Gatehouse MCP Server

mcp-gatehouse

Permission tiers, approval gates, and audit logging for MCP servers. The server is the gatekeeper: you decide what an AI can read, what it can write, and what's off-limits — and every action gets logged.

Most MCP servers hand the model every tool at full strength and keep no record of what it did. That's fine for a demo. It's not fine the day an agent has write access to your CRM, your books, or your order system. mcp-gatehouse is the missing gate, enforced inside the server — no proxy, no external policy service, no dependencies beyond the official mcp SDK.

pip install mcp-gatehouse

What you get

Quickstart

from mcp.server.mcpserver import MCPServer
from mcp_gatehouse import AccessTier, AuditLog, Gatehouse, Policy, terminal_approver

mcp = MCPServer("order-desk")
gatehouse = Gatehouse(
    mcp,
    policy=Policy(approver=terminal_approver),
    audit=AuditLog(path="~/order-desk/audit.jsonl"),
)

@gatehouse.tool(tier=AccessTier.READ)
def lookup_order(order_id: str) -> str:
    """Look up an order's status."""
    ...

@gatehouse.tool(tier=AccessTier.DESTRUCTIVE)
def cancel_order(order_id: str) -> str:
    """Cancel an order. Runs only if the approver says yes."""
    ...

mcp.run()

That's the whole integration: build your MCPServer exactly as the SDK docs show, but register tools through the gatehouse. Schema generation (including Annotated[..., Field(description=...)] parameter docs), transports, and everything else work unchanged — the guard preserves the function's signature. Sync tools still run in a worker thread, exactly as they would on a bare MCPServer.

terminal_approver asks on the controlling terminal (/dev/tty), never stdin — over stdio, stdin is the protocol pipe, so a plain input() approver would corrupt it. An approver is any callable, sync or async, that takes an ApprovalRequest and returns True to allow; sync ones run in a worker thread, so a human taking their time doesn't stall other calls. If the approver raises (Slack down, ticket API timing out), the call is denied and the failure is logged.

Under the default policy, DESTRUCTIVE requires approval and everything is audited. Gate writes too with one line:

Policy(require_approval=frozenset({AccessTier.WRITE, AccessTier.DESTRUCTIVE}), ...)

Add your own secret-bearing keys without losing the defaults:

from mcp_gatehouse import DEFAULT_REDACT
Policy(redact=DEFAULT_REDACT | {"card_pin"}, ...)

What the audit trail looks like:

{"ts": "2026-07-16T14:02:11+00:00", "tool": "lookup_order", "tier": "read", "outcome": "ok", "arguments": {"order_id": "4417"}, "duration_ms": 0.42}
{"ts": "2026-07-16T14:02:38+00:00", "tool": "add_note", "tier": "write", "outcome": "ok", "arguments": {"order_id": "4417", "note": "call back", "api_key": "«redacted»"}, "duration_ms": 1.08}
{"ts": "2026-07-16T14:03:05+00:00", "tool": "cancel_order", "tier": "destructive", "outcome": "denied", "reason": "approver refused", "arguments": {"order_id": "4417"}}

Try the demo

The package ships a runnable order-desk server with all three tiers wired up and a terminal-prompt approver. Add it to any MCP client that speaks stdio — for Claude Desktop, in claude_desktop_config.json:

{
  "mcpServers": {
    "gatehouse-demo": {
      "command": "uvx",
      "args": ["mcp-gatehouse", "--audit-log", "/tmp/gatehouse-audit.jsonl"]
    }
  }
}

Or run mcp-gatehouse-demo yourself after pip install mcp-gatehouse. Ask the model to list the orders and cancel one, then watch the verdict land in the audit log either way. The audit log goes to --audit-log, else $MCP_GATEHOUSE_AUDIT_LOG, else ./audit.jsonl — falling back to ~/.mcp-gatehouse/audit.jsonl when the working directory isn't writable (desktop clients often launch servers from /). The server prints the path it chose to stderr.

The approval prompt needs a terminal: a client that launches the server in the background has none, so cancel_order is denied — the gate failing closed, as designed. Run the server from a terminal to approve interactively. examples/orders_server.py is the same server as a copyable template.

Design notes

• Enforcement lives inside the server, at the tool boundary. A proxy can't see your tools' semantics, and a policy service is one more thing to deploy. A 40-person plant doesn't have a platform team; this is a few small classes and a JSONL file.
• Fail closed. Security defaults that quietly allow are worse than none. That includes redaction: argument values the scrubber can't take apart (arbitrary objects, bytes) are replaced with an opaque placeholder rather than passed through, and exception messages stay out of the log — only the exception type is recorded, because error text loves to embed the very values you just redacted.
• The audit log records denials and errors, not just successes — the calls that didn't happen are half the story.
• A blocking terminal approver and the stdio transport don't mix — stdout/stdin are the protocol pipe. terminal_approver prompts on /dev/tty for exactly that reason (and denies when no terminal exists). Real deployments should approve out-of-band: Slack, a ticket, a queue.
• What this is not: authentication, transport encryption, or a sandbox. It's a gate inside your server, not a perimeter around it. See SECURITY.md.

Compatibility

Targets the official mcp Python SDK v2.x (mcp>=2,<3) and Python 3.10–3.14. Ships type information (py.typed). Release notes: CHANGELOG.md.

The public API (Gatehouse, Policy, AuditLog, AccessTier) is unchanged across that line, as promised. Porting a v1 server is two import edits — FastMCP became MCPServer and moved to mcp.server.mcpserver; see the SDK's migration guide.

Staying on SDK v1 needs no action: 0.1.x pins mcp<2, so pip keeps resolving it. That line is closed to features but still gets security fixes.

Who built this

Nick George — I design and run MCP servers in production for a mid-market reverse logistics-tech company, and build them for businesses at nickgeorgeai.com. This library is the permission-and-audit discipline from those builds, extracted.

If you're an owner or operator wondering what MCP even is, start with the plain-English guide: What is an MCP server?

License

MIT