Fixing Japanese Input (Fcitx5/Mozc) in GPT4All on Ubuntu 24.04
2026-02-21
Translations: ja

I have been using Ollama for my local LLM needs, but I read about GPT4All by Nomic and wanted to give it a try. The thing is, if you're like me and write in Japanese and if you use fcitx5 with Mozc for Japanese input on Ubuntu, you'll find that Ctrl+Space does nothing — you simply can't toggle your input method. So I looked it up and here's how I fixed it.

The Problem

GPT4All is a Qt based application and bundles its own Qt 6.8 libraries, which don't include the fcitx5 input method plugin. I tried copying over my system's fcitx5 plugin which is built against Ubuntu's Qt (6.4 on Ubuntu 24.04), but it fails with symbol mismatches. (duuuh) Setting QT_IM_MODULE=fcitx alone doesn't help because there's no compatible plugin to load.

The Solution

Build the fcitx5 Qt input context plugin from source against a matching Qt 6.8 SDK, then drop it into GPT4All's plugin directory.

Environment

  • Ubuntu 24.04 LTS
  • fcitx5 + Mozc
  • GPT4All (standalone installer here)

Step-by-Step

1. Install build dependencies

sudo apt install cmake extra-cmake-modules \
  libfcitx5core-dev libfcitx5config-dev libfcitx5utils-dev \
  libxkbcommon-dev libwayland-dev wayland-protocols libvulkan-dev

2. Install a matching Qt 6.8 SDK

GPT4All bundles Qt 6.8 but only ships runtime libraries, not development files. We use aqtinstall to grab the SDK:

python3 -m venv /tmp/aqt-env
/tmp/aqt-env/bin/pip install aqtinstall
/tmp/aqt-env/bin/aqt install-qt linux desktop 6.8.0 -O /tmp/qt6.8

3. Clone and build fcitx5-qt

cd /tmp
git clone https://github.com/fcitx/fcitx5-qt.git
cd fcitx5-qt
mkdir build && cd build

cmake .. \
  -DCMAKE_PREFIX_PATH=/tmp/qt6.8/6.8.0/gcc_64/lib/cmake \
  -DENABLE_QT4=OFF \
  -DENABLE_QT5=OFF \
  -DENABLE_QT6=ON \
  -DENABLE_QT6_WAYLAND_WORKAROUND=OFF \
  -DENABLE_QT6_GUI_WRAPPER=OFF

make -j$(nproc) -C qt6/platforminputcontext

We only build the platforminputcontext target — the full build will fail because it tries to compile GUI wrapper components that need headers we don't have, and we don't need them anyway.

4. Copy the plugin into GPT4All

cp /tmp/fcitx5-qt/build/qt6/platforminputcontext/libfcitx5platforminputcontextplugin.so \
  ~/gpt4all/plugins/platforminputcontexts/

Adjust the destination path if you installed GPT4All elsewhere.

5. Launch GPT4All

QT_IM_MODULE=fcitx ~/gpt4all/bin/chat

Ctrl+Space should now toggle fcitx5/Mozc and you can type Japanese.

6. Make it permanent

If you use the desktop shortcut, edit ~/.local/share/applications/GPT4All.desktop and update the Exec line:

Exec=env QT_IM_MODULE=fcitx "/home/YOUR_USERNAME/gpt4all/bin/chat"

Verifying the plugin loads

If something goes wrong, check with debug output:

QT_DEBUG_PLUGINS=1 QT_IM_MODULE=fcitx ~/gpt4all/bin/chat 2>&1 | grep -i fcitx

You should see loaded library in the output. If you see undefined symbol errors instead, it likely means the Qt version of the SDK you built against doesn't match what GPT4All bundles — check with strings ~/gpt4all/lib/libQt6Core.so.6 | grep "^6\." and install the matching version via aqt.

Cleanup

After confirming everything works, you can safely remove the build artifacts:

rm -rf /tmp/qt6.8 /tmp/fcitx5-qt

The -dev packages can also be removed if you don't need them:

sudo apt remove extra-cmake-modules libfcitx5core-dev libfcitx5config-dev libfcitx5utils-dev

Notes

  • This approach applies to any Qt app that bundles its own Qt and lacks the fcitx5 plugin — the same technique should work.
  • If GPT4All updates and ships a different Qt version, you may need to rebuild the plugin against the new version.
  • This was tested on Ubuntu 24.04 with kernel 6.17, GPT4All bundling Qt 6.8, and fcitx5 5.1.7 with Mozc.

Directory: 2026 Tagged: japanese ubuntu ai tool

Translations: ja
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