feat: implement user message creation via Data API in StreamingService

- Added a new method `createUserMessage` in StreamingService to handle user message creation through the Data API, generating server-side message IDs while preserving client-generated block IDs.
- Updated `sendMessage` thunk to utilize the new method for normal topics, ensuring proper message handling based on the active agent session.
- Refactored block conversion logic to streamline the process of preparing message data for the API.

src/renderer/src/services/messageStreaming/StreamingService.ts

@@ -24,7 +24,7 @@ import { loggerService } from '@logger'
 import type { Message, MessageBlock } from '@renderer/types/newMessage'
 import { AssistantMessageStatus, MessageBlockStatus } from '@renderer/types/newMessage'
 import { isAgentSessionTopicId } from '@renderer/utils/agentSession'
-import type { UpdateMessageDto } from '@shared/data/api/schemas/messages'
+import type { CreateMessageDto, UpdateMessageDto } from '@shared/data/api/schemas/messages'
 import type { MessageDataBlock, MessageStats } from '@shared/data/types/message'
 
 import { dbService } from '../db'
@@ -465,8 +465,64 @@ class StreamingService {
     return nextGroupId
   }
 
+  // ============ User Message Creation ============
+
+  /**
+   * Create a user message via Data API
+   *
+   * The message ID is generated by the server, not locally.
+   * Block IDs remain client-generated for Redux store use.
+   *
+   * TRADEOFF: Not passing parentId - Data API will use topic.activeNodeId as parent.
+   * In multi-window/multi-branch scenarios, this may cause incorrect associations
+   * if activeNodeId was changed by another window.
+   * TODO: In the future, parentId should come from the full message tree
+   * maintained in the topic UI, not from topic.activeNodeId.
+   *
+   * @param topicId - Topic ID
+   * @param message - Renderer format message (message.id will be ignored, server generates ID)
+   * @param blocks - Renderer format blocks (block IDs preserved for Redux)
+   * @returns Message with server-generated ID and original block IDs
+   */
+  async createUserMessage(topicId: string, message: Message, blocks: MessageBlock[]): Promise<Message> {
+    // Convert blocks to MessageDataBlock format (remove id, status, messageId)
+    const dataBlocks = this.convertBlocksToDataFormat(blocks)
+
+    // Build CreateMessageDto (parentId omitted - API uses topic.activeNodeId)
+    const createDto: CreateMessageDto = {
+      role: 'user',
+      data: { blocks: dataBlocks },
+      status: 'success',
+      traceId: message.traceId ?? undefined
+    }
+
+    // POST to Data API - server generates message ID
+    const sharedMessage = await dataApiService.post(`/topics/${topicId}/messages`, { body: createDto })
+
+    logger.debug('Created user message via Data API', { topicId, messageId: sharedMessage.id })
+
+    // Return message with server ID, preserving other fields from original message
+    return {
+      ...message,
+      id: sharedMessage.id, // Use server-generated ID
+      blocks: blocks.map((b) => b.id) // Preserve client-generated block IDs
+    }
+  }
+
   // ============ Internal Methods ============
 
+  /**
+   * Convert renderer MessageBlock[] to shared MessageDataBlock[]
+   * Removes renderer-specific fields: id, status, messageId
+   */
+  private convertBlocksToDataFormat(blocks: MessageBlock[]): MessageDataBlock[] {
+    return blocks.map((block) => {
+      // oxlint-disable-next-line @typescript-eslint/no-unused-vars
+      const { id, status, messageId, ...blockData } = block as MessageBlock & { messageId?: string }
+      return blockData as unknown as MessageDataBlock
+    })
+  }
+
   /**
    * Convert session data to database update payload
    *
@@ -530,16 +586,8 @@ class StreamingService {
     // TRADEOFF: Using 'as unknown as' because renderer's MessageBlockType and shared's BlockType
     // are structurally identical but TypeScript treats them as incompatible enums.
     const dataBlocks: MessageDataBlock[] = blocks.map((block) => {
-      // Extract only the fields that belong to MessageDataBlock
-      const {
-        id: _id,
-        status: _blockStatus,
-        messageId: _messageId,
-        ...blockData
-      } = block as MessageBlock & {
-        messageId?: string
-      }
-
+      // oxlint-disable-next-line @typescript-eslint/no-unused-vars
+      const { id, status, messageId, ...blockData } = block as MessageBlock & { messageId?: string }
       return blockData as unknown as MessageDataBlock
     })
 

src/renderer/src/store/thunk/messageThunk.ts

@@ -978,8 +978,18 @@ export const sendMessage =
         userMessage.agentSessionId = activeAgentSession.agentSessionId
       }
 
-      await saveMessageAndBlocksToDB(topicId, userMessage, userMessageBlocks)
-      dispatch(newMessagesActions.addMessage({ topicId, message: userMessage }))
+      let finalUserMessage: Message
+
+      if (activeAgentSession) {
+        // Agent session: keep existing Dexie logic
+        await saveMessageAndBlocksToDB(topicId, userMessage, userMessageBlocks)
+        finalUserMessage = userMessage
+      } else {
+        // Normal topic: use Data API, get server-generated message ID
+        finalUserMessage = await streamingService.createUserMessage(topicId, userMessage, userMessageBlocks)
+      }
+
+      dispatch(newMessagesActions.addMessage({ topicId, message: finalUserMessage }))
       if (userMessageBlocks.length > 0) {
         dispatch(upsertManyBlocks(userMessageBlocks))
       }
@@ -989,7 +999,7 @@ export const sendMessage =
 
       if (activeAgentSession) {
         const assistantMessage = createAssistantMessage(assistant.id, topicId, {
-          askId: userMessage.id,
+          askId: finalUserMessage.id,
           model: assistant.model,
           traceId: userMessage.traceId
         })
@@ -1005,25 +1015,25 @@ export const sendMessage =
             assistant,
             assistantMessage,
             agentSession: activeAgentSession,
-            userMessageId: userMessage.id
+            userMessageId: finalUserMessage.id
           })
         })
       } else {
-        const mentionedModels = userMessage.mentions
+        const mentionedModels = finalUserMessage.mentions
 
         if (mentionedModels && mentionedModels.length > 0) {
-          await dispatchMultiModelResponses(dispatch, getState, topicId, userMessage, assistant, mentionedModels)
+          await dispatchMultiModelResponses(dispatch, getState, topicId, finalUserMessage, assistant, mentionedModels)
         } else {
           // Create message via Data API for normal topics
           const createDto: CreateMessageDto = {
-            parentId: userMessage.id,
+            parentId: finalUserMessage.id,
             role: 'assistant',
             data: { blocks: [] },
             status: 'pending',
             siblingsGroupId: 0,
             assistantId: assistant.id,
             modelId: assistant.model?.id,
-            traceId: userMessage.traceId ?? undefined
+            traceId: finalUserMessage.traceId ?? undefined
           }
 
           const sharedMessage = await dataApiService.post(`/topics/${topicId}/messages`, { body: createDto })