mcp-authzen

MCP server that fronts an OpenID AuthZEN 1.0 PDP.

Sends a subject + resource + action + context bundle to a real Policy Decision Point (OPA-AuthZEN, Topaz, your own) and returns the decision. Use it when "can alice delete this?" should be answered by policy code, not by the model's guess.

Conforms to AuthZEN 1.0 §6 (single-evaluation request/response) and §5.5 (decision entity). Batch evaluation (§7) is not yet exposed as a tool — file an issue if you need it.

Install

go install github.com/0-draft/mcp-authzen@latest

Pre-built signed binaries are on the releases page.

Quickstart

# Point at a real PDP (or your local opa-authzen-plugin on :8181)
export AUTHZEN_PDP_URL=http://localhost:8181/access/v1/evaluation

# Optional: PDP behind auth. Bare values are auto-prefixed with "Bearer ";
# values starting with "Bearer " or "Basic " pass through verbatim.
# export AUTHZEN_PDP_TOKEN=eyJhbGciOi...

# Run the smoke test (spins up an in-process fake PDP, exercises the
# full MCP handshake, asserts the decision is forwarded correctly)
make smoke

Wire it to Claude Code

AUTHZEN_PDP_URL=http://localhost:8181/access/v1/evaluation \
  claude mcp add authzen -- mcp-authzen

Then in a session:

Can alice (role=admin) delete doc-42 (owner=bob)?

The model builds the AuthZEN bundle, calls authzen_evaluate, returns the PDP's decision.

Wire it to Cursor / other clients

{
  "mcpServers": {
    "authzen": {
      "command": "mcp-authzen",
      "env": { "AUTHZEN_PDP_URL": "http://localhost:8181/access/v1/evaluation" }
    }
  }
}

Tool: authzen_evaluate

Param	Required	Description
subject	yes	JSON object. AuthZEN §5.1, e.g. {"type":"user","id":"alice"}.
resource	yes	JSON object. AuthZEN §5.2, e.g. {"type":"doc","id":"doc-1"}.
action	yes	JSON object. AuthZEN §5.3, e.g. {"name":"read"}.
context	no	JSON object with runtime context. AuthZEN §5.4.
pdp_url	no	Override AUTHZEN_PDP_URL for this call.

Returns AuthZEN's {"decision": <bool>, "context": {...}} as JSON.

Test against opa-authzen-plugin

kanywst/opa-authzen-plugin is a reference AuthZEN PDP built on OPA.

# In one terminal — start the PDP on :8181
git clone https://github.com/kanywst/opa-authzen-plugin
cd opa-authzen-plugin && make run

# In another — wire mcp-authzen
export AUTHZEN_PDP_URL=http://localhost:8181/access/v1/evaluation
mcp-authzen

Layout

Flat by design. A single-binary MCP server with one tool does not need cmd/, internal/, or pkg/. When batch (/access/v1/evaluations) or the search APIs (§8) land, they get sibling files, not subpackages.

.
├── main.go         # server bootstrap + tool registration
├── main_test.go    # httptest-driven PDP round-trips
├── scripts/smoke.sh
├── .goreleaser.yml
└── .github/

Verify a release

Releases ship a cosign-signed checksum file (Sigstore keyless via GitHub OIDC) and a CycloneDX SBOM per archive.

TAG=v0.1.0
gh release download "$TAG" -R 0-draft/mcp-authzen -p '*-checksums.txt*'

cosign verify-blob \
  --certificate "mcp-authzen-${TAG#v}-checksums.txt.pem" \
  --signature   "mcp-authzen-${TAG#v}-checksums.txt.sig" \
  --certificate-identity-regexp 'https://github.com/0-draft/mcp-authzen/.github/workflows/release.yml@refs/tags/' \
  --certificate-oidc-issuer 'https://token.actions.githubusercontent.com' \
  "mcp-authzen-${TAG#v}-checksums.txt"

License

MIT.