GrowthBook MCP Thin

A thin MCP server for GrowthBook with four tools:

Competence lives in the skills repo and is bundled at build time. Capability is split into read vs write API tools (no per-endpoint formatters) so clients can honor readOnlyHint / destructiveHint correctly.

Tools are prefixed with growthbook_ so they stay unambiguous when a client has multiple MCP servers loaded.

Install / run

npm install
npm run build

Point your MCP client at the compiled entrypoint:

{
  "mcpServers": {
    "growthbook": {
      "command": "node",
      "args": ["/absolute/path/to/growthbook-mcp/server/index.js"],
      "env": {
        "GB_API_KEY": "your_api_key_or_pat",
        "GB_API_URL": "https://api.growthbook.io"
      }
    }
  }
}

Or run the published package:

npx @growthbook/mcp

Environment variables

HTTP + OAuth mode

OAUTH_AS_ENABLED=1  # on the GrowthBook API
GB_MCP_TRANSPORT=http GB_API_URL=http://localhost:3100 GB_MCP_PORT=3333 npm start

Clients connect to:

• http://127.0.0.1:3333/mcp — full (skills + API read/write)
• http://127.0.0.1:3333/mcp/api — capability-only (growthbook_api_read + growthbook_api_write)

Unauthenticated requests receive 401 with WWW-Authenticate pointing at /.well-known/oauth-protected-resource, which advertises the GrowthBook Authorization Server.

Before handling MCP, the server probes GrowthBook REST (GET /api/v1/) with the bearer. A 401 from that probe (or later from an API tool) yields HTTP 401 with error="invalid_token" so the MCP client can refresh — instead of surfacing "This API key has expired" as a tool error. A 403 is treated as an accepted bearer (permission denied ≠ invalid token) so clients are not forced into a refresh loop.

Capability-only mode

HTTP (recommended for remote): point the client at /mcp/api instead of /mcp:

{
  "mcpServers": {
    "growthbook": {
      "url": "http://127.0.0.1:3333/mcp/api"
    }
  }
}

stdio / process-wide: set env so skills are never registered:

"env": {
  "GB_API_KEY": "...",
  "GB_SKILLS_ENABLED": "false"
}

When skills are disabled, only the API read/write tools are registered. growthbook_list_skills and growthbook_read_skill are not exposed.

How skills are bundled

npm run build   # tsc && bundle-skills

scripts/bundle-skills.mjs copies every skills/*/SKILL.md from the canonical skills checkout into server/skills/<name>.md.

Source path resolution:

1. SKILLS_SRC env var (path to the skills repo root), or
2. ../skills (sibling directory)

The skills repo stays the source of truth — this package never forks skill content.

Using skills with the API tools

Bundled skills still show workflows as:

gb-call GET /api/v1/projects
gb-call POST /api/v2/features ./payload.json

This MCP server does not shell out to gb-call. Map GET → growthbook_api_read and POST/PUT/PATCH/DELETE → growthbook_api_write with the same path and optional JSON body string. Server instructions and growthbook_read_skill output include this bridge note.

Tools detail

growthbook_api_read / growthbook_api_write

{ "path": "/api/v1/projects" }
{ "method": "POST", "path": "/api/v2/features", "body": "{\"id\":\"my-flag\",...}" }

• Read: GET only (readOnlyHint: true)
• Write: POST | PUT | PATCH | DELETE (destructiveHint: true)
• Returns raw response body on 2xx
• On non-2xx, returns an actionable error (isError: true) covering auth failures, self-hosted 404 hints, and rate limits
• Freeform paths target the GrowthBook REST API

growthbook_list_skills / growthbook_read_skill

Only registered when GB_SKILLS_ENABLED is not disabled. growthbook_read_skill returns the full SKILL.md content so the agent can follow workflow steps and guardrails.

Development

# Requires a sibling checkout at ../skills (or SKILLS_SRC)
npm install
npm run build
npm start

Standalone HTTP mode

By default the server runs over stdio. Set GB_MCP_TRANSPORT=http to run it as a standalone HTTP server that exposes MCP at /mcp (skills + API tools) and /mcp/api (capability-only), behind an OAuth 2.0 protected-resource surface (RFC 9728 metadata + RFC 6750 WWW-Authenticate).

• GB_MCP_URL (required in HTTP mode) — the server's public base URL. It is stamped into the OAuth resource (audience) and the protected-resource metadata, so it is never derived from request headers. The server refuses to start without it.
• GB_MCP_PORT (default 3333) and GB_MCP_HOST (default 127.0.0.1).
• Incoming bearers are validated by probing the GrowthBook REST API; a rejected token gets HTTP 401 + WWW-Authenticate so the client can refresh.

Run it on a trusted network or bound to loopback. For a multi-tenant or public deployment, front it with your own gateway/auth.

Releases

Cutting a release is deliberate: bump the version in package.json, then push a matching v* tag:

git tag v2.0.0
git push origin v2.0.0

That tagged commit (with skills frozen at cut time) publishes:

• @growthbook/mcp to npm — prereleases (versions with a -, e.g. 2.0.0-beta.1) go under the beta dist-tag; stable versions become latest
• a multi-arch (amd64 + arm64) image to ghcr.io/growthbook/growthbook-mcp (:<version>, plus :<major>, :<major>.<minor>, and :latest for stable releases)
• an entry in the MCP registry
• a GitHub Release

Install a release with npx @growthbook/mcp@<version> or pull ghcr.io/growthbook/growthbook-mcp:<version>.