feat(gateway): PreferencesService + /preferences REST + /system Valkey override (P8-011)

- PreferencesService: platform defaults, user overrides, IMMUTABLE_KEYS enforcement
- PreferencesController: GET /api/preferences, POST /api/preferences, DELETE /api/preferences/:key
- PreferencesModule: global module exporting PreferencesService and SystemOverrideService
- SystemOverrideService: Valkey-backed session-scoped system prompt override with 5-min TTL + renew
- CommandRegistryService: register /system command (socket execution)
- CommandExecutorService: handle /system command via SystemOverrideService
- AgentService: inject system override before each prompt turn, renew TTL; store userId in session
- ChatGateway: pass userId when creating agent sessions
- PreferencesService unit tests: 11 tests covering defaults, overrides, enforcement wins, immutable key errors

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/gateway/src/agent/agent.service.ts

@@ -1,4 +1,4 @@
-import { Inject, Injectable, Logger, type OnModuleDestroy } from '@nestjs/common';
+import { Inject, Injectable, Logger, Optional, type OnModuleDestroy } from '@nestjs/common';
 import {
   createAgentSession,
   DefaultResourceLoader,
@@ -24,6 +24,8 @@ import { createGitTools } from './tools/git-tools.js';
 import { createShellTools } from './tools/shell-tools.js';
 import { createWebTools } from './tools/web-tools.js';
 import type { SessionInfoDto } from './session.dto.js';
+import { SystemOverrideService } from '../preferences/system-override.service.js';
+import { PreferencesService } from '../preferences/preferences.service.js';
 
 export interface AgentSessionOptions {
   provider?: string;
@@ -55,6 +57,8 @@ export interface AgentSessionOptions {
    * take precedence over config values.
    */
   agentConfigId?: string;
+  /** ID of the user who owns this session. Used for preferences and system override lookups. */
+  userId?: string;
 }
 
 export interface AgentSession {
@@ -73,6 +77,8 @@ export interface AgentSession {
   sandboxDir: string;
   /** Tool names available in this session, or null when all tools are available. */
   allowedTools: string[] | null;
+  /** User ID that owns this session, used for preference lookups. */
+  userId?: string;
 }
 
 @Injectable()
@@ -89,6 +95,12 @@ export class AgentService implements OnModuleDestroy {
     @Inject(CoordService) private readonly coordService: CoordService,
     @Inject(McpClientService) private readonly mcpClientService: McpClientService,
     @Inject(SkillLoaderService) private readonly skillLoaderService: SkillLoaderService,
+    @Optional()
+    @Inject(SystemOverrideService)
+    private readonly systemOverride: SystemOverrideService | null,
+    @Optional()
+    @Inject(PreferencesService)
+    private readonly preferencesService: PreferencesService | null,
   ) {}
 
   /**
@@ -285,6 +297,7 @@ export class AgentService implements OnModuleDestroy {
       skillPromptAdditions: promptAdditions,
       sandboxDir,
       allowedTools,
+      userId: mergedOptions?.userId,
     };
 
     this.sessions.set(sessionId, session);
@@ -368,8 +381,20 @@ export class AgentService implements OnModuleDestroy {
       throw new Error(`No agent session found: ${sessionId}`);
     }
     session.promptCount += 1;
+
+    // Prepend session-scoped system override if present (renew TTL on each turn)
+    let effectiveMessage = message;
+    if (this.systemOverride) {
+      const override = await this.systemOverride.get(sessionId);
+      if (override) {
+        effectiveMessage = `[System Override]\n${override}\n\n${message}`;
+        await this.systemOverride.renew(sessionId);
+        this.logger.debug(`Applied system override for session ${sessionId}`);
+      }
+    }
+
     try {
-      await session.piSession.prompt(message);
+      await session.piSession.prompt(effectiveMessage);
     } catch (err) {
       this.logger.error(
         `Prompt failed for session=${sessionId}, messageLength=${message.length}`,

apps/gateway/src/app.module.ts

@@ -18,6 +18,7 @@ import { PluginModule } from './plugin/plugin.module.js';
 import { McpModule } from './mcp/mcp.module.js';
 import { AdminModule } from './admin/admin.module.js';
 import { CommandsModule } from './commands/commands.module.js';
+import { PreferencesModule } from './preferences/preferences.module.js';
 import { ThrottlerGuard, ThrottlerModule } from '@nestjs/throttler';
 
 @Module({
@@ -39,6 +40,7 @@ import { ThrottlerGuard, ThrottlerModule } from '@nestjs/throttler';
     PluginModule,
     McpModule,
     AdminModule,
+    PreferencesModule,
     CommandsModule,
   ],
   controllers: [HealthController],

apps/gateway/src/chat/chat.gateway.ts

@@ -87,10 +87,12 @@ export class ChatGateway implements OnGatewayInit, OnGatewayConnection, OnGatewa
     try {
       let agentSession = this.agentService.getSession(conversationId);
       if (!agentSession) {
+        const userId = (client.data.user as { id: string } | undefined)?.id;
         agentSession = await this.agentService.createSession(conversationId, {
           provider: data.provider,
           modelId: data.modelId,
           agentConfigId: data.agentId,
+          userId,
         });
       }
     } catch (err) {

apps/gateway/src/commands/command-executor.service.ts

@@ -2,6 +2,7 @@ import { Inject, Injectable, Logger } from '@nestjs/common';
 import type { SlashCommandPayload, SlashCommandResultPayload } from '@mosaic/types';
 import { AgentService } from '../agent/agent.service.js';
 import { CommandRegistryService } from './command-registry.service.js';
+import { SystemOverrideService } from '../preferences/system-override.service.js';
 
 @Injectable()
 export class CommandExecutorService {
@@ -10,6 +11,7 @@ export class CommandExecutorService {
   constructor(
     @Inject(CommandRegistryService) private readonly registry: CommandRegistryService,
     @Inject(AgentService) private readonly agentService: AgentService,
+    @Inject(SystemOverrideService) private readonly systemOverride: SystemOverrideService,
   ) {}
 
   async execute(payload: SlashCommandPayload, _userId: string): Promise<SlashCommandResultPayload> {
@@ -31,6 +33,8 @@ export class CommandExecutorService {
           return await this.handleModel(args ?? null, conversationId);
         case 'thinking':
           return await this.handleThinking(args ?? null, conversationId);
+        case 'system':
+          return await this.handleSystem(args ?? null, conversationId);
         case 'new':
           return {
             command,
@@ -124,4 +128,28 @@ export class CommandExecutorService {
       message: `Thinking level set to "${level}".`,
     };
   }
+
+  private async handleSystem(
+    args: string | null,
+    conversationId: string,
+  ): Promise<SlashCommandResultPayload> {
+    if (!args || args.trim().length === 0) {
+      // Clear the override when called with no args
+      await this.systemOverride.clear(conversationId);
+      return {
+        command: 'system',
+        conversationId,
+        success: true,
+        message: 'Session system prompt override cleared.',
+      };
+    }
+
+    await this.systemOverride.set(conversationId, args.trim());
+    return {
+      command: 'system',
+      conversationId,
+      success: true,
+      message: `Session system prompt override set (expires in 5 minutes of inactivity).`,
+    };
+  }
 }

apps/gateway/src/commands/command-registry.service.ts

@@ -156,6 +156,22 @@ export class CommandRegistryService implements OnModuleInit {
         execution: 'rest',
         available: true,
       },
+      {
+        name: 'system',
+        description: 'Set session-scoped system prompt override',
+        aliases: [],
+        args: [
+          {
+            name: 'override',
+            type: 'string',
+            optional: false,
+            description: 'System prompt text to inject for this session',
+          },
+        ],
+        scope: 'core',
+        execution: 'socket',
+        available: true,
+      },
       {
         name: 'status',
         description: 'Show session and connection status',

apps/gateway/src/preferences/preferences.controller.ts

@@ -0,0 +1,44 @@
+import {
+  Body,
+  Controller,
+  Delete,
+  Get,
+  HttpCode,
+  HttpStatus,
+  Inject,
+  Param,
+  Post,
+  UseGuards,
+} from '@nestjs/common';
+import { PreferencesService } from './preferences.service.js';
+import { AuthGuard } from '../auth/auth.guard.js';
+import { CurrentUser } from '../auth/current-user.decorator.js';
+
+@Controller('api/preferences')
+@UseGuards(AuthGuard)
+export class PreferencesController {
+  constructor(@Inject(PreferencesService) private readonly preferences: PreferencesService) {}
+
+  @Get()
+  async show(@CurrentUser() user: { id: string }): Promise<Record<string, unknown>> {
+    return this.preferences.getEffective(user.id);
+  }
+
+  @Post()
+  @HttpCode(HttpStatus.OK)
+  async set(
+    @CurrentUser() user: { id: string },
+    @Body() body: { key: string; value: unknown },
+  ): Promise<{ success: boolean; message: string }> {
+    return this.preferences.set(user.id, body.key, body.value);
+  }
+
+  @Delete(':key')
+  @HttpCode(HttpStatus.OK)
+  async reset(
+    @CurrentUser() user: { id: string },
+    @Param('key') key: string,
+  ): Promise<{ success: boolean; message: string }> {
+    return this.preferences.reset(user.id, key);
+  }
+}

apps/gateway/src/preferences/preferences.module.ts

@@ -0,0 +1,12 @@
+import { Global, Module } from '@nestjs/common';
+import { PreferencesService } from './preferences.service.js';
+import { PreferencesController } from './preferences.controller.js';
+import { SystemOverrideService } from './system-override.service.js';
+
+@Global()
+@Module({
+  controllers: [PreferencesController],
+  providers: [PreferencesService, SystemOverrideService],
+  exports: [PreferencesService, SystemOverrideService],
+})
+export class PreferencesModule {}

apps/gateway/src/preferences/preferences.service.spec.ts

@@ -0,0 +1,167 @@
+import { describe, it, expect, vi } from 'vitest';
+import { PreferencesService, PLATFORM_DEFAULTS, IMMUTABLE_KEYS } from './preferences.service.js';
+import type { Db } from '@mosaic/db';
+
+/**
+ * Build a mock Drizzle DB where the select chain supports:
+ *   db.select().from().where()            → resolves to `listRows`
+ *   db.select().from().where().limit(n)   → resolves to `singleRow`
+ */
+function makeMockDb(
+  listRows: Array<{ key: string; value: unknown }> = [],
+  singleRow: Array<{ id: string }> = [],
+): Db {
+  const chainWithLimit = {
+    limit: vi.fn().mockResolvedValue(singleRow),
+    then: (resolve: (v: typeof listRows) => unknown) => Promise.resolve(listRows).then(resolve),
+  };
+  const selectFrom = {
+    from: vi.fn().mockReturnThis(),
+    where: vi.fn().mockReturnValue(chainWithLimit),
+  };
+  const updateResult = {
+    set: vi.fn().mockReturnThis(),
+    where: vi.fn().mockResolvedValue([]),
+  };
+  const deleteResult = {
+    where: vi.fn().mockResolvedValue([]),
+  };
+  const insertResult = {
+    values: vi.fn().mockResolvedValue([]),
+  };
+
+  return {
+    select: vi.fn().mockReturnValue(selectFrom),
+    update: vi.fn().mockReturnValue(updateResult),
+    delete: vi.fn().mockReturnValue(deleteResult),
+    insert: vi.fn().mockReturnValue(insertResult),
+  } as unknown as Db;
+}
+
+describe('PreferencesService', () => {
+  describe('getEffective', () => {
+    it('returns platform defaults when user has no overrides', async () => {
+      const db = makeMockDb([]);
+      const service = new PreferencesService(db);
+      const result = await service.getEffective('user-1');
+
+      expect(result['agent.thinkingLevel']).toBe('auto');
+      expect(result['agent.streamingEnabled']).toBe(true);
+      expect(result['session.autoCompactEnabled']).toBe(true);
+      expect(result['session.autoCompactThreshold']).toBe(0.8);
+    });
+
+    it('applies user overrides for mutable keys', async () => {
+      const db = makeMockDb([
+        { key: 'agent.thinkingLevel', value: 'high' },
+        { key: 'response.language', value: 'es' },
+      ]);
+
+      const service = new PreferencesService(db);
+      const result = await service.getEffective('user-1');
+
+      expect(result['agent.thinkingLevel']).toBe('high');
+      expect(result['response.language']).toBe('es');
+    });
+
+    it('ignores user overrides for immutable keys — enforcement always wins', async () => {
+      const db = makeMockDb([
+        { key: 'limits.maxThinkingLevel', value: 'high' },
+        { key: 'limits.rateLimit', value: 9999 },
+      ]);
+
+      const service = new PreferencesService(db);
+      const result = await service.getEffective('user-1');
+
+      // Should still be null (platform default), not the user-supplied values
+      expect(result['limits.maxThinkingLevel']).toBeNull();
+      expect(result['limits.rateLimit']).toBeNull();
+    });
+  });
+
+  describe('set', () => {
+    it('returns error when attempting to override an immutable key', async () => {
+      const db = makeMockDb();
+      const service = new PreferencesService(db);
+
+      const result = await service.set('user-1', 'limits.maxThinkingLevel', 'high');
+      expect(result.success).toBe(false);
+      expect(result.message).toContain('platform enforcement');
+    });
+
+    it('returns error when attempting to override limits.rateLimit', async () => {
+      const db = makeMockDb();
+      const service = new PreferencesService(db);
+
+      const result = await service.set('user-1', 'limits.rateLimit', 100);
+      expect(result.success).toBe(false);
+      expect(result.message).toContain('platform enforcement');
+    });
+
+    it('upserts a mutable preference and returns success — insert path', async () => {
+      // singleRow=[] → no existing row → insert path
+      const db = makeMockDb([], []);
+      const service = new PreferencesService(db);
+      const result = await service.set('user-1', 'agent.thinkingLevel', 'high');
+      expect(result.success).toBe(true);
+      expect(result.message).toContain('"agent.thinkingLevel"');
+    });
+
+    it('upserts a mutable preference and returns success — update path', async () => {
+      // singleRow has an id → existing row → update path
+      const db = makeMockDb([], [{ id: 'existing-id' }]);
+      const service = new PreferencesService(db);
+      const result = await service.set('user-1', 'agent.thinkingLevel', 'low');
+      expect(result.success).toBe(true);
+      expect(result.message).toContain('"agent.thinkingLevel"');
+    });
+  });
+
+  describe('reset', () => {
+    it('returns error when attempting to reset an immutable key', async () => {
+      const db = makeMockDb();
+      const service = new PreferencesService(db);
+
+      const result = await service.reset('user-1', 'limits.rateLimit');
+      expect(result.success).toBe(false);
+      expect(result.message).toContain('platform enforcement');
+    });
+
+    it('deletes user override and returns default value in message', async () => {
+      const db = makeMockDb();
+      const service = new PreferencesService(db);
+      const result = await service.reset('user-1', 'agent.thinkingLevel');
+
+      expect(result.success).toBe(true);
+      expect(result.message).toContain('"auto"'); // platform default for agent.thinkingLevel
+    });
+  });
+
+  describe('IMMUTABLE_KEYS', () => {
+    it('contains only the enforcement keys', () => {
+      expect(IMMUTABLE_KEYS.has('limits.maxThinkingLevel')).toBe(true);
+      expect(IMMUTABLE_KEYS.has('limits.rateLimit')).toBe(true);
+      expect(IMMUTABLE_KEYS.has('agent.thinkingLevel')).toBe(false);
+    });
+  });
+
+  describe('PLATFORM_DEFAULTS', () => {
+    it('has all expected keys', () => {
+      const expectedKeys = [
+        'agent.defaultModel',
+        'agent.thinkingLevel',
+        'agent.streamingEnabled',
+        'response.language',
+        'response.codeAnnotations',
+        'safety.confirmDestructiveTools',
+        'session.autoCompactThreshold',
+        'session.autoCompactEnabled',
+        'limits.maxThinkingLevel',
+        'limits.rateLimit',
+      ];
+      for (const key of expectedKeys) {
+        expect(Object.prototype.hasOwnProperty.call(PLATFORM_DEFAULTS, key)).toBe(true);
+      }
+    });
+  });
+});

apps/gateway/src/preferences/preferences.service.ts

@@ -0,0 +1,119 @@
+import { Inject, Injectable, Logger } from '@nestjs/common';
+import { eq, and, type Db, preferences as preferencesTable } from '@mosaic/db';
+import { DB } from '../database/database.module.js';
+
+export const PLATFORM_DEFAULTS: Record<string, unknown> = {
+  'agent.defaultModel': null,
+  'agent.thinkingLevel': 'auto',
+  'agent.streamingEnabled': true,
+  'response.language': 'auto',
+  'response.codeAnnotations': true,
+  'safety.confirmDestructiveTools': true,
+  'session.autoCompactThreshold': 0.8,
+  'session.autoCompactEnabled': true,
+  'limits.maxThinkingLevel': null,
+  'limits.rateLimit': null,
+};
+
+export const IMMUTABLE_KEYS = new Set<string>(['limits.maxThinkingLevel', 'limits.rateLimit']);
+
+@Injectable()
+export class PreferencesService {
+  private readonly logger = new Logger(PreferencesService.name);
+
+  constructor(@Inject(DB) private readonly db: Db) {}
+
+  /**
+   * Returns the effective preference set for a user:
+   * Platform defaults → user overrides (mutable keys only) → enforcements re-applied last
+   */
+  async getEffective(userId: string): Promise<Record<string, unknown>> {
+    const userPrefs = await this.getUserPrefs(userId);
+    const result: Record<string, unknown> = { ...PLATFORM_DEFAULTS };
+
+    for (const [key, value] of Object.entries(userPrefs)) {
+      if (!IMMUTABLE_KEYS.has(key)) {
+        result[key] = value;
+      }
+    }
+
+    // Re-apply immutable keys (enforcements always win)
+    for (const key of IMMUTABLE_KEYS) {
+      result[key] = PLATFORM_DEFAULTS[key];
+    }
+
+    return result;
+  }
+
+  async set(
+    userId: string,
+    key: string,
+    value: unknown,
+  ): Promise<{ success: boolean; message: string }> {
+    if (IMMUTABLE_KEYS.has(key)) {
+      return {
+        success: false,
+        message: `Cannot override "${key}" — this is a platform enforcement. Contact your admin.`,
+      };
+    }
+
+    await this.upsertPref(userId, key, value);
+    return { success: true, message: `Preference "${key}" set to ${JSON.stringify(value)}.` };
+  }
+
+  async reset(userId: string, key: string): Promise<{ success: boolean; message: string }> {
+    if (IMMUTABLE_KEYS.has(key)) {
+      return { success: false, message: `Cannot reset "${key}" — it is a platform enforcement.` };
+    }
+
+    await this.deletePref(userId, key);
+    const defaultVal = PLATFORM_DEFAULTS[key];
+    return {
+      success: true,
+      message: `Preference "${key}" reset to default: ${JSON.stringify(defaultVal)}.`,
+    };
+  }
+
+  private async getUserPrefs(userId: string): Promise<Record<string, unknown>> {
+    const rows = await this.db
+      .select({ key: preferencesTable.key, value: preferencesTable.value })
+      .from(preferencesTable)
+      .where(eq(preferencesTable.userId, userId));
+
+    const result: Record<string, unknown> = {};
+    for (const row of rows) {
+      result[row.key] = row.value;
+    }
+    return result;
+  }
+
+  private async upsertPref(userId: string, key: string, value: unknown): Promise<void> {
+    const existing = await this.db
+      .select({ id: preferencesTable.id })
+      .from(preferencesTable)
+      .where(and(eq(preferencesTable.userId, userId), eq(preferencesTable.key, key)))
+      .limit(1);
+
+    if (existing.length > 0) {
+      await this.db
+        .update(preferencesTable)
+        .set({ value: value as never, updatedAt: new Date() })
+        .where(and(eq(preferencesTable.userId, userId), eq(preferencesTable.key, key)));
+    } else {
+      await this.db.insert(preferencesTable).values({
+        userId,
+        key,
+        value: value as never,
+        mutable: true,
+      });
+    }
+    this.logger.debug(`Upserted preference "${key}" for user ${userId}`);
+  }
+
+  private async deletePref(userId: string, key: string): Promise<void> {
+    await this.db
+      .delete(preferencesTable)
+      .where(and(eq(preferencesTable.userId, userId), eq(preferencesTable.key, key)));
+    this.logger.debug(`Deleted preference "${key}" for user ${userId}`);
+  }
+}

apps/gateway/src/preferences/system-override.service.ts

@@ -0,0 +1,33 @@
+import { Injectable, Logger } from '@nestjs/common';
+import { createQueue, type QueueHandle } from '@mosaic/queue';
+
+const SESSION_SYSTEM_KEY = (sessionId: string) => `mosaic:session:${sessionId}:system`;
+const TTL_SECONDS = 5 * 60; // 5 minutes, renewed on each turn
+
+@Injectable()
+export class SystemOverrideService {
+  private readonly logger = new Logger(SystemOverrideService.name);
+  private readonly handle: QueueHandle;
+
+  constructor() {
+    this.handle = createQueue();
+  }
+
+  async set(sessionId: string, override: string): Promise<void> {
+    await this.handle.redis.setex(SESSION_SYSTEM_KEY(sessionId), TTL_SECONDS, override);
+    this.logger.debug(`Set system override for session ${sessionId} (TTL=${TTL_SECONDS}s)`);
+  }
+
+  async get(sessionId: string): Promise<string | null> {
+    return this.handle.redis.get(SESSION_SYSTEM_KEY(sessionId));
+  }
+
+  async renew(sessionId: string): Promise<void> {
+    await this.handle.redis.expire(SESSION_SYSTEM_KEY(sessionId), TTL_SECONDS);
+  }
+
+  async clear(sessionId: string): Promise<void> {
+    await this.handle.redis.del(SESSION_SYSTEM_KEY(sessionId));
+    this.logger.debug(`Cleared system override for session ${sessionId}`);
+  }
+}