feat(ai): generate page title from content (#199)

Add an AI button in the page byline that generates a note's title from the
live editor content (including unsaved edits) and applies it immediately.

Server: one-shot, non-streaming POST /ai-chat/generate-page-title mirroring
the chat generateTitle path — gated by settings.ai.generative, throttled via
AI_CHAT_THROTTLER, resolves the workspace chat model and returns { title }.
The endpoint never touches the page; the client applies the title through the
existing /pages/update route (which enforces edit permission).

Client: ai-chat-service.generatePageTitle, a useGeneratePageTitle hook that
converts the editor HTML to markdown, calls the endpoint, applies the title
via updateTitle + updatePageData, reflects it in the unfocused title editor,
and broadcasts the UpdateEvent (mirroring TitleEditor.saveTitle). A sparkles
button (GenerateTitleGroup) renders next to dictation, edit-mode + flag gated.

Tests: pure cleanGeneratedTitle helper + controller gate/delegation/error-map.
i18n: en-US + ru-RU strings.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/core/ai-chat/ai-chat.controller.ts

@@ -32,6 +32,7 @@ import { AiTranscriptionService } from './ai-transcription.service';
 import {
   ChatIdDto,
   ExportChatDto,
+  GeneratePageTitleDto,
   GetChatMessagesDto,
   RenameChatDto,
 } from './dto/ai-chat.dto';
@@ -316,6 +317,43 @@ export class AiChatController {
     return { text };
   }
 
+  /**
+   * Generate a page title from supplied note content (#199). One-shot,
+   * non-streaming. Gated by the workspace AI flag (reusing settings.ai.generative,
+   * the same flag that gates the on-page generative AI menu); returns { title }.
+   * The endpoint NEVER writes the page — the client applies the title via the
+   * existing /pages/update route (which enforces edit permission), so access
+   * checks are not duplicated here. Throttled per user via AI_CHAT_THROTTLER.
+   */
+  @HttpCode(HttpStatus.OK)
+  @UseGuards(JwtAuthGuard, UserThrottlerGuard)
+  @Throttle({ [AI_CHAT_THROTTLER]: { limit: 20, ttl: 60000 } })
+  @Post('generate-page-title')
+  async generatePageTitle(
+    @Body() dto: GeneratePageTitleDto,
+    @AuthWorkspace() workspace: Workspace,
+  ): Promise<{ title: string }> {
+    const settings = (workspace.settings ?? {}) as {
+      ai?: { generative?: boolean };
+    };
+    if (settings.ai?.generative !== true) {
+      throw new ForbiddenException('AI title generation is disabled');
+    }
+    try {
+      const title = await this.aiChatService.generatePageTitle(
+        workspace.id,
+        dto.content,
+      );
+      return { title };
+    } catch (err) {
+      // Preserve meaningful HTTP errors (e.g. AiNotConfiguredException -> 503).
+      if (err instanceof HttpException) throw err;
+      // Surface the real provider/transport reason instead of an opaque 500.
+      this.logger.error('AI title generation failed', err as Error);
+      throw new ServiceUnavailableException(describeProviderError(err));
+    }
+  }
+
   /**
    * Ensure the chat exists, belongs to this workspace, AND was created by the
    * requesting user (per-user isolation). Throws ForbiddenException otherwise.

apps/server/src/core/ai-chat/ai-chat.generate-page-title.spec.ts

@@ -0,0 +1,122 @@
+import {
+  ForbiddenException,
+  HttpException,
+  ServiceUnavailableException,
+} from '@nestjs/common';
+import { AiChatController } from './ai-chat.controller';
+import { cleanGeneratedTitle } from './ai-chat.service';
+import type { Workspace } from '@docmost/db/types/entity.types';
+
+/**
+ * Pure post-processing of a model-generated title (#199): trims, strips a single
+ * pair of surrounding quotes, drops a trailing period, and hard-caps the length.
+ */
+describe('cleanGeneratedTitle', () => {
+  it('trims surrounding whitespace', () => {
+    expect(cleanGeneratedTitle('  Hello world  ')).toBe('Hello world');
+  });
+
+  it('strips a single pair of surrounding double quotes', () => {
+    expect(cleanGeneratedTitle('"My note"')).toBe('My note');
+  });
+
+  it('strips surrounding single quotes', () => {
+    expect(cleanGeneratedTitle("'My note'")).toBe('My note');
+  });
+
+  it('drops a trailing period', () => {
+    expect(cleanGeneratedTitle('A complete sentence.')).toBe(
+      'A complete sentence',
+    );
+  });
+
+  it('caps the result at 255 characters (the page-title column bound)', () => {
+    expect(cleanGeneratedTitle('x'.repeat(400))).toHaveLength(255);
+  });
+
+  it('returns an empty string for blank/garbage input', () => {
+    expect(cleanGeneratedTitle('   ')).toBe('');
+    expect(cleanGeneratedTitle('""')).toBe('');
+  });
+});
+
+/**
+ * Wiring spec for the #199 `POST /ai-chat/generate-page-title` endpoint. It must:
+ * gate on settings.ai.generative (403 when off), delegate to the service when on,
+ * rethrow HttpExceptions verbatim (e.g. AiNotConfiguredException -> 503), and map
+ * any other provider/transport fault to a 503. Exercised by instantiating the
+ * controller with hand-rolled mocks — no Nest graph, no DB.
+ */
+describe('AiChatController.generatePageTitle', () => {
+  const enabledWorkspace = {
+    id: 'ws1',
+    settings: { ai: { generative: true } },
+  } as unknown as Workspace;
+
+  function makeController(generate: jest.Mock) {
+    const aiChatService = { generatePageTitle: generate };
+    const controller = new AiChatController(
+      aiChatService as never,
+      {} as never,
+      {} as never,
+      {} as never,
+    );
+    return { controller, aiChatService };
+  }
+
+  it('forbids when the generative AI flag is off', async () => {
+    const generate = jest.fn();
+    const { controller } = makeController(generate);
+    const disabled = { id: 'ws1', settings: {} } as unknown as Workspace;
+    await expect(
+      controller.generatePageTitle({ content: 'body' }, disabled),
+    ).rejects.toBeInstanceOf(ForbiddenException);
+    expect(generate).not.toHaveBeenCalled();
+  });
+
+  it('forbids when settings.ai.generative is anything but exactly true', async () => {
+    const generate = jest.fn();
+    const { controller } = makeController(generate);
+    const ws = {
+      id: 'ws1',
+      settings: { ai: { generative: 'yes' } },
+    } as unknown as Workspace;
+    await expect(
+      controller.generatePageTitle({ content: 'body' }, ws),
+    ).rejects.toBeInstanceOf(ForbiddenException);
+  });
+
+  it('returns { title } from the service when enabled', async () => {
+    const generate = jest.fn().mockResolvedValue('Generated Title');
+    const { controller } = makeController(generate);
+    const res = await controller.generatePageTitle(
+      { content: 'some markdown body' },
+      enabledWorkspace,
+    );
+    expect(generate).toHaveBeenCalledWith('ws1', 'some markdown body');
+    expect(res).toEqual({ title: 'Generated Title' });
+  });
+
+  it('rethrows an HttpException from the service verbatim (e.g. 503 not configured)', async () => {
+    const notConfigured = new ServiceUnavailableException('AI not configured');
+    const generate = jest.fn().mockRejectedValue(notConfigured);
+    const { controller } = makeController(generate);
+    await expect(
+      controller.generatePageTitle({ content: 'body' }, enabledWorkspace),
+    ).rejects.toBe(notConfigured);
+  });
+
+  it('maps a non-HTTP provider error to a 503', async () => {
+    const generate = jest.fn().mockRejectedValue(new Error('socket hang up'));
+    const { controller } = makeController(generate);
+    // Silence the expected error log.
+    jest
+      .spyOn((controller as unknown as { logger: { error: () => void } }).logger, 'error')
+      .mockImplementation(() => undefined);
+    const err = await controller
+      .generatePageTitle({ content: 'body' }, enabledWorkspace)
+      .catch((e) => e);
+    expect(err).toBeInstanceOf(ServiceUnavailableException);
+    expect(err).toBeInstanceOf(HttpException);
+  });
+});

apps/server/src/core/ai-chat/ai-chat.service.ts

@@ -75,6 +75,18 @@ export function prepareAgentStep(
 
 export { MAX_AGENT_STEPS, FINAL_STEP_INSTRUCTION };
 
+// Pure, unit-testable post-processing for a model-generated title (#199): trim
+// whitespace, strip a single pair of surrounding quotes the model often adds,
+// drop a trailing period, and hard-cap the length to the page-title column.
+export function cleanGeneratedTitle(text: string): string {
+  return text
+    .trim()
+    .replace(/^["']|["']$/g, '')
+    .replace(/\.+$/, '')
+    .trim()
+    .slice(0, 255);
+}
+
 /**
  * Payload accepted from the client `useChat` POST body. We do NOT bind a strict
  * DTO (the global ValidationPipe whitelist would strip the useChat-specific
@@ -793,6 +805,27 @@ export class AiChatService implements OnModuleInit {
     }
   }
 
+  /**
+   * One-shot page-title generation from a note's content (#199). No tools, no
+   * streaming — mirrors generateTitle() but for an arbitrary note body supplied
+   * by the client, and RETURNS the title instead of writing it (the client
+   * applies it via the existing /pages/update route, which enforces edit
+   * permission). The content is truncated to keep the prompt cheap and within
+   * context limits. Throws AiNotConfiguredException (503) if AI is unconfigured.
+   */
+  async generatePageTitle(workspaceId: string, content: string): Promise<string> {
+    const model = await this.ai.getChatModel(workspaceId);
+    const { text } = await generateText({
+      model,
+      system:
+        'You generate a single concise, descriptive title for a note based on ' +
+        'its content. Reply with the title only — at most 8 words, no quotes, ' +
+        'no trailing punctuation, written in the same language as the note.',
+      prompt: content.slice(0, 8000),
+    });
+    return cleanGeneratedTitle(text);
+  }
+
   /**
    * Cheap, non-blocking title generation from the first user message. Uses
    * generateText (async) and writes the result back onto the chat row. Any

apps/server/src/core/ai-chat/dto/ai-chat.dto.ts

@@ -17,6 +17,16 @@ export class RenameChatDto {
   title: string;
 }
 
+/** One-shot page-title generation from note content (#199). */
+export class GeneratePageTitleDto {
+  // Note body as markdown/plain text. Capped to bound the prompt cost and
+  // reject abusive payloads; the service truncates again before the model call.
+  @IsString()
+  @MinLength(1)
+  @MaxLength(20000)
+  content: string;
+}
+
 /** Optional chat id for listing messages of a specific chat. */
 export class GetChatMessagesDto {
   @IsString()