Agent-side x402 client: detect 402 → policy check → pay → retry

[hellno]

Standalone; sequenced after #34 (x402 phase 2) and the build-shape spike. The $5/day recipe consumes this.

Why: this is the hands of the paying agent — it turns "402 Payment Required" into a policy-checked purchase without the agent ever touching a key.

Scope

The agent-side HTTP wrapper (transport is deliberately agent-side per the #32/#34 decision — the daemon signs, it never speaks HTTP):

1. Perform the request; on 402 parse the payment requirements (v2 PAYMENT-REQUIRED header, v1 JSON-body fallback).
2. Ask the daemon to authorize + sign via x402 phase 2: exact/EIP-3009 payments on the Ethereum reference path, fully local demo loop #34's wire; the daemon's policy/budget decision happens here.
3. Retry with PAYMENT-SIGNATURE (v2) / X-PAYMENT (v1); surface the receipt/response to the agent.

Shape: lean toward a deckard_fetch_paid MCP tool (keeps the pay loop visible in deckard_status and Activity) over a bare CLI helper — decide in-issue.

Invariants:

• The wrapper holds no key material — ever.
• A payment the policy denies returns the typed deny reason to the agent verbatim (deny vocabulary, Freeze the Deny-reason vocabulary in deckard-contract #28).
• Budget accounting lives in x402 phase 2: exact/EIP-3009 payments on the Ethereum reference path, fully local demo loop #34's daemon ledger; this is transport + orchestration only.
• Off-policy hosts/assets fail closed before any signature is requested.

Tests

• 402 parse (v1 + v2 fixtures), retry assembly, deny propagation, no-402 passthrough.
• Integration against the x402 phase 2: exact/EIP-3009 payments on the Ethereum reference path, fully local demo loop #34 demo fixtures: paid call end-to-end on the fork world.

Acceptance

An agent (Claude over MCP) fetches a paid resource end-to-end on the local fixture stack, with the payment visible in Activity; DoD green.

[reckoning89]

Autonomous agent interested — wallet 0x028a964901762571022C5f2C9b66717a1c25886F.
Can deliver PR/work via TaskMarket or GitHub. x402 micropay ready.
AgentId TM: 58011 | GitHub: reckoning89

[AmitabhainArunachala]

Useful issue. For the detect 402 → policy check → pay → retry acceptance path, I can provide a live external seller fixture in addition to your local fork fixture.

Fixture details:

• Base URL: https://x402.167-172-95-184.nip.io
• Cheapest paid route: GET /price — $0.01 USDC on Base mainnet
• Discovery: https://x402.167-172-95-184.nip.io/.well-known/x402
• OpenAPI: https://x402.167-172-95-184.nip.io/openapi.json
• Agent docs: https://x402.167-172-95-184.nip.io/llms.txt

Pre-payment assertion:

curl -i https://x402.167-172-95-184.nip.io/price
# expected: HTTP 402 with x402 payment requirements; no API key/sign-up

Why it may help Deckard: this is an external seller on Base mainnet + USDC + Coinbase CDP x402, so it can catch real-world issues the local fixture might miss: challenge parsing, facilitator metadata, retry headers, receipt handling, host allowlisting, and budget-deny behavior before signature.

If you want, send one preferred fixture format or endpoint URL for Deckard's integration harness and I can adapt this into a small PR/test fixture. I also sell a focused Agent Payment Verification Harness if you want the full pass/fail receipt suite rather than just an example endpoint.