Karellen Jdb

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Overview

karellen-jdb-mcp is an MCP (Model Context Protocol) server that enables any MCP-compliant LLM client to use JDB (the Java Debugger) for debugging JVM processes. The LLM can attach to a running JVM, set breakpoints, step through code, evaluate expressions, inspect threads, and analyze concurrency issues, all through structured JSON tool calls over the JDWP protocol.

Requirements

• Java Development Kit (JDK) with jdb on PATH (JDK 8 through 24+ supported)
• Python >= 3.10
• Works on Linux, macOS, and Windows (anywhere JDB runs)

Installing a JDK

Fedora / RHEL / CentOS:

sudo dnf install java-21-openjdk-devel

Ubuntu / Debian:

sudo apt install openjdk-21-jdk

macOS (Homebrew):

brew install openjdk@21

Verify the setup

jdb -version

This should print something like This is jdb version 21.0.4.

Installation

pip install karellen-jdb-mcp

Or with pipx for an isolated environment:

pipx install karellen-jdb-mcp

Claude Code Integration

Claude Code plugin (recommended)

The plugin automatically configures the MCP server and includes:

• Exception detection hook that suggests JDB when a Bash command output contains Java exception stack traces (Exception in thread, NullPointerException, etc.)
• /karellen-jdb-mcp:jdb-debug skill that walks through the full launch-attach-debug workflow step by step, with build-tool-specific JDWP stanzas
• jdb-investigator agent that Claude can spawn to autonomously investigate Java exceptions, deadlocks, and logic bugs using JDB

From the Karellen plugins marketplace:

claude plugin marketplace add karellen/claude-plugins
claude plugin install karellen-jdb-mcp@karellen-plugins

Or from the official Anthropic marketplace (if accepted):

claude plugin install karellen-jdb-mcp@claude-plugins-official

Or load directly from a local checkout for testing:

claude --plugin-dir /path/to/karellen-jdb-mcp

Manual MCP server configuration

If you prefer not to use the plugin, you can configure the MCP server directly. This gives you the MCP tools but not the skill, agent, or exception detection hook.

Using the CLI:

claude mcp add --transport stdio karellen-jdb-mcp -- karellen-jdb-mcp

Or manually add to ~/.claude.json (user scope) or .mcp.json in your project root (project scope, shared via version control):

{
  "mcpServers": {
    "karellen-jdb-mcp": {
      "type": "stdio",
      "command": "karellen-jdb-mcp"
    }
  }
}

If installed with pipx:

claude mcp add --transport stdio karellen-jdb-mcp -- pipx run karellen-jdb-mcp

or manually:

{
  "mcpServers": {
    "karellen-jdb-mcp": {
      "type": "stdio",
      "command": "pipx",
      "args": ["run", "karellen-jdb-mcp"]
    }
  }
}

Auto-approve jdb tools

By default Claude Code will prompt for confirmation before each jdb_* tool call. You can approve individually by selecting "Yes, and don't ask again" when prompted.

To auto-approve all tools upfront, add a permission rule to your user settings (~/.claude/settings.json):

{
  "permissions": {
    "allow": [
      "mcp__plugin_karellen-jdb-mcp_karellen-jdb-mcp__*",
      "mcp__karellen-jdb-mcp__*"
    ]
  }
}

The first rule covers plugin-loaded tools, the second covers manual MCP configuration.

Or for a project-scoped setting, add the same rule to .claude/settings.json in your project root (this file can be committed to version control so all team members get it).

Quick Start

With jdb_launch (recommended)

Launch the JVM with ${JDB_PORT} substitution — a random free port is allocated:

jdb_launch(["java", "-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=*:${JDB_PORT}",
            "-cp", "target/classes", "com.example.Main"])

Connect (port auto-resolves when there's one launched process):

jdb_connect(wait_timeout=30)

Debug:

jdb_breakpoint_set("com.example.Main:42")
jdb_run()
jdb_where()
jdb_locals()

Clean up:

jdb_disconnect()
jdb_launch_stop(port=<port>)

With a manually started JVM

Start the JVM yourself with JDWP on a known port, then connect:

jdb_connect(port=5005, wait_timeout=30)

Available Tools

Process Launch

Tool	Description
jdb_launch	Launch a JVM with JDWP on a random port. ${JDB_PORT} is substituted in the command.
jdb_launch_list	List all launched JVM processes with status.
jdb_launch_status	Get status of a launched process by port.
jdb_launch_stop	Stop a launched process by port.

Session Lifecycle

Tool	Description
jdb_connect	Attach to a running JVM via JDWP. Auto-resolves port from single launched process, or defaults to 5005.
jdb_disconnect	Disconnect and clean up. Port optional if only one session active.
jdb_session_list	List all active debug sessions with port, connection status, and JDK version.
jdb_version	Get JDB version info and available features.

Execution Control

Tool	Description
jdb_run	Start execution of the application's main class. Returns immediately once execution resumes.
jdb_cont	Continue execution. Returns immediately once the JVM resumes (fire-and-forget). Returns stop event if a breakpoint/exception is hit within TIMEOUT_RESUME.
jdb_step	Step into (enter method calls). Returns stop event on completion or returns immediately if execution resumes.
jdb_next	Step over (skip method calls). Same return behavior as step.
jdb_step_up	Step out (run until current method returns). Same return behavior as step.
jdb_wait_for_event	Wait for a stop event after jdb_cont/jdb_run returned reason="resumed". Blocks until breakpoint/exception/exit or timeout (default 120s).

Breakpoints

Tool	Description
jdb_breakpoint_set	Set breakpoint at class:line or class.method. Supports thread filters and suspend policy on JDK 13+.
jdb_breakpoint_clear	Clear a breakpoint.
jdb_breakpoint_list	List all breakpoints.
jdb_catch	Break on exception (caught/uncaught/all). Supports wildcard patterns.
jdb_ignore	Cancel an exception breakpoint.

Watchpoints

Tool	Description
jdb_watch	Watch field access/modifications.
jdb_unwatch	Remove a field watchpoint.

Thread Management

Tool	Description
jdb_threads	List all threads by group with status.
jdb_thread	Set default thread for subsequent commands.
jdb_suspend	Suspend threads (all or specific).
jdb_resume	Resume threads (all or specific).

Stack Navigation

Tool	Description
jdb_where	Get call stack (stack trace). Use "all" for all threads.
jdb_up	Move up the call stack (toward caller).
jdb_down	Move down the call stack (toward callee).

State Inspection

Tool	Description
jdb_print	Evaluate Java expression (fields, locals, method calls, arithmetic).
jdb_dump	Dump object showing all fields (static and instance).
jdb_eval	Evaluate expression (alias for print).
jdb_set	Assign value to variable, field, or array element.
jdb_locals	List local variables with values.

Class Introspection

Tool	Description
jdb_classes	List all loaded classes.
jdb_class_info	Show class details (superclass, interfaces).
jdb_methods	List class methods.
jdb_fields	List class fields.

Source

Tool	Description
jdb_list	List source code at current position or specified location.
jdb_sourcepath	Display or change source path for .java files.
jdb_classpath	Print classpath info from target JVM.

Concurrency Analysis

Tool	Description
jdb_lock	Object lock/monitor info (owner thread, waiting threads).
jdb_threadlocks	Thread lock info (owned monitors, lock being waited on).

Frame Manipulation

Tool	Description
jdb_pop	Pop current frame (return to caller, allows re-execution).
jdb_reenter	Re-enter current method from the beginning.

Tracing

Tool	Description
jdb_trace	Trace method entries/exits (optionally per-thread).
jdb_untrace	Stop tracing.

Monitors (Auto-execute)

Tool	Description
jdb_monitor	Auto-execute a command on every stop (e.g. "locals", "where").
jdb_monitor_list	List active monitors.
jdb_unmonitor	Remove a monitor.

Configuration

Tool	Description
jdb_exclude	Set/display step exclusion filter (skip library classes when stepping).

Note: All debugging tools accept an optional port parameter. When only one session is active, it auto-resolves. When multiple sessions are active, port is required.

Configuration

Timeouts

All timeouts are configurable via environment variables (in seconds). Set them in your MCP server configuration:

{
  "mcpServers": {
    "karellen-jdb-mcp": {
      "type": "stdio",
      "command": "karellen-jdb-mcp",
      "env": {
        "JDB_MCP_TIMEOUT_EXECUTION": "300"
      }
    }
  }
}

Variable	Default	Description
JDB_MCP_TIMEOUT_CONNECT	60	JDB attach and initial prompt
JDB_MCP_TIMEOUT_COMMAND	30	Non-execution commands (breakpoints, inspection, etc.)
JDB_MCP_TIMEOUT_EXECUTION	120	Prompt-based commands (used internally by send_command)
JDB_MCP_TIMEOUT_RESUME	5	Execution commands (run, cont, step, next, step up). These return immediately when output arrives or after this timeout if execution resumes with no output (fire-and-forget).

Build Tool Debug Stanzas

To debug a JVM process, it must be started with the JDWP agent. The exact syntax varies by build tool:

Build Tool	Command
Plain java	java -agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=*:5005 ...
Maven Surefire	mvn -Dmaven.surefire.debug test
Maven Failsafe	mvn -Dmaven.failsafe.debug verify
Gradle tests	./gradlew test --debug-jvm
Tycho Surefire	mvn verify -Dtycho.testArgLine="-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=*:5005"
Spring Boot	mvn spring-boot:run -Dspring-boot.run.jvmArguments="-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=*:5005"
Any (fallback)	JAVA_TOOL_OPTIONS="-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=*:5005" <command>

License

Apache-2.0

Privacy

See PRIVACY.md. This software does not collect or transmit any data.