diff --git a/x-pack/platform/plugins/shared/agent_builder/server/services/conversation/client/client.test.ts b/x-pack/platform/plugins/shared/agent_builder/server/services/conversation/client/client.test.ts
new file mode 100644
index 0000000000000..190b319eb2b31
--- /dev/null
+++ b/x-pack/platform/plugins/shared/agent_builder/server/services/conversation/client/client.test.ts
@@ -0,0 +1,166 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+import { loggerMock } from '@kbn/logging-mocks';
+import { createClient, type ConversationClient } from './client';
+import type { Document } from './converters';
+
+const testSpace = 'default';
+
+interface MockEsClient {
+  search: jest.Mock;
+  index: jest.Mock;
+  delete: jest.Mock;
+}
+
+const mockEsClient: MockEsClient = {
+  search: jest.fn(),
+  index: jest.fn(),
+  delete: jest.fn(),
+};
+
+jest.mock('./storage', () => ({
+  createStorage: jest.fn(() => ({
+    getClient: jest.fn(() => mockEsClient),
+  })),
+}));
+
+describe('ConversationClient', () => {
+  let client: ConversationClient;
+
+  const createConversationDocument = ({
+    id = 'conversation-1',
+    agentId = 'agent-1',
+    userId = 'user-1',
+    username = 'test-user',
+  }: {
+    id?: string;
+    agentId?: string;
+    userId?: string;
+    username?: string;
+  } = {}): Document =>
+    ({
+      _id: id,
+      _seq_no: 1,
+      _primary_term: 1,
+      _source: {
+        agent_id: agentId,
+        user_id: userId,
+        user_name: username,
+        space: testSpace,
+        title: 'Conversation 1',
+        created_at: '2024-09-04T06:44:17.944Z',
+        updated_at: '2025-08-04T06:44:19.123Z',
+        conversation_rounds: [],
+      },
+    } as Document);
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+
+    client = createClient({
+      space: testSpace,
+      logger: loggerMock.create(),
+      esClient: {} as never,
+      user: {
+        id: 'user-1',
+        username: 'test-user',
+      },
+    });
+  });
+
+  describe('exists', () => {
+    it('returns true when the document exists, even when owned by another user', async () => {
+      mockEsClient.search.mockResolvedValue({
+        hits: {
+          hits: [
+            createConversationDocument({
+              userId: 'other-user-id',
+              username: 'other-user',
+            }),
+          ],
+        },
+      });
+
+      await expect(client.exists('conversation-1')).resolves.toBe(true);
+    });
+
+    it('returns false when no document exists', async () => {
+      mockEsClient.search.mockResolvedValue({
+        hits: {
+          hits: [],
+        },
+      });
+
+      await expect(client.exists('conversation-1')).resolves.toBe(false);
+    });
+
+    it('propagates search failures', async () => {
+      const error = new Error('search unavailable');
+      mockEsClient.search.mockRejectedValue(error);
+
+      await expect(client.exists('conversation-1')).rejects.toBe(error);
+    });
+  });
+
+  describe('create', () => {
+    beforeEach(() => {
+      mockEsClient.index.mockResolvedValue({ result: 'created' });
+      mockEsClient.search.mockResolvedValue({
+        hits: {
+          hits: [createConversationDocument()],
+        },
+      });
+    });
+
+    it('indexes with op_type create so existing conversations are never overwritten', async () => {
+      await client.create({
+        id: 'conversation-1',
+        title: 'Conversation 1',
+        agent_id: 'agent-1',
+        rounds: [],
+      });
+
+      expect(mockEsClient.index).toHaveBeenCalledWith(
+        expect.objectContaining({
+          id: 'conversation-1',
+          op_type: 'create',
+        })
+      );
+    });
+
+    it('throws a not found error when the id already exists', async () => {
+      const conflictError = Object.assign(new Error('version conflict'), { statusCode: 409 });
+      mockEsClient.index.mockRejectedValueOnce(conflictError);
+
+      await expect(
+        client.create({
+          id: 'conversation-1',
+          title: 'Conversation 1',
+          agent_id: 'agent-1',
+          rounds: [],
+        })
+      ).rejects.toMatchObject({
+        message: 'Conversation conversation-1 not found',
+      });
+    });
+
+    it('propagates non-conflict index failures', async () => {
+      const error = new Error('index unavailable');
+      mockEsClient.index.mockRejectedValueOnce(error);
+
+      await expect(
+        client.create({
+          id: 'conversation-1',
+          title: 'Conversation 1',
+          agent_id: 'agent-1',
+          rounds: [],
+        })
+      ).rejects.toBe(error);
+    });
+  });
+});
diff --git a/x-pack/platform/plugins/shared/agent_builder/server/services/conversation/client/client.ts b/x-pack/platform/plugins/shared/agent_builder/server/services/conversation/client/client.ts
index 2084b60d08176..60117e91e9450 100644
--- a/x-pack/platform/plugins/shared/agent_builder/server/services/conversation/client/client.ts
+++ b/x-pack/platform/plugins/shared/agent_builder/server/services/conversation/client/client.ts
@@ -111,10 +111,7 @@ class ConversationClientImpl implements ConversationClient {
 
   async exists(conversationId: string): Promise<boolean> {
     const document = await this._get(conversationId);
-    if (!document) {
-      return false;
-    }
-    return hasAccess({ conversation: document, user: this.user });
+    return document !== undefined;
   }
 
   async create(conversation: ConversationCreateRequest): Promise<Conversation> {
@@ -128,10 +125,19 @@ class ConversationClientImpl implements ConversationClient {
       space: this.space,
     });
 
-    await this.storage.getClient().index({
-      id,
-      document: attributes,
-    });
+    try {
+      await this.storage.getClient().index({
+        id,
+        document: attributes,
+        op_type: 'create',
+      });
+    } catch (error) {
+      if (error?.statusCode === 409) {
+        throw createConversationNotFoundError({ conversationId: id });
+      }
+
+      throw error;
+    }
 
     return this.get(id);
   }