diff --git a/src/main/runtime/relay/relay-host-proof.test.ts b/src/main/runtime/relay/relay-host-proof.test.ts
new file mode 100644
index 000000000..39e47e322
--- /dev/null
+++ b/src/main/runtime/relay/relay-host-proof.test.ts
@@ -0,0 +1,225 @@
+import { createHmac, randomBytes } from 'node:crypto'
+import { describe, expect, it } from 'vitest'
+import nacl from 'tweetnacl'
+import {
+  answerRelayHostChallenge,
+  type RelayHostChallenge,
+  type RelayHostProofContext
+} from './relay-host-proof'
+
+const encoder = new TextEncoder()
+const HOST_PROOF_DOMAIN = 'orca-relay-host-proof/v1'
+const CHALLENGE_DOMAIN = 'orca-relay-host-challenge/v1'
+const CLOCK_SKEW_MS = 30_000
+
+function concat(parts: readonly Uint8Array[]): Uint8Array {
+  const output = new Uint8Array(parts.reduce((total, part) => total + part.byteLength, 0))
+  let offset = 0
+  for (const part of parts) {
+    output.set(part, offset)
+    offset += part.byteLength
+  }
+  return output
+}
+
+function uint32(value: number): Uint8Array {
+  const bytes = new Uint8Array(4)
+  new DataView(bytes.buffer).setUint32(0, value, false)
+  return bytes
+}
+
+function uint64(value: number): Uint8Array {
+  const bytes = new Uint8Array(8)
+  new DataView(bytes.buffer).setBigUint64(0, BigInt(value), false)
+  return bytes
+}
+
+function field(name: string, value: Uint8Array): Uint8Array {
+  const encodedName = encoder.encode(name)
+  return concat([uint32(encodedName.byteLength), encodedName, uint32(value.byteLength), value])
+}
+
+function text(value: string): Uint8Array {
+  return encoder.encode(value)
+}
+
+function buildTranscript(input: {
+  origin: string
+  relayKey: Uint8Array
+  nonce: Uint8Array
+  challengeId: string
+  issuedAt: number
+  expiresAt: number
+  relayHostId: string
+  hostKey: Uint8Array
+  userId?: string
+  profileId?: string
+  organizationId?: string
+  assignmentEpoch?: number
+}): Uint8Array {
+  return concat([
+    field('protocol', text(HOST_PROOF_DOMAIN)),
+    field('version', new Uint8Array([1])),
+    field('relayOrigin', text(input.origin)),
+    field('relayEphemeralPublicKey', input.relayKey),
+    field('challengeNonce', input.nonce),
+    field('challengeId', text(input.challengeId)),
+    field('issuedAt', uint64(input.issuedAt)),
+    field('expiresAt', uint64(input.expiresAt)),
+    field('userId', text(input.userId ?? 'user-1')),
+    field('profileId', text(input.profileId ?? 'profile-1')),
+    field('organizationId', text(input.organizationId ?? 'org-1')),
+    field('relayHostId', text(input.relayHostId)),
+    field('hostPublicKey', input.hostKey),
+    field('assignmentEpoch', uint64(input.assignmentEpoch ?? 3)),
+    field('previousGeneration', new Uint8Array()),
+    field('resumeRequested', new Uint8Array([0]))
+  ])
+}
+
+function buildChallengeFixture(options: {
+  issuedAt: number
+  expiresAt: number
+  localNow: number
+}): {
+  challenge: RelayHostChallenge
+  context: RelayHostProofContext
+  expectedProof: string
+} {
+  const hostKeys = nacl.box.keyPair()
+  const relayKeys = nacl.box.keyPair()
+  const nonce = randomBytes(24)
+  const secret = randomBytes(32)
+  const origin = 'https://c2.relay.onorca.dev'
+  const relayHostId = 'host-abc123'
+  const challengeId = 'challenge-skew'
+  const transcript = buildTranscript({
+    origin,
+    relayKey: relayKeys.publicKey,
+    nonce,
+    challengeId,
+    issuedAt: options.issuedAt,
+    expiresAt: options.expiresAt,
+    relayHostId,
+    hostKey: hostKeys.publicKey
+  })
+  const plaintext = concat([
+    text(`${CHALLENGE_DOMAIN}\0`),
+    uint32(transcript.byteLength),
+    transcript,
+    secret
+  ])
+  const challenge: RelayHostChallenge = {
+    challengeId,
+    relayEphemeralPublicKeyB64: Buffer.from(relayKeys.publicKey).toString('base64'),
+    nonceB64: nonce.toString('base64'),
+    ciphertextB64: Buffer.from(
+      nacl.box(plaintext, nonce, hostKeys.publicKey, relayKeys.secretKey)
+    ).toString('base64'),
+    expiresAt: options.expiresAt
+  }
+  const context: RelayHostProofContext = {
+    relayOrigin: origin,
+    userId: 'user-1',
+    profileId: 'profile-1',
+    organizationId: 'org-1',
+    relayHostId,
+    hostPublicKey: hostKeys.publicKey,
+    hostSecretKey: hostKeys.secretKey,
+    assignmentEpoch: 3,
+    resumeRequested: false,
+    now: () => options.localNow
+  }
+  const expectedProof = createHmac('sha256', secret)
+    .update(text(`${HOST_PROOF_DOMAIN}\0ack\0`))
+    .update(transcript)
+    .digest('base64')
+  return { challenge, context, expectedProof }
+}
+
+describe('answerRelayHostChallenge clock skew (#10401)', () => {
+  it('accepts a challenge when local clock is a few seconds behind (issuedAt in the near future)', () => {
+    const serverNow = 1_700_000_000_000
+    const skewBehindMs = 4_400
+    const localNow = serverNow - skewBehindMs
+    const issuedAt = serverNow
+    const expiresAt = issuedAt + 10_000
+    const { challenge, context, expectedProof } = buildChallengeFixture({
+      issuedAt,
+      expiresAt,
+      localNow
+    })
+
+    expect(answerRelayHostChallenge(challenge, context)).toBe(expectedProof)
+  })
+
+  it('accepts a challenge when local clock is a few seconds ahead of expiresAt', () => {
+    const serverNow = 1_700_000_000_000
+    const issuedAt = serverNow
+    const expiresAt = issuedAt + 10_000
+    const localNow = expiresAt + 4_400
+    const { challenge, context, expectedProof } = buildChallengeFixture({
+      issuedAt,
+      expiresAt,
+      localNow
+    })
+
+    expect(answerRelayHostChallenge(challenge, context)).toBe(expectedProof)
+  })
+
+  it('accepts challenges at the clock-skew boundaries', () => {
+    const issuedAt = 1_700_000_000_000
+    const expiresAt = issuedAt + 10_000
+    for (const localNow of [issuedAt - CLOCK_SKEW_MS, expiresAt + CLOCK_SKEW_MS]) {
+      const { challenge, context, expectedProof } = buildChallengeFixture({
+        issuedAt,
+        expiresAt,
+        localNow
+      })
+      expect(answerRelayHostChallenge(challenge, context)).toBe(expectedProof)
+    }
+  })
+
+  it('still rejects challenges outside the allowed skew window', () => {
+    const serverNow = 1_700_000_000_000
+    const issuedAt = serverNow
+    const expiresAt = issuedAt + 10_000
+    const localNowTooBehind = issuedAt - CLOCK_SKEW_MS - 1
+    const behind = buildChallengeFixture({
+      issuedAt,
+      expiresAt,
+      localNow: localNowTooBehind
+    })
+    expect(answerRelayHostChallenge(behind.challenge, behind.context)).toBeNull()
+
+    const localNowTooAhead = expiresAt + CLOCK_SKEW_MS + 1
+    const ahead = buildChallengeFixture({
+      issuedAt,
+      expiresAt,
+      localNow: localNowTooAhead
+    })
+    expect(answerRelayHostChallenge(ahead.challenge, ahead.context)).toBeNull()
+  })
+
+  it('still rejects an oversized server challenge window', () => {
+    const issuedAt = 1_700_000_000_000
+    const expiresAt = issuedAt + 10_001
+    const { challenge, context } = buildChallengeFixture({
+      issuedAt,
+      expiresAt,
+      localNow: issuedAt
+    })
+    expect(answerRelayHostChallenge(challenge, context)).toBeNull()
+  })
+
+  it('rejects a challenge that expires before it is issued', () => {
+    const issuedAt = 1_700_000_000_000
+    const expiresAt = issuedAt - 1
+    const { challenge, context } = buildChallengeFixture({
+      issuedAt,
+      expiresAt,
+      localNow: issuedAt
+    })
+    expect(answerRelayHostChallenge(challenge, context)).toBeNull()
+  })
+})
diff --git a/src/main/runtime/relay/relay-host-proof.ts b/src/main/runtime/relay/relay-host-proof.ts
index 94db5b2f1..da17f8a15 100644
--- a/src/main/runtime/relay/relay-host-proof.ts
+++ b/src/main/runtime/relay/relay-host-proof.ts
@@ -3,6 +3,9 @@ import nacl from 'tweetnacl'
 
 const HOST_PROOF_TRANSCRIPT_DOMAIN = 'orca-relay-host-proof/v1'
 const HOST_CHALLENGE_PLAINTEXT_DOMAIN = 'orca-relay-host-challenge/v1'
+// Covers routine NTP drift without extending the signed challenge window.
+const RELAY_HOST_PROOF_CLOCK_SKEW_MS = 30_000
+const MAX_HOST_PROOF_CHALLENGE_WINDOW_MS = 10_000
 const textEncoder = new TextEncoder()
 const textDecoder = new TextDecoder()
 
@@ -102,9 +105,10 @@ function validateTranscript(
     context.previousGeneration === undefined ? new Uint8Array() : uint64(context.previousGeneration)
   return (
     issuedAt !== null &&
-    issuedAt <= now &&
-    now <= challenge.expiresAt &&
-    challenge.expiresAt - issuedAt <= 10_000 &&
+    issuedAt - RELAY_HOST_PROOF_CLOCK_SKEW_MS <= now &&
+    now - RELAY_HOST_PROOF_CLOCK_SKEW_MS <= challenge.expiresAt &&
+    issuedAt <= challenge.expiresAt &&
+    challenge.expiresAt - issuedAt <= MAX_HOST_PROOF_CHALLENGE_WINDOW_MS &&
     expiresAt === challenge.expiresAt &&
     equal(fields.get('protocol'), textEncoder.encode(HOST_PROOF_TRANSCRIPT_DOMAIN)) &&
     equal(fields.get('version'), new Uint8Array([1])) &&