x402trace

A local CLI debugger for x402 — the HTTP-402-based agentic-payment protocol — on Base. Verified against real Base Sepolia + the production x402.org/facilitator, three independent live reconciliations on-chain (latest: 0x116ccf73…ba52).

Real-time capture of the RECONCILED ⚠ settled-but-server-thinks-not detection against live Base Sepolia + x402.org/facilitator. ~17 seconds, real on-chain tx. Cast file replayable with asciinema play examples/cast/e2e-timeout-reconciliation.cast.

When to use x402trace

• Your buyer's wallet was debited but the server says payment failed — the canonical coinbase/x402#1062 reconciliation gap. x402trace proxy --reconcile detects it in seconds against live USDC Transfer events; x402trace inspect replays captured logs offline.
• You want to pre-flight a wallet before signing — USDC balance, EIP-3009 nonce status, wallet kind (EOA vs Smart Wallet). x402trace validate <wallet> <service-url> is read-only and exits non-zero if the payment would fail.
• You got a cryptic Bad Gateway or a generic 402 and need plain-English diagnosis. x402trace explain <jsonl-log> runs 10 diagnostic rules against captured payment state and tells you what was wrong, with actionable fixes per failure.
• You're shipping an agent that pays for HTTP APIs and want a JSONL audit trail of every 402 / X-PAYMENT / settlement your client or server produced. x402trace proxy records it.

The four subcommands at a glance

     BEFORE SIGNING         DURING PAYMENT          POST-SETTLEMENT          ON FAILURE
  ────────────────────  ──────────────────────  ──────────────────────  ─────────────────────
  ┌──────────────────┐  ┌──────────────────┐    ┌────────────────────┐  ┌───────────────────┐
  │  validate        │  │  proxy           │    │  inspect           │  │  explain          │
  │  ─────────       │  │  ─────           │    │  ───────           │  │  ───────          │
  │  Pre-flight a    │  │  Live capture    │    │  Offline replay    │  │  Plain-English    │
  │  wallet against  │  │  of every        │    │  of a captured     │  │  diagnosis of     │
  │  a service's     │  │  X-PAYMENT       │    │  log + re-run      │  │  every failed     │
  │  402 challenge   │  │  exchange        │    │  reconciliation    │  │  exchange         │
  └──────────────────┘  └──────────────────┘    └────────────────────┘  └───────────────────┘
     USDC balance         JSONL audit log          settled_on_chain        actionable fix
     EIP-3009 nonce       Joins on-chain           not_settled              per failed rule
     Wallet kind          USDC Transfers           value/recipient_         (10 rules)
                          by EIP-3009 nonce        mismatch

The problem

You wire up x402 on Base Sepolia. A buyer sends X-PAYMENT. The facilitator broadcasts the EIP-3009 transferWithAuthorization — and then hangs. Your paymentMiddleware times out and your client gets back:

HTTP/1.1 502 Bad Gateway
content-type: text/plain

Bad Gateway

The buyer sees a failure. The buyer's wallet was actually debited. The on-chain receipt exists, but nothing in your logs points at it. You have no programmatic path to reconcile.

This is the #1062 gap. x402trace closes it.

30-second quickstart

Requires Node ≥ 20, pnpm, and a Base Sepolia test wallet funded with USDC + dust ETH. See examples/README.md for the full prereq list.

# 1. Install
git clone https://github.com/fardinvahdat/x402trace.git
cd x402trace
pnpm install

# 2. Populate .env (one-time)
cp .env.example .env
$EDITOR .env   # set PAYER_PRIVATE_KEY + RECEIVER_ADDRESS

# 3. Run the canonical #1062 demo (real Base Sepolia, ~17s)
./examples/e2e-timeout-reconciliation.sh

The last line of output is the detection x402trace was built for:

RECONCILED ⚠ settled-but-server-thinks-not  id=35d9aea1…
  tx=0x116ccf73…  value=1000  payer=0xADEe…B895 → payee=0xADEe…B895  gap=11904ms

That tx= field is a real Base Sepolia settlement — view it on basescan. An asciinema replay of the full run is committed at examples/cast/e2e-timeout-reconciliation.cast — asciinema play it locally.

Install

npm install -g x402trace      # or `pnpm add -g x402trace`, `npx x402trace --help`

Requires Node ≥ 20.

Node 22 + npx users: if npx x402trace hangs with no output, install globally instead (npm i -g x402trace) or pin to Node 20 (nvm use 20). This is a known Node 22 ESM-resolver interaction with a transitive dep that anchor-x402 / x402-fetch consumers ride on; npm i -g and pnpm add -g are not affected. Tracked at X402-40.

How it works

┌────────┐     ┌────────────┐     ┌──────────┐     ┌─────────────────────┐
│ client │ ──► │ x402trace  │ ──► │  your    │ ──► │ x402.org/facilitator│
└────────┘     │   proxy    │     │  server  │     └──────────┬──────────┘
               └──────┬─────┘     └──────────┘                │
                      │                 ▲                     │ /settle
                      │                 │ slow/timeout      broadcasts
                      │                 │                     ▼
                      │           ┌─────┴────────┐     ┌──────────────┐
                      │           │ reconcile    │ ◄── │ Base Sepolia │
                      └──────────►│ engine       │     │ USDC Transfer│
                                  └──────────────┘     └──────────────┘

• Proxy — sits between the client and your x402 server. Captures every X-PAYMENT / X-PAYMENT-RESPONSE header to a JSONL log.
• Decoder — turns each captured request into structured PaymentRequirements / PaymentPayload / FacilitatorResponse records.
• Chain client — subscribes to Base Sepolia USDC Transfer events, enriches each with the matching EIP-3009 AuthorizationUsed.nonce.
• Reconciliation engine — joins facilitator-rejected exchanges against on-chain transfers by (payer, payee, value, nonce) and emits settled_on_chain / not_settled / value_mismatch / recipient_mismatch.

Full architecture: ARCHITECTURE.md. Wedge rationale: DECISIONS.md → ADR-001. On-disk schema: src/decoder/schema.md.

CLI

# v0.1 — during/after payment
x402trace proxy    --upstream <url> [--reconcile] [--log human|json] …
x402trace inspect  <jsonl-log-file> [--log human|json] …

# v0.2 — before/explaining payment
x402trace validate <wallet> <service-url> [--strict] [--log human|json]
x402trace explain  <jsonl-log-file> [--log human|json]

The full pre/during/post-payment debugger:

• validate <wallet> <service> — read-only pre-flight before signing. Fetches the 402, queries USDC balance + EIP-3009 nonce + wallet kind, runs 10 diagnostic rules, prints a plain-English report. Exits 0 if the payment would succeed, 2 if it would fail. Closes pain rank #4 (wallet-state pre-flight gap).
• explain <jsonl-log> — read a JSONL log produced by proxy --reconcile, find every exchange that didn't settled_on_chain, run the same rule engine against the captured state, print per-failure prose with actionable fixes. CI-friendly: exits 2 if any failures, 0 if clean. Closes pain rank #3 (generic 402 with no error reason).

The authoritative flag list is x402trace --help (or per-subcommand --help) — wired into the unit tests so it can't drift.

versions (v0.3+) — SDK skew audit

Compares your local package.json @x402/* versions and the service's 402 version hints against a bundled known-skew table. Catches the kind of multi-day debugging chase Myceliaman14 (Python SDK pre-v2 → TS V2 refactor) and Poteshniy (@x402/fetch 2.10.0 extension-echo bug) hit on Discord.

x402trace versions https://your-service.example/api/route
x402trace versions https://… --log json

Output names each match with severity (warning / blocking), the upstream evidence link (#2157, #2207), and a concrete fix. Exit 0 = no skew, 2 = at least one match.

validate --diff (v0.3+) — cross-facilitator drift detection

When a payment works on one facilitator but fails on another (TerraDeed's CDP → xpay switch in the Discord transcript), --diff runs the same synthesised payload through both /verify endpoints in parallel and shows you exactly where they disagree.

# Built-in aliases: cdp, xpay, payai, x402.org. Full URLs also accepted.
x402trace validate 0xYourWallet https://your-service.example/api/route --diff cdp,xpay

# JSON output for CI; per-facilitator timeout knob
x402trace validate 0xYourWallet https://… --diff cdp,xpay --log json --diff-timeout-ms 5000

Each row reports the per-facilitator HTTP status + body, captured rejection reason, and any X402-33 facilitator-aware rules firing (e.g. CDP minimum amount, throttling 403/429). Exit codes: 0 = at least one accepts, 2 = all reject, 3 = all timeout.

bazaar-check (v0.3+) — pre-ship Bazaar / agentic.market validator

The headline of v0.3. Answers the question Discord operators are asking each other in real-time: "is my Bazaar / agentic.market integration implemented correctly, or is the bug upstream of me?"

# Default — Base Sepolia
x402trace bazaar-check https://your-service.example/api/route

# Mainnet discovery query (banner printed; still read-only)
x402trace bazaar-check https://your-service.example/api/route --chain base

# With a payer-hint (enables the self-payment guard)
x402trace bazaar-check https://your-service.example/api/route --payer-hint 0xYourPayer

# Shorten the per-request HTTP timeout for CI/smoke checks
x402trace bazaar-check https://your-service.example/api/route --timeout-ms 5000

Four read-only checks compose into a single bottom-line verdict:

| Check | What it validates | | -------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | well-known | /.well-known/x402 manifest exists with name + description + accepts[] | | challenge | The protected endpoint returns 402 with a valid x402 v1/v2 body AND extensions.bazaar.{name, description} | | self-payment | When --payer-hint is supplied, flags payer == payTo (CDP rejects this with a generic invalid_payload) | | indexing | CDP discovery (/v2/x402/discovery/resources?payTo=…) returns non-empty resources (else: matches the #2207 upstream pattern) |

Exit codes:

• 0 — looks correct
• 2 — found issues in your implementation (fix the failed check)
• 3 — your code looks correct; the bug is upstream (e.g. the canonical #2207 Bazaar indexing failure — 94 reports). The verdict prose names the GitHub issue so you don't have to map the symptom.

Scope notes:

• Read-only. Never signs, never broadcasts.
• Each HTTP probe has a 10s default timeout; use --timeout-ms to shorten it in CI or when checking flaky endpoints.
• The opt-in paid-pass mode (--with-wallet) is deferred — see ADR-003. The static-analysis-only checks shipped here cover the dominant Discord pain (Bazaar indexing failure) without needing signing infrastructure.
• --endpoint <paid-url> (v0.3.2+) — skip the root /.well-known/x402 probe and read the 402 challenge directly from a paid route. Use this for services that publish per-route only (the #2207 shape — AsaiShota's test-echo-cdp, evanatpizzarobot's TensorFeed, 0xdespot's hyperD).

JSON API (v0.3.2+) — --log json is a public contract

bazaar-check --log json emits a JSON envelope downstream consumers can take a runtime dependency on. Per ADR-004 Pillar 2, the maintainer commits to:

• Additive changes (new optional fields, new optional facets) — ship in MINOR versions
• Shape-breaking changes (renames, removals, type changes, fixed-position reordering) — require a MAJOR version + integrator notice
• Every change to the --log json output is documented in CHANGELOG.md under a ### JSON API subsection

The full contract — envelope shape, per-check detail keys, verdict discriminator, regeneration workflow — lives at src/bazaar/json-api.md. A frozen exemplar lives at tests/fixtures/bazaar/json-api-snapshot.json and is enforced by tests/integration/bazaar-check-json-api.test.ts.

# Pipe the JSON into your own consumer
x402trace bazaar-check https://your-service.example.com --log json | jq '.verdict.kind'
# → "looks_correct"  (or "implementation_issue" / "upstream_issue")

Chain selection (Base mainnet, v0.3+)

x402trace v0.3 enables Base mainnet alongside the default Base Sepolia. The --chain flag (or BASE_CHAIN_ID env) selects the chain; default stays base-sepolia for backward compatibility.

# Default — Base Sepolia (unchanged)
x402trace proxy --upstream <url> --reconcile

# Base mainnet — RPC URL must be supplied; there is no built-in mainnet endpoint
x402trace proxy --upstream <url> --reconcile --chain base --rpc-url https://your-mainnet-rpc

# Validate a mainnet wallet (read-only)
x402trace validate 0xYourWallet https://your-service --chain base --rpc-url https://your-mainnet-rpc

Mainnet safety notes:

• x402trace never signs transactions and never broadcasts. The worst case from a misconfigured mainnet run is a failed read or a misleading reconciliation verdict.
• The buyer's wallet (which signs the EIP-3009 authorization) is upstream of x402trace. The buyer's risk model is unchanged.
• No mainnet RPC URL is shipped — supply your own via --rpc-url or BASE_RPC_URL env. Mainnet startup prints a ⚠ MAINNET banner so accidental mainnet runs are visible at a glance.
• CI never uses mainnet (CLAUDE.md hard rule #2). Don't commit a populated mainnet URL.

validate — example output

$ x402trace validate 0xADEeaf70…B895 https://example.com/api/weather

diagnose: ✓ would succeed

  ✓ network-match:    network matches: base-sepolia
  ✓ recipient-match:  recipient matches: 0x1111…1111
  ✓ value-sufficient: signed 1000 >= required 1000
  ✓ valid-before:     validBefore=1778573803 is 300s in the future
  ✓ payer-balance:    wallet has 5000000 USDC (raw), needs 1000
  ✓ nonce-fresh:      nonce 0x000000… is fresh
  ✓ wallet-kind:      wallet kind: eoa
  ✓ asset-address:    asset is canonical Base Sepolia USDC

A failing run flips the headline to ✗ would fail and prints a fix: line under each failed rule.

explain — example output

$ x402trace explain ./x402trace.jsonl

─── exchange 94c15089… not_settled at 2026-05-12T08:13:18Z ───
diagnose: ✗ would fail

  ✗ valid-before:    validBefore=1778573803 expired 97s ago (now=1778573900)
    fix: re-sign the authorization with a later validBefore (typical: now + 300s)

explained 1 failed exchange(s), 0 decoder error(s) from 24 lines

FAQ

Q: I see Bad Gateway from x402-fetch but my wallet was debited. What do I do? That's the canonical coinbase/x402#1062. Run x402trace proxy --reconcile --upstream <your-server> between the buyer and your server; when the chain client matches the EIP-3009 nonce against a failed exchange you'll get a RECONCILED ⚠ settled-but-server-thinks-not record with the live tx hash.

Q: The facilitator returned invalid_payload with no explanation. How do I figure out why? Save the captured 402 (proxy does this automatically) and run x402trace explain <log>. It runs 10 rules against the captured state — most invalid_payload cases turn out to be a validBefore expiry, value mismatch, or recipient mismatch, each rendered as a single failed rule with an actionable fix.

Q: Can I check whether a wallet can pay a service without actually signing? Yes — x402trace validate <wallet> <service-url> is read-only. It fetches the 402, queries chain state (USDC balance, EIP-3009 nonce, wallet kind), runs the same rules explain uses. Exits 0 if would-succeed, 2 if would-fail.

Q: Does this work on mainnet? Yes — Base mainnet shipped in v0.3.0 per ADR-003. Pass --chain base (or set BASE_CHAIN_ID=base) to switch. You must supply your own mainnet RPC URL via --rpc-url or BASE_RPC_URL env — x402trace ships no default mainnet endpoint (CLAUDE.md hard rule #2). Default stays base-sepolia for backward compatibility. See Chain selection (Base mainnet, v0.3+) above for examples and safety notes.

Q: My supply-chain scanner shows transitive alerts on dependencies. Are these in x402trace? No. As of v0.3.2 the runtime tree is commander + dotenv + viem + x402 only — the published dist/ imports nothing else (enforced by pnpm check:publish-surface in CI). The test-tooling deps (hono, x402-fetch, x402-hono) are devDependencies and are not installed by npm i x402trace. Any wallet-SDK / WalletConnect / MetaMask transitives a scanner shows on the package come in via viem (read-only chain client). See SECURITY.md to report a vulnerability in x402trace itself.

Roadmap

• v0.1.0 (2026-05-12) — local proxy + timeout reconciliation. ADR-001.
• v0.2.0 — v0.2.3 (2026-05-12 → 2026-05-13) — validate + explain on a shared diagnostic rule engine. ADR-002.
• v0.3.0 (2026-05-17) — bazaar-check (headline), validate --diff cross-facilitator, 5 facilitator-aware diagnose rules, Base mainnet support, versions SDK-skew audit. ADR-003.
• v0.3.1 (2026-05-20) — hotfix: v2 challenge body parsing (#66 by @hypeprinter007-stack — first external contributor), bazaar-check HTTP probe timeouts (#67 by @peterxing), npm package description audit-gap fix.
• v0.3.2 (current, 2026-05-23) — metadata-propagation cut. D.2 propagation diff (manifest vs CDP discovery render — @zev's api.lastlookdata.com pattern), D.3 indexer-state probe + new upstream_stuck verdict (Max's polyodds.bet pattern + the canonical #2207 cluster), D.4 --endpoint <paid-url> per-route probe (AsaiShota / evanatpizzarobot / 0xdespot's per-route services), D.5 variant-aware extensions.bazaar (fixes the #72 Body-discovery false-positive), public JSON API contract + snapshot tests, facilitator-aware verdict semantics. ADR-004. 511 tests.
• v0.3.3+ planned — TomSmart cdp-mature + 4 other contributor fixtures wired into the captured-responses harness; processing_fresh vs processing_stale distinction on D.3 (operator-supplied settle-timestamp); facilitator-status if a 2nd CDP outage cluster lands.
• Deferred (kept, not killed) — ERC-6492 Smart Wallet kind, extensions.diagnostic decoder (gated on #1875), --watch daemon, reconciliation actions (webhook / auto-retry), hosted SaaS, multi-chain non-Base, bazaar-check --with-wallet paid-pass mode. See SPEC.md § 5b "v0.3.2+ deferred".

How x402trace compares

| Capability | x402trace | xpay | x402scan | x402lint | | -------------------------------------------------------------------------------------- | :-------: | :-----: | :------: | :------: | | Local proxy + JSONL audit log | ✅ | — | — | — | | Timeout reconciliation (catches #1062) | ✅ | partial | — | — | | Pre-flight wallet check (no signing) | ✅ | — | — | — | | Plain-English 402 diagnosis | ✅ | — | — | partial | | Static config validation | partial | — | — | ✅ | | Network explorer / discovery | — | — | ✅ | — | | Spending controls | — | ✅ | — | — |

x402trace is the debugger in the x402 toolbox — built for the narrow, expensive moment when a payment fell into the gap between facilitator and chain. The other tools target adjacent jobs (routing, explorer, lint, controls) and compose well. Full comparison in SPEC.md § 8.

Contributing

Personal project. PRs and bug reports welcome. Read in this order:

1. CLAUDE.md — operating manual + hard rules
2. TESTING.md — testing is a hard requirement, not a nice-to-have
3. CONTRIBUTING.md — branching + PR workflow

Donate

If x402trace saved you time and you want to send something back, the maintainer accepts USDC or ETH on Base or Ethereum mainnet:

0x180671C40c5e28Ba0cF88D57B761A8ccE97710De

Verify on Basescan or Etherscan. All amounts welcome — donations fund maintainer time on the next release (currently: v0.3.2 metadata-propagation sub-checks per the v0.3.1 release notes).

Acknowledgments

External contributors who've shaped x402trace:

• @hypeprinter007-stack — first external contributor; fixed v2 challenge body parsing (#65, #66); manifest-hygiene empty-string detection (#70)
• @peterxing — bazaar-check HTTP probe timeouts (#67)
• @TomSmart_ai — first production-scale validation against a 19-endpoint mapper catalog (Coinbase Developer Discord, 2026-05-20); coined the "listing-readiness gap" framing for bazaar-check; named integrator for the v0.3.2 JSON API contract
• @AsaiShota — origin of #72 (Body-discovery extension shape, drove D.5 / X402-43) + #2207 per-route services corroboration (drove D.4 / X402-42)
• @evanatpizzarobot — second-voice corroboration on D.4 per-route shape (#2207, TensorFeed)
• @0xdespot — D.4 third-voice corroboration (hyperD.ai) + D.5 BodyDiscoveryExtension corroboration + bucket taxonomy that informs the D.3 indexer_state enum on #2207
• @Cryptor + @Ferj (Coinbase Developer Discord, 2026-05-21) — CDP-only Bazaar-indexing clarification that anchors ADR-004 Pillar 3 (facilitator-aware verdict attribution)
• @zev (Discord) — D.2 propagation-diff pain pattern (api.lastlookdata.com)
• Max (Discord) — D.3 indexer-state probe pain pattern (polyodds.bet)

Per-release credit lives in the CHANGELOG.

License

Apache 2.0