# The Six-Layer Stack

Brain is a layered protocol. Information flows up; control flows down.

```
Raw → Ledger → Wiki → Policy → Agent → Audit
```

Each tenant has its own logical instance of every layer, with hard isolation at the database, KMS, and policy boundaries. Off-chain state lives in Postgres with pgvector and Azure Blob-backed raw artifacts. On-chain commitments and smart-account execution live on Base L2.

### The six layers at a glance

<table data-view="cards"><thead><tr><th></th><th></th><th data-type="content-ref"></th><th data-hidden data-card-target data-type="content-ref"></th></tr></thead><tbody><tr><td><strong>1️⃣ Raw</strong></td><td>Lossless ingestion of evidence from any authorized source.</td><td><a href="/pages/pPTXzUZ6cZ8LCgvMmMRO">/pages/pPTXzUZ6cZ8LCgvMmMRO</a></td><td></td></tr><tr><td><strong>2️⃣ Ledger</strong></td><td>Deterministic structuring into immutable records with provenance.</td><td><a href="/pages/pPTXzUZ6cZ8LCgvMmMRO">/pages/pPTXzUZ6cZ8LCgvMmMRO</a></td><td></td></tr><tr><td><strong>3️⃣ Wiki</strong></td><td>Continuously updated memory graph per tenant.</td><td><a href="/pages/IDbmiD3RRs6QlgR8UT8z">/pages/IDbmiD3RRs6QlgR8UT8z</a></td><td></td></tr><tr><td><strong>4️⃣ Policy</strong></td><td>Plain-English rules compiled to deterministic guards.</td><td><a href="/pages/GSe2ntE9CLqxoQAiQuzG">/pages/GSe2ntE9CLqxoQAiQuzG</a></td><td></td></tr><tr><td><strong>5️⃣ Agent</strong></td><td>Internal and external agents executing within policy.</td><td><a href="/pages/t9xBO8sSjFElp8ecB6i5">/pages/t9xBO8sSjFElp8ecB6i5</a></td><td></td></tr><tr><td><strong>6️⃣ Audit</strong></td><td>Per-tenant Merkle tree anchored on Base L2.</td><td><a href="/pages/PIgNXssgtEUZDLnC4b4d">/pages/PIgNXssgtEUZDLnC4b4d</a></td><td></td></tr></tbody></table>

### Why six layers, in this order

The stack is not a stylistic choice. Each layer enforces a property the layer above it requires.

| Layer      | What It Enforces                               | Why It Matters                                                             |
| ---------- | ---------------------------------------------- | -------------------------------------------------------------------------- |
| **Raw**    | Lossless, replayable storage                   | Higher layers can be rebuilt deterministically if extraction logic changes |
| **Ledger** | Deterministic structure with provenance        | Reasoning never reinterprets raw documents on the fly                      |
| **Wiki**   | Continuously refreshed memory linked to Ledger | Answers compound over time; citations are always traceable                 |
| **Policy** | Tenant-signed deterministic rules              | No agent action runs unchecked                                             |
| **Agent**  | Scoped, attestable execution                   | Internal and external agents share one verified substrate                  |
| **Audit**  | Hash-chained, Merkle-anchored events           | History cannot be silently rewritten                                       |

{% hint style="success" %}
This is the same separation that exists in any serious system between the database and the cache: structure first, reasoning second, memory bound to citations.
{% endhint %}

### Information flow

Information flows **up**. Each layer enriches the one below.

```
Source webhook
   ↓
[ Raw ]            artifact stored, content-addressed by SHA-256
   ↓
[ Ledger ]         deterministic extractor produces structured records
   ↓
[ Wiki ]           entity resolution, narrative summarization, embeddings
   ↓
[ Policy ]         action evaluated against active policy version
   ↓
[ Agent ]          execution dispatched to off-chain rail or on-chain account
   ↓
[ Audit ]          every step hashed, Merkle root anchored on Base
```

### Control flow

Control flows **down**. Higher layers gate lower ones.

| Layer Above | Gates                    | Layer Below                    |
| ----------- | ------------------------ | ------------------------------ |
| **Audit**   | requires hash links from | every other layer              |
| **Agent**   | requires verdict from    | Policy                         |
| **Policy**  | reads from               | Wiki and Ledger                |
| **Wiki**    | rebuilds from            | Ledger                         |
| **Ledger**  | replays from             | Raw                            |
| **Raw**     | sources from             | the tenant's connected systems |

### Off-chain and on-chain split

Most logic is off-chain by design. On-chain contracts exist to anchor state, register identity, validate ERC-4337 UserOps, and route agent execution.

| Tier                   | What Lives Here                                                                                                         |
| ---------------------- | ----------------------------------------------------------------------------------------------------------------------- |
| **Off-chain**          | Raw artifacts (Azure Blob), Ledger records (Postgres), Wiki graph (Postgres + pgvector), Policy compiler, Agent runtime |
| **On-chain (Base L2)** | `BrainAuditAnchor`, `BrainPolicyRegistry`, `BrainSmartAccount`, `BrainMCPAgentRegistry`                                 |

[**→ Smart contract reference**](/smart-contracts/overview.md)

### What's next

<table data-view="cards"><thead><tr><th></th><th></th><th data-type="content-ref"></th><th data-hidden data-card-target data-type="content-ref"></th></tr></thead><tbody><tr><td><strong>📥 Raw and Ledger</strong></td><td>How evidence becomes deterministic structure.</td><td><a href="/pages/pPTXzUZ6cZ8LCgvMmMRO">/pages/pPTXzUZ6cZ8LCgvMmMRO</a></td><td></td></tr><tr><td><strong>🧠 The Wiki</strong></td><td>The memory graph per tenant.</td><td><a href="/pages/IDbmiD3RRs6QlgR8UT8z">/pages/IDbmiD3RRs6QlgR8UT8z</a></td><td></td></tr><tr><td><strong>📋 Policy and Permissioning</strong></td><td>Plain-English rules to deterministic guards.</td><td><a href="/pages/GSe2ntE9CLqxoQAiQuzG">/pages/GSe2ntE9CLqxoQAiQuzG</a></td><td></td></tr></tbody></table>


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