MCPSentry

🛡️ mcp-rampart

Security ramparts for FastAPI apps exposed as MCP servers.
Pre-flight audit. Runtime prompt-injection guardrail. One package.

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from fastapi import FastAPI
from mcp_rampart import MCPRampart

app = FastAPI()
# ... your existing routes ...

rampart = MCPRampart(app)                       # 1. Speak MCP.
report = rampart.audit()                        # 2. Audit what you'd expose.
if report.has_blockers():
    report.print_text(); raise SystemExit(1)

rampart.enable_guardrails(policy="block")       # 3. Block prompt-injection at runtime.

TL;DR. MCP security tooling is fragmenting into layers. mcp-rampart is the only library that lives inside your MCP server — auditing the routes you're about to expose and scanning the arguments of every tools/call request. Everything else (gateways, firewalls, config scanners) lives elsewhere on the wire.

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The 4 layers of MCP security

MCP went from "experiment" to 97M+ installs per month in 2026. Security tooling caught up only recently, and most of it solves a different problem than the one you have. Here's the map:

┌─────────────────────────────────────────────────────────────────┐
│  Layer 1 — The LLM itself (Claude, GPT, Gemini)                  │
│  Worry: hallucination, jailbreaks at the model level             │
│  → out of scope for everyone — model provider's problem          │
└──────────────────────┬──────────────────────────────────────────┘
                       │
                       ▼  (JSON-RPC over MCP transport)
┌─────────────────────────────────────────────────────────────────┐
│  Layer 2 — The MCP CLIENT (Claude Desktop, Cursor, agents)       │
│  Worry: the LLM calls something risky or exfiltrates data        │
│  Tools: pipelock, mcp-firewall, SecretiveShell/MCP-Bridge        │
└──────────────────────┬──────────────────────────────────────────┘
                       │
                       ▼  (HTTP / SSE)
┌─────────────────────────────────────────────────────────────────┐
│  Layer 3 — The GATEWAY / proxy in front of the MCP server        │
│  Worry: who's allowed to talk to this server, with what auth     │
│  Tools: apache/casbin-gateway, hyprmcp/mcp-gateway               │
└──────────────────────┬──────────────────────────────────────────┘
                       │
                       ▼
┌─────────────────────────────────────────────────────────────────┐
│  Layer 4 — The MCP SERVER itself ← 🛡️ mcp-rampart                │
│  Worry: did I expose dangerous routes? is an injection hiding    │
│         inside the arguments of every tools/call?                │
│  Tools: mcp-rampart (this project)                               │
└──────────────────────┬──────────────────────────────────────────┘
                       │
                       ▼  (which servers are even installed?)
┌─────────────────────────────────────────────────────────────────┐
│  Layer 5 — The USER's MCP config (~/.mcp.json, etc.)             │
│  Worry: am I installing a malicious server on my machine         │
│  Tools: apisec-inc/mcp-audit, ModelContextProtocol-Security/     │
│         mcpserver-audit                                          │
└─────────────────────────────────────────────────────────────────┘

Layer	Question it answers	Representative tool
1	"Is the model itself safe?"	(model provider)
2	"Is my agent leaking / calling something risky?"	pipelock (583⭐)
3	"Who's allowed to talk to my server, with what auth?"	casbin-gateway (559⭐), hyprmcp/mcp-gateway (92⭐)
4	"Did I just hand a language model access to my admin endpoints? Is an injection sneaking into the args of every call?"	mcp-rampart
5	"Is this MCP server I'm installing actually safe?"	apisec mcp-audit (149⭐), mcpserver-audit (16⭐)

You probably need more than one layer. mcp-rampart is the only library that operates at layer 4 — the layer that solves the MCP-server author's problem rather than the operator's or the user's.

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Why "framework-aware", not "MCP-generic"

You'll notice every tool at layers 2, 3, and 5 is framework-agnostic — they intercept the wire (HTTP, JSON-RPC, config files) and don't care what's behind. That works for them because they don't need to.

mcp-rampart needs to look behind. The pre-flight audit literally cannot exist as a proxy:

What mcp-rampart can see	Why (it's installed in the app)
@app.get("/api/admin/users/...") decorators	reads app.routes directly
Pydantic response models declaring email, phone, ssn	introspects route.response_model
Missing docstrings on tool handlers	reads route.endpoint.__doc__
Untyped parameters that fall back to str	reads inspect.signature(handler)
Path patterns that look like /auth/, /oauth/, /internal/	pattern-matches route.path

A proxy at layer 3 sees POST /mcp {"method":"tools/call","name":"delete_user","args":{...}}. It does not see @app.delete("/api/admin/users/{user_id}") three steps upstream. So it can't tell you "you're about to expose your admin to an LLM" — it can only tell you "someone just called delete_user".

The trade-off: mcp-rampart is currently FastAPI-only. We're paying that price on purpose for now, because deep introspection is what makes the audit valuable. Node.js and Flask/Django are next on the roadmap.

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Where mcp-rampart is uniquely positioned

Five concrete cells where mcp-rampart is the only ✅:

	mcp-rampart	pipelock	casbin-gw	apisec mcp-audit	hyprmcp
Runs inside your FastAPI app (no extra process)	✅	❌	❌	❌	❌
Audits the routes you are about to expose (not someone else's)	✅	❌	❌	❌	❌
Refuses to start the server on CRITICAL findings	✅	❌	❌	❌	❌
Prompt-injection detection on every tools/call	✅	partial	❌	❌	❌
Three-policy model (block / alert / log) + pluggable callbacks	✅	❌	❌	❌	❌

Different shape, different question, different price tag.

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Quick start

pip install mcp-rampart

from fastapi import FastAPI
from mcp_rampart import MCPRampart

app = FastAPI(title="My App")

@app.get("/api/users/{user_id}")
async def get_user(user_id: int):
    """Get a user by their ID."""
    return {"id": user_id, "name": "Alice"}

rampart = MCPRampart(app)                       # auto-discovers routes, mounts /mcp
print(rampart.summary())

# Pre-flight audit — refuses to start the server on CRITICAL findings
report = rampart.audit()
report.print_text()
if report.has_blockers():
    raise SystemExit(1)

# Runtime guardrail — every incoming tools/call is scanned
rampart.enable_guardrails(policy="block")

Your app now exposes:

• GET /mcp — server info and tool listing
• POST /mcp — MCP JSON-RPC endpoint (Streamable HTTP transport)

Any MCP client (Claude Desktop, ChatGPT, Gemini, Cursor, Codex) can connect.

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The pre-flight audit, in detail

rampart.audit() walks every exposed tool and runs 8 checks. Each finding gets a severity tag, a suggestion, and a category code you can match in CI.

Severity	Check	Triggers when…
🔴 CRITICAL	EXPOSED_AUTH	route path matches /auth/, /login, /token, /oauth, …
🔴 CRITICAL	EXPOSED_ADMIN	route path matches /admin/, /internal/, /debug/, …
🟠 HIGH	MISSING_DOCSTRING	no description → LLM will guess and call the wrong tool
🟠 HIGH	SENSITIVE_PARAM_NAME	parameter name contains password, token, api_key, …
🟠 HIGH	PII_IN_RESPONSE	response schema declares fields like email, phone, ssn, …
🟡 MEDIUM	DESTRUCTIVE_METHOD	DELETE / PUT / PATCH exposed without an explicit consent flow
🔵 LOW	UNTYPED_PARAMETER	3+ parameters falling back to str — LLMs may send malformed inputs
🔵 LOW	WILDCARD_RESPONSE	GET route with no response_model declared — LLM can't anticipate the output shape

Sample output on a deliberately bad app:

🛡️  MCPRampart audit report
   13 tools from 13 routes
   🔴 2 critical · 🟠 4 high · 🟡 3 medium · 🔵 1 low

   🔴 [CRITICAL] POST   /api/auth/login           Authentication endpoint exposed to LLM clients
      ↳ Add '/api/auth/login' to exclude_paths
   🔴 [CRITICAL] DELETE /api/admin/users/{user_id}  Admin / internal endpoint exposed to LLM clients
      ↳ Exclude this route from MCP exposure
   🟠 [HIGH]     GET    /api/users/me              Response may leak PII fields: email, phone, address
   …

Use it in CI:

- run: python -c "from myapp import rampart; r = rampart.audit(); r.print_text(); exit(1 if r.has_blockers() else 0)"

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The runtime guardrail, in detail

The audit happens once, at startup. The guardrail runs forever — on every tools/call request.

It scans the call's arguments (recursively, in dicts and lists) against a curated catalogue of prompt-injection patterns:

Confidence	What gets caught
🔴 HIGH	ignore previous instructions, you are now …, developer/admin/jailbreak mode, chat-template control tokens (<|im_start|>), [[system]] markers, SYSTEM: do …
🟠 MEDIUM	system prompt, act as …, pretend to be …, "reveal your instructions", "repeat everything above", "begin new session as"
🔵 LOW	exfiltration verbs (send your tokens to …), <script> payloads, embedded curl/wget https://…, base64 obfuscation

Aggregate decision:

• any HIGH match → BLOCK
• 2+ MEDIUM matches → BLOCK
• 1 MEDIUM or LOW → WARN (allowed, logged)
• nothing → ALLOW

Enable in one line

rampart.enable_guardrails(policy="block")     # default
rampart.enable_guardrails(policy="alert")     # let through, log loudly, call on_alert
rampart.enable_guardrails(policy="log")       # observability / shadow mode

Plug your alerting in

def to_security_team(decision):
    slack.post(f"⚠️ mcp-rampart blocked {decision.tool_name}: {decision.reason}")

rampart.enable_guardrails(policy="block", on_block=to_security_team)

Inspect what happened

rampart.guardrail.stats()
# → {"total": 1284, "blocked": 7, "alerted": 23, "clean": 1254}

for entry in rampart.guardrail.recent(10):
    print(entry.tool_name, entry.decision.allowed, entry.decision.reason)

What an MCP client sees when blocked:

{
  "isError": true,
  "content": [{
    "type": "text",
    "text": "🛡️ Blocked by MCPRampart runtime guardrail.\nReason: Prompt-injection detected (HIGH:1)\nTop matches: high instruction_override @ arguments.query"
  }]
}

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🎯 Real-world findings — see case-studies/

We ran rampart.audit() against the official examples of tadata-org/fastapi_mcp (the most popular FastAPI→MCP library, 11.9k ⭐):

Example	🔴 Crit	🟠 High	🟡 Med	🔵 Low	Verdict
01_basic_usage_example	0	0	2	0	✅
02_full_schema_description	0	0	2	0	✅
04_separate_server	0	0	2	0	✅
08_auth_token_passthrough	0	1	2	0	✅
09_auth_example_auth0	3	6	0	1	❌ BLOCK

Headline: the official Auth0 example exposes /oauth/authorize, /oauth/register, and /.well-known/oauth-authorization-server to LLM clients. mcp-rampart catches all three and refuses to start the server. Full breakdown in case-studies/01-fastapi-mcp-examples.md.

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How it works

┌─────────────────────────────────────────────────────┐
│                  Your FastAPI app                    │
│                                                     │
│   @app.get("/api/recipes")     ← Existing routes    │
│   @app.post("/api/recipes")                         │
│   @app.delete("/api/admin/...")  ← BAD              │
│                                                     │
│   ┌─────────────────────────────────────────────┐   │
│   │            mcp-rampart (embedded)            │   │
│   │                                             │   │
│   │  1. Introspect routes at startup            │   │
│   │  2. Extract Pydantic schemas + type hints   │   │
│   │  3. ⚡ Pre-flight security audit             │   │
│   │     ↳ refuse to start on CRITICAL findings  │   │
│   │  4. Generate MCP tool definitions           │   │
│   │  5. Mount JSON-RPC at /mcp                  │   │
│   │  6. 🛡️  Scan every tools/call for injection │   │
│   │     ↳ block / alert / log per policy        │   │
│   └─────────────────────────────────────────────┘   │
└─────────────────────────────────────────────────────┘
         │
         ▼
   ┌───────────┐ ┌───────────┐ ┌───────────┐ ┌─────────┐
   │  Claude   │ │  ChatGPT  │ │  Gemini   │ │ Cursor  │
   └───────────┘ └───────────┘ └───────────┘ └─────────┘

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Roadmap

• v0.1 — FastAPI introspection, MCP Streamable HTTP transport, examples
• v0.2 — rampart.audit() with 7 security checks, severity levels, JSON/text output
• v0.3 — Runtime guardrails: prompt-injection detection + block/alert/log policy + structured callbacks
• v0.4 — 56-test pytest suite, GitHub Actions CI (py 3.10/3.11/3.12 + ruff + build), SECURITY.md, +1 audit check (WILDCARD_RESPONSE), +11 injection patterns (27 total: 10 HIGH / 8 MEDIUM / 9 LOW), issue & PR templates, FUNDING
• v0.5 — Node.js / TypeScript port (mcp-rampart-js for Hono / Express / Fastify). Half of new MCP servers in 2026 ship in JS.
• v0.6 — Flask + Django adapters (the rest of the Python web ecosystem)
• v0.7 — Custom audit & guardrail rules (decorators / config / plugins) + tunable confidence thresholds
• v0.8 — Auth passthrough (OAuth2 / API keys / JWT), stdio transport
• v1.0 — Smart tool grouping (collapse CRUD into fewer tools), policy-as-code, OpenAPI/Asyncapi spec ingestion

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FAQ

Why not just be a generic MCP proxy that works with any framework? Because the audit needs to read your code — Pydantic models, decorators, type hints, docstrings. A proxy on the wire can't see those. See Why framework-aware.

Is this the same as pipelock / casbin-gateway / hyprmcp / apisec mcp-audit? No. They live at layers 2, 3, and 5. mcp-rampart lives at layer 4. See The 4 layers of MCP security.

Do I still need a gateway / firewall if I use mcp-rampart? Probably yes. mcp-rampart catches code-side issues and runtime injection. A gateway adds auth + rate-limiting + network policy. They're complementary.

What happens if mcp-rampart blocks a legitimate call? The MCP client receives an isError: true response with the diagnostic in the response body. Switch to policy="alert" while you tune patterns. You can also bypass per-route with rampart.exclude(...).

Can I add my own rules? Custom rules land in v0.6. Until then, subclass Auditor or InjectionDetector and pass it via custom_detector=.

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Contributing

git clone https://github.com/miloudbelarebia/mcp-rampart
cd mcp-rampart
pip install -e ".[dev]"
pytest

See CONTRIBUTING.md for guidelines.

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License

MIT — see LICENSE.

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Built with ❤️ by Miloud Belarebia
97M MCP installs per month. Someone has to audit what they expose.