Intercept — open-source policy enforcement for MCP tool calls

[s-a-m-a-i]

Sharing something we built that's relevant to anyone running MCP servers in production: Intercept is a transparent proxy that enforces YAML policies on every tool call before it reaches the upstream server.

The problem: MCP gives agents access to every tool on a server with no granular controls. No rate limits, no argument validation, no audit trail at the protocol level.

Intercept wraps any MCP server and adds:

• Hard blocks (deny specific tools entirely)
• Rate limits (sliding window counters)
• Argument validation (regex, allowlists, numeric bounds)
• Spend caps (stateful daily/hourly counters)
• Full audit logging

All defined in a single YAML file. Enforcement is at the transport layer — below the model, invisible to the agent.

intercept -c policy.yaml -- npx -y @modelcontextprotocol/server-github

Works with any of the MCP servers in this repo. Open source, MIT licensed.

• GitHub: https://github.com/policylayer/intercept
• Site: https://intercept.policylayer.com