dejared-mcp: Java decompiler MCP server

dejared-mcp is a Java decompiler and JAR analyzer that speaks the Model Context Protocol. It lets an AI coding agent (Claude Code, Codex, Cursor, VS Code) open a .jar, .war, .ear, or .aar, list what is inside, search its bytecode, and decompile any class back to Java source using CFR, Vineflower, or Procyon.

Table of Contents

• Overview
• Prerequisites
• Quick Start
• Install as a plugin (recommended)
• Install as an MCP server
• Features
• Decompiler Engines
• Custom Java Path
• Server Configuration
• How It Works
• FAQ and Troubleshooting
• Contributing
• Third-Party Licenses
• License

Overview

dejared-mcp is a Java-based MCP server distributed as an npm package. It provides nine tools organized into three categories: discovery, hunting, and deep analysis. AI assistants use these tools to navigate JAR file structures, search for classes and string literals in bytecode, and decompile .class files back to readable Java source code.

Reach for it whenever a dependency ships without sources: reading what a third-party library actually does, recovering an application.yml or MANIFEST.MF embedded in a JAR, following a stack trace into a library you have no source for, or inspecting an obfuscated plugin or mod. It replaces hand-running jar tf, unzip, javap, or a desktop decompiler.

Any ZIP-based Java archive works: .jar, .war, .ear, .aar, and Spring Boot fat-jars.

The npm package acts as a thin wrapper that downloads and caches the server JAR on first run, then spawns it via java -jar using stdio transport.

Prerequisites

• Node.js 20 or later
• Java 17 or later (JRE is sufficient)

Quick Start

The best install is the plugin. One command pair gives your agent both halves: the nine MCP tools, and the jar-analysis skill that tells it when to reach for them instead of shelling out to jar tf, unzip, or javap.

claude plugin marketplace add hqkh4nh/dejared-mcp
claude plugin install dejared@dejared-mcp-marketplace

Codex carries the same bundle. See Install as a plugin. Every other client takes the plain MCP config.

Install as a plugin (recommended)

The MCP server on its own gives an agent nine tools. It does not tell the agent when to use them, so most agents keep reaching for jar tf, unzip -p, and javap -p out of habit and never touch the tools you installed.

A plugin ships both parts together:

| Part | What it does | |------|--------------| | MCP server | The nine tools listed under Features | | jar-analysis skill | Routes JAR questions to those tools, picks the cheap tool before the expensive one, and knows which decompiler to retry with |

Two tools support that bundle today. Everything else is MCP-only.

Claude Code

claude plugin marketplace add hqkh4nh/dejared-mcp
claude plugin install dejared@dejared-mcp-marketplace

Inside a session, the same commands work as /plugin marketplace add ... and /plugin install .... If the install summary says Run /reload-plugins to activate, run it.

Codex

codex plugin marketplace add hqkh4nh/dejared-mcp
codex plugin add dejared@dejared-mcp-marketplace

Start a new session afterwards so the bundled skill and server load. Check with codex mcp list. dejared should be listed as enabled.

Install as an MCP server

For clients without a plugin system. You get the tools, but not the skill that steers the agent toward them.

Most clients accept this standard config:

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

From the command line

If your tool has a CLI, one line is enough:

claude mcp add dejared -- npx -y dejared-mcp     # Claude Code
codex mcp add dejared -- npx -y dejared-mcp      # Codex
amp mcp add dejared -- npx -y dejared-mcp        # Amp
gemini mcp add dejared npx -y dejared-mcp        # Gemini CLI

Where the config file lives

Drop the standard config into the file for your tool:

| Tool | Location | |------|----------| | Claude Desktop | %APPDATA%\Claude\claude_desktop_config.json (Windows) · ~/Library/Application Support/Claude/claude_desktop_config.json (macOS) | | Claude Code | .mcp.json in the project root | | Cursor | .cursor/mcp.json (project) · ~/.cursor/mcp.json (global) | | Windsurf | MCP servers panel in settings, or mcpServers in your settings file | | Cline | MCP settings file, via the extension's MCP Servers panel | | Antigravity Editor | ~/.gemini/antigravity/mcp_config.json | | Kiro | .kiro/settings/mcp.json | | Copilot CLI | ~/.copilot/mcp-config.json, or run /mcp add | | Gemini CLI | ~/.gemini/settings.json (global) · .gemini/settings.json (project) | | Goose | Advanced settings → Extensions → Add custom extension, type STDIO | | JetBrains IDEs | Settings → Tools → AI Assistant → Model Context Protocol (MCP) | | Qodo Gen | Chat panel → Connect more tools → + Add new MCP |

A few setups need more than the standard config. Expand those below.

codex mcp add dejared -- npx -y dejared-mcp
codex mcp list

The -- matters: everything after it is the server's launch command.

Or edit ~/.codex/config.toml (global) or .codex/config.toml (project):

[mcp_servers.dejared]
command = "npx"
args = ["-y", "dejared-mcp"]

# Only if Java is not on your PATH
[mcp_servers.dejared.env]
DEJARED_JAVA_PATH = "/path/to/java"

Create or edit .vscode/mcp.json:

{
  "servers": {
    "dejared": {
      "command": "npx",
      "args": ["-y", "dejared-mcp"]
    }
  }
}

After saving, click Start above the server entry, then use Agent mode in Copilot Chat.

Edit ~/.config/opencode/opencode.json:

{
  "$schema": "https://opencode.ai/config.json",
  "mcp": {
    "dejared": {
      "type": "local",
      "command": ["npx", "-y", "dejared-mcp"],
      "enabled": true
    }
  }
}

If you prefer to run the server JAR directly without Node.js:

1. Download the latest JAR from GitHub Releases.

2. Run it:

   java -jar dejared-mcp-<version>.jar

3. Configure your MCP client to use the JAR directly instead of npx:

   {
     "mcpServers": {
       "dejared": {
         "command": "java",
         "args": ["-jar", "/path/to/dejared-mcp-<version>.jar"]
       }
     }
   }

Features

Discovery

Browse and read JAR contents.

| Tool | Description | |------|-------------| | dejared_list_packages | List all packages with class counts | | dejared_list_classes | List classes in a specific package | | dejared_list_resources | List non-class resource files | | dejared_read_resource | Read text resources (YAML, XML, properties, JSON, and others) |

Hunting

Search inside JAR files.

| Tool | Description | |------|-------------| | dejared_search_class | Search classes by name | | dejared_search_string | Search string literals in bytecode (URLs, SQL, error messages) |

Deep Analysis

Inspect metadata and decompile classes.

| Tool | Description | |------|-------------| | dejared_get_metadata | Extract class metadata via ASM (fast, no decompilation) | | dejared_dump_package_metadata | Batch metadata extraction for entire packages | | dejared_decompile_class | Decompile .class files to Java source code |

Decompiler Engines

dejared-mcp supports three decompiler engines. The engine can be specified per request via the dejared_decompile_class tool.

| Engine | Description | |--------|-------------| | CFR (default) | Reliable general-purpose decompiler | | Vineflower | Modern fork of FernFlower, handles newer Java features well | | Procyon | Alternative engine, can handle some edge cases better |

Custom Java Path

If Java is not in your system PATH, set the DEJARED_JAVA_PATH environment variable in your MCP config. This applies to all npx-based configurations:

{
  "mcpServers": {
    "dejared": {
      "command": "npx",
      "args": ["-y", "dejared-mcp"],
      "env": {
        "DEJARED_JAVA_PATH": "/path/to/java"
      }
    }
  }
}

Server Configuration

| Property | Default | Description | |----------|---------|-------------| | dejared.cache.max-size | 500 | Max entries in the decompilation LRU cache | | dejared.security.max-resource-size | 5242880 | Max resource file size (bytes) | | dejared.security.decompile-timeout-seconds | 30 | Timeout per decompilation |

How It Works

The npm package is a thin Node.js wrapper. On first run it:

1. Checks the platform cache directory for a cached JAR matching the current version.
   • Linux: $XDG_CACHE_HOME/dejared-mcp (defaults to ~/.cache/dejared-mcp)
   • macOS: ~/Library/Caches/dejared-mcp
   • Windows: %LOCALAPPDATA%\dejared-mcp
2. Downloads the JAR from GitHub Releases if not cached.
3. Spawns java -jar with stdio inherited for MCP transport.

The server communicates over stdio using the Model Context Protocol.

FAQ and Troubleshooting

Q: Java is installed but the server cannot find it.

Set the DEJARED_JAVA_PATH environment variable in your MCP configuration. See Custom Java Path.

Q: The server fails to start with a permission error.

Ensure that the cached JAR file is readable. The cache location depends on your platform. See How It Works for the cache directory paths.

Q: Decompilation times out or returns an error.

Some classes are difficult to decompile. Try a different engine by specifying vineflower or procyon in the dejared_decompile_class tool. The default timeout is 30 seconds and can be adjusted via dejared.security.decompile-timeout-seconds.

Q: Which Java version do I need?

Java 17 or later. A JRE is sufficient; you do not need a full JDK.

Q: Can I run the server without Node.js?

Yes. Download the JAR from GitHub Releases and run it directly with java -jar. See the "Pure Java" section under Install as an MCP server.

Contributing

Contributions are welcome. Please follow these guidelines:

1. Fork the repository and create a feature branch from master.
2. Ensure your changes compile and pass existing tests.
3. Write clear commit messages describing the purpose of each change.
4. Open a pull request against master with a description of what the change does and why.

Every manifest listed under Project Structure carries a version field, and CI fails the build if any of them drifts from package.json. Bump them together.

Project Structure

dejared-mcp/
├── bin/                    # Node.js CLI entry point
├── lib/                    # Node.js wrapper (JAR download and process management)
├── java-mcp/               # Java MCP server (Spring Boot, Gradle)
│   └── src/
├── skills/                 # Agent skills, shared by every plugin manifest below
├── .mcp.json               # MCP server definition, shared the same way
├── .claude-plugin/         # Claude Code plugin + marketplace manifests
├── .codex-plugin/          # Codex plugin manifest
├── .agents/plugins/        # Codex marketplace manifest
├── package.json            # npm package manifest
└── server.json             # MCP Registry manifest

Building from Source

cd java-mcp
./gradlew build

Reporting Issues

Open an issue on GitHub Issues with steps to reproduce the problem, your Java version, and your Node.js version.

Third-Party Licenses

This project uses the following open-source libraries:

| Library | License | |---------|---------| | Spring Boot | Apache 2.0 | | Spring AI | Apache 2.0 | | ASM | BSD 3-Clause | | CFR | MIT | | Vineflower | Apache 2.0 | | Procyon | Apache 2.0 |

License

This project is licensed under the MIT License.