State Sync

Research Foundation

This library addresses two failure modes recently formalized in academic research:

"Your LLM Agents are Temporally Blind" Cheng, Moakhar, Fan, Hosseini, Faghih, Sodagar, Wang & Feizi — University of Maryland, 2025 arXiv:2510.23853

Coined the term Temporal Blindness: LLM agents assume a stationary context and fail to account for real-world time elapsed between messages. The study proves that "agents frequently over-rely on stale context and skip needed tool calls" — no model tested achieved a normalized alignment rate better than 65% with human temporal perception.

"State Drift in Language-Conditioned Autonomous Agents" Singh, S.K. — Preprints.org, January 2026 Preprints:202601.0910

Formalized State Drift: the gradual, hidden misalignment between an agent's internal textual representation and the true environment state. The key finding: "increasing context capacity alone does not prevent its emergence or persistence" — giving an LLM 2 million tokens does not solve drift if the database changes externally.

mcp-state-sync is an engineering response to both findings. It uses RFC 7234 Cache-Control vocabulary — deeply embedded in LLM pre-training data — to signal data volatility (no-store) and causal invalidation ([System: Cache invalidated...]) directly through the MCP protocol layer.

The Problem

Temporal Blindness

LLMs have no concept of time. They cannot distinguish between data read 2ms ago and data read 20 minutes ago — both exist as equally valid facts inside the context window. This is not a bug; it is a fundamental architectural property of transformer-based models, formally proven by Cheng et al. Every token in the context window has equal epistemological weight regardless of when it entered.

When an LLM reads sprint data via MCP at turn 3, and a human moves a task via the mobile app at turn 5, the LLM continues planning at turn 7 using the turn-3 snapshot as ground truth. It has no mechanism to detect that the world has changed.

Causal State Drift

The problem compounds when the LLM itself causes mutations — a phenomenon formalized by Singh (2026) as State Drift:

Turn 1: LLM reads sprint S1 → context now contains { tasks: [T1, T2, T3] }
Turn 2: LLM calls sprints.update(S1, { tasks: [T1, T2] })  → removes T3
Turn 3: LLM reads sprint S1 again → still sees { tasks: [T1, T2, T3] } in context

At turn 3, the LLM has two contradictory representations of the same entity. The stale read from turn 1 and the fresh read from turn 3 coexist. The model has no causal reasoning to prefer the newer one. Worse, the original read often has more surrounding context tokens (the full planning conversation), making it more salient to the attention mechanism despite being less accurate.

This is Causal State Drift: the LLM's internal state drifts from the actual system state, and it cannot self-correct because it lacks the temporal metadata to detect the drift.

Why This Matters

In agentic systems where LLMs execute multi-step workflows — managing infrastructure, processing payments, prescribing medications — Causal State Drift leads to:

The failure mode is insidious: the LLM behaves confidently and produces syntactically correct tool calls. The errors only manifest as incorrect data in the database — invisible to the model and often to the user until downstream consequences emerge.

The Solution

mcp-state-sync introduces two machine-to-machine signaling mechanisms, both designed to exploit the LLM's deep training on HTTP infrastructure semantics:

1 — Cache-Control Directives (Static)

Appended to tool descriptions during tools/list:

Tool: sprints.get
Description: Get sprint details. [Cache-Control: no-store]

The model doesn't need to "understand" caching theory — it has seen Cache-Control: no-store millions of times in HTTP documentation, API specs, and developer discussions. The association between no-store and "do not reuse this response" is deeply embedded in the model's weights.

2 — Causal Domain Invalidation (Dynamic)

Injected as a System content block at index 0 of successful write responses:

Content[0]: [System: Cache invalidated for sprints.* — caused by sprints.update]
Content[1]: {"ok": true, "sprint_id": 123}

This creates an explicit causal link: "your write just invalidated these domains, re-read before acting on cached data."

Binary Cache Vocabulary

There is no max-age. LLMs have no clock — time-based expiration is meaningless inside a context window.

Safety Guards

Quick Start

npm install @vinkius-core/mcp-state-sync

Peer dependency: @modelcontextprotocol/sdk >= 1.12.1

With mcp-fusion (Fusion Mode)

import { ToolRegistry } from '@vinkius-core/mcp-fusion';
import { StateSync } from '@vinkius-core/mcp-state-sync';

const registry = new ToolRegistry();
// ... register tool modules ...

const sync = new StateSync({
  defaults: { cacheControl: 'no-store' },
  policies: [
    { match: 'sprints.update', invalidates: ['sprints.*'] },
    { match: 'tasks.update',   invalidates: ['tasks.*', 'sprints.*'] },
    { match: 'countries.*',    cacheControl: 'immutable' },
  ],
});

sync.attachToServer(server, registry);

Without mcp-fusion (Manual Mode)

import { StateSync } from '@vinkius-core/mcp-state-sync';

const sync = new StateSync({
  defaults: { cacheControl: 'no-store' },
  policies: [
    { match: 'update_sprint', invalidates: ['get_sprint', 'list_sprints'] },
    { match: 'countries',     cacheControl: 'immutable' },
  ],
});

sync.attachToServer(server, {
  tools: myToolDefinitions,
  handler: async (name, args, extra) => myHandler(name, args, extra),
});

Real-World Examples

Each example includes a detailed explanation (.md) of the threat model and how StateSync neutralizes it, alongside a complete TypeScript implementation (.ts).

🏦 Fintech Wire Transfer Guard

An LLM reads a $12,340 balance. A customer transfers $10,000 via mobile banking. The LLM approves a $5,000 transfer based on the stale balance → overdraft.

StateSync forces no-store on all account tools and invalidates accounts.*, transfers.*, transactions.* after every successful transfer.

→ Explanation · Code

🏥 Healthcare Patient Record Sync

An LLM reads allergies: ["Penicillin"]. A nurse adds "Sulfonamides" via the hospital EMR. The LLM recommends Sulfamethoxazole → adverse drug reaction.

StateSync marks all patient data as no-store and invalidates patient.*, prescriptions.*, interactions.* after every prescription write.

→ Explanation · Code

🛒 E-Commerce Inventory Protection

An LLM reads "3 units in stock." A customer buys 2 via the website. The LLM promises 3 units to another customer → oversell.

StateSync forces no-store on inventory tools and invalidates inventory.*, orders.*, products.* after every order creation.

→ Explanation · Code

☁️ DevOps Infrastructure State

An LLM reads 4 running instances. Auto-scaling terminates 2. The LLM reports "capacity sufficient" → outage. Worse: the LLM overwrites a security group with stale rules → silently deletes SSH access.

StateSync marks all infrastructure tools as no-store and invalidates cross-resource domains (instances, security groups, costs) after mutations.

→ Explanation · Code

Architecture

StateSync (Facade)
  ├─ PolicyEngine           ← First-match-wins resolution + Map cache (O(1) repeat)
  │    ├─ GlobMatcher           ← Dot-separated glob matching (* and **)
  │    └─ PolicyValidator       ← Fail-fast eager validation at construction
  └─ ServerWrapper          ← MCP Server interception (tools/list + tools/call)
       ├─ DescriptionDecorator  ← Append [Cache-Control: X] to descriptions
       ├─ CausalEngine          ← isError guard + invalidation resolution
       └─ ResponseDecorator     ← Prepend [System: ...] at index 0

UpstreamFactory             ← Fusion / Manual upstream adapters
ServerResolver              ← Duck-type Server vs McpServer resolution

12 modules. Single responsibility each. Zero any. Pure functions where possible. Classes only where state is needed.

Documentation

Compatibility

Optional integration: Works with @vinkius-core/mcp-fusion for multi-module MCP servers. Not required — Manual Mode provides full functionality without mcp-fusion.