Maven Decoder MCP Server

Your agent guesses at library APIs it has never read. This makes it read them.

Lets AI agents read the actual source of any Maven dependency — decompiles jars from ~/.m2 or Maven Central, and diffs versions for breaking changes.

Ask an agent "I'm upgrading org.jsoup:jsoup from 1.17.2 to 1.23.2 — what breaks?" and without a way to read the jars it will answer from memory. With this server, compare_versions reads both jars and reports what actually changed:

Members are compared as declared, so one that moved to a supertype is reported as removed even though it may still be callable. The tool states this in its own output.

It works on artifacts that have no sources jar too: extract_class_info falls back to javap and returns parsed fields, methods, and bytecode version — which is exactly the case for the internal artifacts in a corporate Nexus.

Try it in one command

npx skills add https://github.com/salitaba/maven-decoder-mcp --skill maven-code-search

That installs the maven-code-search agent skill, which tells your agent when to reach for these tools. For a raw MCP server setup instead, see Installation.

🚀 Features

Core Functionality

• Jar File Analysis: Deep inspection of jar files including metadata, manifests, and structure

• Dependency Resolution: Complete dependency tree analysis with transitive dependencies

• Source Code Extraction: Extract source code from source jars or decompile bytecode

• Class Information: Detailed class signatures, methods, fields, and annotations

• Search Capabilities: Find classes, methods, and dependencies across all artifacts

• Version Management: Compare versions, find dependents, and track version conflicts

Online Maven Support

• Maven Central Search: Find artifacts and classes that are not installed locally

• Remote Version Listing: See every published version, not just the ones you have

• On-Demand Download: Fetch any artifact (jar, sources, POM) into a local cache

• Transparent Fallback: Every analysis tool automatically downloads a missing artifact, so decompiling a dependency you never installed just works

• Mirror Friendly: Point it at a corporate Nexus/Artifactory, with optional credentials

• Offline Mode: A single env var restores fully local, network-free behavior

Advanced Features

• Decompilation Support: Integrated support for multiple Java decompilers (CFR, Fernflower, Procyon)

• Conflict Analysis: Detect and analyze dependency version conflicts

• Repository Navigation: Browse and explore the local Maven repository structure

• Metadata Parsing: Extract and parse Maven POM files and metadata

• Service Discovery: Find and analyze Java services and SPI implementations

• Response Management: Intelligent pagination and summarization for large responses

• Method Extraction: Extract specific methods from large Java classes

• Integrity Checking: Downloads are verified against the repository's SHA-1 checksums

Related MCP server: Code Understanding MCP Server

📦 Installation

Prerequisites

• Java 8+ (for decompilation features)

• Maven local repository (~/.m2/repository)

• One of: Python 3.8+, Node.js 14+, or Docker

🚀 Quick Install

One-Line Install (Recommended)

curl -fsSL https://raw.githubusercontent.com/salitaba/maven-decoder-mcp/main/install.sh | bash

📋 Installation Methods

Method 1: uvx (Recommended)

# Install uv (if not installed)
curl -Ls https://astral.sh/uv/install.sh | sh
# Ensure your shell PATH is updated (restart shell or eval as printed by installer)

# Run the server via uvx (isolated, fast, no venv needed)
uvx maven-decoder-mcp

# Optional: pick a specific Python
# uvx --python 3.12 maven-decoder-mcp

Method 2: Node.js/npm

# Install globally
npm install -g maven-decoder-mcp

# Or install locally
npm install maven-decoder-mcp

# Run the server
maven-decoder-mcp
# or if installed locally: npx maven-decoder-mcp

Method 3: Docker

# Pull and run
docker run --rm -it \
  -v ~/.m2:/home/mcpuser/.m2 \
  -v $(pwd):/workspace \
  ali79taba/maven-decoder-mcp:latest

Method 4: From Source (Development)

# Clone repository
git clone https://github.com/salitaba/maven-decoder-mcp.git
cd maven-decoder-mcp

# Option A: Using Virtual Environment
python3 -m venv .venv
source .venv/bin/activate  # On Windows: .venv\Scripts\activate
pip install -r requirements.txt
pip install "git+https://github.com/modelcontextprotocol/python-sdk.git"
./setup_decompilers.sh

# Option B: System-wide Installation (not recommended)
./setup_decompilers.sh

Windows

For a source checkout, use Python 3.10 or newer and a JDK on PATH. From the repository root, create and activate a virtual environment in PowerShell:

python -m venv .venv
.\.venv\Scripts\Activate.ps1
python -m pip install -e ".[dev]"

# Point to your existing local Maven repository (drive-letter paths are supported)
$env:MAVEN_REPOSITORY = 'F:\data\repository'
maven-decoder-mcp

In Command Prompt (cmd.exe), activate with .venv\Scripts\activate.bat and set the repository with set "MAVEN_REPOSITORY=F:\data\repository" instead. These environment settings apply to programs launched from that terminal; set them in your MCP client's environment when it launches the server separately.

Remote downloads use a separate cache, resolved in this order (empty values are skipped):

1. MAVEN_DECODER_CACHE_DIR: the complete cache directory; no subdirectory is appended.

2. XDG_CACHE_HOME: append maven-decoder-mcp\repository.

3. LOCALAPPDATA: append maven-decoder-mcp\repository.

4. Otherwise, ~/.cache/maven-decoder-mcp/repository under your home directory.

On Windows, this usually means %LOCALAPPDATA%\maven-decoder-mcp\repository; XDG_CACHE_HOME still takes precedence if set. The cache uses Maven's directory layout but stays separate from the real local repository so downloads do not interfere with Maven builds. Setting MAVEN_REPOSITORY does not change the cache location.

🔧 Configuration

For Cursor IDE

Add to your ~/.cursor/mcp_servers.json:

{
  "maven-decoder": {
    "command": "uvx",
    "args": ["maven-decoder-mcp"]
  }
}

For Other MCP Clients

The server runs as a standard MCP server and can be integrated with any MCP-compatible client.

🧠 AI Agent Skill

This repository includes a maven-code-search agent skill that tells AI coding agents when and how to use this MCP for searching installed Maven package code.

npx skills add https://github.com/salitaba/maven-decoder-mcp --skill maven-code-search

The skill is located at skills/maven-code-search and is ready for skills.sh indexing after the repository is pushed.

🛠️ Available Tools

Local Analysis

Online (Maven Central)

Tool parameters

* Required — the parameter appears in the tool's inputSchema required array. Parameters without a * are optional and may be omitted.

💡 Usage Examples

Finding Dependencies

"Show me all dependencies of org.springframework:spring-core:5.3.21"

Decompiling Classes

"Decompile the class com.example.MyService from my Maven repository"

Analyzing Conflicts

"Find all version conflicts in my Maven repository"

Checking an Upgrade for Breaking Changes

"Compare org.jsoup:jsoup 1.17.2 with 1.23.2 and tell me what would break"

compare_versions diffs the public and protected members of every class the two versions share, and reports removals separately from additions. Removed members and removed classes are counted as breaking changes. Members are compared as declared, so one that moved to a supertype is reported as removed even though it may still be callable.

Exploring APIs

"Show me all public methods in the Jackson ObjectMapper class"

Finding Real Callers

"Find classes that call ObjectMapper.readValue, including examples from test jars"

find_usage_examples scans compiled class constant pools, so it finds real callers rather than simple name matches. Pass method_name (such as readValue) to narrow the results to callers of a specific method. Test jars are included by default and ranked first because they often contain the clearest examples; set search_tests to false to exclude them. The scan stops after MCP_USAGE_SCAN_LIMIT classes (default: 200000) to stay responsive on large repositories.

Inspecting Compiled-Only Artifacts

"The sources jar is missing. Use extract_class_info for bytecode-backed fields and methods."
"Find and read .proto resources from com.example:protobuf-lib:1.0.0"

When a dependency has no sources jar, extract_class_info uses javap internally and returns parsed fields, methods, bytecode version, and optional verbose bytecode output. Agents should use analyze_jar, extract_class_info, extract_source_code, and extract_jar_resource through this MCP instead of running jar or javap directly.

Working with Large Responses

"List all Spring classes with pagination (page 2, 10 items per page)"
"Extract source code for a large class with summarization"
"Get method information for specific patterns in a class"

Searching Maven Central (Online)

"Which Maven artifact contains the class HikariDataSource?"
"Search Maven Central for retrofit"
"What is the newest published version of org.apache.commons:commons-lang3?"
"Download com.google.code.gson:gson:latest and show me the JsonParser class"

🌐 Online Maven Support

The server works against the local repository and remote repositories. Online access is enabled by default.

How it works

1. Every tool first looks in your local repository (~/.m2/repository).

2. On a miss, the artifact is downloaded from Maven Central into a cache (~/.cache/maven-decoder-mcp/repository) that uses the standard Maven layout.

3. All existing analysis (decompilation, class info, dependencies) then runs on the cached artifact exactly as it would on an installed one.

The cache is deliberately separate from ~/.m2 so downloads never interfere with your Maven or Gradle builds. Responses include an origin field (local-repository or remote-cache) so you always know where a result came from.

Going offline

MAVEN_OFFLINE=true   # no network access at all; original local-only behavior
MAVEN_AUTO_DOWNLOAD=false   # keep online search, but never auto-download

Using a private mirror

MAVEN_REMOTE_REPOS="https://nexus.corp/repository/maven-public"
MAVEN_REMOTE_USERNAME=builder
MAVEN_REMOTE_PASSWORD=secret

A note on the search index

Artifact downloads use repo1.maven.org, which is fast and reliable. Artifact search uses search.maven.org, the only public index that answers class-level (c: / fc:) queries correctly. That index rate-limits bursts, so requests are retried with backoff; a busy period can still surface as a timeout. Downloads and version listing are unaffected, because they read maven-metadata.xml directly from the repository.

🔄 Response Management

Pagination Support

The server automatically handles large responses through intelligent pagination:

• Automatic Detection: Responses exceeding 50KB are automatically paginated

• Configurable Page Size: Default 20 items per page, customizable per request

• Pagination Metadata: Each response includes pagination information

• Supported Tools: list_artifacts, extract_class_info, search_classes, get_dependencies, find_dependents, get_version_info

Summarization Features

Large text content is automatically summarized to improve readability:

• Smart Summarization: Preserves important parts (package declarations, method signatures, closing braces)

• Configurable Limits: Default 10KB text limit, customizable

• Java-Specific: Optimized for Java source code structure

• Metadata Preservation: Original structure and metadata are maintained

Method Extraction

New tool for targeted access to specific methods:

• Pattern Matching: Use regex patterns to find specific methods

• Limited Results: Control the number of methods returned

• Full Context: Includes method signatures, bodies, and line numbers

• Efficient Processing: Only extracts requested methods, not entire classes

🏗️ Architecture

The server is built with a modular architecture:

• MavenDecoderServer: Main MCP server implementation

• ResponseManager: Handles pagination and summarization

• JavaDecompiler: Handles multiple decompilation strategies

• MavenDependencyAnalyzer: Analyzes Maven dependencies and metadata

• MavenCentralClient: Remote search, version listing, and artifact downloads

• Decompilers: CFR, Procyon, Fernflower, and javap integration

🧪 Development

Running Tests

# Install development dependencies
pip install -e ".[dev]"

# Run tests
pytest

# Run specific test
python test_startup.py

Building Package

# Build distribution
python setup.py sdist bdist_wheel

# Install locally
pip install dist/maven_decoder_mcp-*.whl

Docker Development

# Build Docker image
docker build -t maven-decoder-mcp .

# Run container
docker run --rm -it maven-decoder-mcp

📝 Configuration Options

Environment Variables

Local repository

• MAVEN_REPOSITORY / MAVEN_REPO: direct path to local Maven repository (e.g. F:\data\repository). Highest precedence.

• MAVEN_HOME / M2_HOME: Maven install dir or repository dir. A nested repository/ subdir wins when it exists; conf/settings.xml <localRepository> honored.

• ~/.m2/settings.xml <localRepository> honored when no env var set. Fallback: ~/.m2/repository.

Online access

• MAVEN_OFFLINE: set to true to disable all network access (default: false)

• MAVEN_AUTO_DOWNLOAD: auto-fetch artifacts missing locally (default: true)

• MAVEN_REMOTE_REPOS / MAVEN_REMOTE_REPO: comma/space separated repository base URLs (default: https://repo1.maven.org/maven2)

• MAVEN_SEARCH_URL: comma/space separated Solr search endpoints (default: https://search.maven.org/solrsearch/select)

• MAVEN_DECODER_CACHE_DIR: where downloaded artifacts are cached (default: ~/.cache/maven-decoder-mcp/repository)

• MAVEN_REMOTE_USERNAME / MAVEN_REMOTE_PASSWORD: basic-auth credentials for a private mirror

• MAVEN_HTTP_TIMEOUT: per-request timeout in seconds (default: 30)

• MAVEN_HTTP_RETRIES: retries for transient network failures (default: 3)

• MAVEN_MAX_DOWNLOAD_SIZE: maximum download size in bytes (default: 104857600)

• MAVEN_VERIFY_CHECKSUM: verify downloads against published SHA-1 (default: true)

Responses

• MCP_LOG_LEVEL: Logging level (DEBUG, INFO, WARNING, ERROR)

• MCP_MAX_RESPONSE_SIZE: Maximum response size in bytes (default: 50000)

• MCP_MAX_ITEMS_PER_PAGE: Default items per page (default: 20)

• MCP_MAX_TEXT_LENGTH: Maximum text length before summarization (default: 10000)

• MCP_MAX_LINES: Maximum lines before summarization (default: 500)

• MCP_USAGE_SCAN_LIMIT: Max classes scanned by find_usage_examples (default: 200000)

• MCP_API_DIFF_LIMIT: Max classes compared by compare_versions (default: 2000)

• MAVEN_DECODER_DECOMPILER_DIR: Directory holding cfr.jar / procyon-decompiler.jar

Advanced Configuration

The server automatically detects and configures:

• Maven repository location

• Available Java decompilers

• System capabilities

🔍 Troubleshooting

Common Issues

Server won't start

# Check Python installation
python --version

# Check Maven repository
ls ~/.m2/repository

# Check logs
maven-decoder-mcp --debug

Decompilation fails

# Check the environment: Java, repository, cache and available decompilers
maven-decoder-setup status

# Install the optional CFR and Procyon decompilers
maven-decoder-setup decompilers

From a source checkout you can instead run ./setup_decompilers.sh, which downloads CFR and Procyon into a decompilers/ directory beside the script, as decompilers/cfr.jar and decompilers/procyon-decompiler.jar.

If maven-decoder-setup status reports no decompilers even though you have the jars, they are somewhere the server does not look. It searches these roots in order, taking the first match:

1. $MAVEN_DECODER_DECOMPILER_DIR, if set

2. decompilers/ inside the installed package

3. decompilers/ beside the package — the source-checkout layout

4. ~/.cache/maven-decoder-mcp/decompilers

5. decompilers/ in the current working directory

6. the current working directory itself

Your MCP client launches the server from an arbitrary working directory, so the last two are unreliable in practice. If your jars live anywhere else, point at them explicitly:

export MAVEN_DECODER_DECOMPILER_DIR=~/tools/decompilers

The directory must contain the jars under the exact names cfr.jar and procyon-decompiler.jar. The value expands ~ and environment variables.

Without CFR or Procyon the server still works, falling back to javap from the JDK for signatures, fields and methods.

No artifacts found

# Verify Maven repository location
ls ~/.m2/repository

# Run a Maven build to populate repository
mvn dependency:resolve

Maven Central search times out

The public search index rate-limits bursts of requests. Retries with backoff are built in, but during heavy throttling a search can still fail. Workarounds:

# Wait a moment and retry, or raise the retry budget
MAVEN_HTTP_RETRIES=5

# Downloads and version listing do not use the search index, so these keep
# working even while search is throttled:
#   get_remote_versions, download_artifact

Downloads fail behind a proxy or firewall

# requests honors the standard proxy variables
export HTTPS_PROXY=http://proxy.corp:8080

# Or point at an internal mirror
export MAVEN_REMOTE_REPOS="https://nexus.corp/repository/maven-public"

# Or turn the network off entirely
export MAVEN_OFFLINE=true

🤝 Contributing

1. Fork the repository

2. Create a feature branch (git checkout -b feature/amazing-feature)

3. Commit your changes (git commit -m 'Add amazing feature')

4. Push to the branch (git push origin feature/amazing-feature)

5. Open a Pull Request

📄 License

This project is licensed under the MIT License - see the LICENSE file for details.

🙏 Acknowledgments

• Model Context Protocol - The protocol that powers this server

• CFR - Java decompiler

• Procyon - Java decompiler

• Maven - Dependency management

📊 Stats

Made with ❤️ for the Java development community