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Unleash MCP Server

A purpose-driven Model Context Protocol (MCP) server for managing Unleash feature flags. This server enables LLM-powered coding assistants to create and manage feature flags following Unleash best practices.

Experimental feature

The Unleash MCP server is an experimental feature. Functionality may change, and we do not yet recommend using it in production environments.

To share feedback, join our community Slack, open an issue on GitHub, or email us at beta@getunleash.io.

Overview​

This MCP server provides tools that integrate with the Unleash Admin API, allowing AI coding assistants to:

  • Create feature flags with proper validation and typing.
  • Detect existing flags to prevent duplicates or encourage reuse.
  • Evaluate changes to decide when a feature flag is needed.
  • Stream progress for visibility during operations.
  • Handle errors gracefully with helpful hints.
  • Follow best practices from the Unleash documentation.

Available tools​

The MCP server exposes the following tools:

  • create_flag: Creates flags via the Admin API.
  • evaluate_change: Scores risk and recommends feature flag usage.
  • detect_flag: Discovers existing flags to avoid duplicates.
  • wrap_change: Guides the LLM on how to guard code paths.
  • set_flag_rollout: Configures rollout strategies (does not enable environments).
  • get_flag_state: Surfaces feature metadata and environment strategies.
  • toggle_flag_environment: Enables or disables environments on demand.
  • remove_flag_strategy: Deletes strategies from an environment.
  • cleanup_flag: Generates instructions for safely removing flagged code paths.

Core workflow​

The core workflow for an AI assistant is designed to be:

  1. evaluate_change: First, assess a code change to see if a flag is needed.
  2. detect_flag: This is often called automatically by evaluate_change to prevent creating duplicate flags.
  3. create_flag: If a new flag is required, this tool creates it in Unleash.
  4. wrap_change: Finally, this tool provides the language-specific code to implement the new flag.

See more information on the core workflow tools in the Tool reference section.

Prerequisites​

Before you can run the server, you need the following:

  • Node.js 18 or higher
  • Yarn package manager or npm
  • An Unleash instance (hosted or self-hosted)
  • A personal access token with permissions to create feature flags

Get started​

This section covers the different ways to install and run the Unleash MCP server. You can either follow a setup for agents (such as Claude Code and Codex), run the MCP as a standalone process using npx, or use a local development setup.

Agent setup​

You can add the MCP server directly to Claude Code or Codex. Agent configurations are path-specific. You must run the following command from the root directory of the project where you want to use the MCP.

For Claude Code:

claude mcp add unleash \
    --env UNLEASH_BASE_URL={{your-instance-url}} \
    --env UNLEASH_PAT={{your-personal-access-token}} \
    -- npx -y @unleash/mcp@latest --log-level error

For Codex:

codex mcp add unleash \
    --env UNLEASH_BASE_URL={{your-instance-url}} \
    --env UNLEASH_PAT={{your-personal-access-token}} \
    -- npx -y @unleash/mcp@latest --log-level error

Quickstart with npx​

You can run the MCP server as a standalone process without cloning the repository using npx. Provide configuration through environment variables or a local .env file in the directory where you run the command:

UNLEASH_BASE_URL={{your-instance-url}} \
UNLEASH_PAT={{your-personal-access-token}} \
UNLEASH_DEFAULT_PROJECT={{default_project_id}} \
npx unleash-mcp --log-level debug

The CLI supports the same flags as the local build (for example, --dry-run, --log-level).

Local development setup​

Follow these steps to set up the project for local development.

  1. Install dependencies

Clone the repository and install dependencies using Yarn.

git clone https://github.com/Unleash/unleash-mcp.git
cd unleash-mcp
yarn install
  1. Configure environment variables

Copy .env.example to .env and fill in your Unleash credentials:

cp .env.example .env

Edit .env:

UNLEASH_BASE_URL={{your-instance-url}}
UNLEASH_PAT={{your-personal-access-token}}
UNLEASH_DEFAULT_PROJECT={{default_project_id}}  # Optional: the project the MCP should use by default
  1. Build the project
yarn build

Output will be in the dist/ directory.

  1. (Optional) Run checks

Run type checking, linting, and tests:

# Type checking and linting
yarn lint

# Run tests (the Vitest framework is configured, but no test suites yet)
yarn test

Running the server​

Development mode with hot reload

yarn dev

Production mode

node dist/index.js

With CLI flags

# Dry run mode (simulates API calls without actually creating flags)
node dist/index.js --dry-run

# Custom log level
node dist/index.js --log-level debug

# Combine flags
node dist/index.js --dry-run --log-level debug

Tool reference​

This section describes each of the core tools in detail, including its purpose, parameters, and output.

Create flag​

The create_flag tool creates a new feature flag in Unleash with comprehensive validation and progress tracking.

When to use​

Use this tool when you have already determined that a feature flag is required (for example, after running evaluate_change) and you are ready to create it with the correct type and metadata.

Parameters​

The tool accepts the following parameters:

  • name (required): Unique feature flag name within the project.
  • type (required): Feature flag type indicating lifecycle and intent.
    • release: Gradual feature rollouts to users.
    • experiment: A/B tests and experiments.
    • operational: System behavior and operational toggles.
    • kill-switch: Emergency shutdowns or circuit breakers.
    • permission: Control feature access based on user roles or entitlements.
  • description (required): Clear explanation of what the flag controls and why it exists.
  • projectId (optional): Target project (defaults to UNLEASH_DEFAULT_PROJECT).
  • impressionData (optional): Enable analytics tracking (defaults to false).

Usage example​

Agent prompt

Use create_flag with:
- name: "new-checkout-flow"
- type: "release"
- description: "Gradual rollout of the redesigned checkout experience"
- projectId: "ecommerce"

Tool payload

{
  "name": "new-checkout-flow",
  "type": "release",
  "description": "Gradual rollout of the redesigned checkout experience with improved conversion tracking",
  "projectId": "ecommerce",
  "impressionData": true
}

Tool output

On success, the tool returns a JSON object containing the new feature flag's URL in the Unleash Admin UI, an MCP resource link for programmatic access, creation timestamp, and configuration details.

Evaluate change​

The evaluate_change tool evaluates whether a code change should be behind a feature flag. It examines the structure, context, and potential risk of the change and returns a recommendation with an explanation and next steps.

When to use​

Use evaluate_change at the beginning of a feature or modification when you want to understand whether the work requires a feature flag. This tool is also helpful when you are unsure which flag type to use or want guidance on rollout planning.

How it works​

The tool returns detailed, markdown-formatted guidance for the LLM assistant based on Unleash best practices.

The guidance includes:

  • Parent flag detection: Checks if code is already protected by existing flags.
  • Risk assessment: Analyzes code patterns to identify risky operations.
  • Code type evaluation: Classifies the change (for example, test, config, feature, or bug fix).
  • Recommendation: Suggests whether to create a flag, use an existing flag, or skip the flag.
  • Next actions: Provides specific instructions on what to do next.

When evaluate_change determines a flag is needed, it provides explicit instructions to:

  1. Call create_flag tool to create the feature flag.
  2. Call wrap_change tool to get language-specific code wrapping guidance.
  3. Implement the wrapped code following the detected patterns.

The evaluation process

The tool follows a clear evaluation process:

Step 1: Gather code changes (git diff, read files)
        ↓
Step 2: Check for parent flags (avoiding nesting)
        ↓
Step 3: Assess code type (test? config? feature?)
        ↓
Step 4: Evaluate risk (auth? payments? API changes?)
        ↓
Step 5: Calculate risk score
        ↓
Step 6: Make recommendation
        ↓
Step 7: Take action (create flag or proceed without)

Risk assessment

The tool uses language-agnostic patterns to score risk:

  • Critical risk (Score +5): For example, auth, payments, security, and database operations.
  • High risk (Score +3): For example, API changes, external services, or new classes.
  • Medium risk (Score +2): For example, async operations or state management.
  • Low risk (Score +1): For example, bug fixes, refactors, or small changes.

Parent flag detection

The tool looks for common patterns across languages, such as:

  • Conditionals: if (isEnabled('flag')), if client.is_enabled('flag'):
  • Assignments: const enabled = useFlag('flag')
  • Hooks: const enabled = useFlag('flag') → {enabled && <Component />}
  • Guards: if (!isEnabled('flag')) return;
  • Wrappers: withFeatureFlag('flag', () => {...})

Parameters​

All parameters are optional, but more context leads to better recommendations:

  • repository (string): Repository name or path.
  • branch (string): Current branch name.
  • files (array): List of files being changed.
  • description (string): Description of the change.
  • riskLevel (enum): low, medium, high, or critical, as assessed by the user.
  • codeContext (string): Surrounding code for parent flag detection.

Usage example​

Agent prompt

Simple usage where you let the agent gather context:

Use evaluate_change to help me determine if I need a feature flag

Explicit instructions:

Use evaluate_change with:
- description: "Add Stripe payment processing"
- riskLevel: "high"

Tool payload

{
  "repository": "my-app",
  "branch": "feature/stripe-integration",
  "files": ["src/payments/stripe.ts"],
  "description": "Add Stripe payment processing",
  "riskLevel": "high",
  "codeContext": "surrounding code for parent flag detection"
}

Tool output

Returns a JSON object with the evaluation result, including a needsFlag boolean, a recommendation (e.g., "create_new"), a suggested flag name, risk level, and a detailed explanation.

{
  "needsFlag": true,
  "reason": "new_feature",
  "recommendation": "create_new",
  "suggestedFlag": "stripe-payment-integration",
  "riskLevel": "critical",
  "riskScore": 5,
  "explanation": "This change integrates Stripe payments, which is critical risk...",
  "confidence": 0.9
}

Detect flag​

The detect_flag tool finds existing feature flags in the codebase so you can reuse them instead of creating duplicates. This tool is automatically integrated into the evaluate_change workflow but can also be used manually.

When to use​

Use this tool before creating a new feature flag or during code evaluation to check for existing flags that might already cover your use case. This helps prevent flag duplication.

How it works​

The tool returns comprehensive search instructions and uses multiple detection strategies:

  • File-based detection: Search in files you're modifying for existing flags.
  • Git history analysis: Look for recently added flags in commit history.
  • Semantic name matching: Match descriptions to existing flag names.
  • Code context analysis: Inspect code around the change.

The tool then follows a scoring process:

Step 1: Execute file-based search (grep for flag patterns in target files)
        ↓
Step 2: Search git history for recent flag additions
        ↓
Step 3: Perform semantic matching (description → flag names)
        ↓
Step 4: Analyze code context (if provided)
        ↓
Step 5: Combine scores from all methods
        ↓
Step 6: Return best candidate with confidence score

Confidence levels

The tool returns candidates with confidence scores:

  • High ≥0.7: Strong match; reuse is recommended.
  • Medium 0.4-0.7: Possible match; review manually.
  • Low <0.4: Weak match; likely create a new flag.

Parameters​

  • description (required): Description of the change or feature. For example, "payment processing with Stripe", "new checkout flow".
  • files (optional): Files being modified. For example, ["src/payments/stripe.ts", "src/checkout/flow.ts"].
  • codeContext (optional): Nearby code to scan for flags.

Usage example​

Agent prompt

Check for existing flags before creating a flag:

Use detect_flag with description "payment processing with Stripe"

Integrated automatically in evaluation:

Use evaluate_change - automatically searches for existing flags

Tool payload

{
  "description": "payment processing with Stripe",
  "files": ["src/payments/stripe.ts"]
}

Tool output

Returns a JSON object indicating if a flag was found. If flagFound is true, it includes a candidate object with the flag's name, location, confidence score, and the reason for the match.

Match found:

{
  "flagFound": true,
  "candidate": {
    "name": "stripe-payment-integration",
    "location": "src/payments/stripe.ts:42",
    "context": "if (client.isEnabled('stripe-payment-integration')) {",
    "confidence": 0.85,
    "reasoning": "Found in same file you're modifying, added 2 days ago",
    "detectionMethod": "file-based"
  }
}

No match found:

{
  "flagFound": false,
  "candidate": null
}

Wrap change​

The tool wrap_change generates language-specific code snippets and guidance for wrapping code with feature flags. It helps LLMs and developers follow existing patterns in the codebase and use flags correctly.

When to use​

Use this tool after you have created a feature flag (with create_flag) and need to implement it in your code. It's especially useful when you want to ensure you are following existing codebase patterns or need framework-specific examples (e.g., React, Django).

How it works​

This tool is the final step in the evaluate_change → create_flag → wrap_change workflow.

The tool provides the following guidance in its response:

  1. Search instructions: Step-by-step guide for finding existing flag patterns in your codebase using grep.
  2. Pattern detection: Identifies common patterns (for example, imports, client variable names, method names, or wrapping styles).
  3. Default templates: Fallback code snippets if no patterns are found.
  4. Framework-specific examples: Specialized patterns for React, Express, Django, and others.
  5. Multiple patterns: If-blocks, guard clauses, hooks, decorators, middleware, and more.

Supported languages and frameworks:

  • TypeScript/JavaScript: Node.js, React Hooks, Express middleware.
  • Python: FastAPI, Django, Flask decorators.
  • Go: Standard if-blocks, HTTP middleware.
  • Ruby: Rails controllers.
  • PHP: Laravel controllers.
  • C#: .NET/ASP.NET controllers.
  • Java: Spring Boot.
  • Rust: Actix/Rocket handlers.

Parameters​

  • flagName (required): Feature flag name to wrap the code with. For example: "new-checkout-flow", or "stripe-integration".
  • language (optional): Programming language (auto-detected from fileName if not provided). Supported: typescript, javascript, python, go, ruby, php, csharp, java, rust
  • fileName (optional): File name being modified (helps detect language), For example: "checkout.ts", "payment.py", or "handler.go".
  • codeContext (optional): Surrounding code to help detect existing patterns.
  • frameworkHint (optional): Framework for specialized templates. For example, "React", "Express", "Django", "Rails", or "Spring Boot".

Usage example​

Agent prompt

Use wrap_change with:
- flagName: "new-checkout-flow"
- fileName: "src/components/checkout.ts"
- frameworkHint: "React"

Tool payload

{
  "flagName": "new-checkout-flow",
  "fileName": "checkout.ts",
  "frameworkHint": "React"
}

Tool output

Returns a comprehensive, markdown-formatted string that guides the user on how to wrap their code. This includes a quickstart, search instructions, wrapping instructions with placeholders, all available templates for the language, and links to SDK documentation.

# Feature Flag Wrapping Guide: "new-checkout-flow"

**Language:** TypeScript
**Framework:** React

## Quick Start
[Recommended pattern with import and usage]

## How to Search for Existing Flag Patterns
[Step-by-step Grep instructions]

## How to Wrap Code with Feature Flag
[Wrapping instructions with examples]

## All Available Templates
[If-block, guard clause, hooks, ternary, etc.]

Architecture​

The server follows a focused, purpose-driven design.

Structure​

src/
├── index.ts                     # Main server entry point
├── config.ts                    # Configuration loading and validation
├── context.ts                   # Shared runtime context
├── unleash/
│   └── client.ts                # Unleash Admin API client
├── tools/
│   ├── createFlag.ts            # create_flag tool
│   ├── evaluateChange.ts        # evaluate_change tool
│   ├── detectFlag.ts            # detect_flag tool
│   └── wrapChange.ts            # wrap_change tool
├── prompts/
│   └── promptBuilder.ts         # Markdown formatting utilities
├── evaluation/
│   ├── riskPatterns.ts          # Risk assessment patterns
│   └── flagDetectionPatterns.ts # Parent flag detection patterns
├── detection/
│   ├── flagDiscovery.ts         # Flag discovery strategies
│   └── flagScoring.ts           # Scoring and ranking logic
├── knowledge/
│   └── unleashBestPractices.ts  # Best practices knowledge base
├── templates/
│   ├── languages.ts             # Language detection and metadata
│   ├── wrapperTemplates.ts      # Code wrapping templates
│   └── searchGuidance.ts        # Pattern search instructions
└── utils/
    ├── errors.ts                # Error normalization
    └── streaming.ts             # Progress notifications

Design principles​

  • Thin surface area: Only the endpoints needed for the core capabilities.
  • Purpose-driven: Each module serves a specific, well-defined purpose.
  • Explicit validation: Zod schemas validate all inputs before API calls.
  • Error normalization: All errors converted to {code, message, hint} format.
  • Progress streaming: Long-running operations provide visibility.
  • Best practices integration: Guidance from Unleash docs embedded in tool descriptions.

Configuration​

This section provides a quick reference for all configuration options.

Environment variables:

  • UNLEASH_BASE_URL: Your Unleash instance URL (required).
  • UNLEASH_PAT: Personal access token (required).
  • UNLEASH_DEFAULT_PROJECT: The default project ID the MCP should use (optional).

CLI flags:

  • --dry-run: Simulate operations without making actual API calls.
  • --log-level: Set logging verbosity (debug, info, warn, error).

Best practices​

This server encourages Unleash best practices from the official documentation:

Flag lifecycle​

  1. Create with intent: Choose the right flag type to signal purpose.
  2. Document clearly: Write descriptions that explain the "why".
  3. Plan for cleanup: Feature flags are temporary; plan their removal.
  4. Monitor usage: Enable impression data for important flags.

Flag types​

  • Release flags: For gradual feature rollouts (remove after full rollout).
  • Experiment flags: For A/B tests (remove after analysis).
  • Operational flags: For system behavior (longer-lived, review periodically).
  • Kill switches: For emergency controls (maintain until feature is stable).
  • Permission flags: For access control (longer-lived, review permissions).

Naming conventions​

  • Use kebab-case: new-checkout-flow
  • Be descriptive: enable-ai-recommendations not flag1.
  • Include scope when needed: mobile-push-notifications.

API reference​

This server uses the Unleash Admin API. For complete API documentation, see:

Endpoints used​

  • POST /api/admin/projects/{projectId}/features - Create feature flag

Troubleshooting​

Configuration issues​

Error: "UNLEASH_BASE_URL must be a valid URL": Ensure your base URL is complete, including protocol. For example, https://app.unleash-hosted.com/instance. Remove any trailing slashes.

Error: "UNLEASH_PAT is required": Check that your .env file exists and contains UNLEASH_PAT={{your-personal-access-token}}. Verify that the token is valid in Unleash.

API issues​

Error: "HTTP_401": Your personal access token may be invalid or expired. Generate a new token under Profile > View Profile settings > Personal API tokens > New token.

Error: "HTTP_403": Your token doesn't have permission to create flags in this project. Review your role and permissions in Unleash.

Error: "HTTP_404": The project ID doesn't exist. Confirm the project ID in Unleash Admin UI.

Error: "HTTP_409": A flag with this name already exists in the project. Use a different name or reuse the existing flag.

License​

MIT

Contributing​

This is a purpose-driven project with a focused scope. Contributions should:

  • Align with the three core capabilities (create, evaluate, wrap).
  • Maintain the thin, purpose-driven architecture.
  • Follow Unleash best practices.
  • Include clear documentation.

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