SINT Protocol

SINT Protocol

Security-enforcing MCP proxy that sits between an AI agent and any number of downstream MCP servers, intercepting every tool call through a capability-token policy gateway that can allow, deny, or escalate to human approval before the call reaches any real tool. It also exposes built-in operator tools for approval workflows, audit trail queries, token management, voice/HUD output, and hierarchical

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SINT Protocol

CI Node.js TypeScript License

Security, permission, and economic enforcement layer for physical AI.

SINT is the missing governance layer between AI agents and the physical world. Every tool call, robot command, and actuator movement flows through a single Policy Gateway that enforces capability-based permissions, graduated approval tiers, and tamper-evident audit logging.

Academic grounding: SINT is designed with reference to IEC 62443 FR1–FR7, EU AI Act Article 13, and NIST AI RMF. The evaluation framework references the ROSClaw empirical safety study (arXiv:2603.26997) and MCP security analysis (arXiv:2601.17549).

Agent ──► SINT Bridge ──► Policy Gateway ──► Allow / Deny / Escalate
                               │
                       Evidence Ledger (SHA-256 hash-chained)
                               │
                    ProofReceipt (pluggable attestation)

Why SINT?

AI agents can now control robots, execute code, move money, and operate machinery. But there's no standard security layer between "the LLM decided to do X" and "X happened in the physical world."

SINT vs. Other Frameworks

Capability SINT Protocol Microsoft AGT MCP Baseline SROS2
Physical constraint enforcement (velocity, force, geofence) ✅ In token ❌ ❌ ❌
Tier-based human oversight (T0–T3) ✅ 4-tier ⚠️ Execution rings ❌ ❌
Append-only hash-chained audit ✅ SHA-256 ⚠️ Logging ❌ ❌
ROS 2 / MAVLink / industrial bridges ✅ 12 bridges ❌ Digital only ❌ ⚠️ ROS only
OWASP ASI01–ASI10 coverage ✅ 10/10 Full ✅ 10/10 ❌ ❌
Economic routing + budgets ✅ bridge-economy ❌ ❌ ❌
Swarm collective constraints ✅ SwarmCoordinator ❌ ❌ ❌
E-stop / CircuitBreaker ✅ EU AI Act Art. 14 ✅ Kill switch ❌ ❌

SINT is the only framework purpose-built for physical AI — where actions are irreversible and have real-world consequences. Microsoft AGT targets digital/software agents; SINT targets robots, drones, and actuators.

The empirical case for SINT:

  • ROSClaw (IROS 2026): Up to 4.8× spread in out-of-policy LLM action proposals across frontier models under identical safety envelopes. The 3.4× divergence between frontier backends is measurable, reproducible, and persistent.
  • MCP security (arXiv:2601.17549): 10 documented real-world MCP breaches in under 8 months, including a CVSS 9.6 command injection affecting 437,000 downloads.
  • SROS2: Formally demonstrated to contain 4 critical vulnerabilities at ACM CCS 2022, including access-control bypasses permitting arbitrary command injection.
  • Unitree BLE worm (September 2025): Hardcoded crypto keys enabled wormable BLE/Wi-Fi command injection across robot fleets — precisely the scenario SINT's per-agent token scoping and real-time revocation prevent.

Core guarantees:

  • No agent action ever bypasses the Policy Gateway (invariant I-G1: No Bypass)
  • Every decision is recorded in a tamper-evident SHA-256 hash-chained ledger (invariant I-G3: Ledger Primacy)
  • Physical constraints (velocity, force, geofence) are enforced at the protocol level — in the token, not in config
  • Tier-gated verifiable compute hooks support provable-execution evidence on critical actions
  • E-stop is universal across all non-terminal DFA states (invariant I-G2: E-stop Universality)
  • Per-agent capability tokens with real-time revocation

Quick Start

# Prerequisites: Node.js >= 22, pnpm >= 9
pnpm install
pnpm run build
pnpm run test        # full workspace test suite

Start the Gateway Server

pnpm --filter @sint/gateway-server dev
# → http://localhost:3100/v1/health
# → http://localhost:3100/v1/ready
# → http://localhost:3100/v1/docs

Start Production-Like Stacks (One Command)

pnpm run stack:dev
pnpm run stack:edge
pnpm run stack:prod-lite
pnpm run stack:gazebo-validation
pnpm run stack:isaac-sim-validation

Compose profiles:

Developer Docs Site (docs.sint.gg)

pnpm run docs:dev
pnpm run docs:build
pnpm run docs:preview

Docs source lives in docs/, VitePress config is in docs/.vitepress/config.mts, and deployment is handled by docs-site.yml.

Community/adoption assets:

Run a Single Package

pnpm --filter @sint/gate-policy-gateway test
pnpm --filter @sint/bridge-mcp test

SINT Operator Interface

A voice-first, HUD-based control surface for SINT operators. Every command flows through the Policy Gateway.

pnpm run stack:interface  # starts gateway + interface + postgres + redis
# Opens: http://localhost:3202

Features:

  • 🎙️ Voice input — Web Speech API (zero external deps), real-time transcript
  • 🖥️ Command HUD — 3-panel grid: approvals | action stream | context
  • 💾 Operator memory — ledger-backed persistent context (@sint/memory)
  • 🔔 Proactive notifications — sint__notify (T2 tier, requires confirmation)
  • ✅ T2/T3 approvals — one-click approve/deny with timeout countdown

See docs/guides/sint-interface.md for full setup and usage.

For AI Agents

If you are an AI agent (Claude, GPT, Gemini, Cursor, etc.) working in this repo, read AGENTS.md first. It covers key invariants, common mistakes, and entry points for the most common tasks. For deeper implementation details, see CLAUDE.md.

Architecture

┌──────────────────────────────────────────────────────────────┐
│  AI Agents / Foundation Models                               │
│  (Claude, GPT, Gemini, open-source)                         │
└──────────────────┬───────────────────────────────────────────┘
                   │
┌──────────────────▼───────────────────────────────────────────┐
│  SINT Bridge Layer (L1)                                      │
│  ┌────────────┐ ┌────────────┐ ┌────────────┐ ┌──────────┐  │
│  │ bridge-mcp │ │ bridge-ros2│ │ bridge-a2a │ │ bridge-  │  │
│  │ MCP tools  │ │ ROS topics │ │ Google A2A │ │ open-rmf │  │
│  └────────────┘ └────────────┘ └────────────┘ └──────────┘  │
│  ┌──────────────────────┐ ┌───────────────────────────────┐  │
│  │ bridge-mqtt-sparkplug│ │ bridge-opcua                  │  │
│  │ Industrial IoT       │ │ PLC / OT control plane bridge │  │
│  └──────────────────────┘ └───────────────────────────────┘  │
│  Per-resource state: UNREGISTERED→PENDING_AUTH→AUTHORIZED    │
│  →ACTIVE→SUSPENDED (real-time revocation without restart)    │
└──────────────────┬───────────────────────────────────────────┘
                   │ SintRequest (UUIDv7, Ed25519, resource, action, physicalContext)
┌──────────────────▼───────────────────────────────────────────┐
│  SINT Gate (L2) — THE choke point                           │
│  ┌─────────────────────────────────────────────────────────┐ │
│  │  PolicyGateway.intercept()                              │ │
│  │  1. Schema validation (Zod)                             │ │
│  │  2. Token validation (Ed25519 + expiry + revocation)    │ │
│  │  3. Resource scope check                                │ │
│  │  4. Per-token rate limiting (sliding window)            │ │
│  │  5. Physical constraint enforcement                     │ │
│  │  6. Forbidden action sequence detection                 │ │
│  │  7. Tier assignment: max(BaseTier, Δ_human, Δ_trust...) │ │
│  │  8. T2/T3 → escalate to approval queue                 │ │
│  │  9. T0/T1 + approved T2/T3 → allow                     │ │
│  │  10. Bill via EconomyPlugin (if configured)             │ │
│  └─────────────────────────────────────────────────────────┘ │
│                          ↓                                   │
│  EvidenceLedger (SHA-256 hash chain + ProofReceipt)        │
└──────────────────────────────────────────────────────────────┘

APS vs SINT Primitives

APS Concept SINT Implementation
Principal agentId (Ed25519 public key) + W3C DID identity
Capability SintCapabilityToken (Ed25519-signed, scoped, attenuatable)
Authority PolicyGateway.intercept() — single choke point
Confinement Per-token resource scope + physical constraints (velocity, force, geofence)
Revocation RevocationStore + ConsentPass endpoint (real-time)
Audit EvidenceLedger — append-only, SHA-256 hash-chained

Packages

Gate (Security Core)

Package Description Tests
@sint/core Types, Zod schemas, tier constants, formal DFA states —
@sint/gate-capability-tokens Ed25519 tokens, delegation, W3C DID identity 55
@sint/gate-policy-gateway Authorization engine: tiers, constraints, rate limiting, M-of-N quorum 256
@sint/gate-evidence-ledger SHA-256 hash-chained append-only audit log with pluggable attestation 45

Bridges (12 bridges)

Package Description Tests
@sint/bridge-mcp MCP tool call interception and risk classification 66
@sint/bridge-ros2 ROS 2 topic/service/action interception with physics extraction 20
@sint/bridge-a2a Google A2A Protocol bridge for multi-agent coordination 38
@sint/bridge-iot Generic MQTT/CoAP edge IoT bridge with gateway session interception 21
@sint/bridge-mqtt-sparkplug MQTT Sparkplug profile mapping with industrial command tiering defaults 8
@sint/bridge-opcua OPC UA node/method mapping with safety-critical write/call promotion 6
@sint/bridge-open-rmf Open-RMF fleet/facility mapping for warehouse dispatch workflows 5
@sint/bridge-grpc gRPC service/method profile mapping with default tier assignment 5
@sint/bridge-economy Economy bridge: balance, budget, trust, billing ports 47
@sint/bridge-mavlink MAVLink drone/UAV command bridge 15
@sint/bridge-swarm Multi-robot swarm coordination bridge 9

Engine (AI Execution Layer)

Package Description Tests
@sint/engine-system1 Neural perception: sensor fusion, ONNX inference, anomaly detection 42
@sint/engine-system2 Symbolic reasoning: behavior trees, task planning, System 1/2 arbitration 86
@sint/engine-hal Hardware Abstraction Layer: auto-detect hardware, select deployment profile 26
@sint/engine-capsule-sandbox WASM/TS capsule loading, validation, and sandboxed execution 36
@sint/avatar Avatar Layer (L5): behavioral identity profiles, CSML-driven tier escalation 25

Reference Capsules

Package Description Tests
@sint/capsule-navigation Waypoint following navigation reference capsule 11
@sint/capsule-inspection Visual anomaly detection for manufacturing QA 8
@sint/capsule-pick-and-place Gripper control for pick-and-place tasks 12

Persistence

Package Description Tests
@sint/persistence Storage interfaces + in-memory/PG/Redis implementations 26
@sint/persistence-postgres Production PostgreSQL adapters for ledger, revocation, and rate-limit durability 14

Apps & SDKs

Package Description Tests
@sint/gateway-server Hono HTTP API with approvals, SSE streaming, A2A routes —
@sint/mcp Security-first multi-MCP proxy server —
@sint/dashboard Real-time approval dashboard with operator auth 29
@sint/client TypeScript SDK for the Gateway API (delegation, SSE) —
@sint/sdk Zero-dependency public TypeScript SDK aligned to gateway v0.2 contracts 9
@sint/conformance-tests Security regression suite — all phases —

Total: 41 workspace members · 1,772 tests passing

Note: Run pnpm test to get the current exact passing test count.

Approval Tiers

Graduated authorization mapped to physical consequence severity:

Tier Name DFA States Auto-approved? Example
T0 OBSERVE → OBSERVING Yes (logged) Read sensor data, query database
T1 PREPARE → PREPARING Yes (audited) Write file, save waypoint, stage plan
T2 ACT ESCALATING → ACTING Requires review Move robot, operate gripper, publish /cmd_vel
T3 COMMIT ESCALATING → COMMITTING Requires human + optional M-of-N Execute trade, novel environment entry, irreversible action

Tier escalation triggers (Δ factors):

  • Δ_human: Human presence sensor active in workspace → +1 tier
  • Δ_trust: Agent trust score below threshold or recent failures → +1 tier
  • Δ_env: Robot near physical boundary or unstructured environment → +1 tier
  • Δ_novelty: Action outside validated distribution (novelty detector) → +1 tier

Formal Specification

Request Lifecycle DFA

SINT models every request as a deterministic finite automaton with 12 states:

IDLE → PENDING → POLICY_EVAL → PLANNING → OBSERVING/PREPARING/ACTING → COMMITTING → COMPLETED
                     ↓                              ↓
                ESCALATING                      ROLLEDBACK  (estop, execution failure)
                     ↓
                  FAILED     (approval denied, timeout)

The ACTING state is only reachable via POLICY_EVAL with a valid token. Physical actuation is structurally impossible without a valid capability token.

Tier Assignment Function

Tier(r) = max(BaseTier(r), Δ_human(r), Δ_trust(r), Δ_env(r), Δ_novelty(r))

Formal Invariants

Invariant Description
I-T1 (Attenuation) scope(child_token) ⊆ scope(parent_token) — delegation can only reduce permissions
I-T2 (Unforgeability) Capability tokens are Ed25519-signed; valid tokens are computationally unforgeable
I-T3 (Physical Constraint Primacy) Physical constraints (velocity, force, geofence) in a token cannot be weakened by any downstream layer
I-G1 (No Bypass) Physical actuation is only reachable from the ACTING DFA state, which is only reachable via POLICY_EVAL
I-G2 (E-stop Universality) The estop event transitions any non-terminal state to ROLLEDBACK unconditionally
I-G3 (Ledger Primacy) COMMITTING → COMPLETED requires ledger_committed; no action completes without a ledger record

Benchmark Results

PolicyGateway latency (measured on M3 MacBook Pro, pnpm run bench):

Tier p50 p99
T0 (OBSERVE) ~1ms ~3ms
T1 (PREPARE) ~1ms ~3ms
T2 (ACT) ~1ms ~3ms
T3 (COMMIT) ~1ms ~3ms

The gateway adds sub-3ms overhead at p99 for all tiers. Run benchmarks: pnpm run bench.

ROS2 control-loop target benchmark:

Path SLA Target Command
ROS2 command path (/cmd_vel) p99 < 10ms pnpm run benchmark:ros2-loop

Industrial benchmark artifacts:

Compliance mapping assets:

Key Concepts

Capability Tokens

Ed25519-signed capability tokens — the only authorization primitive. Unlike RBAC (ambient authority to principals), OCap requires explicit token presentation for every operation.

Token fields:

  • Resource scoping — what the agent can access (ros2:///cmd_vel, mcp://filesystem/*, a2a://agents.example.com/*)
  • Action restriction — what operations are allowed (publish, call, subscribe, a2a.send)
  • Physical constraints — max velocity (m/s), max force (N), geofence polygon, time window, rate limit
  • Verifiable compute requirements — optional proof type/verifier/freshness/public-input constraints for T2/T3 actions
  • Delegation chains — max 3 hops, attenuation only (invariant I-T1)
  • Revocation — instant invalidation via revocation store (ConsentPass endpoint)
  • W3C DID identity — did:key:z6Mk... format for agent portability

Evidence Ledger

Every policy decision is recorded in a SHA-256 hash-chained append-only log. Chain integrity: ℓ_k.previousHash = SHA256(canonical(ℓ_{k-1})). A gap or hash mismatch constitutes tamper evidence.

Retention policy:

Tier Retention
T0 (OBSERVE) 30 days
T1 (PREPARE) 90 days
T2 (ACT) 180 days
T3 (COMMIT) 365 days (indefinite if legal hold)

CSML: Composite Safety-Model Latency

A deployment metric that fuses behavioral and physical safety dimensions:

CSML(m, p, t) = α·AR_m + β·BP_m + γ·SV_m - δ·CR_m + ε·𝟙[ledger_intact(t)]

CSML above a deployment threshold θ automatically escalates all subsequent requests from that model backend to the next tier.

Compliance Mapping

IEC 62443 FR1–FR7

FR Title SINT Mechanism
FR1 Identification & Authentication SintCapabilityToken with Ed25519 agent identity; W3C DID portability
FR2 Use Control Four-tier Approval Gate; maxRepetitions constraint; per-resource action allowlists
FR3 System Integrity SHA-256 hash-chained Evidence Ledger; ProofReceipt for T2/T3 (TEE attestation planned)
FR4 Data Confidentiality Zenoh TLS transport; capability scope prevents sensor access without explicit token
FR5 Restricted Data Flow Policy Gateway allowlists; geofence constraint; SINT Bridge per-topic DFA
FR6 Timely Response safety.estop.triggered event; E-stop universality invariant I-G2
FR7 Resource Availability Per-token rate limiting; maxRepetitions; budget enforcement in capsule sandbox

EU AI Act Article 13

Requirement SINT Approach
Logging and traceability SHA-256 hash-chained Evidence Ledger — tamper detection is cryptographic
Human oversight Dynamic Consent + T3 approval gate — T3 actions cannot execute without recorded human approval
Risk management Tier escalation based on real-time physical context (Δ_human, Δ_env, Δ_novelty)

Tier Crosswalk (NIST AI RMF / ISO 42001 / EU AI Act)

SINT Tier NIST AI RMF ISO/IEC 42001 EU AI Act
T0 Observe MAP + MEASURE + MANAGE monitoring controls Clause 9 + Clause 8 controls Article 12 + Article 13
T1 Prepare GOVERN + MANAGE controlled write path Clause 8.1/8.2 operational risk treatment Article 9 + Article 12
T2 Act MANAGE risk response with accountable oversight Clause 8 + Clause 6 operational controls Article 14 + Article 15
T3 Commit Highest-consequence GOVERN + MANAGE controls Clause 8.3 + Clause 10 corrective governance Article 14(4)(e) + Articles 9/12/15

Machine-readable crosswalk endpoint: GET /v1/compliance/tier-crosswalk

API Endpoints

Method Endpoint Description
GET /.well-known/sint.json Public protocol discovery (version, bridges, profiles, schemas)
GET /v1/health Health check
POST /v1/intercept Evaluate a single request
POST /v1/intercept/batch Evaluate multiple requests (207 Multi-Status)
POST /v1/tokens Issue a capability token
POST /v1/tokens/delegate Delegate (attenuate) a token
POST /v1/tokens/revoke Revoke a token
GET /v1/ledger Query audit ledger events
GET /v1/approvals/pending List pending approval requests
POST /v1/approvals/:id/resolve Approve or deny a request (M-of-N quorum)
GET /v1/approvals/events SSE stream for real-time approval events
GET /v1/approvals/ws WebSocket stream for low-latency approval events
POST /v1/a2a JSON-RPC 2.0 A2A protocol endpoint
GET /v1/metrics Prometheus metrics
GET /v1/openapi.json OpenAPI surface for gateway integration
GET /v1/compliance/tier-crosswalk SINT tier mapping to NIST AI RMF / ISO 42001 / EU AI Act controls
POST /v1/economy/route Cost-aware route selection with optional x402 pay-per-call quotes

Development Phases

Phase Description Tests
Phase 1 (complete) Security Wedge — capability tokens, PolicyGateway, EvidenceLedger 425
Phase 2 (complete) Engine Core — bridge-mcp, bridge-ros2, engine packages, persistence, gateway-server +221 (646)
Phase 3 (complete) Economy Bridge — @sint/bridge-economy with port/adapter pattern, EconomyPlugin +91 (737)
Phase 4 (complete) Standards Alignment — A2A bridge, rate limiting, M-of-N quorum, W3C DID identity +78
Phase 5 (complete) Protocol Surface v0.2 — discovery/OpenAPI/schema endpoints, industrial profiles shipped
Phase 6 (complete) Engine layer — System1/2 engines, HAL, capsule sandbox, Avatar/CSML, reference capsules shipped

Deployment

Railway (Recommended)

brew install railway
railway login
./scripts/railway-setup.sh
railway variables --set SINT_STORE=postgres SINT_CACHE=redis SINT_API_KEY=$(openssl rand -hex 32)
railway up

Docker Compose

docker-compose up
# Gateway:   http://localhost:3100
# Dashboard: http://localhost:3201
# Postgres:  localhost:5432
# Redis:     localhost:6379

Tech Stack

  • Runtime: Node.js 22+
  • Language: TypeScript 5.7 (strict mode)
  • Monorepo: pnpm workspaces + Turborepo
  • HTTP: Hono
  • Validation: Zod
  • Crypto: @noble/ed25519, @noble/hashes (audited, zero-dependency)
  • MCP SDK: @modelcontextprotocol/sdk
  • Dashboard: React 19, Vite 6
  • Testing: Vitest (run pnpm test for current count)
  • Infra: Docker, PostgreSQL 16+, Redis 7, GitHub Actions CI, Railway

Docs & Artifacts

Design Principles

  1. Single choke point — Every agent action flows through PolicyGateway.intercept(); no bridge adapter makes authorization decisions independently
  2. Result<T, E> over exceptions — All fallible operations return discriminated unions, never throw
  3. Attenuation only — Delegated tokens can only reduce permissions, never escalate (I-T1)
  4. Append-only audit — The evidence ledger is INSERT-only with SHA-256 hash chain integrity (I-G3)
  5. Physical safety first — Velocity, force, and geofence constraints live in the token, not in external config
  6. Interface-first persistence — Storage adapters implement clean interfaces; swap in-memory for Postgres/Redis
  7. Fail-open on infrastructure — Economy/rate-limit infrastructure failures do not block the safety path
  8. E-stop universality — The hardware E-stop bypasses all token checks and is unconditional (I-G2)

References

  • ROSClaw: Empirical safety analysis of LLM-controlled physical AI — arXiv:2603.26997 (IROS 2026)
  • MCP Security Analysis: Architectural vulnerabilities in the Model Context Protocol — arXiv:2601.17549
  • IEC 62443: Industrial automation and control systems cybersecurity standard
  • EU AI Act Article 13: Transparency requirements for AI systems
  • NIST AI RMF: AI Risk Management Framework
  • W3C DID Core: Decentralized Identifiers specification

Roadmap

Feature Status Target
npm package publishing (8 core packages) 🔧 In progress April 2026
Python SDK (PyNaCl + Pydantic) 🔧 In progress April 2026
Production gateway deployment 📋 Planned April 2026
Getting Started tutorial ✅ Complete docs/getting-started.md
TEE proof receipts (Intel SGX / ARM TrustZone) 📋 Planned Q2 2026
Hardware-in-the-loop ROS 2 testing 📋 Planned Q2 2026
Formal verification (TLA+ / Alloy) 📋 Planned Q3 2026

License

Apache-2.0

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