This content originally appeared on DEV Community and was authored by jl03
How MCP Interceptors Stop Your “Autonomous” Agent from Installing
Nmap
Everyone’s busy making “autonomous AI agents.”
Nobody’s asking what they can actually do once they get a shell.
Spoiler: everything you forgot to lock down.
1. The New Problem
Developers built containers, then forgot to pin them.
Ops built pipelines, then forgot to verify them.
Now people are wiring LLMs into real systems — with no pre-call policy checks.
You’ve seen it: the “agent” reads a JSON manifest, loads 20 tools from the internet, and starts running commands on your infra.
It’s not AI.
It’s a remote code execution framework with better marketing.
2. Enter MCP (Model Context Protocol)
MCP is supposed to standardize how agents talk to tools and data sources — discovery, schemas, structured calls.
That’s fine.
The issue?
It trusts whatever tool the agent registers.
If it claims to be crane, it’s allowed to act like crane.
There’s no proof it’s pinned, signed, or safe.
It’s Docker Hub :latest, all over again — but this time your LLM is running the pull.
3. The Fix: Client Interceptors
Think of them as the firewall inside the agent runtime.
An MCP client interceptor hooks every tool call before it leaves the client, and every result after it returns.
[Agent/LLM] → [MCP Client] → (Before Interceptor) → [Tool Server] → (After Interceptor) → [Result]
It’s your single choke point to enforce policy.
- Before: Verify tool name, version, digest, policy.
- After: Log, attest, and sign what actually ran.
Without it, your “autonomous agent” is a glorified curl | bash.
4. The Pre-Call Rule
Every call goes through one gate:
def before_tool_call(tool, args):
verify_manifest(tool)
verify_sha256(tool)
check_opa_policy(tool, args)
if not allowed:
raise PermissionError("Policy denied this stupidity.")
If it isn’t declared in the local manifest,
If it doesn’t have a pinned digest,
If the OPA policy says no —
It doesn’t run.
Period.
5. The After-Call Receipt
When a tool finishes, the interceptor records an execution receipt:
{
"tool": "crane",
"version": "0.20.2",
"sha256": "26a5235f...",
"args": {"image": "nginx:1.27.2"},
"timestamp": "2025-11-12T15:00Z",
"exit_code": 0
}
Then it signs it.
Now you can prove what actually ran instead of what you think ran.
6. The Manifest
Stop calling the network every time.
Agents should have a signed local manifest of approved tools.
mcp_tools:
- name: crane
provider: https://registry.example.com
version: 0.20.2
sha256: 26a5235f43a2...
net_allow:
- "^https://registry\\.example\\.com/"
No manifest, no call.
This isn’t optional — it’s provenance hygiene.
7. Why This Works
You’re finally moving the “policy enforcement point” upstream — into the agent’s own runtime.
Every agent call becomes verifiable, auditable, and reproducible.
It’s not “AI safety.”
It’s system integrity.
You can’t attest a hallucination,
but you can stop it from running rm -rf /.
8. Summary
| Phase | Legacy | EnvSecOps |
|---|---|---|
| Tool discovery | Anything goes | Signed manifest only |
| Invocation | Blind trust | Pre-call OPA policy |
| Execution | Shell free-for-all | Digest-verified binaries |
| Logging | Best-effort | Signed receipts |
| Blame | Everyone else | Nobody, because it never ran |
9. The Exit Line
Your agent doesn’t need a bigger model.
It needs a shorter leash.
Pre-call interceptors.
Pinned manifests.
No exceptions.
Pin it, or bin it.
This content originally appeared on DEV Community and was authored by jl03
jl03 | Sciencx (2025-11-12T22:29:33+00:00) Pin It or Bin It Pt 3: The Agent Firewall. Retrieved from https://www.scien.cx/2025/11/12/pin-it-or-bin-it-pt-3-the-agent-firewall/
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