diff --git a/Dockerfile b/Dockerfile
index a2f5f70..6efd963 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -13,6 +13,7 @@ RUN apk add --no-cache openssl
 # them via build args (Coolify: mark them as build variables) or the browser
 # ClerkProvider / public URLs will be missing.
 ARG NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY
+ARG NEXT_PUBLIC_CLERK_FAPI_HOST
 ARG NEXT_PUBLIC_CLERK_SIGN_IN_URL
 ARG NEXT_PUBLIC_CLERK_SIGN_UP_URL
 ARG NEXT_PUBLIC_CLERK_SIGN_IN_FALLBACK_REDIRECT_URL
diff --git a/docs/coolify.md b/docs/coolify.md
index 612989a..a9f1465 100644
--- a/docs/coolify.md
+++ b/docs/coolify.md
@@ -108,6 +108,9 @@ DATABASE_URL=postgresql://postgres:YOUR_PASSWORD@spomeniqr-db:5432/monuments
 # Clerk
 NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY=pk_live_...
 CLERK_SECRET_KEY=sk_live_...
+# Custom Clerk frontend API domain (if configured in Clerk dashboard) — the CSP
+# allowlist needs it. Build variable. Omit if not using a custom domain.
+NEXT_PUBLIC_CLERK_FAPI_HOST=clerk.testbed.mk
 NEXT_PUBLIC_CLERK_SIGN_IN_URL=/sign-in
 NEXT_PUBLIC_CLERK_SIGN_UP_URL=/sign-up
 NEXT_PUBLIC_CLERK_SIGN_IN_FALLBACK_REDIRECT_URL=/
@@ -148,7 +151,10 @@ NODE_ENV=production
   node -e "import('bcryptjs').then(b => b.default.hash('YOUR_PASSWORD', 12).then(console.log))"
   ```
 - Mark **every `NEXT_PUBLIC_*` variable as a build variable** (checkbox) so it is
-  inlined into the client bundle at `next build`. Server-only vars
+  inlined into the client bundle at `next build` and visible to `next.config.ts`.
+  `NEXT_PUBLIC_CLERK_FAPI_HOST` in particular must be a build variable — the CSP
+  is generated at build time and will otherwise block Clerk JS if you use a
+  custom Clerk frontend API domain. Server-only vars
   (`CLERK_SECRET_KEY`, `DATABASE_URL`, `ADMIN_SESSION_SECRET`,
   `SUPER_ADMIN_PASSWORD_HASH`, `S3_*`) stay as normal runtime variables.
 - The `super` admin row is provisioned automatically on container start by
@@ -344,6 +350,7 @@ npx prisma db push
 |----------|----------|-------------|
 | `DATABASE_URL` | Yes | PostgreSQL connection string (Coolify internal) |
 | `NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY` | Yes | Clerk publishable key (`pk_live_...`) |
+| `NEXT_PUBLIC_CLERK_FAPI_HOST` | No* | Custom Clerk frontend API domain (`clerk.testbed.mk`) — required when using a custom domain; build variable |
 | `CLERK_SECRET_KEY` | Yes | Clerk secret key (`sk_live_...`) |
 | `NEXT_PUBLIC_CLERK_SIGN_IN_URL` | Yes | `/sign-in` |
 | `NEXT_PUBLIC_CLERK_SIGN_UP_URL` | Yes | `/sign-up` |
diff --git a/next.config.ts b/next.config.ts
index dbc420a..51b9fd6 100644
--- a/next.config.ts
+++ b/next.config.ts
@@ -4,9 +4,12 @@ const s3Host = process.env.S3_ENDPOINT
   ? new URL(process.env.S3_ENDPOINT).hostname
   : "";
 
-// Derive the Clerk frontend API host from the publishable key so the
-// CSP allowlist always matches the active environment. Clerk supports
-// two publishable key formats:
+// Derive the Clerk frontend API host so the CSP allowlist always matches the
+// active environment. When a custom Clerk frontend API domain is configured in
+// the Clerk dashboard (e.g. clerk.testbed.mk), it is resolved at runtime by
+// clerk-js and can NOT be derived from the publishable key — so it must be
+// provided explicitly via NEXT_PUBLIC_CLERK_FAPI_HOST. Otherwise Clerk
+// supports two publishable key formats:
 //
 //   1) "Encoded" form (older):  pk_test_<base64slug>$
 //      The base64 portion decodes to "<slug>.clerk.accounts.dev" (test)
@@ -17,6 +20,16 @@ const s3Host = process.env.S3_ENDPOINT
 //      The slug may itself contain hyphens and digits, so only the
 //      final dash-group is captured as the suffix.
 function clerkFrontendApiHost(): string | null {
+  const custom = process.env.NEXT_PUBLIC_CLERK_FAPI_HOST?.trim();
+  if (custom) {
+    try {
+      const host = new URL(custom.includes("://") ? custom : `https://${custom}`).hostname;
+      if (host) return host;
+    } catch {
+      // fall through to derivation from the publishable key
+    }
+  }
+
   const key = process.env.NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY;
   if (!key) return null;