diff --git a/packages/sdk-utils/src/index.js b/packages/sdk-utils/src/index.js
index 371046c6b..1659886d1 100644
--- a/packages/sdk-utils/src/index.js
+++ b/packages/sdk-utils/src/index.js
@@ -10,7 +10,12 @@ import postBuildEvents from './post-build-event.js';
 import flushSnapshots from './flush-snapshots.js';
 import captureAutomateScreenshot from './post-screenshot.js';
 import getResponsiveWidths from './get-responsive-widths.js';
-import { waitForReadyScript, getReadinessConfig, isReadinessDisabled } from './serialize-dom.js';
+import {
+  waitForReadyScript,
+  getReadinessConfig,
+  isReadinessDisabled,
+  runReadinessGate
+} from './serialize-dom.js';
 
 // Iframe depth constants shared with @percy/dom's serialize-frames. Kept
 // here so external Percy SDKs (Capybara, Cypress, Playwright, etc.) can
@@ -47,7 +52,8 @@ export {
   clampIframeDepth,
   waitForReadyScript,
   getReadinessConfig,
-  isReadinessDisabled
+  isReadinessDisabled,
+  runReadinessGate
 };
 
 // export the namespace by default
diff --git a/packages/sdk-utils/src/serialize-dom.js b/packages/sdk-utils/src/serialize-dom.js
index 0d5e8caef..53abfe93e 100644
--- a/packages/sdk-utils/src/serialize-dom.js
+++ b/packages/sdk-utils/src/serialize-dom.js
@@ -1,12 +1,20 @@
 import percy from './percy-info.js';
 
 // Returns the readiness config for a snapshot.
-// Priority: per-snapshot options > global percy.config > empty object (triggers balanced default).
+// Shallow-merge of global .percy.yml config with per-snapshot overrides:
+// per-snapshot keys win, unspecified keys are inherited from the global config.
 // SDKs obtain percy.config via the healthcheck endpoint in isPercyEnabled().
+//
+// Why shallow-merge instead of `||`:
+//   - `options.readiness = {}` would otherwise wipe the global config entirely.
+//   - A partial per-snapshot override like `{ stabilityWindowMs: 500 }` would
+//     drop a global `preset: disabled` kill switch — silently re-enabling the
+//     gate for a snapshot the user thought was opted out.
 export function getReadinessConfig(snapshotOptions = {}) {
-  return snapshotOptions?.readiness ||
-    percy.config?.snapshot?.readiness ||
-    {};
+  return {
+    ...(percy.config?.snapshot?.readiness || {}),
+    ...(snapshotOptions?.readiness || {})
+  };
 }
 
 // Returns true if readiness should be skipped for this snapshot.
@@ -66,4 +74,47 @@ export function waitForReadyScript(readinessConfig = {}, { callback = false } =
   `;
 }
 
+// Runs the readiness gate end-to-end so every JS SDK collapses to a single
+// call. The SDK's only responsibility is to provide an `evalScript` callback
+// that ships the script string to the browser via its driver's evaluator
+// (page.evaluate, driver.executeAsyncScript, b.executeAsync, etc.).
+//
+// Centralised here:
+//   - isReadinessDisabled kill-switch check
+//   - getReadinessConfig shallow-merge of global + per-snapshot config
+//   - waitForReadyScript script generation (callback or promise mode)
+//   - try/catch with debug logging — serialize is never blocked
+//
+// Returns: the diagnostics object from PercyDOM.waitForReady, or null
+// when readiness is disabled / unavailable / failed. The caller attaches
+// the non-null result to domSnapshot.readiness_diagnostics.
+//
+// Usage:
+//   // Puppeteer/Playwright (promise-mode):
+//   const diag = await utils.runReadinessGate(
+//     (script) => page.evaluate(script),
+//     options,
+//     { log }
+//   );
+//
+//   // Selenium-js / WebdriverIO / Nightwatch (callback-mode):
+//   const diag = await utils.runReadinessGate(
+//     (script) => driver.executeAsyncScript(script),
+//     options,
+//     { callback: true, log }
+//   );
+export async function runReadinessGate(evalScript, snapshotOptions = {}, { callback = false, log } = {}) {
+  if (isReadinessDisabled(snapshotOptions)) return null;
+  const config = getReadinessConfig(snapshotOptions);
+  const script = waitForReadyScript(config, { callback });
+  try {
+    return await evalScript(script);
+  } catch (err) {
+    if (log && typeof log.debug === 'function') {
+      log.debug(`waitForReady failed, proceeding to serialize: ${err?.message || err}`);
+    }
+    return null;
+  }
+}
+
 export default waitForReadyScript;
diff --git a/packages/sdk-utils/test/index.test.js b/packages/sdk-utils/test/index.test.js
index 7fb4d64fe..20bd7884a 100644
--- a/packages/sdk-utils/test/index.test.js
+++ b/packages/sdk-utils/test/index.test.js
@@ -779,6 +779,37 @@ describe('SDK Utils', () => {
       expect(getReadinessConfig({ readiness: { preset: 'fast' } })).toEqual({ preset: 'fast' });
       percy.config = undefined;
     });
+
+    it('shallow-merges per-snapshot overrides into global config', () => {
+      percy.config = {
+        snapshot: {
+          readiness: { preset: 'balanced', timeoutMs: 8000, stabilityWindowMs: 200 }
+        }
+      };
+      // Partial override — `preset` and `timeoutMs` are inherited; `stabilityWindowMs` wins.
+      expect(getReadinessConfig({ readiness: { stabilityWindowMs: 500 } })).toEqual({
+        preset: 'balanced',
+        timeoutMs: 8000,
+        stabilityWindowMs: 500
+      });
+      percy.config = undefined;
+    });
+
+    it('inherits global preset: disabled when per-snapshot omits preset', () => {
+      percy.config = { snapshot: { readiness: { preset: 'disabled' } } };
+      // A partial override must NOT silently re-enable the kill switch.
+      expect(getReadinessConfig({ readiness: { stabilityWindowMs: 500 } })).toEqual({
+        preset: 'disabled',
+        stabilityWindowMs: 500
+      });
+      percy.config = undefined;
+    });
+
+    it('empty per-snapshot readiness does not wipe the global config', () => {
+      percy.config = { snapshot: { readiness: { preset: 'strict' } } };
+      expect(getReadinessConfig({ readiness: {} })).toEqual({ preset: 'strict' });
+      percy.config = undefined;
+    });
   });
 
   describe('isReadinessDisabled(snapshotOptions)', () => {
@@ -805,4 +836,99 @@ describe('SDK Utils', () => {
       percy.config = undefined;
     });
   });
+
+  describe('runReadinessGate(evalScript, snapshotOptions[, opts])', () => {
+    let { runReadinessGate, percy } = utils;
+
+    afterEach(() => { percy.config = undefined; });
+
+    it('returns null and skips evalScript when preset is disabled', async () => {
+      let called = false;
+      let result = await runReadinessGate(
+        () => { called = true; return { passed: true }; },
+        { readiness: { preset: 'disabled' } }
+      );
+      expect(result).toBe(null);
+      expect(called).toBe(false);
+    });
+
+    it('returns null and skips evalScript when global preset is disabled', async () => {
+      percy.config = { snapshot: { readiness: { preset: 'disabled' } } };
+      let called = false;
+      let result = await runReadinessGate(() => { called = true; });
+      expect(result).toBe(null);
+      expect(called).toBe(false);
+    });
+
+    it('passes the merged shallow-merge config script to evalScript and returns its result', async () => {
+      percy.config = { snapshot: { readiness: { preset: 'balanced', timeoutMs: 8000, stabilityWindowMs: 200 } } };
+      let captured;
+      let diagnostics = { passed: true, timed_out: false, preset: 'balanced' };
+      let result = await runReadinessGate(
+        (script) => { captured = script; return Promise.resolve(diagnostics); },
+        { readiness: { stabilityWindowMs: 500 } }
+      );
+      expect(result).toEqual(diagnostics);
+      // Shallow-merged: per-snapshot stabilityWindowMs wins, global preset+timeoutMs inherited.
+      expect(captured).toContain('"preset":"balanced"');
+      expect(captured).toContain('"timeoutMs":8000');
+      expect(captured).toContain('"stabilityWindowMs":500');
+    });
+
+    it('emits callback-mode script when opts.callback is true', async () => {
+      let captured;
+      await runReadinessGate(
+        (script) => { captured = script; return null; },
+        {},
+        { callback: true }
+      );
+      expect(captured).toContain('arguments[arguments.length - 1]');
+      expect(captured).toContain('PercyDOM.waitForReady');
+    });
+
+    it('emits promise-mode script by default', async () => {
+      let captured;
+      await runReadinessGate(
+        (script) => { captured = script; return null; },
+        {}
+      );
+      expect(captured).toContain('return PercyDOM.waitForReady');
+      expect(captured).not.toContain('arguments[arguments.length - 1]');
+    });
+
+    it('returns null and never throws when evalScript rejects (with Error)', async () => {
+      let logged;
+      let result = await runReadinessGate(
+        () => Promise.reject(new Error('readiness boom')),
+        {},
+        { log: { debug: (m) => { logged = m; } } }
+      );
+      expect(result).toBe(null);
+      expect(logged).toContain('readiness boom');
+    });
+
+    it('returns null and never throws when evalScript rejects (non-Error)', async () => {
+      let logged;
+      // Exercises the `err?.message || err` second branch where the
+      // rejection value has no `.message`.
+      let result = await runReadinessGate(
+        // eslint-disable-next-line prefer-promise-reject-errors
+        () => Promise.reject('plain-string-rejection'),
+        {},
+        { log: { debug: (m) => { logged = m; } } }
+      );
+      expect(result).toBe(null);
+      expect(logged).toContain('plain-string-rejection');
+    });
+
+    it('returns null and never throws when evalScript throws synchronously', async () => {
+      let result = await runReadinessGate(() => { throw new Error('sync boom'); }, {});
+      expect(result).toBe(null);
+    });
+
+    it('tolerates absent log (no opts.log)', async () => {
+      let result = await runReadinessGate(() => Promise.reject(new Error('no log')), {});
+      expect(result).toBe(null);
+    });
+  });
 });