x402 phase 1: sign-in-with-x — the agent authenticates for free, nothing at risk #32

Why

Agents need to authenticate to HTTP services. x402 (Linux Foundation) specs this as the sign-in-with-x extension: the server's 402 response advertises it, the client signs a CAIP-122/SIWE challenge (eip191 for EOAs) and retries with a SIGN-IN-WITH-X header — zero value at risk, no chain interaction, replay-protected by the server nonce.

Canonical extension spec: https://github.com/x402-foundation/x402/blob/main/specs/extensions/sign-in-with-x.md (signature types eip191/eip1271/eip6492; auth is an extension, not a zero-amount payment).

Corrected 2026-07-10 (codebase re-verified): this issue's original premise — "this is the FIRST message-signing path; the seam lands before payments so parse→policy→card→sign exists with nothing at stake" — is stale. That seam already shipped. So does the general personal_sign path this issue said to "permanently exclude". #32 is still net-new product, but it is NOT a prerequisite for #34, and NOT on the $5/day critical path. Corrections below.

Already shipped (the seam exists — verified 2026-07-10)

Via #133/#155, on main:

• SignerRequest::ProposeMessage / SignMessage wire + SignMessageKind (PersonalSign | TypedDataV4 | EthSign | Authorization7702) in message_signing.rs.
• evaluate_message policy path (policy.rs:861).
• Daemon handlers propose_message / sign_message (daemon.rs:875, :366-372).
• The clear-signing card renders messages (activity_view.rs:1299-1437).

Two consequences for this issue:

1. The "build the message-signing seam first, nothing at stake" ordering vs x402 phase 2: exact/EIP-3009 payments on the Ethereum reference path, fully local demo loop #34 is already satisfied — x402 phase 1: sign-in-with-x — the agent authenticates for free, nothing at risk #32 does not block x402 phase 2: exact/EIP-3009 payments on the Ethereum reference path, fully local demo loop #34.
2. The claim to "permanently exclude raw personal_sign of arbitrary bytes" is contradicted — SignMessageKind::PersonalSign { message: Bytes } already exists and evaluate_message returns NeedsApproval for it. Reconcile this stance in-issue before building. (Note: no MCP tool exposes any message path to agents today — the agent surface is still net-new.)

In scope (the actual net-new work)

• SignIn { domain, challenge: SiweFields } request in deckard-contract under capability name sign-in-with-x (additive per Wire-contract evolution: Hello capability discovery + the five evolution rules #31's evolution rules; capabilities.rs extension point is ready).
• Daemon: CAIP-122/EIP-4361 parse → re-serialize → eip191 sign; malformed/unparseable challenge → typed Deny, nothing signed. (Reuses the existing message evaluate/card seam.)
• Policy: domain_allow: Vec<String>; existing ApprovalMode semantics apply; mainnet guardrail unchanged (on chain 1, auto-Allow downgrades to NeedsApproval).
• MCP tool deckard_x402_sign_in — the first agent-facing message tool — registered only when the daemon advertises the capability.
• Acceptance: X1 sign-in to an allowlisted domain → HTTP 200, zero value moved, clean transcript; X2 domain ∉ domain_allow → typed Deny; X10 transcript scan → no key bytes / passphrase / bearer tokens.

Not in scope

• Payments (x402 phase 2: exact/EIP-3009 payments on the Ethereum reference path, fully local demo loop #34), upto, Permit2, any token/budget policy.
• EIP-712 typed-data signing (already present as TypedDataV4; x402 phase 2: exact/EIP-3009 payments on the Ethereum reference path, fully local demo loop #34 adds the EIP-3009 struct specifically).

Sequencing

Off the $5/day critical path (#34 → #208 → #209 do not depend on #32). A "nothing at risk" warm-up. Since the message seam already exists, the remaining SIWE parse + capability + domain_allow + MCP tool is smaller than the original body implied — run it in parallel off the same foundations if there's spare capacity; skip it if focused on the demo.

Definition of done

• All net-new in-scope items; new Deny reasons from the frozen vocabulary (Freeze the Deny-reason vocabulary in deckard-contract #28).
• cargo fmt --all --check clean; just check green (both feature configs); cargo test --workspace green.
• Any new dep (SIWE parsing may be hand-rolled — the grammar is small) listed and approved before merge.