How do I use karellen-jdb-mcp?

1. Click on "Install Server". 2. Wait a few minutes for the server to deploy. Once ready, it will show a "Started" state. 3. In the chat, type @ followed by the MCP server name and your instructions, e.g., "@karellen-jdb-mcp attach to JVM process 54321 and inspect deadlocked threads" That's it! The server will respond to your query, and you can continue using it as needed. Here is a step-by-step guide with screenshots.

karellen-jdb-mcp

Official

by karellen

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MCP Server for JDB Java Debugging (karellen-jdb-mcp)

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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).

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.

This server cannot be installed

license - permissive license

quality - not tested

maintenance

How are these scores calculated?

Maintenance

–Maintainers

–Response time

2dRelease cycle

5Releases (12mo)

Issues opened vs closed

Resources

GitHub Repository

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MCP Server for JDB Java Debugging (karellen-jdb-mcp)

Overview

Requirements

Installing a JDK

Verify the setup

Installation

Claude Code Integration

Claude Code plugin (recommended)

Manual MCP server configuration

Auto-approve jdb tools

Quick Start

With jdb_launch (recommended)

With a manually started JVM

Available Tools

Process Launch

Session Lifecycle

Execution Control

Breakpoints

Watchpoints

Thread Management

Stack Navigation

State Inspection

Class Introspection

Source

Concurrency Analysis

Frame Manipulation

Tracing

Monitors (Auto-execute)

Configuration

Configuration

Timeouts

Build Tool Debug Stanzas

License

Privacy

Maintenance

Resources

Looking for Admin?

Latest Blog Posts

MCP directory API

With `jdb_launch` (recommended)