feat: add type safety for builder configurations with discriminated unions

.changeset/add-type-safety.md

@@ -0,0 +1,5 @@
+---
+"@workflow/builders": patch
+---
+
+Add type safety for builder configurations with discriminated unions

packages/builders/src/index.ts

@@ -1,7 +1,14 @@
 export { BaseBuilder } from './base-builder.js';
 export { StandaloneBuilder } from './standalone.js';
 export { VercelBuildOutputAPIBuilder } from './vercel-build-output-api.js';
-export type { WorkflowConfig, BuildTarget } from './types.js';
+export type {
+  WorkflowConfig,
+  BuildTarget,
+  StandaloneConfig,
+  VercelBuildOutputConfig,
+  NextConfig,
+  SvelteKitConfig,
+} from './types.js';
 export { validBuildTargets, isValidBuildTarget } from './types.js';
 export type { WorkflowManifest } from './apply-swc-transform.js';
 export { applySwcTransform } from './apply-swc-transform.js';

packages/builders/src/types.ts

@@ -6,14 +6,13 @@ export const validBuildTargets = [
 ] as const;
 export type BuildTarget = (typeof validBuildTargets)[number];
 
-export interface WorkflowConfig {
+/**
+ * Common configuration options shared across all builder types.
+ */
+interface BaseWorkflowConfig {
   watch?: boolean;
   dirs: string[];
   workingDir: string;
-  buildTarget: BuildTarget;
-  stepsBundlePath: string;
-  workflowsBundlePath: string;
-  webhookBundlePath: string;
 
   // Optionally generate a client library for workflow execution. The preferred
   // method of using workflow is to use a loader within a framework (like
@@ -25,6 +24,57 @@ export interface WorkflowConfig {
   workflowManifestPath?: string;
 }
 
+/**
+ * Configuration for standalone (CLI-based) builds.
+ */
+export interface StandaloneConfig extends BaseWorkflowConfig {
+  buildTarget: 'standalone';
+  stepsBundlePath: string;
+  workflowsBundlePath: string;
+  webhookBundlePath: string;
+}
+
+/**
+ * Configuration for Vercel Build Output API builds.
+ */
+export interface VercelBuildOutputConfig extends BaseWorkflowConfig {
+  buildTarget: 'vercel-build-output-api';
+  stepsBundlePath: string;
+  workflowsBundlePath: string;
+  webhookBundlePath: string;
+}
+
+/**
+ * Configuration for Next.js builds.
+ */
+export interface NextConfig extends BaseWorkflowConfig {
+  buildTarget: 'next';
+  // Next.js builder computes paths dynamically, so these are not used
+  stepsBundlePath: string;
+  workflowsBundlePath: string;
+  webhookBundlePath: string;
+}
+
+/**
+ * Configuration for SvelteKit builds.
+ */
+export interface SvelteKitConfig extends BaseWorkflowConfig {
+  buildTarget: 'sveltekit';
+  // SvelteKit builder computes paths dynamically, so these are not used
+  stepsBundlePath: string;
+  workflowsBundlePath: string;
+  webhookBundlePath: string;
+}
+
+/**
+ * Discriminated union of all builder configuration types.
+ */
+export type WorkflowConfig =
+  | StandaloneConfig
+  | VercelBuildOutputConfig
+  | NextConfig
+  | SvelteKitConfig;
+
 export function isValidBuildTarget(
   target: string | undefined
 ): target is BuildTarget {

packages/sveltekit/src/builder.ts

@@ -1,7 +1,7 @@
 import { constants } from 'node:fs';
 import { access, mkdir, readFile, stat, writeFile } from 'node:fs/promises';
 import { join, resolve } from 'node:path';
-import { BaseBuilder, type WorkflowConfig } from '@workflow/builders';
+import { BaseBuilder, type SvelteKitConfig } from '@workflow/builders';
 
 // Helper function code for converting SvelteKit requests to standard Request objects
 const SVELTEKIT_REQUEST_CONVERTER = `
@@ -18,7 +18,7 @@ async function convertSvelteKitRequest(request) {
 `;
 
 export class SvelteKitBuilder extends BaseBuilder {
-  constructor(config?: Partial<WorkflowConfig>) {
+  constructor(config?: Partial<SvelteKitConfig>) {
     super({
       ...config,
       dirs: ['workflows'],