diff --git a/packages/backend/server/src/base/storage/providers/s3.ts b/packages/backend/server/src/base/storage/providers/s3.ts
index c6a693c883..4531fe47d2 100644
--- a/packages/backend/server/src/base/storage/providers/s3.ts
+++ b/packages/backend/server/src/base/storage/providers/s3.ts
@@ -434,7 +434,11 @@ export class S3StorageProvider implements StorageProvider {
 
       this.logger.verbose(`Deleted object \`${key}\``);
     } catch (e) {
-      this.logger.error(`Failed to delete object \`${key}\``);
+      this.logger.error(`Failed to delete object \`${key}\``, {
+        bucket: this.bucket,
+        key,
+        cause: e,
+      });
       throw e;
     }
   }
diff --git a/packages/backend/server/src/models/copilot-session.ts b/packages/backend/server/src/models/copilot-session.ts
index 76e895bd33..4ba6a21403 100644
--- a/packages/backend/server/src/models/copilot-session.ts
+++ b/packages/backend/server/src/models/copilot-session.ts
@@ -113,6 +113,99 @@ export type CleanupSessionOptions = Pick<
 
 @Injectable()
 export class CopilotSessionModel extends BaseModel {
+  private sanitizeString<T extends string | null | undefined>(value: T): T {
+    if (typeof value !== 'string') {
+      return value;
+    }
+    return value.replace(/\u0000/g, '') as T;
+  }
+
+  private sanitizeJsonValue<T>(value: T): T {
+    if (typeof value === 'string') {
+      return this.sanitizeString(value) as T;
+    }
+    if (Array.isArray(value)) {
+      return value.map(v => this.sanitizeJsonValue(v)) as T;
+    }
+    if (
+      value &&
+      typeof value === 'object' &&
+      Object.getPrototypeOf(value) === Object.prototype
+    ) {
+      return Object.fromEntries(
+        Object.entries(value).map(([k, v]) => [k, this.sanitizeJsonValue(v)])
+      ) as T;
+    }
+    return value;
+  }
+
+  private sanitizeStreamObject(stream: ChatStreamObject): ChatStreamObject {
+    switch (stream.type) {
+      case 'text-delta':
+      case 'reasoning':
+        return {
+          ...stream,
+          textDelta: this.sanitizeString(stream.textDelta),
+        };
+      case 'tool-call':
+        return {
+          ...stream,
+          toolCallId: this.sanitizeString(stream.toolCallId) ?? '',
+          toolName: this.sanitizeString(stream.toolName) ?? '',
+          args: this.sanitizeJsonValue(stream.args),
+        };
+      case 'tool-result':
+        return {
+          ...stream,
+          toolCallId: this.sanitizeString(stream.toolCallId) ?? '',
+          toolName: this.sanitizeString(stream.toolName) ?? '',
+          args: this.sanitizeJsonValue(stream.args),
+          result: this.sanitizeJsonValue(stream.result),
+        };
+    }
+  }
+
+  private sanitizeAttachments(
+    attachments?: ChatAttachment[] | null
+  ): ChatAttachment[] | undefined {
+    if (!attachments?.length) {
+      return undefined;
+    }
+
+    return attachments
+      .map(attachment =>
+        typeof attachment === 'string'
+          ? (this.sanitizeString(attachment) ?? '')
+          : {
+              attachment:
+                this.sanitizeString(attachment.attachment) ??
+                attachment.attachment,
+              mimeType:
+                this.sanitizeString(attachment.mimeType) ?? attachment.mimeType,
+            }
+      )
+      .filter(attachment => {
+        if (typeof attachment === 'string') {
+          return !!attachment;
+        }
+        return !!attachment.attachment && !!attachment.mimeType;
+      });
+  }
+
+  private sanitizeMessage(message: ChatMessage): ChatMessage {
+    return {
+      ...message,
+      content: this.sanitizeString(message.content) ?? '',
+      attachments: this.sanitizeAttachments(message.attachments),
+      params: this.sanitizeJsonValue(
+        omit(message.params, ['docs']) || undefined
+      ),
+      streamObjects: message.streamObjects?.map(o =>
+        this.sanitizeStreamObject(o)
+      ),
+    };
+  }
+
   getSessionType(session: Pick<ChatSession, 'docId' | 'pinned'>): SessionType {
     if (session.pinned) return SessionType.Pinned;
     if (!session.docId) return SessionType.Workspace;
@@ -401,6 +494,7 @@ export class CopilotSessionModel extends BaseModel {
     internalCall = false
   ): Promise<string> {
     const { userId, sessionId, docId, promptName, pinned, title } = options;
+    const sanitizedTitle = this.sanitizeString(title);
     const session = await this.getExists(
       sessionId,
       {
@@ -448,7 +542,7 @@ export class CopilotSessionModel extends BaseModel {
 
     await this.db.aiSession.update({
       where: { id: sessionId },
-      data: { docId, promptName, pinned, title },
+      data: { docId, promptName, pinned, title: sanitizedTitle },
     });
 
     return sessionId;
@@ -509,19 +603,23 @@ export class CopilotSessionModel extends BaseModel {
     }
 
     if (messages.length) {
-      const tokenCost = this.calculateTokenSize(messages, state.prompt.model);
+      const sanitizedMessages = messages.map(m => this.sanitizeMessage(m));
+      const tokenCost = this.calculateTokenSize(
+        sanitizedMessages,
+        state.prompt.model
+      );
       await this.db.aiSessionMessage.createMany({
-        data: messages.map(m => ({
+        data: sanitizedMessages.map(m => ({
           ...m,
           attachments: m.attachments || undefined,
-          params: omit(m.params, ['docs']) || undefined,
+          params: m.params || undefined,
           streamObjects: m.streamObjects || undefined,
           sessionId,
         })),
       });
 
       // only count message generated by user
-      const userMessages = messages.filter(m => m.role === 'user');
+      const userMessages = sanitizedMessages.filter(m => m.role === 'user');
       await this.db.aiSession.update({
         where: { id: sessionId },
         data: {
diff --git a/packages/backend/server/src/plugins/copilot/embedding/job.ts b/packages/backend/server/src/plugins/copilot/embedding/job.ts
index b4880b22b6..5769939314 100644
--- a/packages/backend/server/src/plugins/copilot/embedding/job.ts
+++ b/packages/backend/server/src/plugins/copilot/embedding/job.ts
@@ -533,7 +533,15 @@ export class CopilotEmbeddingJob {
       workspaceId
     );
     if (!snapshot) {
-      this.logger.warn(`workspace snapshot ${workspaceId} not found`);
+      // maybe local workspace or empty workspace
+      this.logger.verbose(`workspace root snapshot ${workspaceId} not found`);
+      // mark last check time to avoid repeated checking
+      await this.models.workspace.update(
+        workspaceId,
+        { lastCheckEmbeddings: new Date() },
+        false
+      );
+
       return;
     } else if (
       // always check if never cleared
diff --git a/packages/backend/server/src/plugins/copilot/session.ts b/packages/backend/server/src/plugins/copilot/session.ts
index a2021516ee..ab8afefa0a 100644
--- a/packages/backend/server/src/plugins/copilot/session.ts
+++ b/packages/backend/server/src/plugins/copilot/session.ts
@@ -320,6 +320,20 @@ export class ChatSessionService {
     return messages.data;
   }
 
+  private stripNullBytes(value?: string | null): string {
+    if (!value) return '';
+    return value.replace(/\u0000/g, '');
+  }
+
+  private isNullByteError(error: unknown): boolean {
+    return (
+      error instanceof Error &&
+      (error.message.includes('\\u0000') ||
+        error.message.includes('unsupported Unicode escape sequence') ||
+        error.message.includes('22P05'))
+    );
+  }
+
   private async getHistory(session: Session): Promise<SessionHistory> {
     const prompt = await this.prompt.get(session.promptName);
     if (!prompt) throw new CopilotPromptNotFound({ name: session.promptName });
@@ -655,7 +669,13 @@ export class ChatSessionService {
         );
         return;
       }
-      const { userId, title, messages } = session;
+      const { userId, title } = session;
+      const messages =
+        session.messages?.map(m => ({
+          ...m,
+          content: this.stripNullBytes(m.content),
+        })) ?? [];
+
       if (
         title ||
         !messages.length ||
@@ -665,18 +685,41 @@ export class ChatSessionService {
         return;
       }
 
-      {
-        const title = await this.chatWithPrompt('Summary as title', {
-          content: session.messages
-            .map(m => `[${m.role}]: ${m.content}`)
-            .join('\n'),
-        });
-        await this.models.copilotSession.update({ userId, sessionId, title });
+      const promptContent = messages
+        .map(m => `[${m.role}]: ${m.content}`)
+        .join('\n');
+      const generatedTitle = this.stripNullBytes(
+        await this.chatWithPrompt('Summary as title', {
+          content: promptContent,
+        })
+      ).trim();
+
+      if (!generatedTitle) {
+        this.logger.warn(
+          `Generated empty title for session ${sessionId}, skip updating`
+        );
+        return;
       }
+      await this.models.copilotSession.update({
+        userId,
+        sessionId,
+        title: generatedTitle,
+      });
     } catch (error) {
-      console.error(
+      const context = {
+        sessionId,
+        cause: error instanceof Error ? error.cause : error,
+      };
+      if (this.isNullByteError(error)) {
+        this.logger.warn(
+          `Skip title generation for session ${sessionId} due to invalid null bytes in stored data`,
+          context
+        );
+        return;
+      }
+      this.logger.error(
         `Failed to generate title for session ${sessionId}:`,
-        error
+        context
       );
       throw error;
     }