diff --git a/.github/workflows/check-rfc.yaml b/.github/workflows/check-rfc.yaml
new file mode 100644
index 00000000..e4b44124
--- /dev/null
+++ b/.github/workflows/check-rfc.yaml
@@ -0,0 +1,35 @@
+name: Check RFC
+
+on:
+  push:
+    paths:
+      - "docs/ack-id/rfc/**"
+      - "docs/ack-pay/rfc/**"
+      - "docs/package.json"
+      - "docs/vitest.config.ts"
+      - ".github/workflows/check-rfc.yaml"
+  pull_request:
+    branches:
+      - ack-id-core-rfc
+    paths:
+      - "docs/ack-id/rfc/**"
+      - "docs/ack-pay/rfc/**"
+      - "docs/package.json"
+      - "docs/vitest.config.ts"
+      - ".github/workflows/check-rfc.yaml"
+
+permissions:
+  contents: read
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    env:
+      ANTHROPIC_API_KEY: secret
+      ISSUER_PRIVATE_KEY: "0xa45f5c566918ef954e8c200a96b14092cabcd69cb8a1a132804a2b8cbb8489a1"
+      VERIFIER_PRIVATE_KEY: "0xeeca8f89b2f5196126f7d9199e739153bd43a13f9cdd1099e7191a33143a2059"
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ./.github/actions/setup
+      - run: pnpm run build
+      - run: pnpm run check
diff --git a/docs/ack-id/rfc/core.md b/docs/ack-id/rfc/core.md
index 5ce4089a..00f305ff 100644
--- a/docs/ack-id/rfc/core.md
+++ b/docs/ack-id/rfc/core.md
@@ -367,6 +367,10 @@ ext-revocation.
   exposure with no revocation check at all.
 - **Single-use grants.** For one-shot authority, the RP records the `jti`
   at first acceptance and rejects reuse; the grant is spent when used.
+  For a single-use payment grant, checking the grant before issuing a
+  payment challenge is not acceptance of the paid operation and MUST NOT
+  redeem that grant. ACK-Pay core Section 4.1 defines the ordering between
+  grant checks, challenges, redemption, and paid execution.
   Which actions need single-use authority is the RP's call (a payment
   authorization, a one-shot registration); core supplies the mechanism.
   Redemption MUST be atomic: a check-and-set keyed by (`iss`, `jti`). A
diff --git a/docs/ack-pay/rfc/core.md b/docs/ack-pay/rfc/core.md
index d73d31b7..b1fafa55 100644
--- a/docs/ack-pay/rfc/core.md
+++ b/docs/ack-pay/rfc/core.md
@@ -101,6 +101,39 @@ proofs). This is the full trail from a payment event to the legal entity
 behind the paying agent, walkable by a third party with no callback to any
 participant.
 
+### 4.1 Single-use payment grants and payment challenges
+
+This section applies when a seller verifies a single-use payment grant
+before returning a payment challenge. It distinguishes validation of
+permission from acceptance of the paid operation that consumes it.
+
+- A request that only produces a `402 Payment Required` challenge, without
+  performing the operation authorized by the payment grant, MUST NOT
+  redeem that grant. A separately authorized paid quote-generation
+  operation may consume its own grant; it is not a challenge-only request.
+- The paid retry MUST carry a new request signature and pass the current
+  ACK-ID request and grant checks. A successful earlier check is neither
+  a reservation nor a promise of later acceptance. The seller MUST
+  validate the payment payload before redeeming the grant; validation
+  here does not itself submit settlement or perform the paid operation.
+- Before the first authorized effect, the seller MUST atomically and
+  durably redeem the selected grant by (`iss`, `jti`), using ACK-ID core
+  Section 8's single-use mechanism. Authorized effects include submitting
+  settlement, accepting a deferred payment obligation, executing the paid
+  operation, and delivering the protected resource. Only the request that
+  wins redemption may start those effects. Coordination MUST cover every
+  server accepting that grant for the same RP.
+- Once an authorized effect has started or been submitted, a timeout or
+  uncertain outcome MUST NOT make the grant available for another
+  operation. Recovery must reconcile the existing operation. A redemption
+  record is not a lease that expires on a request timeout.
+
+This ordering adds no wire fields. Request-signature replay checks still
+apply to both attempts. A production implementation needs durable state
+and rail-specific recovery; these rules do not promise exactly-once
+settlement or successful delivery after a crash. The executable examples
+in `vectors/payment-grant-lifecycle.test.ts` model the ordering only.
+
 ## 5. Third-party verification
 
 A third party verifies a receipt (with its offer, when presented together)
diff --git a/docs/ack-pay/rfc/vectors/README.md b/docs/ack-pay/rfc/vectors/README.md
new file mode 100644
index 00000000..4cb5ab44
--- /dev/null
+++ b/docs/ack-pay/rfc/vectors/README.md
@@ -0,0 +1,17 @@
+# Single-use payment grant lifecycle examples
+
+Run from the repository root after installing its pinned dependencies:
+
+```sh
+pnpm --filter @docs/agentcommercekit test
+```
+
+These executable examples specify the ordering in ACK-Pay core Section 4.1.
+Request/grant and payment validation are explicit inputs; the examples do
+not implement cryptography or verify x402 payloads. They cover the unpaid
+challenge, paid retry, concurrent requests, and uncertain execution outcome.
+
+The in-memory claim is atomic within one process only. A production adapter
+needs durable atomic claims shared across the RP's acceptance domain and
+rail-specific recovery. The examples do not test distributed storage,
+settlement, record-retention deadlines, or exactly-once execution.
diff --git a/docs/ack-pay/rfc/vectors/payment-grant-lifecycle.test.ts b/docs/ack-pay/rfc/vectors/payment-grant-lifecycle.test.ts
new file mode 100644
index 00000000..42846f91
--- /dev/null
+++ b/docs/ack-pay/rfc/vectors/payment-grant-lifecycle.test.ts
@@ -0,0 +1,145 @@
+import { describe, expect, it } from "vitest"
+
+// Ordering examples, not an ACK verifier or a settlement implementation.
+// Inputs stand for the results of fresh request/grant and payment validation.
+type Grant = { iss: string; jti: string }
+type Attempt = {
+  grant: Grant
+  authorized: boolean
+  payment: "absent" | "invalid" | "valid"
+  execute?: () => Promise<void>
+}
+
+function createMerchant() {
+  const records = new Map<string, "started" | "completed" | "unknown">()
+  let started = 0
+
+  async function handle(attempt: Attempt) {
+    if (!attempt.authorized) {
+      return "unauthorized"
+    }
+    if (attempt.payment === "absent") {
+      return "challenge"
+    }
+    if (attempt.payment === "invalid") {
+      return "invalid-payment"
+    }
+
+    // The identity comes from the validated grant, not the payment payload.
+    const key = JSON.stringify([attempt.grant.iss, attempt.grant.jti])
+    // Synchronous claim-before-await is atomic only within this model.
+    // Real adapters need a durable atomic store shared by all RP replicas.
+    if (records.has(key)) {
+      return "already-claimed"
+    }
+    records.set(key, "started")
+    started++
+    try {
+      await attempt.execute?.()
+      records.set(key, "completed")
+      return "completed"
+    } catch {
+      records.set(key, "unknown")
+      return "unknown"
+    }
+  }
+
+  return { handle, started: () => started }
+}
+
+const grant = { iss: "did:web:owner.example", jti: "purchase-1" }
+const unpaid: Attempt = { grant, authorized: true, payment: "absent" }
+const paid: Attempt = { grant, authorized: true, payment: "valid" }
+
+describe("single-use payment grant lifecycle", () => {
+  it("preserves a checked grant through the challenge and paid retry", async () => {
+    const merchant = createMerchant()
+    expect(await merchant.handle(unpaid)).toBe("challenge")
+    expect(merchant.started()).toBe(0)
+    expect(await merchant.handle(paid)).toBe("completed")
+    expect(merchant.started()).toBe(1)
+  })
+
+  it("preserves the grant through repeated challenge-only requests", async () => {
+    const merchant = createMerchant()
+    expect(await merchant.handle(unpaid)).toBe("challenge")
+    expect(await merchant.handle(unpaid)).toBe("challenge")
+    expect(await merchant.handle(paid)).toBe("completed")
+    expect(merchant.started()).toBe(1)
+  })
+
+  it("rejects an unauthorized attempt before redeeming the grant", async () => {
+    const merchant = createMerchant()
+    expect(await merchant.handle({ ...paid, authorized: false })).toBe(
+      "unauthorized",
+    )
+    expect(merchant.started()).toBe(0)
+    expect(await merchant.handle(paid)).toBe("completed")
+  })
+
+  it("rejects invalid payment evidence before redeeming the grant", async () => {
+    const merchant = createMerchant()
+    expect(await merchant.handle({ ...paid, payment: "invalid" })).toBe(
+      "invalid-payment",
+    )
+    expect(merchant.started()).toBe(0)
+    expect(await merchant.handle(paid)).toBe("completed")
+  })
+
+  it("uses the paid retry's current authorization result", async () => {
+    const merchant = createMerchant()
+    expect(await merchant.handle(unpaid)).toBe("challenge")
+    // For example, the grant expired or was revoked after the challenge.
+    expect(await merchant.handle({ ...paid, authorized: false })).toBe(
+      "unauthorized",
+    )
+    expect(merchant.started()).toBe(0)
+  })
+
+  it("allows only one concurrent paid request to begin execution", async () => {
+    const merchant = createMerchant()
+    let release: (() => void) | undefined
+    const pending = new Promise<void>((resolve) => {
+      release = resolve
+    })
+    const first = merchant.handle({ ...paid, execute: () => pending })
+    try {
+      expect(await merchant.handle(paid)).toBe("already-claimed")
+      expect(merchant.started()).toBe(1)
+    } finally {
+      release?.()
+    }
+    expect(await first).toBe("completed")
+  })
+
+  it("keeps the claim when a submitted operation has an uncertain outcome", async () => {
+    const merchant = createMerchant()
+    expect(
+      await merchant.handle({
+        ...paid,
+        execute: () => Promise.reject(new Error("settlement timed out")),
+      }),
+    ).toBe("unknown")
+    expect(await merchant.handle(paid)).toBe("already-claimed")
+    expect(merchant.started()).toBe(1)
+  })
+
+  it("rejects another paid operation after completion", async () => {
+    const merchant = createMerchant()
+    expect(await merchant.handle(paid)).toBe("completed")
+    expect(await merchant.handle(paid)).toBe("already-claimed")
+    expect(merchant.started()).toBe(1)
+  })
+
+  it("keeps grants with the same jti from different issuers independent", async () => {
+    const merchant = createMerchant()
+    expect(await merchant.handle(paid)).toBe("completed")
+    expect(
+      await merchant.handle({
+        ...paid,
+        grant: { ...grant, iss: "did:web:another-owner.example" },
+      }),
+    ).toBe("completed")
+    expect(merchant.started()).toBe(2)
+  })
+})
diff --git a/docs/package.json b/docs/package.json
index dfaa38ab..c43faf63 100644
--- a/docs/package.json
+++ b/docs/package.json
@@ -16,7 +16,8 @@
   },
   "scripts": {
     "clean": "git clean -fdX .turbo dist",
-    "docs": "mintlify dev"
+    "docs": "mintlify dev",
+    "test": "vitest run"
   },
   "devDependencies": {
     "mintlify": "4.2.637"
diff --git a/docs/vitest.config.ts b/docs/vitest.config.ts
new file mode 100644
index 00000000..ce9a44f7
--- /dev/null
+++ b/docs/vitest.config.ts
@@ -0,0 +1,8 @@
+import { defineConfig } from "vitest/config"
+
+export default defineConfig({
+  test: {
+    include: ["ack-pay/rfc/vectors/**/*.test.ts"],
+    watch: false,
+  },
+})