Pyjab MCP server

pyjab-mcp

An MCP server that lets an AI agent drive Java desktop applications — Swing and AWT — through pyjab, which reads the JVM's own accessibility tree via Java Access Bridge. JavaFX is not supported: it publishes its accessibility through UI Automation rather than Java Access Bridge, so a JavaFX window is not a Java window as far as JAB is concerned (pyjab#227).

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It never modifies, restarts or injects into the target application. That constraint is the point: launch parameters, classpaths and dependency JARs stay exactly as they are, which is what makes this usable against a production Java client that you cannot change.

The alternative is to put an agent inside the target JVM. An agent is attached to a JVM that is already running, or named on the command line with -javaagent when the application starts, and either way the tool's code sits inside someone else's process — a JVM it did not launch and does not control. This server does neither: it loads nothing into the application and reads the accessibility tree through Java Access Bridge from outside it, which is how it can attach to a process it did not launch.

The dynamic half of that — attaching an agent to a JVM that is already running — is the half the JDK has a direction on, with no release date attached. JEP 451 shipped in JDK 21 as the preparation stage: loading an agent into an already-running JVM now warns. The JEP states its aim as "prepare users for a future release which disallows the dynamic loading of agents by default", a release that has not been named; its own non-goals leave agents named with -javaagent at startup, and any use of the Attach API that is not loading an agent, outside that direction.

Status

Released on PyPI — 16 tools: javadiagnostics, javalistwindows, javaattachwindow, javasnapshot, javafind, javagetelement, javagettext, javagettable, javaclick, javasettext, javaselect, javafillform, javasendkeys, javafocuswindow, javawaitfor, and javascreenshot. It needs pyjab>=1.11.0: 1.9.0 brought the tree walk a snapshot is built on, 1.10.0 brought the public window listing and JABDriver.detach(), which releases a driver's binding without terminating the process — this server has no tool that releases a session, so a binding lasts for the life of the process — and 1.11.0 brought JABDriver.sendkeys, so javasendkeys sends a real chord, and JABDriver.focus(), so javafocus_window can bring a window forward.

javasendkeys sends a chord to whichever window has the keyboard focus, and attaching does not give a window focus (pyjab#180). javafocuswindow is the missing half: it brings the window forward and, because asked for focus and got focus are different facts, it reports whether the window actually came forward rather than assuming it did. It probes for the API rather than assuming it, so an install below the pin is told which release has it instead of failing with an AttributeError.

Two elements can share a name, which is why a handle carries where an element is as well as what it is called: searching by name returns every match, each gets its own handle, and the end-to-end suite checks that clicking one acts on that one and not the other.

Shortened. The full README is on GitHub.