B4X Skill for Claude Code

A focused Agent Skill that helps Claude write, review, and explain idiomatic B4X code for B4A, B4J, and B4i.

It gives Claude practical guidance for B4XPages, XUI, SQLite, JSON, resumable subs, platform-specific APIs, custom views, and .b4xlib libraries. Its main goal is to prevent common AI-generated mistakes such as mixing B4X with VB.NET or Java, using the wrong module scope, and combining incompatible B4A, B4J, and B4i APIs.

Note

B4R (Arduino and ESP boards) is intentionally outside the scope of this skill because it uses a different runtime, type system, and development model.

What this skill improves

• Correct B4X syntax, module types, and variable scope
• Modern B4XPages architecture for new UI projects
• Cross-platform UI code with XUI and B4XView
• Clear separation of B4A-, B4J-, and B4i-specific APIs
• Safe SQLite queries, file handling, and JSON processing
• Correct Sleep, Wait For, resumable-sub, and HttpJob patterns
• Practical checks for initialization, resource cleanup, and event signatures
• Review and modernization of older B4X code

Supported platforms

Platform	Support	Main topics
B4A	Yes	Android, permissions, manifest, JavaObject
B4J	Yes	Desktop UI, JavaFX boundaries, headless servers, jServer
B4i	Yes	iOS UI, build and signing, ATS, NativeObject
Shared B4X	Yes	XUI, B4XPages, SQLite, JSON, I/O, networking
B4R	No	Intentionally out of scope

Installation

Claude Code marketplace

Run these commands inside Claude Code:

/plugin marketplace add Jerryk133/b4x-skill
/plugin install b4x@b4x-marketplace
/reload-plugins

The plugin is then available across the installation scope you selected. Claude can activate the skill automatically when your request involves B4X.

Test locally without installing

Clone the repository and start Claude Code with the plugin directory:

git clone https://github.com/Jerryk133/b4x-skill.git
claude --plugin-dir ./b4x-skill

This is useful when evaluating or developing the skill.

Install as a standalone Claude Code skill

Copy the skills/b4x directory to one of these locations:

Scope	Destination
Personal — all projects	~/.claude/skills/b4x/
Project — current repository	.claude/skills/b4x/

PowerShell:

git clone https://github.com/Jerryk133/b4x-skill.git
New-Item -ItemType Directory -Force "$HOME\.claude\skills" | Out-Null
Copy-Item -Recurse -Force ".\b4x-skill\skills\b4x" "$HOME\.claude\skills\b4x"

Bash:

git clone https://github.com/Jerryk133/b4x-skill.git
mkdir -p ~/.claude/skills
cp -R ./b4x-skill/skills/b4x ~/.claude/skills/b4x

Start a new Claude Code session after copying the skill if it is not detected in the current session.

Usage

The installed plugin can be invoked explicitly as /b4x:b4x:

/b4x:b4x Review this B4A B4XPages code and fix any compile errors.

When installed as a standalone skill, invoke it as /b4x.

In normal use, explicit invocation is optional. Ask Claude naturally:

Create a B4XPages settings page shared by B4A and B4J.

Explain why Process_Globals does not compile in my B4XMainPage class.

Refactor this SQLite query to use ExecQuery2 and close the ResultSet safely.

Convert this B4A-only view code to cross-platform XUI code.

For the most useful result, include:

• the target platform or platforms;
• the module type, such as B4XMainPage, class, code module, or service;
• the relevant libraries;
• the complete compiler error and affected code when debugging.

Knowledge included

The skill uses progressive disclosure: SKILL.md contains the routing rules and essential checks, while detailed guidance is loaded only for the topic being handled.

Reference	Coverage
language-and-modules.md	Syntax, types, scope, modules, collections
b4xpages-and-xui.md	B4XPages, XUI, Designer, views, canvas, bitmap, fonts
data-and-io.md	SQLite, ResultSet, transactions, files, JSON
async-network.md	Sleep, Wait For, resumable subs, timers, HttpJob
custom-views-and-libraries.md	XUI custom views and .b4xlib packaging
bundled-libraries.md	Libraries shipped with the IDE and what they are for
platform-b4a.md	Android manifest, permissions, JavaObject
platform-b4j.md	Desktop, JavaFX, servers, handlers, packaging
platform-b4i.md	iOS build, signing, ATS, NativeObject
common-mistakes.md	Frequent AI and migration errors, version-gated features

Repository structure

b4x-skill/
├── .claude-plugin/
│   ├── marketplace.json
│   └── plugin.json
├── .github/
│   └── workflows/
│       └── validate.yml
├── evals/
│   ├── README.md
│   ├── cases.md
│   └── content-checks.mjs
├── scripts/
│   └── validate.mjs
├── skills/
│   └── b4x/
│       ├── SKILL.md
│       └── references/
│           ├── async-network.md
│           ├── b4xpages-and-xui.md
│           ├── bundled-libraries.md
│           ├── common-mistakes.md
│           ├── custom-views-and-libraries.md
│           ├── data-and-io.md
│           ├── language-and-modules.md
│           ├── platform-b4a.md
│           ├── platform-b4i.md
│           └── platform-b4j.md
├── CHANGELOG.md
├── LICENSE
└── README.md

Limitations

• This is a knowledge skill, not a B4X compiler, IDE extension, or language server.
• It does not install B4X, platform SDKs, libraries, or build tools.
• Code still needs to be compiled and tested in the appropriate B4X IDE.
• B4X and mobile-platform requirements evolve. Claims involving the latest IDE, Android SDK, iOS, or store policies should be verified against current official documentation.
• Some third-party B4X library APIs vary by version. Include the library name and version when asking about them.

Development and validation

Load the repository directly while making changes:

claude --plugin-dir ./b4x-skill

Validate the plugin, marketplace catalog, and skill metadata from Claude Code:

/plugin validate .

The repository also carries its own structural validator, which runs on every push and pull request via GitHub Actions and can be run locally (Node 20+, no dependencies):

node scripts/validate.mjs

It checks SKILL.md frontmatter against the Agent Skills specification and, more importantly, keeps the progressive-disclosure wiring honest: every references/ file named in SKILL.md must exist, every file on disk must be routed to from SKILL.md, backticked cross-file mentions must resolve, and the README reference list must match the files that are actually there. Renaming or deleting a reference file without updating its routing fails the build.

That validator checks structure only — it cannot tell you a claim is wrong. The content checks in evals/ do that, and also run in CI:

node evals/content-checks.mjs

They enforce that no block labelled ' B4X names a single-platform type, and carry one regression assertion per bug that has shipped. evals/cases.md holds the behavioural cases — prompts graded by reading, run before a release that changes guidance.

After editing plugin files, run:

/reload-plugins

When publishing a new release, update the version in .claude-plugin/plugin.json and add a matching entry to CHANGELOG.md. The validator fails if the two disagree, so the changelog cannot silently fall behind.

Versions are read as follows. MAJOR means guidance reversed or the scope changed, so advice you followed before may no longer be recommended. MINOR means new coverage or checks, with existing guidance intact. PATCH means corrections and clarifications. Rearranging files inside references/ is not breaking — the skill's interface is what it tells you about B4X, not how its own files are laid out.

References and credits

• B4X official website
• B4X documentation
• B4X Programming Forum
• Claude Code plugins
• Claude Code plugin marketplaces
• Agent Skills specification

The structure of this skill adapts ideas from multica-ai/andrej-karpathy-skills, licensed under MIT.

B4X, B4A, B4J, B4i, and B4R are products of Anywhere Software.

License

Released under the MIT License.