diff --git a/src/core/packages/elasticsearch/client-server-internal/src/cluster_client.test.ts b/src/core/packages/elasticsearch/client-server-internal/src/cluster_client.test.ts
index 42ed677cc3d3f..72377eeb679fe 100644
--- a/src/core/packages/elasticsearch/client-server-internal/src/cluster_client.test.ts
+++ b/src/core/packages/elasticsearch/client-server-internal/src/cluster_client.test.ts
@@ -19,6 +19,7 @@ import type {
   ElasticsearchClientConfig,
   ElasticsearchClient,
 } from '@kbn/core-elasticsearch-server';
+import { KBN_PROJECT_ROUTING_HEADER } from '@kbn/cps-server-utils';
 import { getRequestHandlerFactory } from './cps_request_handler';
 import { ClusterClient } from './cluster_client';
 import type { OnRequestHandler } from './create_transport';
@@ -344,7 +345,7 @@ describe('ClusterClient', () => {
         );
       });
 
-      it("injects '_alias:_origin' when projectRouting is 'origin-only'", () => {
+      it("injects '_alias:_origin' when no projectRouting opts are provided (default)", () => {
         const onRequest = captureTransportOnRequest();
 
         const clusterClient = new ClusterClient({
@@ -358,7 +359,7 @@ describe('ClusterClient', () => {
         });
 
         const request = httpServerMock.createKibanaRequest();
-        client = clusterClient.asScoped(request, { projectRouting: 'origin-only' }).asCurrentUser;
+        client = clusterClient.asScoped(request).asCurrentUser;
 
         const params = makeSearchParams();
         onRequest.get()({} as never, params, {}, logger);
@@ -366,7 +367,7 @@ describe('ClusterClient', () => {
         expect((params.body as Record<string, unknown>).project_routing).toBe('_alias:_origin');
       });
 
-      it("injects '_alias:*' when projectRouting is 'all'", () => {
+      it("reads routing from the x-kbn-project-routing header when projectRouting is 'request-header'", () => {
         const onRequest = captureTransportOnRequest();
 
         const clusterClient = new ClusterClient({
@@ -379,8 +380,12 @@ describe('ClusterClient', () => {
           onRequestHandlerFactory: mockOnRequestHandlerFactory,
         });
 
-        const request = httpServerMock.createKibanaRequest();
-        client = clusterClient.asScoped(request, { projectRouting: 'all' }).asCurrentUser;
+        const request = httpServerMock.createKibanaRequest({
+          headers: { [KBN_PROJECT_ROUTING_HEADER]: '_alias:*' },
+        });
+        client = clusterClient.asScoped(request, {
+          projectRouting: 'request-header',
+        }).asCurrentUser;
 
         const params = makeSearchParams();
         onRequest.get()({} as never, params, {}, logger);
@@ -388,6 +393,30 @@ describe('ClusterClient', () => {
         expect((params.body as Record<string, unknown>).project_routing).toBe('_alias:*');
       });
 
+      it("falls back to '_alias:_origin' when projectRouting is 'request-header' but header is absent", () => {
+        const onRequest = captureTransportOnRequest();
+
+        const clusterClient = new ClusterClient({
+          config: createConfig(),
+          logger,
+          type: 'custom-type',
+          authHeaders,
+          agentFactoryProvider,
+          kibanaVersion,
+          onRequestHandlerFactory: mockOnRequestHandlerFactory,
+        });
+
+        const request = httpServerMock.createKibanaRequest();
+        client = clusterClient.asScoped(request, {
+          projectRouting: 'request-header',
+        }).asCurrentUser;
+
+        const params = makeSearchParams();
+        onRequest.get()({} as never, params, {}, logger);
+
+        expect((params.body as Record<string, unknown>).project_routing).toBe('_alias:_origin');
+      });
+
       // Note: child() clients that do NOT override Transport inherit the parent's routing
       // automatically, because the ES client propagates the Transport class to child clients.
       // However, callers who pass { Transport: CustomTransport } to child() bypass our
@@ -1386,7 +1415,9 @@ describe('ClusterClient', () => {
         // Even when the scoped client is created with 'space' routing, asSecondaryAuthUser
         // is always a child of asInternalUser, which uses origin-only routing.
         const request = httpServerMock.createKibanaRequest({ path: '/s/my-space/app/discover' });
-        client = clusterClient.asScoped(request, { projectRouting: 'space' }).asSecondaryAuthUser;
+        client = clusterClient.asScoped(request, {
+          projectRouting: 'space',
+        }).asSecondaryAuthUser;
 
         // No Transport override means the child inherits asInternalUser's origin-only Transport.
         expect(internalClient.child).toHaveBeenCalledWith(
@@ -1568,7 +1599,7 @@ describe('ClusterClient', () => {
       });
 
       const request = httpServerMock.createKibanaRequest();
-      client = clusterClient.asScoped(request, { projectRouting: 'origin-only' }).asCurrentUser;
+      client = clusterClient.asScoped(request).asCurrentUser;
 
       const params = makeSearchParamsWithRouting();
       onRequest.get()({} as never, params, {}, logger);
@@ -1590,7 +1621,7 @@ describe('ClusterClient', () => {
       });
 
       const request = httpServerMock.createKibanaRequest();
-      client = clusterClient.asScoped(request, { projectRouting: 'origin-only' }).asCurrentUser;
+      client = clusterClient.asScoped(request).asCurrentUser;
 
       const params = makeSearchParams();
       onRequest.get()({} as never, params, {}, logger);
diff --git a/src/core/packages/elasticsearch/client-server-internal/src/cluster_client.ts b/src/core/packages/elasticsearch/client-server-internal/src/cluster_client.ts
index 63bbaa0d45f01..464247b67914e 100644
--- a/src/core/packages/elasticsearch/client-server-internal/src/cluster_client.ts
+++ b/src/core/packages/elasticsearch/client-server-internal/src/cluster_client.ts
@@ -9,7 +9,7 @@
 
 import type { Client } from '@elastic/elasticsearch';
 import type { Logger } from '@kbn/logging';
-import type { Headers, IAuthHeadersStorage } from '@kbn/core-http-server';
+import type { Headers, IAuthHeadersStorage, KibanaRequest } from '@kbn/core-http-server';
 import {
   ensureRawRequest,
   filterHeaders,
@@ -24,9 +24,8 @@ import type {
   IScopedClusterClient,
   ElasticsearchClientConfig,
   AsScopedOptions,
-  OriginOnlyRouting,
   SpaceNPRERouting,
-  AllProjectsRouting,
+  RequestHeaderRouting,
 } from '@kbn/core-elasticsearch-server';
 import { HTTPAuthorizationHeader, isUiamCredential } from '@kbn/core-security-server';
 import type { InternalSecurityServiceSetup } from '@kbn/core-security-server-internal';
@@ -47,14 +46,35 @@ import type { AgentFactoryProvider } from './agent_manager';
 
 const noop = () => undefined;
 
+interface CommonFactoryRoutingOpts {
+  logger: Logger;
+}
+
+interface SpaceFactoryRoutingOpts extends CommonFactoryRoutingOpts {
+  projectRouting: 'space';
+  request: ScopeableUrlRequest;
+}
+
+interface RequestHeaderFactoryRoutingOpts extends CommonFactoryRoutingOpts {
+  projectRouting: 'request-header';
+  request: ScopeableRequest;
+}
+
+/**
+ * Discriminated union of routing options passed to {@link OnRequestHandlerFactory}.
+ * Each variant carries exactly the data needed for that routing mode.
+ * @internal
+ */
+export type FactoryRoutingOpts =
+  | CommonFactoryRoutingOpts
+  | SpaceFactoryRoutingOpts
+  | RequestHeaderFactoryRoutingOpts;
+
 /**
  * A factory that produces an {@link OnRequestHandler}, which can be bound to a request context.
  * @internal
  */
-export type OnRequestHandlerFactory = (opts: {
-  projectRouting: 'origin-only' | 'all' | ScopeableUrlRequest;
-  logger: Logger;
-}) => OnRequestHandler;
+export type OnRequestHandlerFactory = (opts: FactoryRoutingOpts) => OnRequestHandler;
 
 /** @internal **/
 export class ClusterClient implements ICustomClusterClient {
@@ -103,10 +123,7 @@ export class ClusterClient implements ICustomClusterClient {
     this.getUnauthorizedErrorHandler = getUnauthorizedErrorHandler;
     this.onRequestHandlerFactory = onRequestHandlerFactory;
 
-    const internalUserOnRequest = onRequestHandlerFactory({
-      projectRouting: 'origin-only',
-      logger,
-    });
+    const internalUserOnRequest = onRequestHandlerFactory({ logger });
 
     this.asInternalUser = configureClient(config, {
       logger,
@@ -127,25 +144,17 @@ export class ClusterClient implements ICustomClusterClient {
     });
   }
 
-  asScoped(request: ScopeableUrlRequest, opts: SpaceNPRERouting): IScopedClusterClient;
-  asScoped(
-    request: ScopeableRequest,
-    opts?: OriginOnlyRouting | AllProjectsRouting
-  ): IScopedClusterClient;
-  asScoped(request: ScopeableRequest, opts: AsScopedOptions = { projectRouting: 'origin-only' }) {
+  asScoped(request: KibanaRequest, opts: AsScopedOptions): IScopedClusterClient;
+  asScoped(request: ScopeableRequest, opts?: RequestHeaderRouting): IScopedClusterClient;
+  asScoped(request: ScopeableUrlRequest, opts?: SpaceNPRERouting): IScopedClusterClient;
+  asScoped(request: ScopeableRequest, opts?: AsScopedOptions): IScopedClusterClient {
     const createScopedClient = () => {
       const scopedHeaders = this.getScopedHeaders(request);
-      const { projectRouting } = opts;
-
       const transportClass = createTransport({
         scoped: true,
         getExecutionContext: this.getExecutionContext,
         getUnauthorizedErrorHandler: this.createInternalErrorHandlerAccessor(request),
-        onRequest: this.onRequestHandlerFactory({
-          projectRouting:
-            projectRouting === 'space' ? (request as ScopeableUrlRequest) : projectRouting,
-          logger: this.logger,
-        }),
+        onRequest: this.onRequestHandlerFactory({ ...opts, logger: this.logger, request }),
         logger: this.logger,
       });
 
diff --git a/src/core/packages/elasticsearch/client-server-internal/src/cps_request_handler/cps_request_handler_factory.test.ts b/src/core/packages/elasticsearch/client-server-internal/src/cps_request_handler/cps_request_handler_factory.test.ts
index bbbe42ea6f604..dd8697423786c 100644
--- a/src/core/packages/elasticsearch/client-server-internal/src/cps_request_handler/cps_request_handler_factory.test.ts
+++ b/src/core/packages/elasticsearch/client-server-internal/src/cps_request_handler/cps_request_handler_factory.test.ts
@@ -10,7 +10,11 @@
 import type { TransportRequestParams } from '@elastic/elasticsearch';
 import type { Logger } from '@kbn/logging';
 import { httpServerMock } from '@kbn/core-http-server-mocks';
-import { PROJECT_ROUTING_ORIGIN, PROJECT_ROUTING_ALL, getSpaceNPRE } from '@kbn/cps-server-utils';
+import {
+  PROJECT_ROUTING_ORIGIN,
+  KBN_PROJECT_ROUTING_HEADER,
+  getSpaceNPRE,
+} from '@kbn/cps-server-utils';
 import { loggerMock } from '@kbn/logging-mocks';
 import { getRequestHandlerFactory } from './cps_request_handler_factory';
 
@@ -29,9 +33,9 @@ describe('getRequestHandlerFactory', () => {
   });
 
   describe('without request (internal user)', () => {
-    it('injects PROJECT_ROUTING_ORIGIN when projectRouting is origin-only', () => {
+    it('injects PROJECT_ROUTING_ORIGIN when no opts are passed', () => {
       const factory = getRequestHandlerFactory(true);
-      const handler = factory({ projectRouting: 'origin-only', logger: mockLogger });
+      const handler = factory({ logger: mockLogger });
       const params = makeSearchParams();
 
       handler({ scoped: false }, params, {}, mockLogger);
@@ -40,52 +44,72 @@ describe('getRequestHandlerFactory', () => {
     });
   });
 
-  describe("projectRouting: 'origin-only'", () => {
-    it('injects PROJECT_ROUTING_ORIGIN when CPS is enabled', () => {
+  describe("projectRouting: 'space'", () => {
+    it('injects the space NPRE derived from a KibanaRequest', () => {
       const factory = getRequestHandlerFactory(true);
-      const handler = factory({ projectRouting: 'origin-only', logger: mockLogger });
+      const request = httpServerMock.createKibanaRequest({ path: '/s/my-space/app/discover' });
+      const handler = factory({ projectRouting: 'space', request, logger: mockLogger });
       const params = makeSearchParams();
 
       handler({ scoped: true }, params, {}, mockLogger);
 
-      expect((params.body as Record<string, unknown>).project_routing).toBe(PROJECT_ROUTING_ORIGIN);
+      expect((params.body as Record<string, unknown>).project_routing).toBe(
+        getSpaceNPRE('my-space')
+      );
     });
+  });
 
-    it('strips project_routing when CPS is disabled', () => {
-      const factory = getRequestHandlerFactory(false);
-      const handler = factory({ projectRouting: 'origin-only', logger: mockLogger });
-      const params = makeSearchParams({ project_routing: 'should-be-removed' });
+  describe("projectRouting: 'request-header'", () => {
+    it('injects the routing value from the x-kbn-project-routing header when present', () => {
+      const factory = getRequestHandlerFactory(true);
+      const request = httpServerMock.createKibanaRequest({
+        headers: { [KBN_PROJECT_ROUTING_HEADER]: '_alias:*' },
+      });
+      const handler = factory({ projectRouting: 'request-header', request, logger: mockLogger });
+      const params = makeSearchParams();
 
       handler({ scoped: true }, params, {}, mockLogger);
 
-      expect((params.body as Record<string, unknown>).project_routing).toBeUndefined();
+      expect((params.body as Record<string, unknown>).project_routing).toBe('_alias:*');
     });
-  });
 
-  describe("projectRouting: 'all'", () => {
-    it('injects PROJECT_ROUTING_ALL when CPS is enabled', () => {
+    it('falls back to PROJECT_ROUTING_ORIGIN when the header is absent', () => {
       const factory = getRequestHandlerFactory(true);
-      const handler = factory({ projectRouting: 'all', logger: mockLogger });
+      const request = httpServerMock.createKibanaRequest();
+      const handler = factory({ projectRouting: 'request-header', request, logger: mockLogger });
       const params = makeSearchParams();
 
       handler({ scoped: true }, params, {}, mockLogger);
 
-      expect((params.body as Record<string, unknown>).project_routing).toBe(PROJECT_ROUTING_ALL);
+      expect((params.body as Record<string, unknown>).project_routing).toBe(PROJECT_ROUTING_ORIGIN);
     });
-  });
 
-  describe("projectRouting: 'space'", () => {
-    it('injects the space NPRE derived from a KibanaRequest', () => {
+    it('takes the first value when the header is an array', () => {
       const factory = getRequestHandlerFactory(true);
-      const request = httpServerMock.createKibanaRequest({ path: '/s/my-space/app/discover' });
-      const handler = factory({ projectRouting: request, logger: mockLogger });
+      // Use a plain FakeRequest so the headers object is mutable, allowing us to set
+      // an array value that simulates multi-value HTTP header parsing by Node.js.
+      const request = {
+        headers: { [KBN_PROJECT_ROUTING_HEADER]: ['_alias:_origin', '_alias:*'] },
+      };
+      const handler = factory({ projectRouting: 'request-header', request, logger: mockLogger });
       const params = makeSearchParams();
 
       handler({ scoped: true }, params, {}, mockLogger);
 
-      expect((params.body as Record<string, unknown>).project_routing).toBe(
-        getSpaceNPRE('my-space')
-      );
+      expect((params.body as Record<string, unknown>).project_routing).toBe('_alias:_origin');
+    });
+
+    it('strips project_routing when CPS is disabled, regardless of header', () => {
+      const factory = getRequestHandlerFactory(false);
+      const request = httpServerMock.createKibanaRequest({
+        headers: { [KBN_PROJECT_ROUTING_HEADER]: '_alias:*' },
+      });
+      const handler = factory({ projectRouting: 'request-header', request, logger: mockLogger });
+      const params = makeSearchParams({ project_routing: 'should-be-removed' });
+
+      handler({ scoped: true }, params, {}, mockLogger);
+
+      expect((params.body as Record<string, unknown>).project_routing).toBeUndefined();
     });
   });
 });
diff --git a/src/core/packages/elasticsearch/client-server-internal/src/cps_request_handler/cps_request_handler_factory.ts b/src/core/packages/elasticsearch/client-server-internal/src/cps_request_handler/cps_request_handler_factory.ts
index dace38fea98e2..d678955b6d811 100644
--- a/src/core/packages/elasticsearch/client-server-internal/src/cps_request_handler/cps_request_handler_factory.ts
+++ b/src/core/packages/elasticsearch/client-server-internal/src/cps_request_handler/cps_request_handler_factory.ts
@@ -7,7 +7,11 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
-import { getSpaceNPRE, PROJECT_ROUTING_ORIGIN, PROJECT_ROUTING_ALL } from '@kbn/cps-server-utils';
+import {
+  getSpaceNPRE,
+  PROJECT_ROUTING_ORIGIN,
+  KBN_PROJECT_ROUTING_HEADER,
+} from '@kbn/cps-server-utils';
 import type { OnRequestHandlerFactory } from '../cluster_client';
 import { getCpsRequestHandler } from './cps_request_handler';
 
@@ -18,15 +22,17 @@ import { getCpsRequestHandler } from './cps_request_handler';
  * @internal
  */
 export function getRequestHandlerFactory(cpsEnabled: boolean): OnRequestHandlerFactory {
-  return ({ projectRouting, logger }) => {
-    switch (projectRouting) {
-      case 'origin-only':
-        return getCpsRequestHandler(cpsEnabled, PROJECT_ROUTING_ORIGIN, logger);
-      case 'all':
-        return getCpsRequestHandler(cpsEnabled, PROJECT_ROUTING_ALL, logger);
-      default:
-        // projectRouting is a ScopeableUrlRequest - derive the NPRE from its URL.
-        return getCpsRequestHandler(cpsEnabled, getSpaceNPRE(projectRouting), logger);
+  return (opts) => {
+    if ('projectRouting' in opts) {
+      if (opts.projectRouting === 'space') {
+        return getCpsRequestHandler(cpsEnabled, getSpaceNPRE(opts.request), opts.logger);
+      } else {
+        const raw = opts.request.headers[KBN_PROJECT_ROUTING_HEADER];
+        const value = Array.isArray(raw) ? raw[0] : raw;
+        return getCpsRequestHandler(cpsEnabled, value ?? PROJECT_ROUTING_ORIGIN, opts.logger);
+      }
+    } else {
+      return getCpsRequestHandler(cpsEnabled, PROJECT_ROUTING_ORIGIN, opts.logger);
     }
   };
 }
diff --git a/src/core/packages/elasticsearch/server/index.ts b/src/core/packages/elasticsearch/server/index.ts
index 05ba282797d0b..467311d29bed8 100644
--- a/src/core/packages/elasticsearch/server/index.ts
+++ b/src/core/packages/elasticsearch/server/index.ts
@@ -14,9 +14,8 @@ export type {
   IClusterClient,
   ICustomClusterClient,
   AsScopedOptions,
-  OriginOnlyRouting,
   SpaceNPRERouting,
-  AllProjectsRouting,
+  RequestHeaderRouting,
   ScopeableRequest,
   UnauthorizedErrorHandlerResult,
   UnauthorizedErrorHandler,
diff --git a/src/core/packages/elasticsearch/server/src/client/cluster_client.ts b/src/core/packages/elasticsearch/server/src/client/cluster_client.ts
index a23e39ae3d2b0..3b7e6bad06598 100644
--- a/src/core/packages/elasticsearch/server/src/client/cluster_client.ts
+++ b/src/core/packages/elasticsearch/server/src/client/cluster_client.ts
@@ -7,6 +7,7 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
+import type { KibanaRequest } from '@kbn/core-http-server';
 import type { ElasticsearchClient } from './client';
 import type { ScopeableRequest, ScopeableUrlRequest } from './types';
 import type { IScopedClusterClient } from './scoped_cluster_client';
@@ -27,32 +28,24 @@ export interface AsScopedOptions {
    * header and Elasticsearch handles execution, security enforcement, and result aggregation.
    *
    * **Options**:
-   * - `'origin-only'`: Requests are routed exclusively to the "origin" Elasticsearch instance
-   *   (i.e., the project that Kibana is directly connected to). Use this for administrative or
-   *   internal operations that must not fan out across other projects.
-   * - `'space'`: Requests are routed to the Named Project Routing Expression (NPRE) configured for
-   *   the current Kibana space. Requires a {@link ScopeableUrlRequest} to be passed to `asScoped`
-   *   so that the space can be extracted from the URL pathname. Use this when the scope of the
-   *   query should match the data boundaries of the active space.
-   * - `'all'`: Requests are broadcast to all CPS-connected Elasticsearch instances. This is the
-   *   broadest option and is appropriate when the intent is to search or aggregate data across
-   *   all connected projects.
+   * - `'space'`: Requests are routed to the Named Project Routing Expression (NPRE) configured
+   *   for the current Kibana space. Requires a {@link ScopeableUrlRequest} to be passed to
+   *   `asScoped` so that the space can be extracted from the URL pathname. Use this when the scope
+   *   of the query should match the data boundaries of the active space (e.g. alerting rules).
+   * - `'request-header'`: The `project_routing` value is read from the `x-kbn-project-routing`
+   *   HTTP header present on the incoming request. Browser-side code sets this header via the
+   *   Kibana HTTP client, eliminating the need to propagate the routing value through request
+   *   bodies or service layers. Falls back to origin-only routing when the header is absent.
+   *
+   * When no options are passed to `asScoped`, requests are always routed to the origin project
+   * (i.e. the Elasticsearch instance Kibana is directly connected to).
    *
    * **Important**: This option only takes effect in CPS-enabled Serverless environments. In all
    * other environments (stateful, non-CPS Serverless), any `project_routing` params are
    * stripped from requests to avoid Elasticsearch rejections and to preserve traditional
    * single-cluster routing behavior.
    */
-  projectRouting: 'origin-only' | 'space' | 'all';
-}
-
-/**
- * {@link AsScopedOptions} variant that locks routing to the origin Elasticsearch instance.
- * Use for administrative or internal operations that must not fan out across CPS-connected projects.
- * @public
- */
-export interface OriginOnlyRouting extends AsScopedOptions {
-  projectRouting: 'origin-only';
+  projectRouting: 'space' | 'request-header';
 }
 
 /**
@@ -66,12 +59,15 @@ export interface SpaceNPRERouting extends AsScopedOptions {
 }
 
 /**
- * {@link AsScopedOptions} variant that broadcasts requests to all CPS-connected Elasticsearch
- * instances. Use when the intent is to search or aggregate data across all available projects.
+ * {@link AsScopedOptions} variant that reads `project_routing` from the `x-kbn-project-routing`
+ * HTTP header present on the incoming request. Browser-side code sets this header via the Kibana
+ * HTTP client (e.g. `http.post('/api/...', { headers: { 'x-kbn-project-routing': value } })`),
+ * eliminating the need to propagate the value through request bodies or service layers.
+ * Falls back to origin-only routing when the header is absent.
  * @public
  */
-export interface AllProjectsRouting extends AsScopedOptions {
-  projectRouting: 'all';
+export interface RequestHeaderRouting extends AsScopedOptions {
+  projectRouting: 'request-header';
 }
 
 /**
@@ -92,35 +88,23 @@ export interface IClusterClient {
    * to `'origin-only'` routing - they will never fan out to other CPS-connected projects.
    */
   readonly asInternalUser: ElasticsearchClient;
+
+  asScoped(request: KibanaRequest, opts: AsScopedOptions): IScopedClusterClient;
+  asScoped(request: ScopeableRequest, opts?: RequestHeaderRouting): IScopedClusterClient;
   /**
    * Creates a {@link IScopedClusterClient | scoped cluster client} bound to the given request,
    * forwarding the request's authentication headers to Elasticsearch.
    *
-   * In CPS-enabled Serverless environments, the `opts` parameter controls how `project_routing`
-   * is injected into outgoing requests. See {@link AsScopedOptions} for details.
-   *
-   * @param request - A {@link ScopeableUrlRequest} whose URL is used to extract the active space
-   *   for space-level CPS routing. Accepts both a real {@link KibanaRequest} (the typical caller
-   *   from route handlers) and a synthetic {@link UrlRequest}.
-   * @param opts - {@link SpaceNPRERouting} options with `projectRouting` set to `'space'`.
-   */
-  asScoped(request: ScopeableUrlRequest, opts: SpaceNPRERouting): IScopedClusterClient;
-  /**
-   * Creates a {@link IScopedClusterClient | scoped cluster client} bound to the given request,
-   * forwarding the request's authentication headers to Elasticsearch.
-   *
-   * In CPS-enabled Serverless environments, the `opts` parameter controls how `project_routing`
-   * is injected into outgoing requests. See {@link AsScopedOptions} for details.
-   *
-   * @param request - The incoming {@link ScopeableRequest | request} whose credentials are used
-   *   to authenticate Elasticsearch calls.
-   * @param opts - Optional {@link AsScopedOptions | options} to configure CPS routing behavior.
-   *   Defaults to `'origin-only'` when not specified.
+   * @param request - The incoming request whose credentials authenticate Elasticsearch calls.
+   *   - {@link KibanaRequest}: supports all routing options via `opts`.
+   *   - {@link ScopeableRequest}: supports `'request-header'` routing or no routing (origin-only).
+   *   - {@link ScopeableUrlRequest}: additionally supports `'space'` routing (space id extracted from URL).
+   * @param opts - Optional {@link AsScopedOptions} to configure CPS routing behavior.
+   *   - 'request-header': Routes the request to the PRE specified in the `x-kbn-project-routing` header.
+   *   - 'space': Routes the request to the NPRE configured for the current Kibana space.
+   *   The client will route the request to the origin project if no options are provided.
    */
-  asScoped(
-    request: ScopeableRequest,
-    opts?: OriginOnlyRouting | AllProjectsRouting
-  ): IScopedClusterClient;
+  asScoped(request: ScopeableUrlRequest, opts?: SpaceNPRERouting): IScopedClusterClient;
 }
 
 /**
diff --git a/src/core/packages/elasticsearch/server/src/client/index.ts b/src/core/packages/elasticsearch/server/src/client/index.ts
index e7611d2522a1a..ef6f22c8e7654 100644
--- a/src/core/packages/elasticsearch/server/src/client/index.ts
+++ b/src/core/packages/elasticsearch/server/src/client/index.ts
@@ -12,9 +12,8 @@ export type {
   IClusterClient,
   ICustomClusterClient,
   AsScopedOptions,
-  OriginOnlyRouting,
   SpaceNPRERouting,
-  AllProjectsRouting,
+  RequestHeaderRouting,
 } from './cluster_client';
 export type { ScopeableRequest, FakeRequest, UrlRequest, ScopeableUrlRequest } from './types';
 export type { IScopedClusterClient } from './scoped_cluster_client';
diff --git a/src/platform/packages/private/kbn-cps-common/index.ts b/src/platform/packages/private/kbn-cps-common/index.ts
index 33ecbc3ffaaf8..7e429f4f317a6 100644
--- a/src/platform/packages/private/kbn-cps-common/index.ts
+++ b/src/platform/packages/private/kbn-cps-common/index.ts
@@ -7,6 +7,6 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
-export { PROJECT_ROUTING } from './src/common/constants';
+export { PROJECT_ROUTING, KBN_PROJECT_ROUTING_HEADER } from './src/common/constants';
 export type { ProjectRoutingValue } from './src/common/constants';
 export { getSpaceDefaultNpreName } from './src/common/get_space_default_npre_name';
diff --git a/src/platform/packages/private/kbn-cps-common/src/common/constants.ts b/src/platform/packages/private/kbn-cps-common/src/common/constants.ts
index 821a622520f97..22719d08ab714 100644
--- a/src/platform/packages/private/kbn-cps-common/src/common/constants.ts
+++ b/src/platform/packages/private/kbn-cps-common/src/common/constants.ts
@@ -19,3 +19,11 @@ export const PROJECT_ROUTING = {
 } as const;
 
 export type ProjectRoutingValue = (typeof PROJECT_ROUTING)[keyof typeof PROJECT_ROUTING];
+
+/**
+ * HTTP header name used to propagate the CPS `project_routing` value from the browser to
+ * Kibana server-side route handlers. App developers set this header on outgoing HTTP requests
+ * (via the Kibana HTTP client) and opt in on the server by calling
+ * `esClient.asScoped(request, { projectRouting: 'request-header' })`.
+ */
+export const KBN_PROJECT_ROUTING_HEADER = 'x-kbn-project-routing' as const;
diff --git a/src/platform/packages/shared/kbn-cps-server-utils/index.ts b/src/platform/packages/shared/kbn-cps-server-utils/index.ts
index 5c9ef617073d7..c613a82c00be3 100644
--- a/src/platform/packages/shared/kbn-cps-server-utils/index.ts
+++ b/src/platform/packages/shared/kbn-cps-server-utils/index.ts
@@ -8,4 +8,8 @@
  */
 
 export { getSpaceNPRE } from './src/get_space_npre';
-export { PROJECT_ROUTING_ORIGIN, PROJECT_ROUTING_ALL } from './src/constants';
+export {
+  PROJECT_ROUTING_ORIGIN,
+  PROJECT_ROUTING_ALL,
+  KBN_PROJECT_ROUTING_HEADER,
+} from './src/constants';
diff --git a/src/platform/packages/shared/kbn-cps-server-utils/src/constants.ts b/src/platform/packages/shared/kbn-cps-server-utils/src/constants.ts
index 98f2138d0450f..52cf67cb9cb85 100644
--- a/src/platform/packages/shared/kbn-cps-server-utils/src/constants.ts
+++ b/src/platform/packages/shared/kbn-cps-server-utils/src/constants.ts
@@ -16,3 +16,11 @@ export const PROJECT_ROUTING_ORIGIN = '_alias:_origin';
  * Project routing expression that allows requests across all projects.
  */
 export const PROJECT_ROUTING_ALL = '_alias:*';
+
+/**
+ * HTTP header name used to propagate the CPS `project_routing` value from the browser to
+ * Kibana server-side route handlers. App developers set this header on outgoing HTTP requests
+ * (via the Kibana HTTP client) and opt in on the server by calling
+ * `esClient.asScoped(request, { projectRouting: 'request-header' })`.
+ */
+export const KBN_PROJECT_ROUTING_HEADER = 'x-kbn-project-routing' as const;
diff --git a/src/platform/packages/shared/kbn-cps-utils/index.ts b/src/platform/packages/shared/kbn-cps-utils/index.ts
index 1aa7b4fac6739..08ad394592246 100644
--- a/src/platform/packages/shared/kbn-cps-utils/index.ts
+++ b/src/platform/packages/shared/kbn-cps-utils/index.ts
@@ -22,5 +22,5 @@ export { ProjectPicker } from './components/project_picker';
 export { ProjectPickerContent } from './components/project_picker_content';
 export { ProjectPickerContainer } from './components/project_picker_container';
 export { useFetchProjects } from './components/use_fetch_projects';
-export { PROJECT_ROUTING } from '@kbn/cps-common';
+export { KBN_PROJECT_ROUTING_HEADER, PROJECT_ROUTING } from '@kbn/cps-common';
 export { ProjectRoutingAccess } from './types';
diff --git a/src/platform/packages/shared/kbn-search-types/src/types.ts b/src/platform/packages/shared/kbn-search-types/src/types.ts
index d2074cab8c011..d52f912b685cf 100644
--- a/src/platform/packages/shared/kbn-search-types/src/types.ts
+++ b/src/platform/packages/shared/kbn-search-types/src/types.ts
@@ -147,5 +147,4 @@ export type ISearchOptionsSerializable = Pick<
   | 'retrieveResults'
   | 'executionContext'
   | 'stream'
-  | 'projectRouting'
 >;
diff --git a/src/platform/plugins/shared/data/public/search/search_interceptor/search_interceptor.test.ts b/src/platform/plugins/shared/data/public/search/search_interceptor/search_interceptor.test.ts
index 11a9adc2768fd..84499a5fc7f6e 100644
--- a/src/platform/plugins/shared/data/public/search/search_interceptor/search_interceptor.test.ts
+++ b/src/platform/plugins/shared/data/public/search/search_interceptor/search_interceptor.test.ts
@@ -32,6 +32,7 @@ import type { Start as InspectorStart } from '@kbn/inspector-plugin/public';
 import { SearchTimeoutError, TimeoutErrorMode } from './timeout_error';
 import { SearchSessionIncompleteWarning } from './search_session_incomplete_warning';
 import { getMockSearchConfig } from '../../../config.mock';
+import { KBN_PROJECT_ROUTING_HEADER } from '@kbn/cps-utils';
 import type { ICPSManager } from '@kbn/cps-utils';
 
 jest.mock('./create_request_hash', () => {
@@ -2274,8 +2275,13 @@ describe('SearchInterceptor', () => {
       mockCoreSetup.http.post.mockResolvedValue(getMockSearchResponse());
     });
 
+    const getProjectRoutingHeader = (call: unknown[]) => {
+      const requestOptions = (call as unknown as [string, HttpFetchOptions])[1];
+      return requestOptions.headers?.[KBN_PROJECT_ROUTING_HEADER];
+    };
+
     describe('ESQL_ASYNC_SEARCH_STRATEGY', () => {
-      test('User passes "_alias:*" with global "_alias:_origin" - sends to ES', async () => {
+      test('User passes "_alias:*" with global "_alias:_origin" - header set to user value', async () => {
         searchInterceptor = getSearchInterceptor({
           getCPSManager: jest.fn().mockReturnValue(createMockCPSManager('_alias:_origin')),
         });
@@ -2287,14 +2293,10 @@ describe('SearchInterceptor', () => {
           )
           .toPromise();
 
-        const requestOptions = (
-          mockCoreSetup.http.post.mock.calls[0] as unknown as [string, HttpFetchOptions]
-        )[1];
-        const requestBody = JSON.parse(requestOptions.body as string);
-        expect(requestBody.projectRouting).toBe('_alias:*');
+        expect(getProjectRoutingHeader(mockCoreSetup.http.post.mock.calls[0])).toBe('_alias:*');
       });
 
-      test('User passes "_alias:*" with global "_alias:*" - sends to ES', async () => {
+      test('User passes "_alias:*" with global "_alias:*" - header set to user value', async () => {
         searchInterceptor = getSearchInterceptor({
           getCPSManager: jest.fn().mockReturnValue(createMockCPSManager('_alias:*')),
         });
@@ -2306,14 +2308,10 @@ describe('SearchInterceptor', () => {
           )
           .toPromise();
 
-        const requestOptions = (
-          mockCoreSetup.http.post.mock.calls[0] as unknown as [string, HttpFetchOptions]
-        )[1];
-        const requestBody = JSON.parse(requestOptions.body as string);
-        expect(requestBody.projectRouting).toBe('_alias:*');
+        expect(getProjectRoutingHeader(mockCoreSetup.http.post.mock.calls[0])).toBe('_alias:*');
       });
 
-      test('User passes "_alias:_origin" with global "_alias:_origin" - sends to ES', async () => {
+      test('User passes "_alias:_origin" with global "_alias:_origin" - header set to user value', async () => {
         searchInterceptor = getSearchInterceptor({
           getCPSManager: jest.fn().mockReturnValue(createMockCPSManager('_alias:_origin')),
         });
@@ -2325,14 +2323,12 @@ describe('SearchInterceptor', () => {
           )
           .toPromise();
 
-        const requestOptions = (
-          mockCoreSetup.http.post.mock.calls[0] as unknown as [string, HttpFetchOptions]
-        )[1];
-        const requestBody = JSON.parse(requestOptions.body as string);
-        expect(requestBody.projectRouting).toBe('_alias:_origin');
+        expect(getProjectRoutingHeader(mockCoreSetup.http.post.mock.calls[0])).toBe(
+          '_alias:_origin'
+        );
       });
 
-      test('User passes "_alias:_origin" with global "_alias:*" - sends to ES', async () => {
+      test('User passes "_alias:_origin" with global "_alias:*" - header set to user value', async () => {
         searchInterceptor = getSearchInterceptor({
           getCPSManager: jest.fn().mockReturnValue(createMockCPSManager('_alias:*')),
         });
@@ -2344,14 +2340,12 @@ describe('SearchInterceptor', () => {
           )
           .toPromise();
 
-        const requestOptions = (
-          mockCoreSetup.http.post.mock.calls[0] as unknown as [string, HttpFetchOptions]
-        )[1];
-        const requestBody = JSON.parse(requestOptions.body as string);
-        expect(requestBody.projectRouting).toBe('_alias:_origin');
+        expect(getProjectRoutingHeader(mockCoreSetup.http.post.mock.calls[0])).toBe(
+          '_alias:_origin'
+        );
       });
 
-      test('User passes nothing with global "_alias:_origin" - sends global to ES', async () => {
+      test('User passes nothing with global "_alias:_origin" - header set to global value', async () => {
         searchInterceptor = getSearchInterceptor({
           getCPSManager: jest.fn().mockReturnValue(createMockCPSManager('_alias:_origin')),
         });
@@ -2360,14 +2354,12 @@ describe('SearchInterceptor', () => {
           .search({ params: {} }, { strategy: ESQL_ASYNC_SEARCH_STRATEGY })
           .toPromise();
 
-        const requestOptions = (
-          mockCoreSetup.http.post.mock.calls[0] as unknown as [string, HttpFetchOptions]
-        )[1];
-        const requestBody = JSON.parse(requestOptions.body as string);
-        expect(requestBody.projectRouting).toBe('_alias:_origin');
+        expect(getProjectRoutingHeader(mockCoreSetup.http.post.mock.calls[0])).toBe(
+          '_alias:_origin'
+        );
       });
 
-      test('User passes nothing with global "_alias:*" - sends global to ES', async () => {
+      test('User passes nothing with global "_alias:*" - header set to global value', async () => {
         searchInterceptor = getSearchInterceptor({
           getCPSManager: jest.fn().mockReturnValue(createMockCPSManager('_alias:*')),
         });
@@ -2376,14 +2368,10 @@ describe('SearchInterceptor', () => {
           .search({ params: {} }, { strategy: ESQL_ASYNC_SEARCH_STRATEGY })
           .toPromise();
 
-        const requestOptions = (
-          mockCoreSetup.http.post.mock.calls[0] as unknown as [string, HttpFetchOptions]
-        )[1];
-        const requestBody = JSON.parse(requestOptions.body as string);
-        expect(requestBody.projectRouting).toBe('_alias:*');
+        expect(getProjectRoutingHeader(mockCoreSetup.http.post.mock.calls[0])).toBe('_alias:*');
       });
 
-      test('User passes nothing with global undefined - does not send to ES', async () => {
+      test('User passes nothing with global undefined - header not set', async () => {
         searchInterceptor = getSearchInterceptor({
           getCPSManager: jest.fn().mockReturnValue(createMockCPSManager(undefined)),
         });
@@ -2392,14 +2380,10 @@ describe('SearchInterceptor', () => {
           .search({ params: {} }, { strategy: ESQL_ASYNC_SEARCH_STRATEGY })
           .toPromise();
 
-        const requestOptions = (
-          mockCoreSetup.http.post.mock.calls[0] as unknown as [string, HttpFetchOptions]
-        )[1];
-        const requestBody = JSON.parse(requestOptions.body as string);
-        expect(requestBody.projectRouting).toBeUndefined();
+        expect(getProjectRoutingHeader(mockCoreSetup.http.post.mock.calls[0])).toBeUndefined();
       });
 
-      test('CPS unavailable - does not send to ES', async () => {
+      test('CPS unavailable - header not set', async () => {
         searchInterceptor = getSearchInterceptor({ getCPSManager: undefined });
 
         await searchInterceptor
@@ -2409,16 +2393,12 @@ describe('SearchInterceptor', () => {
           )
           .toPromise();
 
-        const requestOptions = (
-          mockCoreSetup.http.post.mock.calls[0] as unknown as [string, HttpFetchOptions]
-        )[1];
-        const requestBody = JSON.parse(requestOptions.body as string);
-        expect(requestBody.projectRouting).toBeUndefined();
+        expect(getProjectRoutingHeader(mockCoreSetup.http.post.mock.calls[0])).toBeUndefined();
       });
     });
 
     describe('ENHANCED_ES_SEARCH_STRATEGY', () => {
-      test('User passes "_alias:*" with global "_alias:_origin" - sends to ES', async () => {
+      test('User passes "_alias:*" with global "_alias:_origin" - header set to user value', async () => {
         searchInterceptor = getSearchInterceptor({
           getCPSManager: jest.fn().mockReturnValue(createMockCPSManager('_alias:_origin')),
         });
@@ -2430,14 +2410,10 @@ describe('SearchInterceptor', () => {
           )
           .toPromise();
 
-        const requestOptions = (
-          mockCoreSetup.http.post.mock.calls[0] as unknown as [string, HttpFetchOptions]
-        )[1];
-        const requestBody = JSON.parse(requestOptions.body as string);
-        expect(requestBody.projectRouting).toBe('_alias:*');
+        expect(getProjectRoutingHeader(mockCoreSetup.http.post.mock.calls[0])).toBe('_alias:*');
       });
 
-      test('User passes "_alias:_origin" with global "_alias:*" - sends to ES', async () => {
+      test('User passes "_alias:_origin" with global "_alias:*" - header set to user value', async () => {
         searchInterceptor = getSearchInterceptor({
           getCPSManager: jest.fn().mockReturnValue(createMockCPSManager('_alias:*')),
         });
@@ -2449,14 +2425,12 @@ describe('SearchInterceptor', () => {
           )
           .toPromise();
 
-        const requestOptions = (
-          mockCoreSetup.http.post.mock.calls[0] as unknown as [string, HttpFetchOptions]
-        )[1];
-        const requestBody = JSON.parse(requestOptions.body as string);
-        expect(requestBody.projectRouting).toBe('_alias:_origin');
+        expect(getProjectRoutingHeader(mockCoreSetup.http.post.mock.calls[0])).toBe(
+          '_alias:_origin'
+        );
       });
 
-      test('User passes nothing with global "_alias:_origin" - sends global to ES', async () => {
+      test('User passes nothing with global "_alias:_origin" - header set to global value', async () => {
         searchInterceptor = getSearchInterceptor({
           getCPSManager: jest.fn().mockReturnValue(createMockCPSManager('_alias:_origin')),
         });
@@ -2465,14 +2439,12 @@ describe('SearchInterceptor', () => {
           .search({ params: { body: {} } }, { strategy: ENHANCED_ES_SEARCH_STRATEGY })
           .toPromise();
 
-        const requestOptions = (
-          mockCoreSetup.http.post.mock.calls[0] as unknown as [string, HttpFetchOptions]
-        )[1];
-        const requestBody = JSON.parse(requestOptions.body as string);
-        expect(requestBody.projectRouting).toBe('_alias:_origin');
+        expect(getProjectRoutingHeader(mockCoreSetup.http.post.mock.calls[0])).toBe(
+          '_alias:_origin'
+        );
       });
 
-      test('CPS unavailable - does not send to ES', async () => {
+      test('CPS unavailable - header not set', async () => {
         searchInterceptor = getSearchInterceptor({ getCPSManager: undefined });
 
         await searchInterceptor
@@ -2482,11 +2454,7 @@ describe('SearchInterceptor', () => {
           )
           .toPromise();
 
-        const requestOptions = (
-          mockCoreSetup.http.post.mock.calls[0] as unknown as [string, HttpFetchOptions]
-        )[1];
-        const requestBody = JSON.parse(requestOptions.body as string);
-        expect(requestBody.projectRouting).toBeUndefined();
+        expect(getProjectRoutingHeader(mockCoreSetup.http.post.mock.calls[0])).toBeUndefined();
       });
     });
   });
diff --git a/src/platform/plugins/shared/data/public/search/search_interceptor/search_interceptor.ts b/src/platform/plugins/shared/data/public/search/search_interceptor/search_interceptor.ts
index d356581f27694..603223a8f1fff 100644
--- a/src/platform/plugins/shared/data/public/search/search_interceptor/search_interceptor.ts
+++ b/src/platform/plugins/shared/data/public/search/search_interceptor/search_interceptor.ts
@@ -64,6 +64,7 @@ import type {
 } from '@kbn/search-types';
 import { createEsError, isEsError, renderSearchError } from '@kbn/search-errors';
 import { AbortReason, defaultFreeze } from '@kbn/kibana-utils-plugin/common';
+import { KBN_PROJECT_ROUTING_HEADER } from '@kbn/cps-utils';
 import type { ICPSManager } from '@kbn/cps-utils';
 import {
   EVENT_TYPE_DATA_SEARCH_TIMEOUT,
@@ -285,9 +286,6 @@ export class SearchInterceptor {
     if (combined.executionContext !== undefined) {
       serializableOptions.executionContext = combined.executionContext;
     }
-    if (combined.projectRouting !== undefined) {
-      serializableOptions.projectRouting = combined.projectRouting;
-    }
 
     return serializableOptions;
   }
@@ -479,7 +477,7 @@ export class SearchInterceptor {
     { params, ...request }: IKibanaSearchRequest,
     options?: ISearchOptions
   ): Promise<IKibanaSearchResponse> {
-    const { abortSignal } = options || {};
+    const { abortSignal, projectRouting } = options || {};
 
     const requestHash = params ? createRequestHashForBackgroundSearches(params) : undefined;
 
@@ -496,6 +494,9 @@ export class SearchInterceptor {
           version: '1',
           signal: abortSignal,
           context: this.deps.executionContext.withGlobalContext(executionContext),
+          ...(projectRouting != null && {
+            headers: { [KBN_PROJECT_ROUTING_HEADER]: projectRouting },
+          }),
           body: JSON.stringify({
             ...{ ...request, params: paramsToUse },
             ...searchOptions,
diff --git a/src/platform/plugins/shared/data/server/search/routes/search.ts b/src/platform/plugins/shared/data/server/search/routes/search.ts
index 892cfa6112edb..ce241ebc4dbcd 100644
--- a/src/platform/plugins/shared/data/server/search/routes/search.ts
+++ b/src/platform/plugins/shared/data/server/search/routes/search.ts
@@ -55,7 +55,6 @@ export function registerSearchRoute(
                 retrieveResults: schema.maybe(schema.boolean()),
                 stream: schema.maybe(schema.boolean()),
                 requestHash: schema.maybe(schema.string()),
-                projectRouting: schema.maybe(schema.string()),
               },
               { unknowns: 'allow' }
             ),
@@ -71,7 +70,6 @@ export function registerSearchRoute(
           retrieveResults,
           stream,
           requestHash,
-          projectRouting,
           ...searchRequest
         } = request.body;
         const { strategy, id } = request.params;
@@ -106,7 +104,6 @@ export function registerSearchRoute(
                   retrieveResults,
                   stream,
                   requestHash,
-                  projectRouting,
                 }
               )
               .pipe(first())
diff --git a/src/platform/plugins/shared/data/server/search/search_service.test.ts b/src/platform/plugins/shared/data/server/search/search_service.test.ts
index e56ff93ce59d4..820188fc09c4d 100644
--- a/src/platform/plugins/shared/data/server/search/search_service.test.ts
+++ b/src/platform/plugins/shared/data/server/search/search_service.test.ts
@@ -151,35 +151,13 @@ describe('Search service', () => {
         expect(asScopedSpy).toHaveBeenCalledWith(request, { projectRouting: 'space' });
       });
 
-      it('calls elasticsearch.client.asScoped with request and projectRouting: "all" when opts.projectRouting is "all"', () => {
-        const asScopedSpy = mockCoreStart.elasticsearch.client.asScoped as jest.Mock;
-        asScopedSpy.mockClear();
-
-        const request = {} as any;
-        searchPluginStart.asScoped(request, { projectRouting: 'all' });
-
-        expect(asScopedSpy).toHaveBeenCalledTimes(1);
-        expect(asScopedSpy).toHaveBeenCalledWith(request, { projectRouting: 'all' });
-      });
-
-      it('calls elasticsearch.client.asScoped with request and projectRouting: "origin-only" when opts.projectRouting is "origin-only"', () => {
-        const asScopedSpy = mockCoreStart.elasticsearch.client.asScoped as jest.Mock;
-        asScopedSpy.mockClear();
-
-        const request = {} as any;
-        searchPluginStart.asScoped(request, { projectRouting: 'origin-only' });
-
-        expect(asScopedSpy).toHaveBeenCalledTimes(1);
-        expect(asScopedSpy).toHaveBeenCalledWith(request, { projectRouting: 'origin-only' });
-      });
-
       it('returns a scoped client that can search when called with opts', async () => {
         const asScopedSpy = mockCoreStart.elasticsearch.client.asScoped as jest.Mock;
         asScopedSpy.mockClear();
 
         const request = {} as any;
         const scopedClient = searchPluginStart.asScoped(request, {
-          projectRouting: 'origin-only',
+          projectRouting: 'request-header',
         });
 
         expect(scopedClient).toHaveProperty('search');
diff --git a/src/platform/plugins/shared/data/server/search/search_service.ts b/src/platform/plugins/shared/data/server/search/search_service.ts
index 3ac40703cb863..155bbbe1fdcce 100644
--- a/src/platform/plugins/shared/data/server/search/search_service.ts
+++ b/src/platform/plugins/shared/data/server/search/search_service.ts
@@ -14,7 +14,6 @@ import moment from 'moment';
 import type {
   CoreSetup,
   CoreStart,
-  IClusterClient,
   KibanaRequest,
   Logger,
   PluginInitializerContext,
@@ -292,11 +291,9 @@ export class SearchService {
       asScoped: this.asScoped,
       searchSource: {
         asScoped: async (request: KibanaRequest, opts?: AsScopedOptions) => {
-          const esClient = this.createScopedEsClient({
-            client: elasticsearch.client,
-            request,
-            opts,
-          });
+          const esClient = opts
+            ? elasticsearch.client.asScoped(request, opts)
+            : elasticsearch.client.asScoped(request);
 
           const savedObjectsClient = savedObjects.getScopedClient(request);
           const scopedIndexPatterns = await indexPatterns.dataViewsServiceFactory(
@@ -546,7 +543,9 @@ export class SearchService {
       const deps = {
         searchSessionsClient,
         savedObjectsClient,
-        esClient: this.createScopedEsClient({ client: elasticsearch.client, request, opts }),
+        esClient: opts
+          ? elasticsearch.client.asScoped(request, opts)
+          : elasticsearch.client.asScoped(request),
         uiSettingsClient: new CachedUiSettingsClient(
           uiSettings.asScopedToClient(savedObjectsClient)
         ),
@@ -576,22 +575,4 @@ export class SearchService {
       };
     };
   };
-
-  private createScopedEsClient = ({
-    client,
-    request,
-    opts,
-  }: {
-    client: IClusterClient;
-    request: KibanaRequest;
-    opts?: AsScopedOptions;
-  }) => {
-    return opts?.projectRouting === 'space'
-      ? client.asScoped(request, { projectRouting: 'space' })
-      : opts?.projectRouting === 'all'
-      ? client.asScoped(request, { projectRouting: 'all' })
-      : opts?.projectRouting === 'origin-only'
-      ? client.asScoped(request, { projectRouting: 'origin-only' })
-      : client.asScoped(request);
-  };
 }
diff --git a/src/platform/plugins/shared/data/server/search/strategies/common/async_utils.ts b/src/platform/plugins/shared/data/server/search/strategies/common/async_utils.ts
index 5d1d8be8cc8fa..359a566f4555b 100644
--- a/src/platform/plugins/shared/data/server/search/strategies/common/async_utils.ts
+++ b/src/platform/plugins/shared/data/server/search/strategies/common/async_utils.ts
@@ -29,7 +29,7 @@ export function getCommonDefaultAsyncSubmitParams(
 ): Pick<
   AsyncSearchSubmitRequest,
   'keep_alive' | 'wait_for_completion_timeout' | 'keep_on_completion'
-> & { project_routing?: string } {
+> {
   const useSearchSessions =
     config.sessions.enabled && !!options.sessionId && !overrides?.disableSearchSessions;
   const keepAlive =
@@ -44,8 +44,6 @@ export function getCommonDefaultAsyncSubmitParams(
     keep_on_completion: useSearchSessions,
     // The initial keepalive is as defined in defaultExpiration if search sessions are used or 1m otherwise.
     keep_alive: keepAlive,
-    // Pass project routing for CPS if available
-    ...(options.projectRouting !== undefined && { project_routing: options.projectRouting }),
   };
 }
 
diff --git a/src/platform/plugins/shared/data/server/search/strategies/es_search/es_search_strategy.ts b/src/platform/plugins/shared/data/server/search/strategies/es_search/es_search_strategy.ts
index f68ec28ab2de1..11445e32b8bbd 100644
--- a/src/platform/plugins/shared/data/server/search/strategies/es_search/es_search_strategy.ts
+++ b/src/platform/plugins/shared/data/server/search/strategies/es_search/es_search_strategy.ts
@@ -70,7 +70,6 @@ export const esSearchStrategyProvider = (
           ...defaults,
           ...getShardTimeout(config),
           ...(terminateAfter ? { terminate_after: terminateAfter } : {}),
-          ...(options.projectRouting !== undefined && { project_routing: options.projectRouting }),
           ...requestParams,
         };
         const { body, meta } = await esClient.asCurrentUser.search(params, {
diff --git a/src/platform/plugins/shared/data/server/search/strategies/ese_search/request_utils.ts b/src/platform/plugins/shared/data/server/search/strategies/ese_search/request_utils.ts
index 50e6899b15574..0d0a9db666bca 100644
--- a/src/platform/plugins/shared/data/server/search/strategies/ese_search/request_utils.ts
+++ b/src/platform/plugins/shared/data/server/search/strategies/ese_search/request_utils.ts
@@ -49,7 +49,7 @@ export async function getDefaultAsyncSubmitParams(
     | 'ignore_unavailable'
     | 'track_total_hits'
     | 'keep_on_completion'
-  > & { project_routing?: string }
+  >
 > {
   return {
     // TODO: adjust for partial results