Next.js 15: Environment variables missing in API Routes

I’m experiencing a critical issue where ALL non-NEXT_PUBLIC_ environment variables are unavailable at runtime in Next.js API Routes (Route Handlers), even though they’re correctly configured in the Vercel dashboard. This is blocking production functionality that requires server-side environment variables like SUPABASE_SERVICE_ROLE_KEY.

Current versus Expected behavior:

Current Behavior:

All non-NEXT_PUBLIC_ environment variables are completely missing from process.env at runtime in API Routes
SUPABASE_SERVICE_ROLE_KEY is not available (returns undefined)
TEST_SERVICE_KEY (test variable) is also not available
Only NEXT_PUBLIC_* variables work correctly

Expected Behavior:

All environment variables set in Vercel dashboard should be available in process.env at runtime in API Routes
SUPABASE_SERVICE_ROLE_KEY should be accessible via process.env.SUPABASE_SERVICE_ROLE_KEY
Both NEXT_PUBLIC_* and non-NEXT_PUBLIC_ variables should work

Test Endpoint Results: I created a test endpoint (/api/test-env) that checks available environment variables: { “hasSupabaseServiceRoleKey”: false, “hasTestServiceKey”: false, “availableSupabaseVars”: [“NEXT_PUBLIC_SUPABASE_ANON_KEY”, “NEXT_PUBLIC_SUPABASE_URL”], “allEnvVarNames”: [/* SUPABASE_SERVICE_ROLE_KEY and TEST_SERVICE_KEY are completely missing */] }Code, configuration, and steps that reproduce this issue:

**Test Endpoint Code:**ipt // app/api/test-env/route.ts import { NextRequest, NextResponse } from “next/server”;

export async function GET(request: NextRequest) { const envCheck = { hasSupabaseServiceRoleKey: !!process.env.SUPABASE_SERVICE_ROLE_KEY, hasTestServiceKey: !!process.env.TEST_SERVICE_KEY, availableSupabaseVars: Object.keys(process.env) .filter((key) => key.includes(“SUPABASE”) || key.includes(“TEST”)) .sort(), }; return NextResponse.json(envCheck, { status: 200 }); }API Route Example (failing): // app/api/search-players/route.ts export async function GET(request: NextRequest) { const supabaseServiceKey = process.env.SUPABASE_SERVICE_ROLE_KEY; // Returns undefined // … rest of code fails because key is missing }next.config.ts: import type { NextConfig } from “next”;

const nextConfig: NextConfig = { output: process.env.CAPACITOR_BUILD === ‘true’ ? ‘export’ : undefined, skipTrailingSlashRedirect: true, // No env section - was removed as suggested by support };

export default nextConfig;Steps to Reproduce:

Set SUPABASE_SERVICE_ROLE_KEY in Vercel dashboard (all environments enabled)
Deploy to Vercel
Call /api/test-env endpoint
Observe that hasSupabaseServiceRoleKey: false

Troubleshooting Steps Already Taken:

✅ Verified variables are set in Vercel dashboard for all environments (Production, Preview, Development)
✅ Multiple redeployments performed (with and without build cache)
✅ Removed env section from next.config.ts (was suggested by support)
✅ Downgraded from Next.js 16 RC to stable Next.js 15.5.6
✅ Created test variable TEST_SERVICE_KEY to rule out name-specific issues
✅ Verified Route Handlers are correctly implemented (app/api/*/route.ts)

Project information (URL, framework, environment, project settings):

Project URL: https://neonarc.de
Framework: Next.js 15.5.6 (App Router)
React: 18.3.1
Deployment: Vercel Production
Environment: Production (VERCEL_ENV=production)
Route Handlers: app/api/*/route.ts structure
Variables Set in Dashboard:
- NEXT_PUBLIC_SUPABASE_URL ✅ (works)
- NEXT_PUBLIC_SUPABASE_ANON_KEY ✅ (works)
- SUPABASE_SERVICE_ROLE_KEY ❌ (doesn’t work)
- TEST_SERVICE_KEY ❌ (doesn’t work)

Impact: This is blocking critical production functionality:

Player search API routes
Contact request management
User blocking/unblocking
Direct messaging

Question: Has anyone else experienced this issue where ALL non-NEXT_PUBLIC_ environment variables are unavailable in Next.js Route Handlers on Vercel? Is there a project-specific setting or configuration that could be affecting this?

Any help or insights would be greatly appreciated! Sascha