style → segfault the moment ready() fires and the first model is requested.

Fix: use QApplication (which pulls in Qt6::Widgets). This is consistent with client/main.cpp:38 (the standalone GammaRay client uses QApplication), and gammaray_client.so already runtime-links libQt6Widgets.so.6. Run the bridge headless with QT_QPA_PLATFORM=offscreen.

MCP server skeleton (using qtmcp)

#include <QApplication>               // NOT QCoreApplication — see note above
#include <QtMcpServer/QMcpServer>
#include "scenegraph_tools.h"

int main(int argc, char **argv) {
    QApplication app(argc, argv);     // RemoteModel needs QApplication::style()
    // ... parse --connect <url> ...

    QMcpServer server(QStringLiteral("stdio"));   // backend name in ctor, no default
    server.setInstructions("QML SceneGraph introspection via GammaRay");

    auto *tools = new SceneGraphTools(&server);  // holds GammaRay client connection
    server.registerToolSet(tools, {
        {"listScenegraphNodes", "List all QSGNode tree nodes"},
        {"getNodeVertices", "Get vertex data of a SG node"},
        {"getShaderSource", "Get shader source for a material"},
        {"setRenderMode", "Set SceneGraph render visualization mode"},
        // ...
    });

    QObject::connect(&server, &QMcpServer::finished, &app, &QCoreApplication::quit);

    // Connect to GammaRay probe, then start MCP server
    tools->connectToProbe(url);  // async, uses ClientConnectionManager
    server.start();

    return app.exec();
}

Tool implementation as Q_INVOKABLE methods (auto-registered via registerToolSet):

class SceneGraphTools : public QObject {
    Q_OBJECT
public:
    // NOTE: qtmcp's callTool() only supports void/bool/QString/QStringList/QImage
    // returns (and QFuture<QList<QMcpCallToolResultContent>> for async). A
    // QJsonObject return would hit qFatal() and crash. So structured results
    // are returned as JSON-serialized QString:
    Q_INVOKABLE QString listScenegraphNodes() const;        // returns compact JSON
    Q_INVOKABLE QString listQuickWindows() const;

    // Async tool returns QFuture<QList<QMcpCallToolResultContent>> (the ONLY
    // async return type qtmcp recognizes). NOT QFuture<QString>.
    Q_INVOKABLE QFuture<QList<QMcpCallToolResultContent>> getShaderSource(int row) const;

    // QImage return supported by qtmcp (for texture/screenshot)
    Q_INVOKABLE QImage grabTexture(const QString &nodeId) const;

private:
    GammaRaySession *m_session = nullptr;
};

VERIFIED against src/mcpserver/qmcpserversession.cpp (callTool() switch on mm.returnMetaType().id()): only Void/Bool/QString/QStringList/QImage (sync) + QFuture<QList<QMcpCallToolResultContent>> (async) are handled; the default: branch calls qFatal(). The plan's earlier QJsonObject / QFuture<QString> sketches were wrong and would have crashed.

qtmcp target names — VERIFIED

FetchContent produces targets Qt6::McpServer, Qt6::McpCommon, Qt6::McpClient (with Qt:: alias). Headers generated at ${CMAKE_BINARY_DIR}/include/QtMcpServer/ etc. The include #include <QtMcpServer/QMcpServer> works once Qt6::McpServer is linked. Note QMcpServer ctor takes the backend name explicitly: QMcpServer server(QStringLiteral("stdio")); (no default ctor).

Step 3: Client-side connection flow

Reference: client/clientconnectionmanager.h:43, client/client.h:27, common/objectbroker.h:28.

#include <QApplication>               // QApplication, not QCoreApplication
#include <client/clientconnectionmanager.h>
#include <common/objectbroker.h>

// One-time init (registers stream operators, factory callbacks)
ClientConnectionManager::init();

QApplication app(argc, argv);         // needed for RemoteModel's QApplication::style()
auto *conMan = new ClientConnectionManager(&app, false /*no splash*/);
QObject::connect(conMan, &ClientConnectionManager::ready, [&]() {
    // Connection established, object map received, tool model populated.
    // NOW safe to call ObjectBroker::model(...) etc.
    auto *sgModel = ObjectBroker::model(
        QStringLiteral("com.kdab.GammaRay.QuickSceneGraphModel"));
    // ...
});
QObject::connect(conMan, &ClientConnectionManager::disconnected,
                 &app, &QCoreApplication::quit);

conMan->connectToHost(QUrl("tcp://127.0.0.1:11732"));

ClientConnectionManager::ready() is emitted after protocol handshake + object map sync — models are only available after this. Do NOT access models before ready(). VERIFIED: against a running 3.4.0 probe (qml target), ready() fires and ObjectBroker::model("com.kdab.GammaRay.QuickSceneGraphModel") returns a non-null RemoteModel* — full handshake works with matching protocol version 38.

Step 4: Available data and model registration names

All names verified from plugins/quickinspector/quickinspector.cpp and extension .cpp files. These are the strings to pass to ObjectBroker::model(QString).

Models (tree/tabular data via RemoteModel)

Model name	Source	Description
com.kdab.GammaRay.QuickWindowModel	quickinspector.cpp:347	List of QQuickWindows
com.kdab.GammaRay.QuickItemModel	quickinspector.cpp:352	QQuickItem tree
com.kdab.GammaRay.QuickSceneGraphModel	quickinspector.cpp:371	QSGNode tree (primary target)
com.kdab.GammaRay.QuickSceneGraph.sgGeometryVertexModel	sggeometryextension.cpp:30	Vertex data of selected SG node
com.kdab.GammaRay.QuickSceneGraph.sgGeometryAdjacencyModel	sggeometryextension.cpp:31	Adjacency/draw mode of selected SG node
com.kdab.GammaRay.QuickSceneGraph.shaderModel	materialextension.cpp:39	Shader list for selected material
com.kdab.GammaRay.QuickSceneGraph.materialPropertyModel	materialextension.cpp:38	Material properties
com.kdab.GammaRay.QuickItem.<...>	property controller extensions	Item properties (suffix varies)

Model name pattern for property-controller models: <objectBaseName>.<nameSuffix> where objectBaseName is com.kdab.GammaRay.QuickSceneGraph or com.kdab.GammaRay.QuickItem (quickinspector.cpp:327-329). The suffix is registered via PropertyController::registerModel(model, nameSuffix) (core/propertycontroller.cpp:62-64).

Remote objects (interfaces via ObjectBroker::object())

Interface	Object name pattern	Key methods
QuickInspectorInterface	com.kdab.GammaRay.QuickInspectorInterface (IID-based, use ObjectBroker::object<QuickInspectorInterface*>())	selectWindow(int), setCustomRenderMode(RenderMode), setSlowMode(bool), analyzePainting()
MaterialExtensionInterface	<baseName>.material (e.g. com.kdab.GammaRay.QuickSceneGraph.material)	getShader(int row) → emits gotShader(QString)

Client-side factory registration

The GUI client registers factories via ObjectBroker::registerClientObjectFactoryCallback<T>() (quickinspectorwidget.cpp:82, 268). The bridge must do the same for interfaces it wants to use:

ObjectBroker::registerClientObjectFactoryCallback<QuickInspectorInterface*>(
    [](const QString &, QObject *parent) -> QObject* {
        return new QuickInspectorClient(parent);  // or own impl
    });

Check whether QuickInspectorClient (quickinspectorclient.h) is exported/installed. If not, may need a thin reimplementation calling Endpoint::invokeObject() directly. Verify after install by checking installed headers in $INSTALL_PREFIX/include/gammaray/.

RESOLVED (open question #1): QuickInspectorClient, QuickInspectorInterface, MaterialExtensionInterface and all other plugins/quickinspector/* headers are NOT in install-prefix/include/gammaray/. Only client/ (3 headers: clientconnectionmanager.h, processtracker.h, gammaray_client_export.h), common/, core/, launcher/, ui/ are installed. The plugin client-side classes live only in the GammaRay source tree (plugins/quickinspector/) and are compiled into the per-tool gammaray_quickinspector client plugin .so that the GUI client loads dynamically. The bridge does NOT load these plugins. Good news (verified): Endpoint::invokeObject(name, method, args) is PUBLIC in the installed endpoint.h, and it only needs the name→address mapping (synced at handshake) — NOT a local stub object. So the bridge can call probe-side interface methods directly by IID + bare slot name, with zero stub code. See the "Key implementation findings" in the Verification status section for the exact name (com.kdab.GammaRay.QuickInspectorInterface/1.0) and method ("selectWindow") conventions. Only for receiving signals (e.g. MaterialExtensionInterface::gotShader) would a real client stub be needed — handled case-by-case when implementing those tools.

Step 5: SceneGraph model roles and structure

QuickSceneGraphModel (plugins/quickinspector/quickscenegraphmodel.h:34) inherits ObjectModelBase<QAbstractItemModel> (core/objectmodelbase.h:40).

Columns

• Column 0: node address string (Util::addressToString(node))
• Column 1: node type string ("Node", "Geometry Node", "Transform Node", "Clip Node", "Opacity Node", "Root Node", "Render Node")

Roles (from ObjectModelBase + QuickSceneGraphModel)

• Qt::DisplayRole — address / type (see columns above)
• ObjectModel::ObjectRole — QVariant::fromValue(node) (QSGNode* as void*)
• ObjectModel::ObjectIdRole — ObjectId wrapper
• ObjectModel::DecorationIdRole — icon id
• ObjectModel::CreationLocationRole / DeclarationLocationRole — source locations

ObjectModel roles are defined in common/objectmodel.h (read it for exact enum values).

Tree structure

• Root: the topmost QSGNode (walks up from contentItem's itemNode to find true root, quickscenegraphmodel.cpp:79-90)
• Children: QSGNode::firstChild() / nextSibling()
• Updates: refreshed on QQuickWindow::afterRendering signal
• Node deletion: QuickSceneGraphModel::nodeDeleted(QSGNode*) signal

Caveat: QSGNode* values in the model are raw pointers in the probe's process. They cannot be dereferenced from the client. Use them only as opaque IDs (via ObjectId) for selection and model navigation. Actual geometry/material/texture data must be fetched through the corresponding models/extensions.

Step 6: Suggested MCP tools

Map GammaRay client capabilities to MCP tool calls.

Connection management tools (NEW — implemented)

• connectProbe(host?, port?) → GammaRaySession::connectToHost(tcp://host:port), block up to 6s for handshake, return {connected, state, url, error?}. Defaults 127.0.0.1:11732. URL remembered for auto-reconnect.
• connectProbeDefault() → convenience wrapper for connectProbe(127.0.0.1, 11732) (qtmcp doesn't support arg-overloading, so two tool entries).
• probeStatus() → return {state, ready, url, error?}. No side effects.
• disconnectProbe() → drop connection, clear remembered URL (stops auto-reconnect).

All introspection tools below call session->ensureConnected(timeoutMs) first; if a URL is known and the session is Disconnected/Failed, they transparently reconnect. If no URL is known, they return a JSON error pointing the client at connectProbe().

Navigation tools

• list_quick_windows → read com.kdab.GammaRay.QuickWindowModel, return window addresses
• select_quick_window(index) → QuickInspectorInterface::selectWindow(int)
• list_quick_items → traverse com.kdab.GammaRay.QuickItemModel, return item tree (id, type, geometry)
• list_scenegraph_nodes(parentObjectId?) → traverse com.kdab.GammaRay.QuickSceneGraphModel, return node tree (id, type, address)
• get_scenegraph_node_detail(objectId) → select node via selection model, read property models

Geometry tools

• get_node_vertices(objectId) → select SG node, read sgGeometryVertexModel (vertices: x, y, z, w + render role)
• get_node_adjacency(objectId) → read sgGeometryAdjacencyModel (drawing mode, vertex indices)

Material/Shader tools

• get_material_shaders(objectId) → read shaderModel for selected node
• get_shader_source(row) → MaterialExtensionInterface::getShader(int) (async: listen for gotShader signal)
• get_material_properties(objectId) → read materialPropertyModel

Rendering visualization tools

• set_render_mode(mode) → QuickInspectorInterface::setCustomRenderMode(mode) where mode ∈ {Normal, VisualizeClipping, VisualizeOverdraw, VisualizeBatches, VisualizeChanges, VisualizeTraces} (quickinspectorinterface.h:52-66)
• set_slow_mode(bool) → QuickInspectorInterface::setSlowMode(bool)

Texture tools

• grab_texture(objectId) → TextureExtension (async image grab; image arrives via RemoteViewInterface as TransferImage)

Implementation status

• ✅ list_quick_windows (as listQuickWindows), select_quick_window (as selectQuickWindow), list_scenegraph_nodes (as listScenegraphNodes) — working end-to-end against a live probe.
• ✅ Connection management (connectProbe/connectProbeDefault/disconnectProbe/probeStatus) — implemented; bridge no longer hard-requires --connect.
• ⏳ Geometry/Material/Shader/Render/Texture tools — pending. They require (a) selecting an SG node via the QuickSceneGraphModel selection model so the per-node sgGeometryVertexModel/sgGeometryAdjacencyModel/shaderModel/materialPropertyModel populate, and (b) for shader/texture the bridge needs to receive async signals from MaterialExtensionInterface/TextureExtension (which are NOT in installed headers — will need either a thin custom client stub connected via Endpoint::registerObject on the client side, or a signal-forwarding shim built on Endpoint::invokeObject + a probe-side callback. To be designed.)

Step 7: Launch and run

Connection model (decided)

Phase 1 (now): lazy connect + explicit connectProbe tool + auto-reconnect. The bridge does NOT hard-require a probe URL at startup. A --connect <url> (or GAMMARAY_PROBE_URL env) is treated as a default URL — the bridge tries it best-effort on startup but won't fail if the probe isn't up yet. The MCP client drives connection lifecycle via three tools:

• connectProbe(host?, port?) — establish a connection to a probe (defaults 127.0.0.1:11732). Blocks up to ~6s for the handshake. Returns JSON {connected, state, url, error?}. The URL is remembered for subsequent auto-reconnect.
• probeStatus() — report {state, ready, url, error?} without side effects.
• disconnectProbe() — drop the current connection and forget the URL (auto-reconnect stops until connectProbe is called again).

Every introspection tool (listQuickWindows, selectQuickWindow, listScenegraphNodes, …) calls session->ensureConnected(timeoutMs) first: if a URL is known and the session is Disconnected/Failed, it transparently re-issues connectToHost(url) and waits for ready(). If no URL is known, the tool returns a JSON error pointing the user at connectProbe(). So in practice a client only needs to call connectProbe once after starting the probe; subsequent tool calls keep working across probe restarts.

Rationale: opencode (and other MCP hosts) launch the bridge subprocess at host startup, typically BEFORE the developer has started the target app + probe. A hard --connect either succeeds by luck or leaves the bridge in a permanently-broken state (the original scaffold's failure mode). The lazy-connect model lets the bridge come up clean and wait for the client to point it at a probe — and recover automatically when the probe is restarted mid-session (common during iterative QML debugging).

Why not auto-discovery (mDNS)? GammaRay's probe does broadcast via mDNS, which would let the bridge find probes with zero configuration. But (a) multiple probes on a dev machine are common, and silently picking one is wrong; (b) mDNS on Linux is flaky across sandboxes/containers; (c) the explicit connectProbe is one cheap MCP call and gives the client full control. mDNS can be added later as an optional discoverProbes() tool if needed.

Auto-reconnect details: ClientConnectionManager already retries transientConnectionError (host not yet up) every 1s for 60s before emitting persistentConnectionError — so the INITIAL handshake has built-in resilience. But a disconnected signal (probe killed / target crashed MID-session) is NOT auto-retried by the manager. The bridge adds its own 1s-backoff reconnect on disconnected (guarded against re-entrancy and disabled after explicit disconnectProbe()). On persistentConnectionError the bridge does NOT auto-retry (GammaRay already did for 60s); the user calls connectProbe() again once the probe is back. State machine: Disconnected → Connecting → Ready, with → Failed on persistent error (recoverable only via connectProbe()).

Phase 2 (future, not in scope now): hybrid mode. Bridge could also expose attach(pid) / launch(path) MCP tools for LLM-driven dynamic switching using launcher/core/launcher.h and launcher/core/clientlauncher.h. Out of scope for now.

Phase 1 startup sequence

# 1. Developer starts target app with probe (manual)
install-prefix/bin/gammaray --inject-only --listen tcp://127.0.0.1:11732 \
  /usr/lib/qt6/bin/qml /path/to/app.qml -platform offscreen

# 2. MCP host launches bridge (stdio). --connect is OPTIONAL: if given, it's
#    used as a default URL and tried best-effort at startup; if omitted, the
#    client must call connectProbe() before any introspection tool returns data.
#    Either way, tools auto-reconnect across probe restarts.
./run.sh   # no --connect required
#   or: ./run.sh --connect tcp://127.0.0.1:11732

# 3. (MCP client) call connectProbe(host, port) once the probe is up — or just
#    call listQuickWindows/etc. and the bridge will return a JSON error hinting
#    at connectProbe if no URL is configured.

For headless/CI launching of the probe, systemd-run --user --no-block (with StandardOutput=file:... + Environment=... properties) is a reliable way to fully detach the probe so the launching shell returns immediately — plain &/disown backgrounding tends to hang the parent shell's stdout pipe.

Bridge accepts an optional default probe URL via --connect <url> (or GAMMARAY_PROBE_URL env). At runtime the connection is driven by connectProbe/disconnectProbe MCP tools; the URL is remembered so tools can auto-reconnect after a probe restart. The bridge's GammaRaySession state machine (Disconnected → Connecting → Ready, with → Failed on persistent error) is the single source of truth; probeStatus() reports it to the client.

The bridge acts as an MCP server on stdio. The LLM host (e.g. Claude Desktop, opencode) launches the bridge as a subprocess and communicates via JSON-RPC over stdio.

Key gotchas to remember

1. Protocol version lock: probe and client must be same GammaRay build. System package too old → build from 3.4.0 source. (common/protocol.h:141)
2. GPL-2.0-or-later: bridge linking gammaray_client must be GPL-compatible. Accepted. qtmcp offers GPL-2.0-only option — compatible.
3. Qt private headers: GammaRay build needs qt6-base-private-dev, qt6-declarative-private-dev. The bridge itself only needs public Qt + installed GammaRay headers. qtmcp requires Qt 6.8+ (newer than GammaRay's 6.5+ floor); ensure build environment Qt >= 6.8.
4. C++20: qtmcp requires C++20. Bridge project set to C++20; GammaRay C++17 libs link fine.
5. Models only available after ClientConnectionManager::ready() — do not call ObjectBroker::model() before handshake completes. MCP tool calls arriving before probe connection should return an error or wait.
6. QSGNode pointers are opaque IDs on client side — cannot dereference. Use ObjectId for selection/navigation; fetch real data via property/extension models.
7. Async interfaces: MaterialExtensionInterface::getShader() returns via signal gotShader(QString), not synchronously. Texture grabs return via RemoteViewInterface. Bridge must handle async and correlate to MCP requests. qtmcp supports QFuture<Result> return types for async tool handlers.
8. Target namespace: exported CMake targets may lack GammaRay:: prefix (NAMESPACE commented out at CMakeLists.txt:931). Verify in installed GammaRayTarget.cmake. Same applies to qtmcp installed targets — verify after building qtmcp.
9. OpenGL dependency: SG geometry/material/texture extensions only build when QT_NO_OPENGL is not set (quickinspector/CMakeLists.txt:47-66). If headless/no-GL build, these models won't exist. Ensure GammaRay is built WITH OpenGL (default).
10. In-process mode exists (gammaray --inprocess): probe + client in same process via gammaray_inprocessui module. Not useful for bridge (bridge IS the client). Use TCP/local socket mode.
11. Probe ABI: probes are versioned by Qt ABI. GAMMARAY_MULTI_BUILD=ON (default) builds multiple. The launcher picks the right one. Bridge doesn't need to worry about probe ABI, only client ABI matching the installed gammaray_client.
12. QApplication required (NEW, verified): the client-side RemoteModel ctor calls QApplication::style()->sizeFromContents(). A QCoreApplication-only bridge segfaults on the first ObjectBroker::model() call after ready(). Use QApplication + QT_QPA_PLATFORM=offscreen for the headless bridge.
13. qtmcp shared libs + plugin path (NEW, verified): under FetchContent, libQt6Mcp*.so and the mcpserverbackend/libqmcpserverstdio.so plugin land in the consumer's build tree. Set LD_LIBRARY_PATH and QT_PLUGIN_PATH (or RPATH) so the bridge finds them at runtime, else QMcpServer("stdio") can't load the stdio backend.

Files worth re-reading in GammaRay source

When in doubt, these files have the ground truth:

• plugins/quickinspector/quickinspector.cpp:322-402 — model + object registration names
• plugins/quickinspector/quickscenegraphmodel.{h,cpp} — SG node tree model
• plugins/quickinspector/quickinspectorinterface.h:38-66 — RenderMode enum, Feature flags
• plugins/quickinspector/materialextension/materialextensioninterface.h — shader API
• plugins/quickinspector/geometryextension/sggeometrymodel.h — vertex/adjacency models
• common/objectbroker.h — model/object retrieval API
• common/objectmodel.h — ObjectRole enum values
• client/clientconnectionmanager.h:43 — connection lifecycle
• client/client.h:27 — low-level client endpoint
• client/remotemodel.h — client-side model wrapper
• core/objectmodelbase.h:40 — base model roles/columns
• GammaRayConfig.cmake.in — installed CMake package contents

Open questions to resolve during implementation

1. Are QuickInspectorClient and MaterialExtensionClient classes exported in installed headers? RESOLVED: No. See Step 4 note above — bridge needs own thin client stubs calling Endpoint::invokeObject().
2. Exact ObjectModel role enum values — read common/objectmodel.h when implementing. (Installed at install-prefix/include/gammaray/common/objectmodel.h.)
3. How does RemoteModel (client/remotemodel.h) expose data? It's a QAbstractItemModel proxying the server model. Standard model API applies. VERIFIED: ObjectBroker::model() returns a non-null QAbstractItemModel* that is a RemoteModel — standard QAbstractItemModel API works.
4. Texture image transfer: TransferImage (common/transferimage.h) over the wire, arrives via RemoteViewInterface. Need to trace the exact signal path for texture grabs. Still open.

Verification status (as of this revision)

• ✅ Step 1 done: GammaRay 3.4.0 built + installed to install-prefix/. Protocol version 38.
• ✅ Minimal client built & run: minimal-client/ links gammaray_client + gammaray_common, connects to a live probe, receives ready(), fetches QuickSceneGraphModel / QuickWindowModel via ObjectBroker — clean exit. Proves the client ABI + protocol path is viable for the bridge.
• ✅ qtmcp FetchContent viable: Qt6::McpServer/Qt6::McpCommon build & link from a FetchContent_MakeAvailable() call. No separate clone/install.
• ✅ Bridge scaffold built & working end-to-end (bridge/): qml-sg-mcp-bridge links GammaRay client + qtmcp, runs as a stdio MCP server, and against a live probe returns REAL data:
  • initialize → serverInfo + capabilities + protocolVersion 2024-11-05 ✅
  • tools/list → connectProbe, connectProbeDefault, disconnectProbe, probeStatus, listQuickWindows, selectQuickWindow, listScenegraphNodes ✅
  • connectProbe(host, port) → connects + waits for handshake, returns {connected, state, url, error?} ✅
  • probeStatus() → {state, ready, url, error?} (verified: returns disconnected with empty url when no probe configured; returns ready after connectProbe) ✅
  • listQuickWindows → [{"address":"0x... (QQuickWindowQmlImpl)"}] ✅
  • selectQuickWindow(index) → forwards via Endpoint::invokeObject("com.kdab.GammaRay.QuickInspectorInterface/1.0", "selectWindow", {index}) ✅
  • listScenegraphNodes → nested QSGNode tree (Root Node → Transform Node → ...) ✅
• ✅ Connection model revised: bridge no longer hard-requires --connect at startup. Lazy connect + connectProbe MCP tool + auto-reconnect on dropped session. See "Connection model (decided)" in Step 7. State machine (Disconnected → Connecting → Ready, with → Failed on persistent error) exposed via probeStatus(). Verified via standalone stdio test: with no --connect, probeStatus returns disconnected/empty URL; listQuickWindows returns a JSON error hinting at connectProbe; after connectProbe(127.0.0.1, 11732) against a live probe, tools return real data.

Key implementation findings (new, verified during scaffold work)

• Endpoint::invokeObject() needs the object name == interface IID, NOT the bare name. QuickInspectorInterface is registered via ObjectBroker::registerObject<QuickInspectorInterface*>(this) (quickinspectorinterface.cpp:59), which derives the name from qobject_interface_iid = "com.kdab.GammaRay.QuickInspectorInterface/1.0". The bare "com.kdab.GammaRay.QuickInspectorInterface" is NOT in the client's name map → Q_ASSERT(obj) crash in debug builds. Also the method arg is the bare slot name "selectWindow" (no (int) signature), matching QuickInspectorClient::selectWindow.
• No client-side stub object needed for invokeObject: it only uses the name→address mapping (synced at handshake ready()), not a local object. So selectQuickWindow works without registering any factory/stub — Endpoint::invokeObject is public in the installed endpoint.h. (This resolves open question #1's "approach (b)" more cleanly than expected.)
• RemoteModel is lazily fetched (client/remotemodel.cpp): data(), rowCount(), hasChildren() return placeholders ("Loading..." / 0 / false) and queue server fetches whose results arrive async via dataChanged/rowsInserted. So a tool that reads a model right after ready() gets stale/empty data. The scaffold handles this with: waitForReady() (block until handshake), waitForData() (poll column 0 until non-"Loading..."), and for the SG tree a primeTree() + settle() round-robin (walk touching every cell + rowCount to queue fetches, run the event loop ~600ms, repeat 4x) before snapshotting with walkChildren().
• QApplication required (already noted): the headless bridge runs QApplication + QT_QPA_PLATFORM=offscreen.
• FetchContent + qtmcp CMake scope gotcha: a top-level find_package(Qt6)/find_package(GammaRay) BEFORE FetchContent_MakeAvailable(qtmcp) creates Threads::Threads at top scope; qtmcp's subdirectory then fails to promote it to global ("not built in this directory"). Fix: run FetchContent_MakeAvailable(qtmcp) FIRST (so qtmcp's own find_package(Qt6) creates+promotes 3rd-party targets inside its subdirectory), THEN find_package(GammaRay).
• qtmcp tool return types: only void/bool/QString/QStringList/QImage (sync) + QFuture<QList<QMcpCallToolResultContent>> (async). QJsonObject hits qFatal. Structured results → JSON-serialized QString (see Step 2 tool section).

Running the bridge

cd bridge
cmake -S . -B build -G Ninja -DCMAKE_PREFIX_PATH=$(pwd)/../install-prefix
cmake --build build
# Probe (manual, Phase 1): start a Qt/QML app with the GammaRay probe
../install-prefix/bin/gammaray --inject-only --listen tcp://127.0.0.1:11732 \
  --injector preload /usr/lib/qt6/bin/qml /path/to/app.qml
# Bridge (stdio MCP server) — --connect is OPTIONAL now:
./run.sh
#   or: ./run.sh --connect tcp://127.0.0.1:11732
# Then (MCP client) call connectProbe(127.0.0.1, 11732) once the probe is up.

run.sh sets LD_LIBRARY_PATH (GammaRay + qtmcp libs) and QT_PLUGIN_PATH (qtmcp mcpserverbackend/libqmcpserverstdio.so) and QT_QPA_PLATFORM=offscreen.

• ⏳ Next: implement the remaining Step 6 tools (get_node_vertices/get_node_adjacency → sgGeometryVertexModel/sgGeometryAdjacencyModel, get_material_shaders/get_shader_source → shaderModel + MaterialExtensionInterface::getShader async, get_material_properties, set_render_mode/set_slow_mode via QuickInspectorInterface invokeObject, grab_texture). The geometry/material models require selecting a SG node first (via a selection model on QuickSceneGraphModel), which is the next pattern to wire up.