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mrwulf 6cd08b771d warden: agent-native field research + design decisions (AX/MCP/A2A/AP2/x402/ERC-8004) 
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
2026-07-06 23:20:56 +02:00

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Agent-native design notes — warden, the verified-custody wallet

Field research (July 2026) distilled into the design decisions behind warden, pacta's custody product. Sources at the bottom; each decision names what it borrows and what it deliberately does differently.

The field, in one paragraph

"Agent experience" (AX) graduated in 2026 from a Netlify-blog coinage to job postings and VC theses: the discipline of making products that agents can discover, evaluate, and operate without a human in the loop. The load-bearing standards: MCP (tools/resources over JSON-RPC; the de-facto agent tool bus), A2A with its /.well-known/agent-card.json discovery (v1.0 added signed agent cards, plus the AP2 payments extension whose Intent/Cart/Payment mandates are W3C Verifiable Credentials), x402 (the HTTP 402 payment-required handshake; >150M agentic stablecoin transactions since 2025), ERC-8004 (on-chain identity/reputation/validation registries for agents), and llms.txt (machine-readable site manifests; ~10% adoption, Shopify default). The AX community's design canon: outcome -first tools, machine-parseable errors, deterministic interfaces, progressive scoping, capability metadata at well-known paths.

Where every 2026 agent wallet puts its trust — and where we put ours

Turnkey evaluates signing policy inside AWS Nitro enclaves. Privy gates every signature through a policy engine on its servers. Coinbase AgentKit ships session keys with TEE-enforced spending caps. The common shape: policy is enforced by trusted hardware plus a vendor. The verification of the cryptography itself is nowhere in the trust story — it is assumed.

warden inverts the foundation. Its acceptance boundary is a quorum of independently compiled Ed25519 verifiers, each built from source whose correctness certificates are machine-checked in Lean 4 and replay-attested in a public RFC 9162 transparency log. Classic N-version programming fails because independent implementations share design bugs; here each member is proven to decide accept ⇔ decompress(R) = [k](−A) + [s]B, so members cannot disagree about semantics on the proven domain — runtime disagreement is evidence of build corruption, memory fault, or tampering, and is treated as an incident, not an anomaly to shrug at. No enclave is required, and nothing stops you adding one; proofs and TEEs compose.

The honest asymmetry, stated everywhere the product speaks: theorems cover the verification path. Signing is declared trusted base (mitigated by the firewall below), SHA-512 is an oracle, wire parsing is hypothesis, side channels and reproducible builds are the R5 frontier. ML-DSA remains fail-closed: no proven implementation exists, so the PQC slot reports unavailable rather than pretending.

The four semantic edges (why divergence needs a taxonomy)

The four proven forks do not promise identical verdicts on all inputs — each fork's theorem characterizes its own accept():

  • anza checks A ≠ 0 and rejects a legacy list of excluded small -order R values; its certificate-covered entry point is verify_sha512 (≡ verify_dalek), not the default verify(), which implements different (Zebra-lineage) semantics.
  • dalek / risc0 / betrusted follow upstream VerifyingKey::verify canonical semantics.

So quorum divergence is classified, never ignored:

class example trigger wallet behavior
unanimous-accept honest signature accept, ledger entry
unanimous-reject bad signature reject, ledger entry
semantic-edge small-order R on the legacy list, zero A reject + incident (severity: note)
unexplained anything else reject + incident (severity: tamper) + custody latch

Fail-closed is unconditional: acceptance requires unanimity. The taxonomy only grades the alarm.

Agent-native surfaces (what we borrowed, what we sharpened)

  1. MCP server (warden-mcp, stdio JSON-RPC, stdlib-only). Follows the AX canon: outcome-first tools (verify_inbound, request_signature, posture, custody_card, incidents, explain_refusal), strict schemas, and errors that are machine-actionable objects (code, missing, remediation) rather than prose.
  2. The custody card — A2A's /.well-known/agent-card.json pattern, sharpened: A2A v1.0 cards are signed; warden's card is self-proving. It embeds the LTL inclusion proofs for the exact attested sources its quorum binaries were compiled from, the pinned STH, binary hashes, and the live latch state. A counterparty does not have to believe the card's adjectives; it can recompute the Merkle roots.
  3. Proof-of-posture (PoP) challenge — a counterparty sends a nonce; warden returns a signed posture attestation (capsule hash, quorum membership, ledger head, incident count, STH pin age) whose signature has itself passed the outbound firewall, with the firewall's quorum trail attached. A heartbeat you can audit.
  4. Refusal receipts — the inverse of x402. x402 answers "402 Payment Required" with a structured way to pay; warden answers refusals with EVIDENCE_REQUIRED (or POLICY_DENIED, CUSTODY_LATCHED, …): a signed, hash-chained receipt naming exactly which artifact is missing (e.g. "LTL inclusion proof at tier ≥ R4 for your verifier") and how to remedy. The requesting agent can prove to its principal that it was refused, by whom, and why — an auditable conscience.
  5. Intent envelopes — an AP2 homage, honestly reduced: outbound signing requests carry a structured intent (purpose, constraints, payload digest) that is bound into the ledger entry next to the signature. Not W3C VCs; a documented, simpler binding that an auditor can walk.
  6. Evidence freshness as a liveness rule — via pacta's STH pin store: if the wallet's view of the transparency log goes stale beyond policy, outbound custody refuses until refreshed. A wallet that declines to outlive its evidence.

Roadmap ideas that survived contact with honesty (not built yet)

  • x402 lane: answer 402 challenges and pay per-call — after the outbound path earns more than trusted-base status.
  • zk verification receipts: run the proven verifier inside the RISC Zero guest (the risc0 fork is the proven code) and hand counterparties STARK receipts of verified verification.
  • ERC-8004 registration: put the custody card's hash in the validation registry; the LTL becomes an off-chain validator.
  • The choir: N wardens cross-witnessing each other's ledger heads — gossip-audited custody, the same witness pattern the LTL already uses for its STHs.

Sources