🛡️ Context-Aware RBAC with FastMCP + Vault + File-Based Policy

This demo MCP server supports pluggable policy enforcement using the Adapter pattern.
It implements context-aware RBAC using two interchangeable enforcers:

• ✅ HashiCorpVaultPolicyEnforcer
• ✅ FilePolicyEnforcer

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How It Works

When a tool is called via FastMCP, the PolicyMiddleware intercepts the request using the on_call_tool hook.
It delegates access checks to the active enforcer based on role and context.

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Mermaid Diagram

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Quick Start

1. Vault Mode

Install vault on your local machine

brew tap hashicorp/tap
brew install hashicorp/tap/vault

Start Vault in Dev Mode

vault server -dev

In a new terminal:

export VAULT_ADDR=http://127.0.0.1:8200
export VAULT_TOKEN=root

check the vault status by running vault status

Store Policies

vault kv put secret/mcp-policies/agent_worker tools='["generate_report", "check_status"]'
vault kv put secret/mcp-policies/user tools='["start_cluster", "read_user_data", "check_status"]'
vault kv put secret/mcp-policies/agent_supervisor tools='["terminate_cluster", "start_cluster", "generate_report"]'

So the tool access policy are as follows:

Tool Name	Description	Who Can Access
start_cluster()	Starts a compute cluster	agent_supervisor, user
check_status()	Checks service health	all roles
terminate_cluster()	Terminates compute cluster	agent_supervisor only
generate_report()	Creates compliance report	agent_worker, agent_supervisor
read_user_data()	Fetch PII data (sensitive)	user only

Run the Server

export POLICY_ENFORCER_MODE=vault
uv run test/test_policy.py

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3. File Mode

policy.json

{
  "agent_worker": ["generate_report", "check_status"],
  "user": ["start_cluster", "read_user_data", "check_status"],
  "agent_supervisor": ["terminate_cluster", "start_cluster", "generate_report"]
}

Run the Server

export POLICY_ENFORCER_MODE=file
uv run test/test_policy.py

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Test Example Tool Call

Example context:

{
  "role": "agent_worker"
}

If the tool is generate_report, access is granted ✅
If the tool is terminate_cluster, access is denied ❌

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Tools Provided

• start_cluster
• terminate_cluster
• generate_report
• check_status
• read_user_data

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Extending

You can add new enforcers (e.g., OPA, JWT, Permit) via the Adapter interface in BasePolicyEnforcer.

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Demo with MCP Inspector