diff --git a/.changeset/beta7-async-response-refs.md b/.changeset/beta7-async-response-refs.md
new file mode 100644
index 0000000000..16e55f0516
--- /dev/null
+++ b/.changeset/beta7-async-response-refs.md
@@ -0,0 +1,9 @@
+---
+"adcontextprotocol": patch
+---
+
+Build schema bundles with `core/async-response-refs/` copies so SDK validators
+can pre-register async response refs before compiling
+`core/async-response-data.json` in the next release candidate.
+
+Refs #5161.
diff --git a/package.json b/package.json
index 4b6f4f4ed2..4692452054 100644
--- a/package.json
+++ b/package.json
@@ -68,6 +68,7 @@
     "test:sign-protocol-tarball": "node --test --test-force-exit --test-timeout=30000 tests/sign-protocol-tarball.test.cjs",
     "test:build-schemas-hoist-enums": "node --test --test-force-exit --test-timeout=30000 tests/build-schemas-hoist-enums.test.cjs",
     "test:build-schemas-hoist-marked": "node --test --test-force-exit --test-timeout=30000 tests/build-schemas-hoist-marked.test.cjs",
+    "test:build-schemas-async-response-refs": "node --test --test-force-exit --test-timeout=30000 tests/build-schemas-async-response-refs.test.cjs",
     "test:error-codes": "node scripts/lint-error-codes.cjs",
     "test:error-code-drift": "node scripts/lint-error-code-drift.cjs",
     "test:substitution-vector-names": "node scripts/lint-substitution-vector-names.cjs",
@@ -85,7 +86,7 @@
     "audit:oneof": "node scripts/audit-oneof.mjs",
     "test:schema-utf8": "node scripts/normalize-schema-utf8.mjs --check",
     "fix:schema-utf8": "node scripts/normalize-schema-utf8.mjs",
-    "test": "npm run test:docs-nav && npm run test:schemas && npm run test:examples && npm run test:extensions && npm run test:extension-schemas && npm run test:error-handling && npm run test:json-schema && npm run test:canonical-reference-resolver && npm run test:composed && npm run test:rejection-arm-mutex && npm run test:migrations && npm run test:hmac-vectors && npm run test:hmac-signer-conformance && npm run test:transport-errors && npm run test:targeting-overlay-vectors && npm run test:storyboard-scoping && npm run test:storyboard-branch-sets && npm run test:storyboard-provides-state-for && npm run test:storyboard-contradictions && npm run test:storyboard-context-entity && npm run test:storyboard-auth-shape && npm run test:storyboard-test-kits && npm run test:storyboard-sample-request-schema && npm run test:storyboard-response-schema && npm run test:storyboard-context-output-paths && npm run test:storyboard-validations-paths && npm run test:storyboard-check-enum && npm run test:storyboard-advisory-expiry && npm run test:storyboard-raw-mode-required && npm run test:storyboard-upstream-traffic-paths && npm run test:storyboard-doc-parity && npm run test:pagination-invariant && npm run test:version-envelope && npm run test:test-dynamic-imports && npm run test:callapi-state-change && npm run test:sign-protocol-tarball && npm run test:build-schemas-hoist-enums && npm run test:build-schemas-hoist-marked && npm run test:error-codes && npm run test:substitution-vector-names && npm run test:platform-agnostic && npm run test:oneof-discriminators && npm run test:schema-utf8 && npm run test:unit && npm run test:server-unit && npm run test:openapi && npm run typecheck",
+    "test": "npm run test:docs-nav && npm run test:schemas && npm run test:examples && npm run test:extensions && npm run test:extension-schemas && npm run test:error-handling && npm run test:json-schema && npm run test:canonical-reference-resolver && npm run test:composed && npm run test:rejection-arm-mutex && npm run test:migrations && npm run test:hmac-vectors && npm run test:hmac-signer-conformance && npm run test:transport-errors && npm run test:targeting-overlay-vectors && npm run test:storyboard-scoping && npm run test:storyboard-branch-sets && npm run test:storyboard-provides-state-for && npm run test:storyboard-contradictions && npm run test:storyboard-context-entity && npm run test:storyboard-auth-shape && npm run test:storyboard-test-kits && npm run test:storyboard-sample-request-schema && npm run test:storyboard-response-schema && npm run test:storyboard-context-output-paths && npm run test:storyboard-validations-paths && npm run test:storyboard-check-enum && npm run test:storyboard-advisory-expiry && npm run test:storyboard-raw-mode-required && npm run test:storyboard-upstream-traffic-paths && npm run test:storyboard-doc-parity && npm run test:pagination-invariant && npm run test:version-envelope && npm run test:test-dynamic-imports && npm run test:callapi-state-change && npm run test:sign-protocol-tarball && npm run test:build-schemas-hoist-enums && npm run test:build-schemas-hoist-marked && npm run test:build-schemas-async-response-refs && npm run test:error-codes && npm run test:substitution-vector-names && npm run test:platform-agnostic && npm run test:oneof-discriminators && npm run test:schema-utf8 && npm run test:unit && npm run test:server-unit && npm run test:openapi && npm run typecheck",
     "test:all": "npm run test:schemas && npm run test:examples && npm run test:extensions && npm run test:error-handling && npm run test:snippets && npm run typecheck",
     "precommit": "bash scripts/with-timeout.sh 60 npm run test:unit && npm run test:test-dynamic-imports && npm run test:callapi-state-change && npm run typecheck",
     "test:storyboards": "bash scripts/run-storyboards-matrix.sh",
diff --git a/scripts/build-schemas.cjs b/scripts/build-schemas.cjs
index 5decc9602b..9588fc197e 100644
--- a/scripts/build-schemas.cjs
+++ b/scripts/build-schemas.cjs
@@ -1018,6 +1018,45 @@ function copyAndTransformSchemas(sourceDir, targetDir, version) {
   }
 }
 
+function copyAsyncResponseRefsToCore(targetDir) {
+  const asyncRefDir = path.join(targetDir, 'core', 'async-response-refs');
+
+  if (fs.existsSync(asyncRefDir)) {
+    fs.rmSync(asyncRefDir, { recursive: true, force: true });
+  }
+  ensureDir(asyncRefDir);
+
+  let count = 0;
+
+  function walk(dir) {
+    const entries = fs.readdirSync(dir, { withFileTypes: true });
+    for (const entry of entries) {
+      const fullPath = path.join(dir, entry.name);
+      const rel = path.relative(targetDir, fullPath);
+
+      if (entry.isDirectory()) {
+        if (rel === 'bundled' || rel.startsWith(`bundled${path.sep}`)) continue;
+        if (rel === path.join('core', 'async-response-refs') || rel.startsWith(`${path.join('core', 'async-response-refs')}${path.sep}`)) continue;
+        walk(fullPath);
+        continue;
+      }
+
+      if (!entry.isFile()) continue;
+      if (!entry.name.endsWith('.json')) continue;
+      if (!/-async-response-(submitted|working|input-required)\.json$/.test(entry.name)) continue;
+      if (rel.startsWith(`core${path.sep}`)) continue;
+
+      const targetPath = path.join(asyncRefDir, rel);
+      ensureDir(path.dirname(targetPath));
+      fs.copyFileSync(fullPath, targetPath);
+      count++;
+    }
+  }
+
+  walk(targetDir);
+  return count;
+}
+
 function updateSourceRegistry(version) {
   const registryPath = path.join(SOURCE_DIR, 'index.json');
   const registry = JSON.parse(fs.readFileSync(registryPath, 'utf8'));
@@ -1879,6 +1918,8 @@ async function main() {
     console.log(`📋 Creating release: dist/schemas/${version}/`);
     ensureDir(versionDir);
     copyAndTransformSchemas(SOURCE_DIR, versionDir, version);
+    const asyncRefCount = copyAsyncResponseRefsToCore(versionDir);
+    console.log(`   ✓ Copied ${asyncRefCount} async response schemas to core/async-response-refs/`);
 
     // Build extensions (auto-discovered, filtered by version)
     console.log(`🔌 Building extensions for ${version}`);
@@ -1910,6 +1951,7 @@ async function main() {
     console.log(`📋 Updating latest/ to match release`);
     ensureDir(latestDir);
     copyAndTransformSchemas(SOURCE_DIR, latestDir, 'latest');
+    copyAsyncResponseRefsToCore(latestDir);
 
     // Build extensions for latest (using full version for filtering)
     buildExtensions(SOURCE_DIR, latestDir, version);
@@ -1963,6 +2005,8 @@ async function main() {
     console.log(`📋 Building schemas to dist/schemas/latest/`);
     ensureDir(latestDir);
     copyAndTransformSchemas(SOURCE_DIR, latestDir, 'latest');
+    const asyncRefCount = copyAsyncResponseRefsToCore(latestDir);
+    console.log(`   ✓ Copied ${asyncRefCount} async response schemas to core/async-response-refs/`);
 
     // Build extensions (auto-discovered, filtered by current version)
     console.log(`🔌 Building extensions for ${version}`);
@@ -2016,7 +2060,7 @@ async function main() {
   console.log('📖 See docs/reference/versioning.mdx for guidance on which to use.');
 }
 
-module.exports = { hoistDuplicateInlineEnums, hoistMarkedSchemas, resolveRefs, versionInlineSchemaIds, dedupBundledSchemaIds, stripIdsFromSubtreesWithLocalRefs };
+module.exports = { hoistDuplicateInlineEnums, hoistMarkedSchemas, resolveRefs, versionInlineSchemaIds, dedupBundledSchemaIds, stripIdsFromSubtreesWithLocalRefs, copyAsyncResponseRefsToCore };
 
 if (require.main === module) {
   main().catch(err => {
diff --git a/scripts/stage-sdk-schema-bundle.sh b/scripts/stage-sdk-schema-bundle.sh
index d9968e1763..ec0293dbf2 100755
--- a/scripts/stage-sdk-schema-bundle.sh
+++ b/scripts/stage-sdk-schema-bundle.sh
@@ -79,8 +79,8 @@ mkdir -p "$DST"
 (cd "$SRC" && tar -cf - .) | (cd "$DST" && tar -xf -)
 
 # SDK 8.1.0-beta.12's loader deliberately skips response-tool files during
-# core pre-registration so response root relaxation can happen lazily. Older
-# 3.0.x schema bundles have core/async-response-data.json $ref directly to
+# core pre-registration so response root relaxation can happen lazily. Some
+# published schema bundles have core/async-response-data.json $ref directly to
 # tool async response schemas, so those refs must still be registered before
 # core schemas compile. Duplicating them under core/ keeps the original bundle
 # intact while placing a copy in a directory the loader treats as fragments.
diff --git a/tests/build-schemas-async-response-refs.test.cjs b/tests/build-schemas-async-response-refs.test.cjs
new file mode 100644
index 0000000000..39246c7895
--- /dev/null
+++ b/tests/build-schemas-async-response-refs.test.cjs
@@ -0,0 +1,90 @@
+#!/usr/bin/env node
+
+'use strict';
+
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const os = require('node:os');
+const path = require('node:path');
+const Ajv = require('ajv');
+
+const { copyAsyncResponseRefsToCore } = require('../scripts/build-schemas.cjs');
+
+function writeJson(filePath, value) {
+  fs.mkdirSync(path.dirname(filePath), { recursive: true });
+  fs.writeFileSync(filePath, JSON.stringify(value, null, 2), 'utf8');
+}
+
+function findJsonFiles(dir) {
+  const out = [];
+  for (const entry of fs.readdirSync(dir, { withFileTypes: true })) {
+    const fullPath = path.join(dir, entry.name);
+    if (entry.isDirectory()) {
+      out.push(...findJsonFiles(fullPath));
+    } else if (entry.isFile() && entry.name.endsWith('.json')) {
+      out.push(fullPath);
+    }
+  }
+  return out;
+}
+
+function isOriginalAsyncResponse(root, filePath) {
+  const rel = path.relative(root, filePath);
+  if (rel.startsWith(`core${path.sep}async-response-refs${path.sep}`)) return false;
+  return /-async-response-(submitted|working|input-required)\.json$/.test(path.basename(filePath));
+}
+
+test('core async-response-refs let async-response-data compile when original async response schemas are skipped', () => {
+  const tmpRoot = fs.mkdtempSync(path.join(os.tmpdir(), 'adcp-async-response-refs-'));
+  try {
+    writeJson(path.join(tmpRoot, 'core', 'async-response-data.json'), {
+      $schema: 'http://json-schema.org/draft-07/schema#',
+      $id: '/schemas/latest/core/async-response-data.json',
+      title: 'Async Response Data',
+      description: 'Test async response union.',
+      anyOf: [
+        { $ref: '/schemas/latest/media-buy/example-response.json' },
+        { $ref: '/schemas/latest/media-buy/example-async-response-working.json' },
+      ],
+    });
+
+    writeJson(path.join(tmpRoot, 'media-buy', 'example-response.json'), {
+      $schema: 'http://json-schema.org/draft-07/schema#',
+      $id: '/schemas/latest/media-buy/example-response.json',
+      title: 'Example Response',
+      description: 'Completed response.',
+      type: 'object',
+      properties: { ok: { type: 'boolean' } },
+    });
+
+    writeJson(path.join(tmpRoot, 'media-buy', 'example-async-response-working.json'), {
+      $schema: 'http://json-schema.org/draft-07/schema#',
+      $id: '/schemas/latest/media-buy/example-async-response-working.json',
+      title: 'Example Async Working',
+      description: 'Working response.',
+      type: 'object',
+      properties: { percentage: { type: 'number' } },
+    });
+
+    const copied = copyAsyncResponseRefsToCore(tmpRoot);
+    assert.equal(copied, 1);
+    assert.ok(fs.existsSync(path.join(tmpRoot, 'core', 'async-response-refs', 'media-buy', 'example-async-response-working.json')));
+
+    const ajv = new Ajv({ allErrors: true, strict: false, discriminator: true, validateFormats: false });
+    const target = path.join(tmpRoot, 'core', 'async-response-data.json');
+
+    for (const filePath of findJsonFiles(tmpRoot)) {
+      if (filePath === target) continue;
+      if (isOriginalAsyncResponse(tmpRoot, filePath)) continue;
+      const schema = JSON.parse(fs.readFileSync(filePath, 'utf8'));
+      ajv.addSchema(schema, schema.$id);
+    }
+
+    assert.doesNotThrow(() => {
+      ajv.compile(JSON.parse(fs.readFileSync(target, 'utf8')));
+    });
+  } finally {
+    fs.rmSync(tmpRoot, { recursive: true, force: true });
+  }
+});