Java Debug

jdb-mcp — Let your LLM debug Java (JDB / JDI over MCP, for Claude Code & any MCP client)

jdb-mcp turns an LLM into a Java debugger. It exposes JDI — the same engine behind jdb and the JDWP protocol — as an MCP server, so AI agents like Claude Code can set breakpoints, step through code, capture exceptions, and inspect a live JVM. The core advantage: the AI pre-declares exactly which values it wants before the breakpoint fires, and jdb-mcp returns only those as a compact structured snapshot — no heap dumps, no stack-dredging, no context blowup. Just the data the hypothesis needs, on demand.

Keywords: LLM · AI agent · Java DEBUG · JDB · JDI · JDWP · MCP · Claude Code · breakpoint · live JVM inspection

Why jdb-mcp?

• LLM-friendly by design. Capture-then-snapshot means the model never sees a raw heap — only the few fields/paths it named. Keeps token cost and hallucination risk low.
• Two debug modes. Mode A: fire-and-buffer in the background, poll snapshots later. Mode B: arm a breakpoint non-blocking, then resume blocks until the hit so the AI can explore / step / get_variables interactively.
• Expression engine. Read paths like this.user.id, args[0].items, arr.length, list[i], sorted[mid], obj.getX() — with null-break reporting (you learn where a path went null).
• Anti-addiction. A session counter warns after 8+ debug actions — debugging is costly, so the tool steers the AI toward logs/tests first.
• Zero-config. No config file, no env vars. Pass target info to start_session; the npm launcher auto-detects your JDK 17+.
• Production-safe option. safeMode=true disables all method invocation (no toString/getter side effects) — field reads and array indexing only.

Design

analyze code -> form hypothesis ("I suspect user.id is null") -> set breakpoint + declare captures
       -> tool extracts exactly what was declared -> AI gets structured result -> verify/fix

Two modes:

• Mode A (default, non-blocking): breakpoint fires in the background, capture is stored into a HitBuffer, thread auto-resumes. AI polls list_hits for snapshots.
• Mode B (interactive): set_breakpoint(mode=B) arms non-blocking and returns immediately. The AI triggers its request (curl/manual), then calls resume which blocks until the hit — the JVM suspends for inspection via explore, get_frames, get_variables, step, or eval (dangerous), then resume again to continue. One interactive hit per set_breakpoint(mode=B); later hits degrade to mode A (buffered) — re-arm mode B to inspect again. set_breakpoint never blocks; resume does the waiting.

DEBUG SPARINGLY. Debugging is costly (target-JVM time + context). Only use with a concrete hypothesis that logs, inspection, and tests cannot resolve. The server instructions and every tool description emphasize this with a session-level counter that warns after 8+ debug actions.

Quick Start

Via npm (recommended — no source, no build)

npm install -g jdb-mcp

Requires: JDK 17+ and Node.js 16+. That's it — no Maven, no source, no config file.

The jdb-mcp command auto-detects your JDK (JAVA_HOME → PATH → common install locations) and launches the bundled debug server over stdio. If your JDK isn't on the PATH, point JAVA_HOME at it:

# Linux / macOS
export JAVA_HOME=/path/to/jdk-17
jdb-mcp

# Windows (Git Bash / cmd / PowerShell)
set JAVA_HOME=F:\path\to\jdk-17
jdb-mcp

Via npx (no global install)

npx -y jdb-mcp

From source (for development)

Requires JDK 17+ and Maven 3.6+ on your PATH.

git clone https://github.com/ylw/jdb-mcp.git
cd java-debug
export JAVA_HOME=/path/to/jdk-17
mvn package -DskipTests
# fat jar: jdb-mcp/target/jdb-mcp-1.0.0.jar
java -jar jdb-mcp/target/jdb-mcp-1.0.0.jar

Claude Code Wiring

With npx (auto-install, recommended)

No npm install needed — npx -y fetches jdb-mcp from the registry on first run and caches it.

Default Java is JDK 17+ (auto-detected by the launcher):

{
  "mcpServers": {
    "jdb-mcp": {
      "command": "npx",
      "args": ["-y", "jdb-mcp"]
    }
  }
}

Default Java is NOT JDK 17+ — point JAVA_HOME at your JDK 17 with the env field:

{
  "mcpServers": {
    "jdb-mcp": {
      "command": "npx",
      "args": ["-y", "jdb-mcp"],
      "env": { "JAVA_HOME": "XXX/jdk17" }
    }
  }
}

Replace XXX/jdk17 with your actual JDK 17 path. The launcher checks JAVA_HOME first (highest priority), then java on PATH, then common install locations.

With global install

After npm install -g jdb-mcp:

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

If your default Java is not JDK 17, add "env": { "JAVA_HOME": "XXX/jdk17" } as above.

Direct jar

{
  "mcpServers": {
    "jdb-mcp": {
      "command": "XXX/jdk17/bin/java",
      "args": ["-jar", "XXX/jdb-mcp-1.0.0.jar"]
    }
  }
}

No config file required (optional JDB_MCP_LOG for log level). The AI passes target info directly to start_session.

Tool List (17 tools)

Expression Grammar (captures, explore, expand_paths)

this                        current object
this.field / obj.field      field read
a.b.c.d.e                   nested path (default max 5 dereferences)
obj.getX() / obj.isX()      no-arg getter (get*/is* + size/isEmpty/length/toString)
args[N]                     method parameter by position
arr[i] / list[i]            array index, or java.util.List index (uses get(int))
arr.length                  array length (mirrored as int)
sorted[mid]                 index by named local variable (resolved at runtime)
e.getMessage()              exception root (exception breakpoints only)

The root is one of: this, args[N], e (exception bp), or a named local variable. Null-break: intermediate null -> status=null_break, nullBreakAt="<segment>" — you know WHERE it broke.

Limits (defaults, adjustable via configure or start_session):

• path depth: 5, toString: 500 chars, collection: 20 elements, fields: 50
• render node budget: 1000 per capture (explosion guard; bare this renders shallow — direct fields only)
• Mode B budgets: explore 5, eval 2, step 5 per suspend

Safe mode (safeMode=true): no method invocation at all — no getters, no toString, no List expansion. Only field reads and array indexing. Recommended for production debugging to avoid unpredictable side effects from toString/equals implementations.

Tests

export JAVA_HOME=/path/to/jdk-17
mvn -pl jdb-mcp -am test

• DebugSessionIT (integration, 17 tests): mode A (null-break, collection truncation, getter paths, List indexing, named param, this fields, boxed primitives, safe_mode), mode B (non-blocking arm + resume blocks + explore + get_frames/get_variables/step), resolve_class, exception breakpoint (e.getMessage/e.reason), list_debuggable_jvms, configure, remove_breakpoint clears hits, list_classes substring filter, shallow bare-root render, render-budget truncation, array.length capture, [var] index resolution.
• ExprCaptureTest (3 unit tests): valid expressions, malformed, whitespace.

Project Structure

java-debug/
  pom.xml                     # parent pom (modules: demo, jdb-mcp)
  README.md
  LICENSE                     # MIT
  demo/                       # debug target (executable jar)
  jdb-mcp/
    pom.xml                   # mcp, slf4j-simple; shade fat jar
    src/main/java/com/ylw/jdbmcp/
      Main.java               # entry: stdio MCP server, zero-config
      Defaults.java           # default constants
      StartParams.java        # start_session parameters
      mcp/                    # MCP layer (ToolDefs, McpJson, ToolResponses)
      debug/                  # JDI core (DebugSession, JdiEventLoop, SessionConfig,
                             #   BreakpointManager, Breakpoint, ClassResolver, ProxyFilter,
                             #   ExprCapture, ExprEval, TargetLauncher, TargetSpec, CommandBus)
      snapshot/               # POJO models (Hit, HitBuffer, ValueNode, CaptureResult)
    src/test/java/...         # integration + unit tests

Package naming: com.ylw.jdbmcp (.mcp / .debug / .snapshot).

License

MIT License — the most permissive open-source license. Use it commercially, fork it, modify it, ship it, no strings attached. Attribution appreciated but not required.

See LICENSE for the full text.

Copyright © 2026 ylw.