OPA MCP

OPA MCP Server

A Model Context Protocol (MCP) server that turns any MCP-compatible client (Claude Desktop, Claude Code, Cursor, VS Code, Windsurf, Zed, and others) into a first-class Open Policy Agent and Rego authoring environment.

+--------------------+ MCP/stdio  +-----------------+ spawn/HTTP +---------------------+

| Claude · Cursor · |----------> | @orygn/opa-mcp |----------> | opa · regal | | VS Code · ... |<---------- | |<---------- | conftest · REST API | +--------------------+ 52 tools +-----------------+ +---------------------+

Status: v0.9.1. Tool surface, error codes, and

environment variables follow SemVer from

v0.1.0 forward.

Upgrading to 0.9.0: a tool refuses an argument it does not declare,

and the error names it; it used to be dropped. A policy or query that does

not compile is INVALID_REGO in every tool that runs opa, where several

returned EVAL_ERROR. rego_explain_decision returns the trace as

readable lines, not OPA's raw events. conftest_test reports a run that

evaluated no rule as passed: false, and rego_generate_test_skeleton

writes todo_test_ stubs, which opa test skips until they are filled in.

Responses are compact JSON.

Upgrading to 0.8.0: opa_exec loads dataPaths as opa eval --data

does, so a directory that also holds test fixtures, or JSON and YAML that

is not data, can now fail to load; pass it as bundle to load it as

before. A policy that does not load is INVALID_REGO in opa_exec and

the conftest tools, where it was EVAL_ERROR and UNKNOWN_ERROR.

Upgrading to 0.7.0: Node.js 22 or later is required. The bundled OPA

is 1.21, which reads YAML against the 1.2 schema: bare yes, no, on

and off in data files are strings now, not booleans. If you supply your

own binary via OPA_BINARY or PATH, only the Node requirement applies.

Upgrading to 0.6.0: rego_bench reports iterations, nsPerOp,

allocsPerOp and bytesPerOp. The fields opa prints (N, T, Bytes,

MemAllocs, MemBytes, Extra) were top-level and now sit under raw

for a single run, so anything that read them from the top level has to

look there. With count above one, raw is omitted: every document is in

runs, and fastest indexes the one the top-level figures come from.

Upgrading to 0.4.0: subprocesses no longer inherit the server's

environment. A policy that read a variable through opa.runtime().env

will no longer see it; name the variable in OPA_MCP_PASSTHROUGH_ENV if

it is genuinely needed. See the security section for why.

Upgrading to 0.3.0: the bundled OPA is now 1.19, so Rego v0 policies

no longer parse (if is required before a rule body, contains before a

partial set). Run rego_migrate_v1 to convert them, or pass

v0Compatible: true to load one as it is. If you supply your own binary via

OPA_BINARY or PATH, nothing changes.

Table of contents

• What you can do with it
• Why this MCP
• Install
• Configuration
• Tool reference
• Prompts
• Resources
• Cookbook
• Architecture
• Security
• Troubleshooting
• Development
• Versioning & support
• License

What you can do with it

Once an MCP client is connected, an agent can:

• Author Rego. Generate, format, and refactor policies. The server runs the real opa fmt and opa parse so output is byte-identical to what you'd get on the command line, and regal (optional) surfaces idiomatic suggestions.
• Evaluate against data. Run a query against a policy and an input document. Optional --explain, --profile, and --coverage flags surface execution traces, hot rules, and per-line coverage.
• Debug a deny. rego_explain_decision walks the agent through every rule that fired (and every one that didn't), so it can answer "why was this rejected" without you reading the trace by hand.
• Manage policies on a running OPA. List, get, put, delete policies on an OPA server through its REST API. Works against a local opa run --server or a production deployment with bearer-token auth.
• Build & sign bundles. Package a directory of policies into a deployable bundle, optionally signing it. Output is a regular .tar.gz the agent can hand to your delivery system.
• Lint. rego_lint runs Regal across a directory or a single file and returns each finding with its category, level and location.

A walk-through of a typical session lives in Cookbook.

Why this MCP

OPA already has a perfectly good CLI and REST API. So why an MCP wrapper?

• Schema-shaped tool surface. An agent calling rego_eval gets a validated input schema, a structured output envelope, and stable error codes, instead of parsing free-form CLI text and inventing its own failure taxonomy. That alone makes Rego usable to an agent the way a language server makes a language usable to an IDE.
• Higher-level helpers. rego_explain_decision, rego_generate_test_skeleton, rego_describe_policy, and rego_suggest_fix compose the lower-level primitives into the tasks agents are actually asked to do. They don't exist in the OPA CLI.
• Curated knowledge. The bundled MCP resources expose the OPA built-in function catalog, the official Rego style guide (formatted for LLMs), and a curated pattern library covering RBAC, ABAC, Kubernetes admission, IaC gates, API authz, and rate limiting, so the agent has authoritative context without needing to scrape it.
• Safety boundaries the agent can rely on. Path allow-list, subprocess timeouts, and response-size caps. Defaults are conservative; running the server doesn't quietly grant the agent more reach than the operator intended.

If you've ever watched an agent fight opa eval's argument order, you'll recognize the gap this fills.

Install

The server runs locally over stdio. Pick the install path that matches your client.

Claude Desktop

Edit claude_desktop_config.json directly (or copy from examples/claude-desktop.json):

{

"mcpServers": { "opa": { "command": "npx", "args": ["-y", "@orygn/opa-mcp"], "env": { "OPA_BINARY": "/usr/local/bin/opa", "REGAL_BINARY": "/usr/local/bin/regal", "OPA_URL": "http://localhost:8181", "OPA_MCP_ALLOWED_PATHS": "/path/to/your/policies" } } } }

Replace the /usr/local/bin/... paths with your real ones. See the

first-time install gotcha

below. Windows users substitute C:\\path\\to\\opa.exe.

Or download opa-mcp.mcpb from the latest release and double-click it.

Claude Code (CLI)

Register the server for the current project with claude mcp add:

claude mcp add \

--env OPA_BINARY=/usr/local/bin/opa \ --env REGAL_BINARY=/usr/local/bin/regal \ --env OPA_MCP_ALLOWED_PATHS=/path/to/your/policies \ opa -- npx -y @orygn/opa-mcp

This writes the config into .mcp.json at your project root and is picked up automatically on every claude session in that directory. Add --scope user to register it globally instead.

Replace the paths with your real absolute paths (same caveat as

Claude Desktop above). On Windows use C:\path\to\opa.exe syntax.

Persistent context and auto-checks for policy repos. If you work in an OPA policy repo regularly, two extra files remove repetitive setup from every session:

• examples/CLAUDE.md -- copy to your repo root or .claude/CLAUDE.md. Claude Code loads it every session, so the agent always knows which tools to use and what conventions apply.
• examples/claude-code-hook.json -- merge the hooks block into .claude/settings.json. Runs opa check after Claude writes or edits a .rego file and hands any error back to Claude, without a manual tool call.

Cursor

Drop examples/cursor.json into either .cursor/mcp.json (project-scoped) or ~/.cursor/mcp.json (user-scoped).

VS Code (GitHub Copilot Chat)

Drop examples/vscode.json into .vscode/mcp.json, or paste the servers block into your user settings.json under mcp.servers.

Windsurf, Zed, and others

See examples/ for a full set of drop-in configs.

Manual install (any MCP client)

npm install -g @orygn/opa-mcp

opa-mcp --version

then point your client at the opa-mcp binary.

Docker

docker pull orygn/opa-mcp:latest

docker run --rm -i \ -v /path/to/your/policies:/policies:ro \ -e OPA_MCP_ALLOWED_PATHS=/policies \ orygn/opa-mcp

The image is multi-arch (linux/amd64, linux/arm64), bundles pinned versions of opa and regal, and runs as a non-root user. No host install of OPA or Regal is required.

⚠ If every tool call returns OPA_BINARY_NOT_FOUND

The npm package carries its own opa for the five platforms it is built for, so a client PATH without opa on it does not matter there. The MCPB has no bundled copy, and on any other platform neither does npm: then the server boots but every tool call returns OPA_BINARY_NOT_FOUND. Neither the npm package nor the MCPB bundles regal or conftest, and the Docker image ships regal but not conftest, so their tools need a PATH entry or an explicit path either way.

Fix: add OPA_BINARY and REGAL_BINARY env entries to your client config with the absolute path to each binary. The example configs under examples/ ship with placeholder paths you replace. Find the real paths with:

which opa && which regal                                    # macOS / Linux

Get-Command opa, regal | Select-Object Source              # Windows

This does not affect the Docker install path, which ships opa and regal in the image and bypasses PATH entirely. The MCPB bundle carries neither, and unlike the npm install has no bundled fallback: set OPA_BINARY or put opa on PATH. See Troubleshooting for full detail.

Configuration

The server reads its configuration from environment variables. Every variable is optional; defaults are sensible for a local OPA on http://localhost:8181.

| Variable | Default | Purpose