Solon · Architecture

Non-custodial
middleware.

Solon is the third of five layers — sitting between agents and infrastructure. Adapters connect both ways. We never hold a key.

Explore the stack Browse the modules
01 · The Stack

Five layers.
One sits between.

The agent-finance world settles into a five-layer architecture. Solon occupies the control plane — the layer that decides what gets passed down to be signed.

L 4
Application
Customer agents — finance automation, cross-border payments, subscription management.
Your agents
Your code
↓ payment intent
L 3
Control Plane
Policy engine, approval workflow, audit trail, compliance screening, multi-agent coordination, ERP integration, error recovery.
Solon · this layer
7 modules below
Policy
Engine
Approval
Workflow
Compliance
Screening
Audit
Trail
Multi-Agent
Coordination
ERP
Integration
Error
Recovery
↓ approved intent
L 2
Infrastructure
Wallets, custody, MPC, on-chain settlement, fiat ramps, blacklist screening.
Coinbase
Stripe · Fireblocks
Circle · Anchorage
↓ sign & submit
L 1
Protocol
The "languages" agents speak with the outside world. HTTP-style payment, agent-to-agent commerce.
x402 · A2A
AP2 · ACP · MPP
↓ broadcast
L 0
Settlement
Public blockchains that finalize value transfer.
Base · Solana
Ethereum · Polygon
02 · Design Principles

Two principles.
Everything follows.

The two architectural commitments that determine where Solon sits, what it can do, and — more importantly — what it will never do.

— Principle 01

Adapter Pattern.

Layers 1 and 2 are not built. They are adopted. Solon connects to existing wallets, protocols, custodians, and chains through adapters — never competing with Coinbase, Stripe, Fireblocks, or Circle. They become distribution channels instead.

x402 · A2A · AP2 · ACP · MPP
All major protocols are addressed by adapters.
New protocols → ship a new adapter, not a new core.
— Principle 02

Non-Custodial by Design.

Solon never holds a complete signing key. Policy enforcement is structural: when an evaluation fails, the signing request is never produced. There is no policy-bypass path that Solon can be tricked into taking — because the bypass path doesn't exist in the codepath.

Policy fails → no signing request emitted.
Not a process veto. A structural one.
No keys held = no keys to leak.

Five protocols.
One control plane.

The protocols agents speak with the outside world — HTTP payment, agent-to-agent commerce, multi-party flows. Each one becomes addressable to Solon through a thin adapter.

x402
Coinbase · Cloudflare
HTTP 402 status-code extension. Agents trigger stablecoin payments through the standard HTTP flow — already the de-facto standard.
A2A
Google
Agent-to-agent communication. Lets two agents negotiate tasks, reconcile state, and complete a payment.
AP2
Industry Draft
Agent Payments Protocol. Standardized semantics for subscriptions, metering, and refunds.
ACP
Industry Draft
Agent Commerce Protocol. Discovery, quotation, ordering, fulfillment — the full commerce loop.
MPP
Industry Draft
Multi-Party Payments. Splits, escrow, conditional release — orchestrated across agents.
03 · A Call, End to End

Anatomy of
an x402 call.

One concrete example. An agent buys $0.001 of data. Here's everything that happens between the request and the settlement.

Step 01 · Agent
Receives an x402 request.
GET /api/forecast  ·  X-Payment: 0.001 USDC on base
HTTP 402 · payment intent
◆ Solon
Step 02 · Protocol adapter
Parses the protocol.
→ extracts { amount, recipient, chain, context, agent_id }
structured intent
◆ Solon
Step 03 · Policy engine
Evaluates the rules.
amount ≤ daily-limit recipient ∈ whitelist risk-score < 0.45 sox-policy compliant
verdict
✓
Approve
Forward to wallet for signing
!
Hold
Route to human approval
✕
Reject
Log and block, alert
on approve · signed transaction
◆ Solon
Step 04 · Protocol adapter
Wraps the response.
→ X-Payment-Signature: 0xa7…3f  ·  HTTP 200
signed payment
Step 05 · Agent → Settlement
Broadcasts on-chain.
Base · USDC · settled in ~2 seconds  ·  hash anchored to audit log
Solon never holds your wallet, the protocol, or the chain.
It sits in the request lifecycle, decides, and steps out of the way.

Seven modules in the control plane.

Each of Solon's seven modules solves one of the five gates between agent adoption and production.

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