diff --git a/apps/web/src/server/runtime-env.test.ts b/apps/web/src/server/runtime-env.test.ts
index a629ce8..01df164 100644
--- a/apps/web/src/server/runtime-env.test.ts
+++ b/apps/web/src/server/runtime-env.test.ts
@@ -2,8 +2,54 @@ import { describe, expect, it } from "vitest";
 import { assertSecureRuntimeEnv, getRuntimeEnvViolations } from "./runtime-env";
 
 describe("runtime env validation", () => {
-  it("allows non-production environments without auth runtime settings", () => {
-    expect(getRuntimeEnvViolations({ NODE_ENV: "development" })).toEqual([]);
+  it("allows non-production environments once an auth secret is present", () => {
+    expect(
+      getRuntimeEnvViolations({ NODE_ENV: "development", AUTH_SECRET: "any-dev-secret" }),
+    ).toEqual([]);
+  });
+
+  it("does not apply production secret strength rules outside production", () => {
+    // Short, low-entropy and placeholder secrets stay acceptable in dev — only
+    // their total absence is fatal there.
+    expect(
+      getRuntimeEnvViolations({
+        NODE_ENV: "development",
+        AUTH_SECRET: "dev-secret-change-in-production",
+      }),
+    ).toEqual([]);
+  });
+
+  it("rejects a missing auth secret in development too", () => {
+    // Regression guard: a dev server with no secret starts fine but answers
+    // every /api/auth/* route with an opaque 500, so login bounces back to the
+    // form with no error. It must fail at startup instead.
+    expect(getRuntimeEnvViolations({ NODE_ENV: "development" })).toContain(
+      "AUTH_SECRET or NEXTAUTH_SECRET must be set.",
+    );
+  });
+
+  it("rejects E2E_TEST_MODE on an https deployment outside production", () => {
+    expect(
+      getRuntimeEnvViolations({
+        NODE_ENV: "development",
+        AUTH_SECRET: "any-dev-secret",
+        NEXTAUTH_URL: "https://nexus.example.com",
+        E2E_TEST_MODE: "true",
+      }),
+    ).toContain(
+      "E2E_TEST_MODE must not be 'true' on an https deployment — it disables login rate limiting and session controls.",
+    );
+  });
+
+  it("allows E2E_TEST_MODE against a local http deployment", () => {
+    expect(
+      getRuntimeEnvViolations({
+        NODE_ENV: "development",
+        AUTH_SECRET: "any-dev-secret",
+        NEXTAUTH_URL: "http://localhost:3100",
+        E2E_TEST_MODE: "true",
+      }),
+    ).toEqual([]);
   });
 
   it("accepts a valid production auth secret and https url", () => {
@@ -76,6 +122,6 @@ describe("runtime env validation", () => {
         NEXTAUTH_SECRET: "dev-secret-change-in-production",
         NEXTAUTH_URL: "not-a-url",
       }),
-    ).toThrow(/Invalid production runtime configuration/);
+    ).toThrow(/Invalid runtime configuration/);
   });
 });
diff --git a/apps/web/src/server/runtime-env.ts b/apps/web/src/server/runtime-env.ts
index ad03bc6..957eaa2 100644
--- a/apps/web/src/server/runtime-env.ts
+++ b/apps/web/src/server/runtime-env.ts
@@ -58,22 +58,55 @@ function isLocalhost(hostname: string): boolean {
   return hostname === "localhost" || hostname === "127.0.0.1" || hostname === "::1";
 }
 
-export function getRuntimeEnvViolations(env: RuntimeEnv = process.env): string[] {
-  if (!isProductionLike(env)) {
-    return [];
+function isHttpsUrl(value: string | null): boolean {
+  if (!value) return false;
+  try {
+    return new URL(value).protocol === "https:";
+  } catch {
+    return false;
   }
+}
 
+export function getRuntimeEnvViolations(env: RuntimeEnv = process.env): string[] {
   const violations: string[] = [];
   const authSecret = readEnvValue(env, "AUTH_SECRET", "NEXTAUTH_SECRET");
   const authUrl = readEnvValue(env, "AUTH_URL", "NEXTAUTH_URL");
+  const production = isProductionLike(env);
 
+  // Checked in EVERY environment, not just production: without a secret
+  // Auth.js cannot sign session JWTs, so it answers *every* /api/auth/* route
+  // with an opaque 500 "problem with the server configuration" and the login
+  // form silently bounces the user back to itself with no error shown.
+  // A dev server started outside the monorepo root (which never loads the root
+  // .env) hits this, and the symptom points nowhere near the cause. Failing at
+  // startup turns a day of debugging into one readable line.
   if (!authSecret) {
-    violations.push("AUTH_SECRET or NEXTAUTH_SECRET must be set in production.");
-  } else if (DISALLOWED_PRODUCTION_SECRETS.has(authSecret)) {
+    violations.push(
+      production
+        ? "AUTH_SECRET or NEXTAUTH_SECRET must be set in production."
+        : "AUTH_SECRET or NEXTAUTH_SECRET must be set.",
+    );
+  }
+
+  // An https deployment URL means the instance is reachable off this machine,
+  // whatever NODE_ENV claims. E2E_TEST_MODE disables login rate limiting and
+  // the concurrent-session registry, so it must never be live there. The
+  // production case is covered by getDevBypassViolations() further down.
+  if (!production && env["E2E_TEST_MODE"] === "true" && isHttpsUrl(authUrl)) {
+    violations.push(
+      "E2E_TEST_MODE must not be 'true' on an https deployment — it disables login rate limiting and session controls.",
+    );
+  }
+
+  if (!production) {
+    return violations;
+  }
+
+  if (authSecret && DISALLOWED_PRODUCTION_SECRETS.has(authSecret)) {
     violations.push(
       "AUTH_SECRET or NEXTAUTH_SECRET must not use a known development placeholder in production.",
     );
-  } else {
+  } else if (authSecret) {
     if (authSecret.length < MIN_AUTH_SECRET_LENGTH) {
       violations.push(
         `AUTH_SECRET or NEXTAUTH_SECRET must be at least ${MIN_AUTH_SECRET_LENGTH} characters in production.`,
@@ -110,5 +143,5 @@ export function assertSecureRuntimeEnv(env: RuntimeEnv = process.env): void {
     return;
   }
 
-  throw new Error(`Invalid production runtime configuration: ${violations.join(" ")}`);
+  throw new Error(`Invalid runtime configuration: ${violations.join(" ")}`);
 }