SentinelMCP

Stateless enterprise policy firewall & token-cost proxy for MCP.

Drop it in front of any Model Context Protocol server. Every tool call gets checked for identity, policy, and budget before it reaches your infrastructure — no database, no sticky sessions, nothing new to operate at 3am.

The 2-minute problem

MCP made it trivial to wire an LLM agent up to real tools — databases, payment APIs, internal services. It did not ship a policy layer. If you've run an MCP server in production, you've probably already hit one of these:

Runaway loops. An agent gets stuck retrying a failing call, or a bad prompt sends it into a call-and-recall spiral. Nothing in the MCP spec stops it. You find out from the bill, not an alert.

Token budgets that don't exist until they explode. Nothing in a typical agent loop stops it from calling the same expensive tool hundreds of times in an hour. The common failure mode: a $40/day budget quietly becomes a $40k/month bill, and the first signal anyone gets is the invoice.

DNS-rebinding and confused-deputy risk. The Streamable HTTP transport spec requires Origin validation for exactly this reason — a malicious page can rebind DNS to reach a local MCP server your browser would otherwise block. Most self-hosted servers don't implement it. Stack a compromised or injected tool result on top, and "just add an MCP server" is a bigger attack surface than most teams have budgeted time to secure.

SentinelMCP won't stop a model from being tricked — no proxy can. What it does is make sure a tricked or runaway agent can't reach a tool it isn't allowed to call, blow through a budget nobody approved, or keep running once the numbers look wrong — without a human getting a say.

Related MCP server: arc-gate-mcp

Key features

Zero-trust security

• Origin validation on every request — the DNS-rebinding mitigation the Streamable HTTP spec requires and most self-hosted servers skip. Loopback-only by default; explicit allowlist for anything else, override not extend.

• ID-JAG identity enforcement — real JWKS-backed JWT verification, asymmetric algorithms only (no HMAC-downgrade path), including the resource claim binding that closes the confused-deputy hole: a token issued for one MCP server can't be replayed against another.

Token economics

• Structural token estimation — walks the actual JSON-RPC payload instead of length / 4, pricing natural-language content and JSON structure at different, calibrated rates. Input measured exactly; output projected from a per-tool rolling average and trued up against the real response after every call.

• 7-day sliding-window budgets per team, enforced against a fixed-memory circular buffer (168 hourly buckets) — bounded, no per-request log to prune, no unbounded growth under load.

• Spike detection independent of the hard cap — a sudden burst against a team's own recent baseline gets caught even while the cumulative total is nowhere near the limit. Catches the exponential-growth pattern before the raw budget does.

Human-in-the-loop, not hard failure

• A budget breach doesn't 500 the call. It returns InputRequiredResult — a normal JSON-RPC success — carrying the projected overrun, the run-rate spike, and an HMAC-signed, request-bound resumeToken.

• POST /mcp/resume with { resumeToken, decision: "APPROVE" | "DENY", originalRequest } lets a human clear it. The token is bound to the exact call via a canonical-JSON hash of the request — approve this wire transfer, not whatever an agent decides to substitute next.

• Delivery is transport-aware: the pause notice reaches the client over whichever channel the original call used.

Protocol support

• Streamable HTTP — the current MCP standard (spec 2025-03-26+), and the primary path.

• Legacy HTTP+SSE (spec 2024-11-05) for backward compatibility with clients that haven't migrated. Not the default; don't build new integrations against it.

Quickstart

git clone https://github.com/your-org/sentinelmcp.git
cd sentinelmcp
npm install
cp .env.example .env

Six required values in .env — everything else ships with a sane default:

npm run build && npm start   # production
# or, for local iteration:
npm run dev                  # tsx watch

Confirm it's alive:

curl http://localhost:8080/healthz

How it fits

flowchart LR
    Client(["MCP Client<br/>Claude Desktop · Cursor · custom agent"])

    subgraph Sentinel["SentinelMCP — stateless gateway"]
        direction TB
        Origin["Origin Guard<br/>DNS-rebinding check"]
        Policy["Policy Gate<br/>tool denylist"]
        Identity["Identity Gate<br/>ID-JAG · JWKS verify"]
        Budget["Budget Gate<br/>sliding window + spike detect"]
        Origin --> Policy --> Identity --> Budget
    end

    Upstream["Upstream MCP Server(s)"]
    Resume["POST /mcp/resume<br/>HMAC-signed resumeToken"]

    Client -->|"POST /mcp"| Origin
    Budget -->|"allowed → forward"| Upstream
    Upstream -->|"result"| Client
    Budget -.->|"denied → paused<br/>InputRequiredResult"| Client
    Client -.->|"human decides,<br/>out of band"| Resume
    Resume -->|"APPROVE<br/>budget bypassed only —<br/>identity + policy still enforced"| Upstream

SentinelMCP sits in front of one or more upstream MCP servers as a reverse proxy. Every gate runs on every request; a denial short-circuits before the call ever reaches your tools. Only the budget gate can be bypassed, and only by an explicit, signed, single-call human approval — identity and policy are re-checked even on resume.

Two of the five subsystems keep state in memory (SSE session tracking and the budget ledger) — the request/response path itself is stateless and horizontally scalable behind a plain load balancer. See docs/ENGINEERING.md for exactly which pieces, and what that means for running more than one replica.

Quality

• 63 tests, 19 suites — npm test. Unit coverage for the token estimator, the sliding-window budget algorithm (spike detection included, verified with an injectable clock rather than waiting real days), and the HMAC token service (round-trip, tampering, forged secrets, canonical-hash binding). Integration coverage runs the full request pipeline through fastify.inject() against a real, ephemeral-port mock identity provider and mock upstream — no stubbed JWT verification, no mocked crypto.

• Strict TypeScript — strict, noUncheckedIndexedAccess, noImplicitOverride. No any in the source tree.

• Zero test-framework dependencies — built on Node's own node:test.

• CI runs typecheck, the full suite, and a production build on every push, against Node 20 and 22.

The full engineering log — what's verified, every bug found during development and how it was fixed, and every known limitation stated plainly rather than glossed over — lives in docs/ENGINEERING.md.

License

MIT