deployment error Vercel Error occurred prerendering page Error: @supabase/ssr: Your project’s URL and API key are required to create a Supabase clien

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Summary

The deployment failed during the Static Site Generation (SSG) prerender phase in Next.js, specifically when attempting to render the /admin/Expertise page. The build process crashed because the Supabase client library (@supabase/ssr) attempted to initialize a client instance without the required environment variables (NEXT_PUBLIC_SUPABASE_URL and NEXT_PUBLIC_SUPABASE_ANON_KEY). Because these values were missing or undefined at build time, the framework halted the build to prevent a runtime failure in production.

Root Cause

The immediate technical failure is triggered by the Supabase initialization function asserting that the project URL and API key are present. In the context of Next.js (using the App Router), this typically happens in one of two places:

  • Server Components / Prerendering: The code executes during the build (next build) or during a server-side request.
  • Middleware / Edge Runtimes: The code runs in an environment where process.env or process.env.NEXT_PUBLIC_* is not automatically populated from .env files unless explicitly handled.

The root cause is a misalignment between the Supabase client wrapper logic and the Vercel/Next.js environment configuration. The client wrapper expected these secrets to be present at the exact moment the createClient function was called during the build step.

Why This Happens in Real Systems

This issue is common in modern Jamstack architectures (Next.js + Vercel) interacting with third-party BaaS (Backend as a Service) providers like Supabase.

  1. Build-Time Prerendering: Next.js attempts to pre-render pages at build time for performance. If the Supabase client is instantiated at the module level (top-level of a file) in a Server Component, it runs during the build.
  2. Environment Variable Scoping: In Vercel, environment variables are not automatically injected into the build environment unless they are explicitly defined in the project settings. Locally, a .env.local file might work, but the cloud build pipeline might not have access to the same variables.
  3. Hydration Mismatch Prevention: To prevent security risks (like exposing private keys to the browser), Supabase SDKs strictly validate input. If keys are missing, the library throws an error rather than failing silently, ensuring the developer knows the system is misconfigured before it reaches production users.

Real-World Impact

  • Deployment Blockage: The most immediate impact is a hard stop to the CI/CD pipeline. No changes can be deployed until the configuration is fixed.
  • Downtime: If the error is introduced in a hotfix or minor update, the site can go down entirely, showing a Vercel “Application Error” page to users.
  • Development Velocity Loss: Junior developers often get stuck here for hours, debugging code that is actually correct, because the issue is environmental rather than logical.
  • Security Risks: If a developer panics and hardcodes keys to fix the build, they risk exposing sensitive API keys in client-side code or server logs.

Example or Code

The error specifically references @supabase/ssr. While the exact file content isn’t provided, the issue stems from a client initialization helper that looks similar to this:

import { createServerClient } from '@supabase/ssr'
import { cookies } from 'next/headers'

export function createClient(cookieStore) {
  return createServerClient(
    process.env.NEXT_PUBLIC_SUPABASE_URL, // This is undefined during build
    process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY, // This is undefined during build
    {
      cookies: {
        get() {
          return cookieStore.getAll().map((cookie) => `${cookie.name}=${cookie.value}`).join('; ')
        },
        set() {
          // ...
        },
        remove() {
          // ...
        },
      },
    }
  )
}

If NEXT_PUBLIC_SUPABASE_URL is not defined in the Vercel project settings or .env files, process.env.NEXT_PUBLIC_SUPABASE_URL evaluates to undefined, triggering the Your project's URL and API key are required error.

How Senior Engineers Fix It

Senior engineers approach this as an infrastructure configuration issue, not a code logic issue.

  1. Verify Local Configuration: Ensure .env.local exists with the correct keys and that the .env file is listed in .gitignore.
  2. Configure Vercel Environment Variables: Log in to the Vercel dashboard, navigate to the project’s Settings > Environment Variables, and add:
    • NEXT_PUBLIC_SUPABASE_URL
    • NEXT_PUBLIC_SUPABASE_ANON_KEY
    • Note: In Supabase v2, the project URL is specific to your project, not the generic dashboard URL in the error message.
  3. Trigger a Redeploy: Manually trigger a redeployment in Vercel to inject the new variables into the build environment.
  4. Audit Client Instantiation: Move Supabase client creation inside Server Actions or API Routes if the data is sensitive, or ensure it is compatible with the App Router’s createServerClient pattern, avoiding initialization at the top level if it requires dynamic cookies.

Why Juniors Miss It

Juniors often misdiagnose this error because the error message points to a code line (the createClient function) rather than the configuration.

  • Confusion of Scope: They see “URL and API key are required” and assume they need to type the string literal into the code file, rather than setting environment variables.
  • Local vs. Remote Discrepancy: They verify that the keys exist in their local .env file, assuming those same variables are automatically transferred to the hosting provider (Vercel). They do not realize Vercel requires manual input for security reasons.
  • Framework Complexity: The abstract nature of “Prerendering” and “Server Components” obscures the fact that the code runs twice (once on the server, once in the browser). They might not realize NEXT_PUBLIC_ variables are required even for server-side execution during the build.
  • Misreading the URL: The error message includes a link to https://supabase.com/dashboard/project/_/settings/api. Juniors often click this expecting a solution, rather than realizing the /_/ part is a placeholder and they need to copy the actual URL from their specific project dashboard.