feat(api): add workspace member management endpoints (#556)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/api/src/workspaces/dto/add-member.dto.ts

@@ -0,0 +1,13 @@
+import { WorkspaceMemberRole } from "@prisma/client";
+import { IsEnum, IsUUID } from "class-validator";
+
+/**
+ * DTO for adding a user to a workspace.
+ */
+export class AddMemberDto {
+  @IsUUID("4", { message: "userId must be a valid UUID" })
+  userId!: string;
+
+  @IsEnum(WorkspaceMemberRole, { message: "role must be a valid WorkspaceMemberRole" })
+  role!: WorkspaceMemberRole;
+}

apps/api/src/workspaces/dto/index.ts

@@ -1 +1,3 @@
+export { AddMemberDto } from "./add-member.dto";
+export { UpdateMemberRoleDto } from "./update-member-role.dto";
 export { WorkspaceResponseDto } from "./workspace-response.dto";

apps/api/src/workspaces/dto/update-member-role.dto.ts

@@ -0,0 +1,10 @@
+import { WorkspaceMemberRole } from "@prisma/client";
+import { IsEnum } from "class-validator";
+
+/**
+ * DTO for updating a workspace member's role.
+ */
+export class UpdateMemberRoleDto {
+  @IsEnum(WorkspaceMemberRole, { message: "role must be a valid WorkspaceMemberRole" })
+  role!: WorkspaceMemberRole;
+}

apps/api/src/workspaces/workspaces.controller.spec.ts

@@ -3,6 +3,7 @@ import { Test, TestingModule } from "@nestjs/testing";
 import { WorkspacesController } from "./workspaces.controller";
 import { WorkspacesService } from "./workspaces.service";
 import { AuthGuard } from "../auth/guards/auth.guard";
+import { WorkspaceGuard, PermissionGuard } from "../common/guards";
 import { WorkspaceMemberRole } from "@prisma/client";
 import type { AuthUser } from "@mosaic/shared";
 
@@ -12,6 +13,9 @@ describe("WorkspacesController", () => {
 
   const mockWorkspacesService = {
     getUserWorkspaces: vi.fn(),
+    addMember: vi.fn(),
+    updateMemberRole: vi.fn(),
+    removeMember: vi.fn(),
   };
 
   const mockUser: AuthUser = {
@@ -32,6 +36,10 @@ describe("WorkspacesController", () => {
     })
       .overrideGuard(AuthGuard)
       .useValue({ canActivate: () => true })
+      .overrideGuard(WorkspaceGuard)
+      .useValue({ canActivate: () => true })
+      .overrideGuard(PermissionGuard)
+      .useValue({ canActivate: () => true })
       .compile();
 
     controller = module.get<WorkspacesController>(WorkspacesController);
@@ -72,4 +80,70 @@ describe("WorkspacesController", () => {
       await expect(controller.getUserWorkspaces(mockUser)).rejects.toThrow("Database error");
     });
   });
+
+  describe("POST /api/workspaces/:id/members", () => {
+    it("should call service with workspace id, actor id, and add member dto", async () => {
+      const workspaceId = "ws-1";
+      const addMemberDto = {
+        userId: "user-2",
+        role: WorkspaceMemberRole.MEMBER,
+      };
+      const mockMember = {
+        workspaceId,
+        userId: "user-2",
+        role: WorkspaceMemberRole.MEMBER,
+        joinedAt: new Date("2026-02-01"),
+      };
+      mockWorkspacesService.addMember.mockResolvedValueOnce(mockMember);
+
+      const result = await controller.addMember(workspaceId, addMemberDto, mockUser);
+
+      expect(result).toEqual(mockMember);
+      expect(service.addMember).toHaveBeenCalledWith(workspaceId, mockUser.id, addMemberDto);
+    });
+  });
+
+  describe("PATCH /api/workspaces/:id/members/:userId", () => {
+    it("should call service with workspace id, actor id, target user id, and role dto", async () => {
+      const workspaceId = "ws-1";
+      const targetUserId = "user-2";
+      const updateRoleDto = {
+        role: WorkspaceMemberRole.ADMIN,
+      };
+      const mockMember = {
+        workspaceId,
+        userId: targetUserId,
+        role: WorkspaceMemberRole.ADMIN,
+        joinedAt: new Date("2026-02-01"),
+      };
+      mockWorkspacesService.updateMemberRole.mockResolvedValueOnce(mockMember);
+
+      const result = await controller.updateMemberRole(
+        workspaceId,
+        targetUserId,
+        updateRoleDto,
+        mockUser
+      );
+
+      expect(result).toEqual(mockMember);
+      expect(service.updateMemberRole).toHaveBeenCalledWith(
+        workspaceId,
+        mockUser.id,
+        targetUserId,
+        updateRoleDto
+      );
+    });
+  });
+
+  describe("DELETE /api/workspaces/:id/members/:userId", () => {
+    it("should call service with workspace id, actor id, and target user id", async () => {
+      const workspaceId = "ws-1";
+      const targetUserId = "user-2";
+      mockWorkspacesService.removeMember.mockResolvedValueOnce(undefined);
+
+      await controller.removeMember(workspaceId, targetUserId, mockUser);
+
+      expect(service.removeMember).toHaveBeenCalledWith(workspaceId, mockUser.id, targetUserId);
+    });
+  });
 });

apps/api/src/workspaces/workspaces.controller.ts

@@ -1,9 +1,12 @@
-import { Controller, Get, UseGuards } from "@nestjs/common";
+import { Body, Controller, Delete, Get, Param, Patch, Post, UseGuards } from "@nestjs/common";
 import { WorkspacesService } from "./workspaces.service";
 import { AuthGuard } from "../auth/guards/auth.guard";
 import { CurrentUser } from "../auth/decorators/current-user.decorator";
-import type { AuthUser } from "@mosaic/shared";
-import type { WorkspaceResponseDto } from "./dto";
+import { WorkspaceGuard, PermissionGuard } from "../common/guards";
+import { Permission, RequirePermission } from "../common/decorators";
+import type { WorkspaceMember } from "@prisma/client";
+import type { AuthenticatedUser } from "../common/types/user.types";
+import type { AddMemberDto, UpdateMemberRoleDto, WorkspaceResponseDto } from "./dto";
 
 /**
  * User-scoped workspace operations.
@@ -22,7 +25,61 @@ export class WorkspacesController {
    * Auto-provisions a default workspace if the user has none.
    */
   @Get()
-  async getUserWorkspaces(@CurrentUser() user: AuthUser): Promise<WorkspaceResponseDto[]> {
+  async getUserWorkspaces(@CurrentUser() user: AuthenticatedUser): Promise<WorkspaceResponseDto[]> {
     return this.workspacesService.getUserWorkspaces(user.id);
   }
+
+  /**
+   * POST /api/workspaces/:workspaceId/members
+   * Add a member to a workspace with the specified role.
+   * Requires: ADMIN role or higher.
+   */
+  @Post(":workspaceId/members")
+  @UseGuards(WorkspaceGuard, PermissionGuard)
+  @RequirePermission(Permission.WORKSPACE_ADMIN)
+  async addMember(
+    @Param("workspaceId") workspaceId: string,
+    @Body() addMemberDto: AddMemberDto,
+    @CurrentUser() user: AuthenticatedUser
+  ): Promise<WorkspaceMember> {
+    return this.workspacesService.addMember(workspaceId, user.id, addMemberDto);
+  }
+
+  /**
+   * PATCH /api/workspaces/:workspaceId/members/:userId
+   * Change a member role in a workspace.
+   * Requires: ADMIN role or higher.
+   */
+  @Patch(":workspaceId/members/:userId")
+  @UseGuards(WorkspaceGuard, PermissionGuard)
+  @RequirePermission(Permission.WORKSPACE_ADMIN)
+  async updateMemberRole(
+    @Param("workspaceId") workspaceId: string,
+    @Param("userId") targetUserId: string,
+    @Body() updateMemberRoleDto: UpdateMemberRoleDto,
+    @CurrentUser() user: AuthenticatedUser
+  ): Promise<WorkspaceMember> {
+    return this.workspacesService.updateMemberRole(
+      workspaceId,
+      user.id,
+      targetUserId,
+      updateMemberRoleDto
+    );
+  }
+
+  /**
+   * DELETE /api/workspaces/:workspaceId/members/:userId
+   * Remove a member from a workspace.
+   * Requires: ADMIN role or higher.
+   */
+  @Delete(":workspaceId/members/:userId")
+  @UseGuards(WorkspaceGuard, PermissionGuard)
+  @RequirePermission(Permission.WORKSPACE_ADMIN)
+  async removeMember(
+    @Param("workspaceId") workspaceId: string,
+    @Param("userId") targetUserId: string,
+    @CurrentUser() user: AuthenticatedUser
+  ): Promise<void> {
+    await this.workspacesService.removeMember(workspaceId, user.id, targetUserId);
+  }
 }

apps/api/src/workspaces/workspaces.service.spec.ts

@@ -3,11 +3,19 @@ import { Test, TestingModule } from "@nestjs/testing";
 import { WorkspacesService } from "./workspaces.service";
 import { PrismaService } from "../prisma/prisma.service";
 import { WorkspaceMemberRole } from "@prisma/client";
+import {
+  BadRequestException,
+  ConflictException,
+  ForbiddenException,
+  NotFoundException,
+} from "@nestjs/common";
 
 describe("WorkspacesService", () => {
   let service: WorkspacesService;
 
   const mockUserId = "550e8400-e29b-41d4-a716-446655440001";
+  const mockAdminUserId = "550e8400-e29b-41d4-a716-446655440010";
+  const mockMemberUserId = "550e8400-e29b-41d4-a716-446655440011";
   const mockWorkspaceId = "550e8400-e29b-41d4-a716-446655440002";
 
   const mockWorkspace = {
@@ -36,11 +44,18 @@ describe("WorkspacesService", () => {
   const mockPrismaService = {
     workspaceMember: {
       findMany: vi.fn(),
+      findUnique: vi.fn(),
+      count: vi.fn(),
       create: vi.fn(),
+      update: vi.fn(),
+      delete: vi.fn(),
     },
     workspace: {
       create: vi.fn(),
     },
+    user: {
+      findUnique: vi.fn(),
+    },
     $transaction: vi.fn(),
   };
 
@@ -58,6 +73,11 @@ describe("WorkspacesService", () => {
     service = module.get<WorkspacesService>(WorkspacesService);
 
     vi.clearAllMocks();
+
+    mockPrismaService.$transaction.mockImplementation(
+      async (fn: (tx: typeof mockPrismaService) => Promise<unknown>) =>
+        fn(mockPrismaService as unknown as typeof mockPrismaService)
+    );
   });
 
   describe("getUserWorkspaces", () => {
@@ -226,4 +246,271 @@ describe("WorkspacesService", () => {
       );
     });
   });
+
+  describe("addMember", () => {
+    const addMemberDto = {
+      userId: mockMemberUserId,
+      role: WorkspaceMemberRole.MEMBER,
+    };
+
+    it("should add a new member to the workspace", async () => {
+      const createdMembership = {
+        workspaceId: mockWorkspaceId,
+        userId: mockMemberUserId,
+        role: WorkspaceMemberRole.MEMBER,
+        joinedAt: new Date("2026-02-02"),
+      };
+      mockPrismaService.user.findUnique.mockResolvedValueOnce({ id: mockMemberUserId });
+      mockPrismaService.workspaceMember.findUnique
+        .mockResolvedValueOnce({
+          workspaceId: mockWorkspaceId,
+          userId: mockAdminUserId,
+          role: WorkspaceMemberRole.ADMIN,
+          joinedAt: new Date("2026-01-01"),
+        })
+        .mockResolvedValueOnce(null);
+      mockPrismaService.workspaceMember.create.mockResolvedValueOnce(createdMembership);
+
+      const result = await service.addMember(mockWorkspaceId, mockAdminUserId, addMemberDto);
+
+      expect(result).toEqual(createdMembership);
+      expect(mockPrismaService.workspaceMember.create).toHaveBeenCalledWith({
+        data: {
+          workspaceId: mockWorkspaceId,
+          userId: mockMemberUserId,
+          role: WorkspaceMemberRole.MEMBER,
+        },
+      });
+    });
+
+    it("should throw NotFoundException when user does not exist", async () => {
+      mockPrismaService.workspaceMember.findUnique.mockResolvedValueOnce({
+        workspaceId: mockWorkspaceId,
+        userId: mockAdminUserId,
+        role: WorkspaceMemberRole.ADMIN,
+        joinedAt: new Date("2026-01-01"),
+      });
+      mockPrismaService.user.findUnique.mockResolvedValueOnce(null);
+
+      await expect(
+        service.addMember(mockWorkspaceId, mockAdminUserId, addMemberDto)
+      ).rejects.toThrow(NotFoundException);
+    });
+
+    it("should throw ConflictException when user is already a member", async () => {
+      mockPrismaService.workspaceMember.findUnique.mockResolvedValueOnce({
+        workspaceId: mockWorkspaceId,
+        userId: mockAdminUserId,
+        role: WorkspaceMemberRole.ADMIN,
+        joinedAt: new Date("2026-01-01"),
+      });
+      mockPrismaService.user.findUnique.mockResolvedValueOnce({ id: mockMemberUserId });
+      mockPrismaService.workspaceMember.findUnique.mockResolvedValueOnce({
+        workspaceId: mockWorkspaceId,
+        userId: mockMemberUserId,
+        role: WorkspaceMemberRole.MEMBER,
+        joinedAt: new Date("2026-01-02"),
+      });
+
+      await expect(
+        service.addMember(mockWorkspaceId, mockAdminUserId, addMemberDto)
+      ).rejects.toThrow(ConflictException);
+    });
+
+    it("should throw ForbiddenException when admin tries to assign OWNER role", async () => {
+      mockPrismaService.workspaceMember.findUnique.mockResolvedValueOnce({
+        workspaceId: mockWorkspaceId,
+        userId: mockAdminUserId,
+        role: WorkspaceMemberRole.ADMIN,
+        joinedAt: new Date("2026-01-01"),
+      });
+
+      await expect(
+        service.addMember(mockWorkspaceId, mockAdminUserId, {
+          userId: mockMemberUserId,
+          role: WorkspaceMemberRole.OWNER,
+        })
+      ).rejects.toThrow(ForbiddenException);
+    });
+  });
+
+  describe("updateMemberRole", () => {
+    it("should update a member role", async () => {
+      const updatedMembership = {
+        workspaceId: mockWorkspaceId,
+        userId: mockMemberUserId,
+        role: WorkspaceMemberRole.ADMIN,
+        joinedAt: new Date("2026-01-02"),
+      };
+      mockPrismaService.workspaceMember.findUnique
+        .mockResolvedValueOnce({
+          workspaceId: mockWorkspaceId,
+          userId: mockUserId,
+          role: WorkspaceMemberRole.OWNER,
+          joinedAt: new Date("2026-01-01"),
+        })
+        .mockResolvedValueOnce({
+          workspaceId: mockWorkspaceId,
+          userId: mockMemberUserId,
+          role: WorkspaceMemberRole.MEMBER,
+          joinedAt: new Date("2026-01-02"),
+        });
+      mockPrismaService.workspaceMember.update.mockResolvedValueOnce(updatedMembership);
+
+      const result = await service.updateMemberRole(mockWorkspaceId, mockUserId, mockMemberUserId, {
+        role: WorkspaceMemberRole.ADMIN,
+      });
+
+      expect(result).toEqual(updatedMembership);
+      expect(mockPrismaService.workspaceMember.update).toHaveBeenCalledWith({
+        where: {
+          workspaceId_userId: {
+            workspaceId: mockWorkspaceId,
+            userId: mockMemberUserId,
+          },
+        },
+        data: {
+          role: WorkspaceMemberRole.ADMIN,
+        },
+      });
+    });
+
+    it("should throw NotFoundException when target member does not exist", async () => {
+      mockPrismaService.workspaceMember.findUnique
+        .mockResolvedValueOnce({
+          workspaceId: mockWorkspaceId,
+          userId: mockUserId,
+          role: WorkspaceMemberRole.OWNER,
+          joinedAt: new Date("2026-01-01"),
+        })
+        .mockResolvedValueOnce(null);
+
+      await expect(
+        service.updateMemberRole(mockWorkspaceId, mockUserId, mockMemberUserId, {
+          role: WorkspaceMemberRole.ADMIN,
+        })
+      ).rejects.toThrow(NotFoundException);
+    });
+
+    it("should throw BadRequestException when sole owner attempts self-demotion", async () => {
+      mockPrismaService.workspaceMember.findUnique
+        .mockResolvedValueOnce({
+          workspaceId: mockWorkspaceId,
+          userId: mockUserId,
+          role: WorkspaceMemberRole.OWNER,
+          joinedAt: new Date("2026-01-01"),
+        })
+        .mockResolvedValueOnce({
+          workspaceId: mockWorkspaceId,
+          userId: mockUserId,
+          role: WorkspaceMemberRole.OWNER,
+          joinedAt: new Date("2026-01-01"),
+        });
+      mockPrismaService.workspaceMember.count.mockResolvedValueOnce(1);
+
+      await expect(
+        service.updateMemberRole(mockWorkspaceId, mockUserId, mockUserId, {
+          role: WorkspaceMemberRole.ADMIN,
+        })
+      ).rejects.toThrow(BadRequestException);
+    });
+
+    it("should throw ForbiddenException when actor tries to change role of higher-ranked member", async () => {
+      mockPrismaService.workspaceMember.findUnique
+        .mockResolvedValueOnce({
+          workspaceId: mockWorkspaceId,
+          userId: mockAdminUserId,
+          role: WorkspaceMemberRole.ADMIN,
+          joinedAt: new Date("2026-01-01"),
+        })
+        .mockResolvedValueOnce({
+          workspaceId: mockWorkspaceId,
+          userId: mockUserId,
+          role: WorkspaceMemberRole.OWNER,
+          joinedAt: new Date("2026-01-01"),
+        });
+
+      await expect(
+        service.updateMemberRole(mockWorkspaceId, mockAdminUserId, mockUserId, {
+          role: WorkspaceMemberRole.MEMBER,
+        })
+      ).rejects.toThrow(ForbiddenException);
+    });
+  });
+
+  describe("removeMember", () => {
+    it("should remove a workspace member", async () => {
+      mockPrismaService.workspaceMember.findUnique
+        .mockResolvedValueOnce({
+          workspaceId: mockWorkspaceId,
+          userId: mockUserId,
+          role: WorkspaceMemberRole.OWNER,
+          joinedAt: new Date("2026-01-01"),
+        })
+        .mockResolvedValueOnce({
+          workspaceId: mockWorkspaceId,
+          userId: mockMemberUserId,
+          role: WorkspaceMemberRole.MEMBER,
+          joinedAt: new Date("2026-01-02"),
+        });
+      mockPrismaService.workspaceMember.delete.mockResolvedValueOnce({
+        workspaceId: mockWorkspaceId,
+        userId: mockMemberUserId,
+        role: WorkspaceMemberRole.MEMBER,
+        joinedAt: new Date("2026-01-02"),
+      });
+
+      await service.removeMember(mockWorkspaceId, mockUserId, mockMemberUserId);
+
+      expect(mockPrismaService.workspaceMember.delete).toHaveBeenCalledWith({
+        where: {
+          workspaceId_userId: {
+            workspaceId: mockWorkspaceId,
+            userId: mockMemberUserId,
+          },
+        },
+      });
+    });
+
+    it("should throw BadRequestException when trying to remove the last owner", async () => {
+      mockPrismaService.workspaceMember.findUnique
+        .mockResolvedValueOnce({
+          workspaceId: mockWorkspaceId,
+          userId: mockUserId,
+          role: WorkspaceMemberRole.OWNER,
+          joinedAt: new Date("2026-01-01"),
+        })
+        .mockResolvedValueOnce({
+          workspaceId: mockWorkspaceId,
+          userId: mockUserId,
+          role: WorkspaceMemberRole.OWNER,
+          joinedAt: new Date("2026-01-01"),
+        });
+      mockPrismaService.workspaceMember.count.mockResolvedValueOnce(1);
+
+      await expect(service.removeMember(mockWorkspaceId, mockUserId, mockUserId)).rejects.toThrow(
+        BadRequestException
+      );
+    });
+
+    it("should throw ForbiddenException when admin attempts to remove an owner", async () => {
+      mockPrismaService.workspaceMember.findUnique
+        .mockResolvedValueOnce({
+          workspaceId: mockWorkspaceId,
+          userId: mockAdminUserId,
+          role: WorkspaceMemberRole.ADMIN,
+          joinedAt: new Date("2026-01-01"),
+        })
+        .mockResolvedValueOnce({
+          workspaceId: mockWorkspaceId,
+          userId: mockUserId,
+          role: WorkspaceMemberRole.OWNER,
+          joinedAt: new Date("2026-01-01"),
+        });
+
+      await expect(
+        service.removeMember(mockWorkspaceId, mockAdminUserId, mockUserId)
+      ).rejects.toThrow(ForbiddenException);
+    });
+  });
 });

apps/api/src/workspaces/workspaces.service.ts

@@ -1,7 +1,22 @@
-import { Injectable, Logger } from "@nestjs/common";
-import { WorkspaceMemberRole } from "@prisma/client";
+import {
+  BadRequestException,
+  ConflictException,
+  ForbiddenException,
+  Injectable,
+  Logger,
+  NotFoundException,
+} from "@nestjs/common";
+import { Prisma, WorkspaceMemberRole } from "@prisma/client";
+import type { WorkspaceMember } from "@prisma/client";
 import { PrismaService } from "../prisma/prisma.service";
-import type { WorkspaceResponseDto } from "./dto";
+import type { AddMemberDto, UpdateMemberRoleDto, WorkspaceResponseDto } from "./dto";
+
+const WORKSPACE_ROLE_RANK: Record<WorkspaceMemberRole, number> = {
+  [WorkspaceMemberRole.GUEST]: 1,
+  [WorkspaceMemberRole.MEMBER]: 2,
+  [WorkspaceMemberRole.ADMIN]: 3,
+  [WorkspaceMemberRole.OWNER]: 4,
+};
 
 @Injectable()
 export class WorkspacesService {
@@ -94,4 +109,237 @@ export class WorkspacesService {
       },
     ];
   }
+
+  /**
+   * Add a member to a workspace.
+   */
+  async addMember(
+    workspaceId: string,
+    actorUserId: string,
+    addMemberDto: AddMemberDto
+  ): Promise<WorkspaceMember> {
+    const actorMembership = await this.prisma.workspaceMember.findUnique({
+      where: {
+        workspaceId_userId: {
+          workspaceId,
+          userId: actorUserId,
+        },
+      },
+      select: {
+        role: true,
+      },
+    });
+
+    if (!actorMembership) {
+      throw new ForbiddenException("You are not a member of this workspace");
+    }
+
+    this.assertCanAssignRole(actorMembership.role, addMemberDto.role);
+
+    const user = await this.prisma.user.findUnique({
+      where: { id: addMemberDto.userId },
+      select: { id: true },
+    });
+
+    if (!user) {
+      throw new NotFoundException(`User with ID ${addMemberDto.userId} not found`);
+    }
+
+    const existingMembership = await this.prisma.workspaceMember.findUnique({
+      where: {
+        workspaceId_userId: {
+          workspaceId,
+          userId: addMemberDto.userId,
+        },
+      },
+      select: {
+        workspaceId: true,
+        userId: true,
+      },
+    });
+
+    if (existingMembership) {
+      throw new ConflictException("User is already a member of this workspace");
+    }
+
+    try {
+      return await this.prisma.workspaceMember.create({
+        data: {
+          workspaceId,
+          userId: addMemberDto.userId,
+          role: addMemberDto.role,
+        },
+      });
+    } catch (error) {
+      if (this.isUniqueConstraintError(error)) {
+        throw new ConflictException("User is already a member of this workspace");
+      }
+      throw error;
+    }
+  }
+
+  /**
+   * Update the role of an existing workspace member.
+   */
+  async updateMemberRole(
+    workspaceId: string,
+    actorUserId: string,
+    targetUserId: string,
+    updateMemberRoleDto: UpdateMemberRoleDto
+  ): Promise<WorkspaceMember> {
+    return this.prisma.$transaction(async (tx) => {
+      const actorMembership = await tx.workspaceMember.findUnique({
+        where: {
+          workspaceId_userId: {
+            workspaceId,
+            userId: actorUserId,
+          },
+        },
+        select: {
+          role: true,
+        },
+      });
+
+      if (!actorMembership) {
+        throw new ForbiddenException("You are not a member of this workspace");
+      }
+
+      const targetMembership = await tx.workspaceMember.findUnique({
+        where: {
+          workspaceId_userId: {
+            workspaceId,
+            userId: targetUserId,
+          },
+        },
+        select: {
+          role: true,
+        },
+      });
+
+      if (!targetMembership) {
+        throw new NotFoundException(`User ${targetUserId} is not a member of this workspace`);
+      }
+
+      this.assertCanManageTargetMember(actorMembership.role, targetMembership.role);
+      this.assertCanAssignRole(actorMembership.role, updateMemberRoleDto.role);
+
+      if (targetMembership.role === WorkspaceMemberRole.OWNER) {
+        const isDemotion = updateMemberRoleDto.role !== WorkspaceMemberRole.OWNER;
+        if (isDemotion) {
+          const ownerCount = await tx.workspaceMember.count({
+            where: {
+              workspaceId,
+              role: WorkspaceMemberRole.OWNER,
+            },
+          });
+          if (ownerCount <= 1) {
+            if (actorUserId === targetUserId) {
+              throw new BadRequestException("Cannot self-demote if you are the sole owner");
+            }
+            throw new BadRequestException("Cannot remove the last owner from a workspace");
+          }
+        }
+      }
+
+      return tx.workspaceMember.update({
+        where: {
+          workspaceId_userId: {
+            workspaceId,
+            userId: targetUserId,
+          },
+        },
+        data: {
+          role: updateMemberRoleDto.role,
+        },
+      });
+    });
+  }
+
+  /**
+   * Remove a member from a workspace.
+   */
+  async removeMember(
+    workspaceId: string,
+    actorUserId: string,
+    targetUserId: string
+  ): Promise<void> {
+    await this.prisma.$transaction(async (tx) => {
+      const actorMembership = await tx.workspaceMember.findUnique({
+        where: {
+          workspaceId_userId: {
+            workspaceId,
+            userId: actorUserId,
+          },
+        },
+        select: {
+          role: true,
+        },
+      });
+
+      if (!actorMembership) {
+        throw new ForbiddenException("You are not a member of this workspace");
+      }
+
+      const targetMembership = await tx.workspaceMember.findUnique({
+        where: {
+          workspaceId_userId: {
+            workspaceId,
+            userId: targetUserId,
+          },
+        },
+        select: {
+          role: true,
+        },
+      });
+
+      if (!targetMembership) {
+        throw new NotFoundException(`User ${targetUserId} is not a member of this workspace`);
+      }
+
+      this.assertCanManageTargetMember(actorMembership.role, targetMembership.role);
+
+      if (targetMembership.role === WorkspaceMemberRole.OWNER) {
+        const ownerCount = await tx.workspaceMember.count({
+          where: {
+            workspaceId,
+            role: WorkspaceMemberRole.OWNER,
+          },
+        });
+        if (ownerCount <= 1) {
+          throw new BadRequestException("Cannot remove the last owner from a workspace");
+        }
+      }
+
+      await tx.workspaceMember.delete({
+        where: {
+          workspaceId_userId: {
+            workspaceId,
+            userId: targetUserId,
+          },
+        },
+      });
+    });
+  }
+
+  private assertCanAssignRole(
+    actorRole: WorkspaceMemberRole,
+    requestedRole: WorkspaceMemberRole
+  ): void {
+    if (WORKSPACE_ROLE_RANK[actorRole] < WORKSPACE_ROLE_RANK[requestedRole]) {
+      throw new ForbiddenException("You cannot assign a role higher than your own");
+    }
+  }
+
+  private assertCanManageTargetMember(
+    actorRole: WorkspaceMemberRole,
+    targetRole: WorkspaceMemberRole
+  ): void {
+    if (WORKSPACE_ROLE_RANK[actorRole] < WORKSPACE_ROLE_RANK[targetRole]) {
+      throw new ForbiddenException("You cannot manage a member with a higher role");
+    }
+  }
+
+  private isUniqueConstraintError(error: unknown): error is Prisma.PrismaClientKnownRequestError {
+    return error instanceof Prisma.PrismaClientKnownRequestError && error.code === "P2002";
+  }
 }