Jdwp

Debug live JVMs through JDWP — from any MCP-compatible agent.

License: MIT

Attach to a running Java process, pause threads, inspect stacks and objects, set breakpoints, and evaluate state — through natural language. One prompt starts an agent-driven diagnosis loop with live runtime data.

Works with Claude Code, Codex, Cursor, or any MCP-compatible agent.

See it in action

A service query is hanging. Find the root cause:

> Attach to localhost:5005 and find out why a query is stuck.

The agent attaches, pauses all threads, and scans for the problem:

connected localhost:5005
paused
24 threads, 2 blocked

Thread pool-3-thread-7 is waiting for a monitor lock:
#0 RolapResult.loadMembers:142
  monitor=@3f2a  state=BLOCKED
#1 RolapResult.execute:89

Lock is held by pool-3-thread-2, which is running:
#0 SqlStatement.execute:218
  sql="SELECT ... FROM fact_table"   -- full scan on 36M rows

Root cause: the query bypassed the aggregate table and fell back to
a full fact-table scan. Thread-7 is waiting for thread-2 to finish.

One prompt. Six tool calls. Lock contention and root cause identified.

Quick Start

1. Install

pip install jdwp-mcp

# Pre-built binary
curl -fsSL https://raw.githubusercontent.com/dronsv/jdwp-mcp/main/install.sh | sh

# Cargo (requires Rust)
cargo install --git https://github.com/dronsv/jdwp-mcp

# From source
git clone https://github.com/dronsv/jdwp-mcp && cd jdwp-mcp && cargo build --release

2. Configure your agent

claude mcp add jdwp jdwp-mcp

{
  "mcpServers": {
    "jdwp": {
      "command": "jdwp-mcp"
    }
  }
}

3. Start your Java app with JDWP

java -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=*:5005 -jar app.jar

4. Debug

Attach to localhost:5005 and set a breakpoint at com.example.MyService line 42

5. Auto-approve (optional)

Debugging involves many rapid tool calls. Auto-approve eliminates confirm prompts:

# Allow all jdwp tools for this project
claude config set --project allowedTools 'mcp__jdwp__*'

claude config set allowedTools 'mcp__jdwp__*'

Prompt packs

Pick the pack that matches your situation:

App hangs or is slow

Attach to localhost:5005
Pause the JVM and find all blocked or waiting threads
Show the stack for the blocked thread with variables
Who holds the lock? Show their stack too

Exception in logs

Attach to localhost:5005
Set an exception breakpoint for NullPointerException
Wait for the exception to fire
Show the stack and all local variables at the throw site

Need to understand a code path

Attach to localhost:5005
Trace method calls on com.example.service
[send your HTTP request]
Show the trace result — which methods were called?

Breakpoint-driven debugging

Attach to localhost:5005
Find classes matching UserService
List methods of UserService with line numbers
Set a breakpoint at UserService line 45
When it hits, show the stack with all variables
Step over to the next line

Claude Code commands

If you clone this repo, you get ready-made slash commands:

• /investigate-hang — diagnose a hanging JVM (pause, find blocked threads, trace locks)
• /investigate-exception — catch a live exception and inspect the throw site
• /trace-request — trace which methods a request passes through

And an autonomous investigator agent (.claude/agents/jdwp-investigator.md) that can be spawned to diagnose hangs, deadlocks, exceptions, and unexpected code paths.

See .claude/settings.example.json for recommended auto-approve and update-check config.

Best first use cases

• Hung requests and deadlocks
• Blocked thread pools
• Suspicious SQL or runtime state mismatch
• Breakpoint-driven diagnosis without IDE access
• Remote debugging via kubectl port-forward

Why this instead of jstack or an IDE?

• Works inside your agent — no tool switching, no separate debugger window
• Combines attach + inspect + reasoning in one loop — the agent decides what to look at next
• Conversational — describe the problem, the agent runs the debug session
• Ground truth for large codebases — in complex projects with deep framework stacks (Spring, Hibernate, OLAP engines), agents can get lost tracing code paths statically. Live debugging gives the agent actual runtime state: which thread holds the lock, what SQL was generated, what value a variable actually has right now

Tools

Connection and control attach, disconnect, pause, continue, step into/over/out

Breakpoints and events set_breakpoint (with conditions), clear, list, exception_breakpoint, watch (field modification), wait_for_event

Inspection get_stack (auto-resolves objects), get_variable, inspect, eval, set_value, snapshot, find_class, list_methods, list_threads, vm_info

Tracing trace (arm method-level tracing on a package), trace_result (get the call path)

Don't use it for

• Postmortem heap analysis
• Always-on production observability
• Environments where JDWP attach or thread pausing is operationally unsafe

Operational note

JDWP changes runtime behavior. Pausing threads and setting breakpoints may be disruptive. Use carefully in production; prefer staging environments or controlled maintenance windows.

Deploy scenarios

See docs/deploy.md for setup with Maven, Gradle, Tomcat, Docker, Kubernetes (port-forward), and SSH tunnels.

Examples

• Debugging a hanging query — full walkthrough: lock contention, thread analysis, root cause identification
• Observability debugging — investigating Spring Boot ObservationRegistry issues

Architecture

Agent  -->  MCP Server  -->  JDWP Client  -->  TCP  -->  JVM
              |
        Translates tool calls to JDWP,
        tracks session state, summarizes
        runtime objects for the agent.

Building from source

cargo build --release
cargo test

License

MIT