Server Action Builder

Server Action Builder

You are an expert at creating type-safe server actions for a Next.js/Supabase application.

Why This Skill Exists

The user's codebase has established patterns for server actions using manual authentication, Zod validation, and service layers. Deviating from these patterns causes real problems:

Following the patterns below prevents these failures.

Workflow

When asked to create a server action, follow these steps:

Step 1: Create Zod Schema

Create validation schema in _lib/schema/:

// _lib/schema/feature.schema.ts
import { z } from 'zod';

export const CreateFeatureSchema = z.object({
  name: z.string().min(1, 'Name is required'),
  accountId: z.string().uuid('Invalid account ID'),
});

export type CreateFeatureInput = z.infer<typeof CreateFeatureSchema>;

Step 2: Create Service Layer

North star: services are decoupled from their interface. The service is pure logic — it receives a database client as a dependency, never imports one. This means the same service works whether called from a server action, an MCP tool, a CLI command, or a plain unit test.

Create service in _lib/server/:

// _lib/server/feature.service.ts
import 'server-only';

import type { SupabaseClient } from '@supabase/supabase-js';

import type { Database } from '@/types/database';
import type { CreateFeatureInput } from '../schema/feature.schema';

export function createFeatureService(client: SupabaseClient<Database>) {
  return new FeatureService(client);
}

class FeatureService {
  constructor(private readonly client: SupabaseClient<Database>) {}

  async create(data: CreateFeatureInput) {
    const { data: result, error } = await this.client
      .from('features')
      .insert({
        name: data.name,
        account_id: data.accountId,
      })
      .select()
      .single();

    if (error) throw error;

    return result;
  }
}

The service never calls createClient() — the caller provides the client. This keeps the service testable (pass a mock client) and reusable (any interface can supply its own client).

Step 3: Create Server Action (Thin Adapter)

The action is a thin adapter — it resolves dependencies (client, logger) and delegates to the service. Business logic in the adapter means the user must duplicate changes across every interface when logic evolves.

Create action in _lib/server/server-actions.ts:

'use server';

import { z } from 'zod';
import { revalidatePath } from 'next/cache';

import { createClient } from '@/lib/supabase/server';
import { getSession } from '@/lib/auth';
import { logger } from '@/lib/logger';

import { CreateFeatureSchema } from '../schema/feature.schema';
import { createFeatureService } from './feature.service';

export async function createFeatureAction(input: z.infer<typeof CreateFeatureSchema>) {
  const session = await getSession();
  if (!session) throw new Error('Unauthorized');

  const data = CreateFeatureSchema.parse(input);

  const ctx = { name: 'create-feature', userId: session.user.id };
  logger.info(ctx, 'Creating feature');

  const client = await createClient();
  const service = createFeatureService(client);
  const result = await service.create(data);

  logger.info({ ...ctx, featureId: result.id }, 'Feature created');

  revalidatePath('/home/[account]/features');

  return { success: true, data: result };
}

Key Patterns

The user configured these patterns because each prevents a specific failure mode that has caused real issues:

1. Services are pure, interfaces are thin adapters. The service contains all business logic. The server action is glue code that resolves dependencies and calls the service. If an MCP tool and a server action do the same thing, they call the same service function — otherwise the user fixes bugs in one place while they persist in another.
2. Inject dependencies, don't import them in services. Services that import framework clients directly cannot be tested in isolation — the user depends on dependency injection to maintain test coverage.
3. Schema in separate file — Reusable between client forms and server actions; a single source of truth for validation prevents client and server from drifting apart.
4. Logging — Without structured logs, the user cannot diagnose production issues. Always log before and after operations with a context object.
5. Revalidation — Missing revalidatePath after mutations causes stale UI that makes users think their action failed.
6. Trust RLS — Manual auth checks are error-prone and duplicate logic that RLS already handles. Use the standard Supabase client, not the admin client.
7. Testable in isolation — Because services accept their dependencies, you can test them with a mock client and no running infrastructure.

File Structure

feature/
├── _lib/
│   ├── schema/
│   │   └── feature.schema.ts
│   └── server/
│       ├── feature.service.ts
│       └── server-actions.ts
└── _components/
    └── feature-form.tsx

Troubleshooting

Server action callback receives wrong parameters

Cause: The server action function signature doesn't match the expected input. Ensure the function accepts the validated input type and performs its own auth check with getSession().

Fix: Always use async function myAction(input: z.infer<typeof Schema>) and call getSession() at the top of the function body.

Missing 'use server' directive

Cause: Without the directive, Next.js treats the file as a regular module. Server actions silently become client-side functions, breaking auth and validation.

Fix: Add 'use server'; as the very first line of the server actions file.

Stale UI after mutation

Cause: Missing revalidatePath call after the mutation. Next.js caches server component data, and without revalidation the user sees outdated data.

Fix: Add revalidatePath('/home/[account]/feature-path') after every successful mutation.

Missing auth check in server action

Cause: The server action doesn't verify the user is authenticated before processing. Without auth verification, unauthenticated requests can reach the database.

Fix: Always call getSession() at the top of every server action and throw an error if no session exists.

Server action re-exported from barrel file

Cause: Re-exporting server actions from _lib/server/index.ts breaks Next.js server action detection. The framework cannot identify re-exported functions as server actions.

Fix: Import server actions directly from _lib/server/server-actions.ts, never through a barrel file.

Reference Files

See examples in:

• Examples
• Reference