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Weights & Biases

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MCP server for querying and analyzing Weights & Biases data and visualizing results.

Weights & Biases

Weights & Biases MCP Server

A Model Context Protocol (MCP) server for querying Weights & Biases data. This server allows a MCP Client to:

  • query W&B Models runs and sweeps
  • query W&B Weave traces, evaluations and datasets
  • query wandbot, the W&B support agent, for general W&B feature questions
  • run python code in isolated E2B or Pyodide sandboxes for data analysis
  • write text and charts to W&B Reports

Installation

1. Install uv

Please first install uv with either:

curl -LsSf https://astral.sh/uv/install.sh | sh

or

brew install uv

2. Code sandbox setup (optional)

The wandb MCP server exposes a secure, isolated python code sandbox tool to the client to let it send code (e.g. pandas) for additional data analysis to be run on queried W&B data.

Option 1: Local Pyodide sandbox - Install Deno

The local Pyodide sandbox uses Deno to run Python in a WebAssembly environment, providing secure isolation from the host system. This option is automatically used if Deno is installed and no E2B API key is found.

# One-line install for macOS/Linux:
curl -fsSL https://deno.land/install.sh | sh -s -- -y

# Add Deno to your PATH (if not done automatically):
echo 'export PATH="$HOME/.deno/bin:$PATH"' >> ~/.bashrc  # for bash
echo 'export PATH="$HOME/.deno/bin:$PATH"' >> ~/.zshrc   # for zsh
source ~/.bashrc  # or ~/.zshrc

# Or on Windows (PowerShell):
irm https://deno.land/install.ps1 | iex

After installation, verify Deno is available:

# Restart your terminal or source your shell config
source ~/.bashrc  # or ~/.zshrc

# Verify installation
deno --version

If deno --version doesn't work, you may need to manually add Deno to your PATH:

echo 'export PATH="$HOME/.deno/bin:$PATH"' >> ~/.bashrc  # for bash
echo 'export PATH="$HOME/.deno/bin:$PATH"' >> ~/.zshrc   # for zsh
source ~/.bashrc  # or ~/.zshrc

Note, first execution may take longer as Pyodide downloads required packages

Option 2: Hosted E2B sandbox - Set E2B api key

The sandbox tool will use E2B if an E2B API key is detected. E2B provides persistent cloud VMs with full Python environment:

  1. Sign up to E2B at e2b.dev
  2. Get your API key from the E2B dashboard
  3. Set the E2B_API_KEY environment variable in the client settings.json
  • To explicitly disable the sandbox tool completely, set DISABLE_CODE_SANDBOX=1 environment variable

3. Installation helpers

We provide a helper utility below to easily install the Weights & Biases MCP Server into applications that use a JSON server spec - inspired by the OpenMCP Server Registry add-to-client pattern.

Example helper

uvx --from git+https://github.com/wandb/wandb-mcp-server -- add_to_client --config_path <path to the MCP config json file> --add_deno_path && uvx wandb login

Cursor installation

Cursor project

Enable the server for a specific project. Run the following in the root of your project dir:

uvx --from git+https://github.com/wandb/wandb-mcp-server -- add_to_client --config_path .cursor/mcp.json --add_deno_path && uvx wandb login

Cursor global

Enable the server for all Cursor projects, doesn't matter where this is run:

uvx --from git+https://github.com/wandb/wandb-mcp-server -- add_to_client --config_path ~/.cursor/mcp.json --add_deno_path && uvx wandb login

Windsurf installation

uvx --from git+https://github.com/wandb/wandb-mcp-server -- add_to_client --config_path ~/.codeium/windsurf/mcp_config.json --add_deno_path && uvx wandb login

Claude Code

claude mcp add wandb -- uvx --from git+https://github.com/wandb/wandb-mcp-server wandb_mcp_server --add_deno_path && uvx wandb login

Passing an environment variable to Claude Code, e.g. api key:

claude mcp add wandb -e WANDB_API_KEY=your-api-key -- uvx --from git+https://github.com/wandb/wandb-mcp-server wandb_mcp_server

Claude Desktop installation

First ensure uv is installed, you might have to use brew to install depite uv being available in your terminal.

Then run the below:

uvx --from git+https://github.com/wandb/wandb-mcp-server -- add_to_client --config_path "~/Library/Application Support/Claude/claude_desktop_config.json" --add_deno_path && uvx wandb login

Writing environment variables to the config file

The add_to_client function accepts a number of flags to enable writing optional environment variables to the server's config file. Below is an example of using the built-in convenience flag, --e2b_api_key, as well as setting other env variables that don't have dedicated flags.

# Write the server config file with additional env vars
uvx --from git+https://github.com/wandb/wandb-mcp-server -- add_to_client \
  --config_path ~/.codeium/windsurf/mcp_config.json \
  --e2b_api_key 12345abcde \
  --add_deno_path \
  --write_env_vars MCP_LOGS_WANDB_ENTITY=my_wandb_entity E2B_PACKAGE_ALLOWLIST=numpy,pandas

# Then login to W&B
uvx wandb login

Arguments passed to --write_env_vars must be space separated and the key and value of each env variable must be separated only by a =.

Manual Installation

  1. Ensure you have uv installed, see above installation instructions for uv.
  2. Get your W&B api key here
  3. Add the following to your MCP client config manually.
{
  "mcpServers": {
    "wandb": {
      "command": "uvx",
      "args": [
        "--from",
        "git+https://github.com/wandb/wandb-mcp-server",
        "wandb_mcp_server"
      ],
      "env": {
        "WANDB_API_KEY": "<insert your wandb key>",
      }
    }
  }
}

Running from Source

Run the server from source by running the below in the root dir:

wandb login && uv run src/wandb_mcp_server/server.py

Environment Variables

The full list of environment variables used to control the server's settings can be found in the .env.example file.

Available MCP tools

1. wandb

  • query_wandb_tool Execute queries against wandb experiment tracking data including Runs & Sweeps.

2. weave

  • query_weave_traces_tool Queries Weave traces with powerful filtering, sorting, and pagination options. Returns either complete trace data or just metadata to avoid overwhelming the LLM context window.

  • count_weave_traces_tool Efficiently counts Weave traces matching given filters without returning the trace data. Returns both total trace count and root traces count to understand project scope before querying.

3. W&B Support agent

  • query_wandb_support_bot Connect your client to wandbot, our RAG-powered support agent for general help on how to use Weigths & Biases products and features.

4. Python code sandbox

  • execute_sandbox_code_tool Execute Python code in secure, isolated sandbox environments, either a hosted E2B sandbox or a local Pyodide sandbox, WebAssembly-based execution that uses Deno to isolate execution from the host system (inspired by Pydantic AI's Run Python MCP). See sandbox setup instructions above.

    Sandbox Behavior:

    • E2B: Maintains a single persistent sandbox instance during the MCP server session. Files written in one execution are available in subsequent executions. The sandbox is automatically terminated when the server stops. Default E2B sandbox lifetime is 15 minutes of inactivity (configurable via E2B_SANDBOX_TIMEOUT_SECONDS), but is kept alive by code executions.
    • Pyodide: Maintains a persistent Pyodide environment for the lifetime of the MCP server. Files written in one execution are available in subsequent executions. The Pyodide process is initialized when the server starts and terminates when the server stops.

    File Operations:

    • Both sandboxes support standard Python file I/O operations
    • Query results from query_wandb_tool and query_weave_traces_tool can be automatically saved as json files in the sandbox if the LLM passes a filename to save_filename to the tool call
    • Use the save_filename parameter to save results: save_filename="my_data.json"
    • Files are saved to /tmp/ directory in the sandbox

5. Saving Analysis

  • create_wandb_report_tool Creates a new W&B Report with markdown text and HTML-rendered visualizations. Provides a permanent, shareable document for saving analysis findings and generated charts.

6. General W&B helpers

  • query_wandb_entity_projects List the available W&B entities and projects that can be accessed to give the LLM more context on how to write the correct queries for the above tools.

Usage tips

Provide your W&B project and entity name

LLMs are not mind readers, ensure you specify the W&B Entity and W&B Project to the LLM. Example query for Claude Desktop:

how many openai.chat traces in the wandb-applied-ai-team/mcp-tests weave project? plot the most recent 5 traces over time and save to a report

Avoid asking overly broad questions

Questions such as "what is my best evaluation?" are probably overly broad and you'll get to an answer faster by refining your question to be more specific such as: "what eval had the highest f1 score?"

Ensure all data was retrieved

When asking broad, general questions such as "what are my best performing runs/evaluations?" its always a good idea to ask the LLM to check that it retrieved all the available runs. The MCP tools are designed to fetch the correct amount of data, but sometimes there can be a tendency from the LLMs to only retrieve the latest runs or the last N runs.

Sandbox Configuration (Optional)

You can configure sandbox behavior using environment variables:

Disable Sandbox

  • DISABLE_CODE_SANDBOX: Set to any value to completely disable the code sandbox tool (e.g., DISABLE_CODE_SANDBOX=1)

Deno path

Use the --add_deno_path when using the add_to_client helper to automatically add Deno to your MCP configuration's PATH if Deno is installed but not in your system PATH. The flag automatically detects Deno installations from common locations including:

  • macOS/Linux: ~/.deno/bin, Homebrew (/opt/homebrew/bin, /usr/local/bin), MacPorts, system packages, Snap, Flatpak, asdf, vfox, Nix, Cargo, npm global
  • Windows: ~/.deno/bin, Scoop, Chocolatey, Winget, npm global, system locations

Package Installation Security

Control which packages can be installed in E2B sandboxes:

  • E2B_PACKAGE_ALLOWLIST: Comma-separated list of allowed packages (e.g., numpy,pandas,matplotlib)
  • E2B_PACKAGE_DENYLIST: Comma-separated list of denied packages (default includes potentially dangerous packages)

Cache Settings

  • E2B_CACHE_TTL_SECONDS: Execution cache TTL in seconds (default: 900 = 15 minutes)

E2B Sandbox Lifetime

  • E2B_SANDBOX_TIMEOUT_SECONDS: Sandbox lifetime in seconds (default: 900 = 15 minutes)
    • The sandbox will automatically shut down after this timeout if no code is executed
    • Each code execution resets the timeout
    • Example: E2B_SANDBOX_TIMEOUT_SECONDS=600 for 10-minute timeout

Troubleshooting

Authentication

Ensure the machine running the MCP server is authenticated to Weights & Biases, either by setting the WANDB_API_KEY or running the below to add the key to the .netrc file:

uvx wandb login

Error: spawn uv ENOENT

If you encounter an error like this when starting the MCP server:

Error: spawn uv ENOENT

This indicates that the uv package manager cannot be found. Fix this with these steps:

  1. Install uv using the official installation script:

    curl -LsSf https://astral.sh/uv/install.sh | sh

    or if using a Mac:

    brew install uv

  2. If the error persists after installation, create a symlink to make uv available system-wide:

    sudo ln -s ~/.local/bin/uv /usr/local/bin/uv

  3. Restart your application or IDE after making these changes.

This ensures that the uv executable is accessible from standard system paths that are typically included in the PATH for all processes.

Code Sandbox Issues

If the code execution tool is not available or failing, here's how to diagnose and fix common issues:

Deno/Pyodide Sandbox Issues

Problem: Deno is installed but not detected by the MCP server

Symptoms:

  • You can run deno --version in your terminal
  • But MCP server logs show "Pyodide not available (Deno not installed)"

Root Cause: Deno is not properly installed or not in the system PATH

Solutions:

  1. Install Deno correctly:

    curl -fsSL https://deno.land/install.sh | sh -s -- -y

  2. Add to your shell configuration:

    echo 'export PATH="$HOME/.deno/bin:$PATH"' >> ~/.bashrc  # for bash
echo 'export PATH="$HOME/.deno/bin:$PATH"' >> ~/.zshrc   # for zsh
source ~/.bashrc  # or ~/.zshrc

  3. Restart your MCP client (Claude Desktop, Cursor, etc.) to pick up the new PATH

  4. Verify your setup:

    # Check if Deno is installed
ls -la ~/.deno/bin/deno

# Test Deno works
deno --version

# Test in a fresh shell
zsh -c "deno --version"  # or bash -c "deno --version"

Quick Fix: If you have Deno installed but it's not being detected, try using the automatic PATH fix with the add_deno_path flag:

uvx --from git+https://github.com/wandb/wandb-mcp-server -- add_to_client \
  --config_path /path/to/your/mcp_config.json \
  --add_deno_path

This automatically searches for Deno in common installation locations across different package managers and installation methods.

Alternative: If PATH issues persist, you can manually specify the environment in your MCP client config:

{
  "mcpServers": {
    "wandb": {
      "command": "uvx",
      "args": ["--from", "git+https://github.com/wandb/wandb-mcp-server", "wandb_mcp_server"],
      "env": {
        "WANDB_API_KEY": "your-key",
        "PATH": "/Users/yourusername/.deno/bin:/usr/local/bin:/usr/bin:/bin"
      }
    }
  }
}
  1. Test the detection directly: 
    # Test if Python can detect Deno the same way the MCP server does
python3 -c "
import subprocess
try:
    result = subprocess.run(['deno', '--version'], capture_output=True, text=True, timeout=5)
    print(f'Success: {result.returncode == 0}')
    print(f'Output: {result.stdout}')
except FileNotFoundError:
    print('Deno not found in PATH')
except Exception as e:
    print(f'Error: {e}')
"

Problem: Deno installation fails or PATH issues persist

Solutions:

  • Reinstall Deno with automatic PATH setup: 
    curl -fsSL https://deno.land/install.sh | sh -s -- -y
source ~/.bashrc  # or ~/.zshrc
  • Verify installation location: 
    ls -la ~/.deno/bin/deno
E2B Sandbox Issues

Problem: E2B sandbox not available

Check: Verify your API key is set:

echo $E2B_API_KEY

Solutions:

  1. Get an API key from e2b.dev
  2. Set it in your environment or MCP configuration
  3. Restart your MCP server
General Debugging Steps
  1. Check MCP server startup logs for sandbox availability messages
  2. Verify environment variables are properly set in your MCP configuration
  3. Test sandbox detection using the Python snippet above
  4. Restart your MCP client (Claude Desktop, Cursor, etc.) after making configuration changes
  5. Check for conflicting installations - ensure you don't have multiple Deno installations
Still Having Issues?

If sandbox detection still fails:

  1. Check the exact error message in your MCP server logs
  2. Verify your shell configuration files (.zshrc, .bashrc) are properly formatted
  3. Try running the MCP server directly from terminal to see if PATH issues persist
  4. Consider using E2B as an alternative if Deno setup is problematic

Testing

The tests include a mix of unit tests and integration tests that test the tool calling reliability of a LLM. For now the integration tets only use claude-sonnet-3.7.

Set LLM provider API key

Set the appropriate api key in the .env file, e.g.

ANTHROPIC_API_KEY=<my_key>

Run 1 test file

Run a single test using pytest with 10 workers

uv run pytest -s -n 10 tests/test_query_wandb_gql.py

Test debugging

Turn on debug logging for a single sample in 1 test file

pytest -s -n 1 "tests/test_query_weave_traces.py::test_query_weave_trace[longest_eval_most_expensive_child]" -v --log-cli-level=DEBUG

Sandbox tests

Run sandbox-specific tests:

# Unit tests (with mocking, no real sandboxes needed)
uv run pytest tests/test_sandbox_execution.py -v

# Integration tests (requires E2B_API_KEY or Deno)
uv run pytest tests/test_sandbox_integration.py -v

# Run all sandbox tests
uv run pytest tests/test_sandbox*.py -v

For E2B tests, ensure E2B_API_KEY is set in your environment or .env file. For Pyodide tests, ensure Deno is installed and available in your PATH.

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