@@ -0,0 +1,16 @@
+---
+'@objectstack/core': minor
+'@objectstack/trigger-api': minor
+---
+
+feat(core,trigger-api): a timestamped `X-Objectstack-Signature` form with a five-minute tolerance window, accepted by the inbound flow hook beside the body-only form
+
+Clause-②: yes
+
+- **The new form.** `x-objectstack-signature: t=<unix seconds>,v1=<hex>`, where the hex is the lowercase HMAC-SHA256 of `<t>.<raw body>` under the flow's secret. The timestamp is inside the signed material, so it cannot be changed without the secret. The inbound hook (`POST /api/v1/automation/hooks/:flowName/:hookId`) refuses the value once `t` is more than 300 seconds from the server's clock, in either direction, so a captured post stops verifying five minutes after it was signed.
+- **Refusals.** A value that starts with `t=` is judged by the timestamped form alone. If it is malformed, carries the wrong HMAC, carries the body-only HMAC, or has a `t` outside the window, the answer is the same `401 INVALID_SIGNATURE` a bad signature gets, and nothing is enqueued.
+- **Body-only signatures keep working.** `sha256=<hex HMAC-SHA256 of the raw body>` is still accepted, so no existing sender breaks. It signs no time, so a captured post can be replayed, and it is now deprecated: the trigger logs one `warn` line per armed hook that names the flow and the hook, the first time that hook accepts a body-only post.
+- **`@objectstack/core` exports.** `signHttpBodyAt(body, secret, timestampSeconds)` signs the new form. `verifyHttpSignature(body, secret, header)` verifies both forms and returns an `HttpSignatureVerdict` (`{ valid: true, scheme: 'v1' | 'body-only' }`, or `{ valid: false, reason }`). `HTTP_SIGNATURE_TOLERANCE_SECONDS` is `300`. They sit beside `signHttpBody` and `HTTP_SIGNATURE_HEADER` in the one definition of the scheme. `@objectstack/trigger-api`'s `verifySignature` keeps its signature and now verifies through it, so it accepts both forms.
+- **Unchanged.** ObjectStack's own senders, the messaging HTTP outbox (webhooks) and the flow `http` node, still send the body-only form. Receivers outside ObjectStack verify exactly that today.
+
+**To adopt it as a sender.** Sign at send time, not at enqueue: `t = Math.floor(Date.now() / 1000)`, then send `t=<t>,v1=<hex HMAC-SHA256 of "<t>.<raw body>">` in `x-objectstack-signature`. From TypeScript, call `signHttpBodyAt(rawBody, secret, t)`.
@@ -1,7 +1,21 @@
 // Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
 
-import { describe, it, expect } from 'vitest';
-import { HTTP_SIGNATURE_HEADER, signHttpBody } from '../index.js';
+import { timingSafeEqual } from 'node:crypto';
+import { afterEach, beforeEach, describe, it, expect, vi } from 'vitest';
+import {
+    HTTP_SIGNATURE_HEADER,
+    HTTP_SIGNATURE_TOLERANCE_SECONDS,
+    signHttpBody,
+    signHttpBodyAt,
+    verifyHttpSignature,
+} from '../index.js';
+
+// The constant-time pin below counts calls to `timingSafeEqual`; the wrapper
+// keeps its behaviour byte for byte.
+vi.mock('node:crypto', async (importOriginal) => {
+    const actual = await importOriginal<typeof import('node:crypto')>();
+    return { ...actual, timingSafeEqual: vi.fn(actual.timingSafeEqual) };
+});
 
 /**
  * The scheme's wire form, pinned by literal values rather than by recomputing
@@ -30,3 +44,146 @@ describe('the outbound HTTP signature scheme', () => {
         expect(signHttpBody('{"a": 1}', 'shh')).not.toBe(signHttpBody('{"a":1}', 'shh'));
     });
 });
+
+/**
+ * The timestamped form: the same literal-value discipline as above. Every
+ * receiver of a `t=…,v1=…` value — ours and anyone else's — relies on these
+ * exact bytes, and on the timestamp being inside the signed material.
+ */
+describe('the timestamped form of the HTTP signature scheme', () => {
+    it('is t=<seconds>,v1= plus the lowercase hex HMAC-SHA256 of "<t>.<body>"', () => {
+        expect(signHttpBodyAt('{"a":1}', 'shh', 1760000000)).toBe(
+            't=1760000000,v1=f5f3d98fe723d34c35649871bfa9eb4530a1b2f197e96e6e8c1b95a984e3d46d',
+        );
+        expect(signHttpBodyAt('', 'flow-hook-secret', 0)).toBe(
+            't=0,v1=6433f1bb31a3f1b7ddbc61b4e6361758e607526ac9953cb6a91389500ffefad0',
+        );
+    });
+
+    it('signs the timestamp: the same body at another second is another signature', () => {
+        const at = signHttpBodyAt('{"a":1}', 'shh', 1760000000).split(',v1=')[1];
+        const next = signHttpBodyAt('{"a":1}', 'shh', 1760000001).split(',v1=')[1];
+        expect(next).not.toBe(at);
+        // And it is not the body-only HMAC with a timestamp beside it.
+        expect(at).not.toBe(signHttpBody('{"a":1}', 'shh').slice('sha256='.length));
+    });
+
+    it('refuses a timestamp that is not a non-negative whole number of seconds', () => {
+        for (const bad of [-1, 1.5, Number.NaN, Number.POSITIVE_INFINITY, Number.MAX_SAFE_INTEGER + 1]) {
+            expect(() => signHttpBodyAt('{}', 'shh', bad)).toThrow(RangeError);
+        }
+    });
+
+    it('has a fixed five-minute tolerance window', () => {
+        expect(HTTP_SIGNATURE_TOLERANCE_SECONDS).toBe(300);
+    });
+});
+
+describe('verifyHttpSignature — the receiver\'s half of both forms', () => {
+    const NOW = 1760000000;
+    const BODY = '{"event":"paid","id":"evt_1"}';
+    const SECRET = 'pin-verify-secret-7c1e';
+
+    beforeEach(() => {
+        vi.useFakeTimers({ toFake: ['Date'] });
+        vi.setSystemTime(NOW * 1000);
+        vi.mocked(timingSafeEqual).mockClear();
+    });
+    afterEach(() => {
+        vi.useRealTimers();
+    });
+
+    it('accepts a fresh timestamped value, as the v1 form', () => {
+        expect(verifyHttpSignature(BODY, SECRET, signHttpBodyAt(BODY, SECRET, NOW))).toEqual({ valid: true, scheme: 'v1' });
+    });
+
+    it('accepts a timestamp at either edge of the window, and refuses one second past either edge', () => {
+        const at = (t: number) => verifyHttpSignature(BODY, SECRET, signHttpBodyAt(BODY, SECRET, t));
+        expect(at(NOW - HTTP_SIGNATURE_TOLERANCE_SECONDS)).toEqual({ valid: true, scheme: 'v1' });
+        expect(at(NOW + HTTP_SIGNATURE_TOLERANCE_SECONDS)).toEqual({ valid: true, scheme: 'v1' });
+        expect(at(NOW - HTTP_SIGNATURE_TOLERANCE_SECONDS - 1)).toEqual({ valid: false, reason: 'outside-window' });
+        expect(at(NOW + HTTP_SIGNATURE_TOLERANCE_SECONDS + 1)).toEqual({ valid: false, reason: 'outside-window' });
+    });
+
+    it('refuses a value it accepted once the clock has moved past the window — a replay', () => {
+        const header = signHttpBodyAt(BODY, SECRET, NOW);
+        expect(verifyHttpSignature(BODY, SECRET, header)).toEqual({ valid: true, scheme: 'v1' });
+        vi.setSystemTime((NOW + HTTP_SIGNATURE_TOLERANCE_SECONDS + 1) * 1000);
+        expect(verifyHttpSignature(BODY, SECRET, header)).toEqual({ valid: false, reason: 'outside-window' });
+    });
+
+    it('refuses a timestamp the HMAC does not cover — never a fallback to the body-only form', () => {
+        const bodyOnlyHex = signHttpBody(BODY, SECRET).slice('sha256='.length);
+        // A captured body-only signature, re-labelled with a fresh timestamp,
+        // in either spelling.
+        expect(verifyHttpSignature(BODY, SECRET, `t=${NOW},v1=${bodyOnlyHex}`)).toEqual({ valid: false, reason: 'mismatch' });
+        expect(verifyHttpSignature(BODY, SECRET, `t=${NOW},sha256=${bodyOnlyHex}`)).toEqual({ valid: false, reason: 'malformed' });
+        // A genuine value whose timestamp was moved into the window.
+        const stale = signHttpBodyAt(BODY, SECRET, NOW - 3600);
+        const moved = stale.replace(`t=${NOW - 3600},`, `t=${NOW},`);
+        expect(moved).not.toBe(stale);
+        expect(verifyHttpSignature(BODY, SECRET, moved)).toEqual({ valid: false, reason: 'mismatch' });
+    });
+
+    it('refuses the wrong secret and the wrong body in the timestamped form', () => {
+        expect(verifyHttpSignature(BODY, SECRET, signHttpBodyAt(BODY, 'another-secret', NOW)))
+            .toEqual({ valid: false, reason: 'mismatch' });
+        expect(verifyHttpSignature(`${BODY} `, SECRET, signHttpBodyAt(BODY, SECRET, NOW)))
+            .toEqual({ valid: false, reason: 'mismatch' });
+    });
+
+    it('refuses every value that claims the timestamped form without its exact shape', () => {
+        const hex = signHttpBodyAt(BODY, SECRET, NOW).split(',v1=')[1]!;
+        for (const malformed of [
+            `t=${NOW}`,
+            `t=${NOW},v1=`,
+            `t=${NOW},v1=${hex.slice(1)}`,
+            `t=${NOW},v1=${hex.toUpperCase()}`,
+            `v1=${hex},t=${NOW}`,
+            `t=0${NOW},v1=${hex}`,
+            `t=+${NOW},v1=${hex}`,
+            `t=${NOW}.5,v1=${hex}`,
+            `t=${NOW},v1=${hex},v0=${hex}`,
+            `t=${NOW},v1=${hex},v1=${hex}`,
+            `t = ${NOW}, v1 = ${hex}`,
+            `t=${NOW},sha256=${hex}`,
+            `T=${NOW},V1=${hex}`,
+            'garbage',
+        ]) {
+            expect(verifyHttpSignature(BODY, SECRET, malformed), malformed).toEqual({ valid: false, reason: 'malformed' });
+        }
+    });
+
+    it('still accepts the body-only form, as body-only, and refuses a wrong one', () => {
+        expect(verifyHttpSignature(BODY, SECRET, signHttpBody(BODY, SECRET))).toEqual({ valid: true, scheme: 'body-only' });
+        expect(verifyHttpSignature(BODY, SECRET, signHttpBody(BODY, 'another-secret'))).toEqual({ valid: false, reason: 'mismatch' });
+        expect(verifyHttpSignature(BODY, SECRET, 'sha256=zz')).toEqual({ valid: false, reason: 'mismatch' });
+        // The body-only form signs no time, so the clock never refuses it.
+        vi.setSystemTime((NOW + 10 * 365 * 24 * 3600) * 1000);
+        expect(verifyHttpSignature(BODY, SECRET, signHttpBody(BODY, SECRET))).toEqual({ valid: true, scheme: 'body-only' });
+    });
+
+    it('trims the header, and reads a missing or blank one as absent', () => {
+        expect(verifyHttpSignature(BODY, SECRET, `  ${signHttpBodyAt(BODY, SECRET, NOW)}  `)).toEqual({ valid: true, scheme: 'v1' });
+        expect(verifyHttpSignature(BODY, SECRET, ` ${signHttpBody(BODY, SECRET)}\t`)).toEqual({ valid: true, scheme: 'body-only' });
+        for (const absent of [undefined, '', '   ']) {
+            expect(verifyHttpSignature(BODY, SECRET, absent)).toEqual({ valid: false, reason: 'absent' });
+        }
+    });
+
+    it('decides every well-formed value of either form by a constant-time comparison', () => {
+        const compare = vi.mocked(timingSafeEqual);
+        const cases: Array<[string, boolean]> = [
+            [signHttpBodyAt(BODY, SECRET, NOW), true],
+            [signHttpBodyAt(BODY, 'another-secret', NOW), false],
+            [signHttpBody(BODY, SECRET), true],
+            [signHttpBody(BODY, 'another-secret'), false],
+        ];
+        for (const [header, valid] of cases) {
+            compare.mockClear();
+            expect(verifyHttpSignature(BODY, SECRET, header).valid, header).toBe(valid);
+            expect(compare, header).toHaveBeenCalledTimes(1);
+            expect(compare.mock.results[0]!.value, header).toBe(valid);
+        }
+    });
+});
@@ -1,6 +1,6 @@
 // Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
 
-import { createHmac } from 'node:crypto';
+import { createHmac, timingSafeEqual } from 'node:crypto';
 
 /**
  * The outbound HTTP signature scheme — the ONE definition every ObjectStack
@@ -22,11 +22,55 @@ import { createHmac } from 'node:crypto';
  *
  * What the scheme does NOT own is WHICH bytes are the body — each sender signs
  * the serialization it actually sends.
+ *
+ * ## The timestamped form, and the receiver's side
+ *
+ * The body-only value signs no time, so a captured request verifies forever: a
+ * replay of it is indistinguishable from the original. The header therefore
+ * has a second, versioned form, {@link signHttpBodyAt}:
+ * `t=<unix seconds>,v1=<hex>`, where the hex is the HMAC-SHA256 of
+ * `<t>.<body>` — the timestamp is INSIDE the signed material, so it cannot be
+ * moved without the secret, and a receiver refuses a `t` outside
+ * {@link HTTP_SIGNATURE_TOLERANCE_SECONDS} of its own clock. The two forms
+ * share the one header and are told apart by their prefix (`sha256=` against
+ * `t=`), so a receiver needs no second header and a sender changes one value.
+ *
+ * {@link verifyHttpSignature} is the receiver's half of both forms, and the
+ * only one: it re-signs with the functions above and compares in constant
+ * time, so the material a receiver checks is, by construction, the material a
+ * sender signs. Our own senders still send the body-only form — a receiver
+ * outside ObjectStack verifies exactly that today, and switching them would
+ * break it — so the body-only form stays accepted; refusing it is a decision
+ * of its own, not this definition's.
  */
 
 /** Header carrying the HMAC-SHA256 signature of the request body. */
 export const HTTP_SIGNATURE_HEADER = 'X-Objectstack-Signature';
 
+/**
+ * How far, in seconds, a timestamped signature's `t` may sit from the
+ * receiver's clock, in either direction, and still verify. Five minutes: wide
+ * enough for ordinary clock skew and a sender's retry, narrow enough that a
+ * captured request stops verifying soon after it was sent. Fixed, not
+ * configured — the window is part of the scheme every sender targets.
+ */
+export const HTTP_SIGNATURE_TOLERANCE_SECONDS = 300;
+
+/** The body-only form's prefix — what {@link signHttpBody} produces. */
+const BODY_ONLY_PREFIX = 'sha256=';
+
+/**
+ * The timestamped form, exactly as {@link signHttpBodyAt} writes it: a
+ * canonical decimal `t` (no sign, no leading zero) and one 64-digit lowercase
+ * hex `v1`. Anything else claiming the form is malformed, never re-read.
+ */
+const TIMESTAMPED_FORM = /^t=(0|[1-9][0-9]{0,14}),v1=[0-9a-f]{64}$/;
+
+/** The lowercase hex HMAC-SHA256 of `material` — the one HMAC both forms use. */
+function hmacSha256Hex(material: string, secret: string): string {
+    return createHmac('sha256', secret).update(material).digest('hex');
+}
+
 /**
  * Compute the {@link HTTP_SIGNATURE_HEADER} value for a body: `sha256=<hex>` of
  * `HMAC-SHA256(body, secret)`.
@@ -35,5 +79,90 @@ export const HTTP_SIGNATURE_HEADER = 'X-Objectstack-Signature';
  * anyway); the `secret` argument is NOT.
  */
 export function signHttpBody(body: string, secret: string): string {
-    return `sha256=${createHmac('sha256', secret).update(body).digest('hex')}`;
+    return `${BODY_ONLY_PREFIX}${hmacSha256Hex(body, secret)}`;
+}
+
+/**
+ * Compute the timestamped {@link HTTP_SIGNATURE_HEADER} value for a body sent
+ * at `timestampSeconds` (Unix time, whole seconds): `t=<timestampSeconds>,v1=<hex>`
+ * where the hex is `HMAC-SHA256(<timestampSeconds>.<body>, secret)`.
+ *
+ * Sign at send time, not at enqueue: a receiver refuses the value once its
+ * clock is more than {@link HTTP_SIGNATURE_TOLERANCE_SECONDS} from `t`, so a
+ * value persisted and replayed later stops verifying — which is the point.
+ *
+ * @throws RangeError when `timestampSeconds` is not a non-negative safe integer.
+ */
+export function signHttpBodyAt(body: string, secret: string, timestampSeconds: number): string {
+    if (!Number.isSafeInteger(timestampSeconds) || timestampSeconds < 0) {
+        throw new RangeError(
+            `signHttpBodyAt: timestampSeconds must be a non-negative whole number of seconds since the Unix epoch, ` +
+                `got ${String(timestampSeconds)} — pass Math.floor(Date.now() / 1000).`,
+        );
+    }
+    return `t=${timestampSeconds},v1=${hmacSha256Hex(`${timestampSeconds}.${body}`, secret)}`;
+}
+
+/**
+ * What {@link verifyHttpSignature} concluded. A receiver answers every
+ * `valid: false` alike; `reason` is for its own tests and logs, never for the
+ * sender. `scheme` says which form verified, so a receiver can signal the
+ * body-only form's deprecation.
+ *
+ *  - `absent` — no header, or a blank one.
+ *  - `malformed` — neither form's shape.
+ *  - `mismatch` — the shape is right, and the HMAC is not the secret's for
+ *    this body (and, for the timestamped form, this `t`).
+ *  - `outside-window` — a timestamped value whose HMAC verified, with a `t`
+ *    more than {@link HTTP_SIGNATURE_TOLERANCE_SECONDS} from the receiver's
+ *    clock: a replay, or a sender clock that is wrong.
+ */
+export type HttpSignatureVerdict =
+    | { readonly valid: true; readonly scheme: 'v1' | 'body-only' }
+    | { readonly valid: false; readonly reason: 'absent' | 'malformed' | 'mismatch' | 'outside-window' };
+
+/** Constant-time compare of two equal-length strings (a length difference is not secret). */
+function constantTimeEqual(expected: string, received: string): boolean {
+    const a = Buffer.from(expected, 'utf8');
+    const b = Buffer.from(received, 'utf8');
+    if (a.length !== b.length) return false;
+    return timingSafeEqual(a, b);
+}
+
+/**
+ * Verify an inbound {@link HTTP_SIGNATURE_HEADER} value against the exact bytes
+ * of the body received and the shared secret, in either form:
+ *
+ *  - `sha256=<hex>` — the body-only form, verified by re-signing with
+ *    {@link signHttpBody}.
+ *  - `t=<seconds>,v1=<hex>` — the timestamped form, verified by re-signing
+ *    with {@link signHttpBodyAt} at the `t` it carries, and then refused unless
+ *    `t` is within {@link HTTP_SIGNATURE_TOLERANCE_SECONDS} of this clock. A
+ *    value that claims this form is judged by it alone: it never falls back to
+ *    the body-only form.
+ *
+ * The header is trimmed first. Both comparisons are constant-time.
+ */
+export function verifyHttpSignature(body: string, secret: string, header: string | undefined): HttpSignatureVerdict {
+    const value = header?.trim();
+    if (!value) return { valid: false, reason: 'absent' };
+
+    if (value.startsWith(BODY_ONLY_PREFIX)) {
+        return constantTimeEqual(signHttpBody(body, secret), value)
+            ? { valid: true, scheme: 'body-only' }
+            : { valid: false, reason: 'mismatch' };
+    }
+
+    const form = TIMESTAMPED_FORM.exec(value);
+    if (!form) return { valid: false, reason: 'malformed' };
+    // At most fifteen digits, so always a safe integer.
+    const timestampSeconds = Number(form[1]);
+    if (!constantTimeEqual(signHttpBodyAt(body, secret, timestampSeconds), value)) {
+        return { valid: false, reason: 'mismatch' };
+    }
+    const nowSeconds = Math.floor(Date.now() / 1000);
+    if (Math.abs(nowSeconds - timestampSeconds) > HTTP_SIGNATURE_TOLERANCE_SECONDS) {
+        return { valid: false, reason: 'outside-window' };
+    }
+    return { valid: true, scheme: 'v1' };
 }
@@ -51,8 +51,17 @@ export {
 // The outbound HTTP signature scheme (`X-Objectstack-Signature`) — one
 // definition shared by the messaging outbox and the flow `http` node's inline
 // arm, which cannot import each other; `@objectstack/service-messaging`
-// re-exports both names unchanged.
-export { HTTP_SIGNATURE_HEADER, signHttpBody } from './http-signature.js';
+// re-exports both names unchanged. Its timestamped form and the receiver's
+// verifier for both forms live beside it (`@objectstack/trigger-api`'s
+// inbound hook verifies through `verifyHttpSignature`).
+export {
+  HTTP_SIGNATURE_HEADER,
+  HTTP_SIGNATURE_TOLERANCE_SECONDS,
+  signHttpBody,
+  signHttpBodyAt,
+  verifyHttpSignature,
+  type HttpSignatureVerdict,
+} from './http-signature.js';
 
 // [#12699 / ADR-0131 D7] The ONE fail-closed reader of the mounted `org-scoping`
 // service's per-deployment keys — shared by the engine's schema registry