diff --git a/infra/relay/src/deploymentConfig.test.ts b/infra/relay/src/deploymentConfig.test.ts
index d7940b803188..44c7627a4daf 100644
--- a/infra/relay/src/deploymentConfig.test.ts
+++ b/infra/relay/src/deploymentConfig.test.ts
@@ -1,4 +1,5 @@
 import { describe, expect, it } from "vite-plus/test";
+import * as Schema from "effect/Schema";
 
 import {
   managedEndpointDigestInput,
@@ -7,11 +8,14 @@ import {
   isManagedEndpointHostname,
   managedEndpointTunnelName,
   relayOwnsManagedEndpointZone,
+  RelayPublicDomainLabelTooLongError,
   relayPublicDomainForStage,
   relayResourceNameForStage,
   relayStageSlug,
 } from "./deploymentConfig.ts";
 
+const isRelayPublicDomainLabelTooLongError = Schema.is(RelayPublicDomainLabelTooLongError);
+
 describe("relayStageSlug", () => {
   it("matches Alchemy physical-name sanitization for default developer stages", () => {
     expect(relayStageSlug("dev_julius")).toBe("dev-julius");
@@ -28,6 +32,29 @@ describe("relayPublicDomainForStage", () => {
       "relay-dev-julius.example.com",
     );
   });
+
+  it("reports the stage and derived DNS label when the label is too long", () => {
+    const stage = `dev_${"x".repeat(60)}`;
+    let error: unknown;
+
+    try {
+      relayPublicDomainForStage(stage, "example.com");
+    } catch (cause) {
+      error = cause;
+    }
+
+    if (!isRelayPublicDomainLabelTooLongError(error)) {
+      throw error;
+    }
+    expect(error).toMatchObject({
+      stage,
+      label: `relay-dev-${"x".repeat(60)}`,
+      maxLength: 63,
+    });
+    expect(error.message).toBe(
+      `Relay stage '${stage}' produces custom domain label 'relay-dev-${"x".repeat(60)}' (70 characters), exceeding the DNS label limit of 63.`,
+    );
+  });
 });
 
 describe("relayOwnsManagedEndpointZone", () => {
diff --git a/infra/relay/src/deploymentConfig.ts b/infra/relay/src/deploymentConfig.ts
index fbb130548220..fe9d37b29988 100644
--- a/infra/relay/src/deploymentConfig.ts
+++ b/infra/relay/src/deploymentConfig.ts
@@ -1,10 +1,24 @@
 import type { RelayManagedEndpoint } from "@t3tools/contracts/relay";
+import * as Schema from "effect/Schema";
 
 const DNS_LABEL_MAX_LENGTH = 63;
 const MANAGED_ENDPOINT_HASH_LENGTH = 16;
 const MANAGED_ENDPOINT_TUNNEL_PREFIX = "t3coderelay-managedendpoint";
 export const MANAGED_ENDPOINT_ZONE_OWNER_STAGE = "prod";
 
+export class RelayPublicDomainLabelTooLongError extends Schema.TaggedErrorClass<RelayPublicDomainLabelTooLongError>()(
+  "RelayPublicDomainLabelTooLongError",
+  {
+    stage: Schema.String,
+    label: Schema.String,
+    maxLength: Schema.Number,
+  },
+) {
+  override get message(): string {
+    return `Relay stage '${this.stage}' produces custom domain label '${this.label}' (${this.label.length} characters), exceeding the DNS label limit of ${this.maxLength}.`;
+  }
+}
+
 function normalizeZoneName(zoneName: string): string {
   return zoneName
     .trim()
@@ -62,7 +76,11 @@ export function relayPublicDomainForStage(stage: string, zoneName: string): stri
   const stageSlug = relayStageSlug(stage);
   const relayLabel = stage === "prod" ? "relay" : `relay-${stageSlug}`;
   if (relayLabel.length > DNS_LABEL_MAX_LENGTH) {
-    throw new Error(`Relay stage is too long for a custom domain: ${stage}`);
+    throw new RelayPublicDomainLabelTooLongError({
+      stage,
+      label: relayLabel,
+      maxLength: DNS_LABEL_MAX_LENGTH,
+    });
   }
   return `${relayLabel}.${normalizeZoneName(zoneName)}`;
 }