diff --git a/src/lib/auth/__tests__/secure-cookie.test.ts b/src/lib/auth/__tests__/secure-cookie.test.ts
index e56d5645e..c329851ab 100644
--- a/src/lib/auth/__tests__/secure-cookie.test.ts
+++ b/src/lib/auth/__tests__/secure-cookie.test.ts
@@ -1,6 +1,13 @@
 import { describe, it, expect, afterEach, vi } from "vitest";
 
-import { shouldEmitSecureCookie } from "../secure-cookie";
+import {
+  shouldEmitSecureCookie,
+  detectInsecureCookieTransport,
+} from "../secure-cookie";
+
+function req(headers: Record<string, string>, url = "http://10.0.0.5:3000/x") {
+  return { headers: new Headers(headers), url };
+}
 
 describe("shouldEmitSecureCookie", () => {
   afterEach(() => {
@@ -47,3 +54,56 @@ describe("shouldEmitSecureCookie", () => {
     expect(shouldEmitSecureCookie()).toBe(false);
   });
 });
+
+describe("detectInsecureCookieTransport", () => {
+  afterEach(() => {
+    vi.unstubAllEnvs();
+  });
+
+  it("returns null when the Secure flag is off (SESSION_COOKIE_SECURE=false)", () => {
+    vi.stubEnv("NODE_ENV", "production");
+    vi.stubEnv("SESSION_COOKIE_SECURE", "false");
+    expect(detectInsecureCookieTransport(req({}))).toBeNull();
+  });
+
+  it("warns when a Secure cookie will be set on a plain-HTTP request (no proxy)", () => {
+    vi.stubEnv("NODE_ENV", "production");
+    vi.stubEnv("SESSION_COOKIE_SECURE", "");
+    const msg = detectInsecureCookieTransport(req({}));
+    expect(msg).toContain("SESSION_COOKIE_SECURE=false");
+  });
+
+  it("returns null behind a TLS proxy that forwards X-Forwarded-Proto: https", () => {
+    vi.stubEnv("NODE_ENV", "production");
+    vi.stubEnv("SESSION_COOKIE_SECURE", "");
+    expect(
+      detectInsecureCookieTransport(req({ "x-forwarded-proto": "https" })),
+    ).toBeNull();
+  });
+
+  it("reads the left-most entry of a comma-separated proto chain", () => {
+    vi.stubEnv("NODE_ENV", "production");
+    vi.stubEnv("SESSION_COOKIE_SECURE", "");
+    expect(
+      detectInsecureCookieTransport(
+        req({ "x-forwarded-proto": "https, http" }),
+      ),
+    ).toBeNull();
+  });
+
+  it("warns when the proxy forwards X-Forwarded-Proto: http explicitly", () => {
+    vi.stubEnv("NODE_ENV", "production");
+    vi.stubEnv("SESSION_COOKIE_SECURE", "");
+    expect(
+      detectInsecureCookieTransport(req({ "x-forwarded-proto": "http" })),
+    ).not.toBeNull();
+  });
+
+  it("returns null when the request URL is already https", () => {
+    vi.stubEnv("NODE_ENV", "production");
+    vi.stubEnv("SESSION_COOKIE_SECURE", "");
+    expect(
+      detectInsecureCookieTransport(req({}, "https://healthlog.example.com/x")),
+    ).toBeNull();
+  });
+});
diff --git a/src/lib/auth/login-response.ts b/src/lib/auth/login-response.ts
index d0998afd8..d9330dd83 100644
--- a/src/lib/auth/login-response.ts
+++ b/src/lib/auth/login-response.ts
@@ -27,6 +27,8 @@ import {
 } from "@/lib/auth/native-client";
 import { issueAccessAndRefresh } from "@/lib/auth/refresh-token";
 import { recordSignInDevice } from "@/lib/auth/login-alert";
+import { detectInsecureCookieTransport } from "@/lib/auth/secure-cookie";
+import { annotate } from "@/lib/logging/context";
 import type { User } from "@/generated/prisma/client";
 
 export interface FinishLoginParams {
@@ -116,8 +118,20 @@ export async function finishLogin(
     });
   }
 
-  // Browser path — session cookie. `createSession` anchors the onboarding
-  // cookie itself; thread the user's onboarding state through.
+  // Browser path — session cookie. Before minting it, surface the single
+  // most common self-host snag: a Secure cookie about to be set on a plain-HTTP
+  // request drops silently and loops the login. Warn-log + wide-event so an
+  // operator sees the cause; the login still completes.
+  const transportWarning = detectInsecureCookieTransport(request);
+  if (transportWarning) {
+    console.warn(
+      `[auth] session-cookie transport mismatch: ${transportWarning}`,
+    );
+    annotate({ meta: { session_cookie_secure_transport_mismatch: true } });
+  }
+
+  // `createSession` anchors the onboarding cookie itself; thread the user's
+  // onboarding state through.
   await createSession(
     user.id,
     user.onboardingCompletedAt == null,
diff --git a/src/lib/auth/secure-cookie.ts b/src/lib/auth/secure-cookie.ts
index c2fb8b931..d200b9af0 100644
--- a/src/lib/auth/secure-cookie.ts
+++ b/src/lib/auth/secure-cookie.ts
@@ -28,3 +28,57 @@ export function shouldEmitSecureCookie(): boolean {
   if (override === "true") return true;
   return process.env.NODE_ENV === "production";
 }
+
+/**
+ * One-line diagnostic for the single most common self-host login-loop cause:
+ * the app is about to set a `Secure` session cookie
+ * (`shouldEmitSecureCookie() === true`) while the request itself arrived over
+ * plain HTTP. The browser silently drops a `Secure` cookie on an `http://`
+ * origin, so the next request is unauthenticated and the user bounces back to
+ * the login page with no error at all — it reads like a wrong password forever.
+ *
+ * Returns a human-readable message when the mismatch is detected, else `null`.
+ * The caller logs it at login time (`console.warn` survives the prod
+ * `console.*` strip) so an operator sees WHY login loops without having to find
+ * the note buried in three docs. Non-fatal: the login still completes; the
+ * cookie just won't stick until `SESSION_COOKIE_SECURE=false` is set (or TLS is
+ * fronted).
+ *
+ * A request counts as HTTPS when `x-forwarded-proto` names https (the shape
+ * every documented TLS reverse proxy sets) OR the request URL is already
+ * https. A proxy that terminates TLS but omits `x-forwarded-proto` can produce
+ * a benign false positive — the message names that case so it stays actionable
+ * either way.
+ */
+export function detectInsecureCookieTransport(req: {
+  headers: Headers;
+  url: string;
+}): string | null {
+  if (!shouldEmitSecureCookie()) return null;
+
+  // `x-forwarded-proto` can be a comma-separated proto chain; the left-most
+  // entry is the client-facing scheme the browser actually spoke.
+  const clientProto = req.headers
+    .get("x-forwarded-proto")
+    ?.split(",")[0]
+    ?.trim()
+    .toLowerCase();
+  if (clientProto === "https") return null;
+
+  let urlIsHttps = false;
+  try {
+    urlIsHttps = new URL(req.url).protocol === "https:";
+  } catch {
+    urlIsHttps = false;
+  }
+  if (urlIsHttps) return null;
+
+  return (
+    "This request arrived over plain HTTP but the session cookie is being " +
+    "issued with the `Secure` flag — the browser will drop it and login will " +
+    "loop back to the sign-in page with no error message. If you serve HTTP " +
+    "directly (LAN / NAS / Tailscale, no TLS reverse proxy), set " +
+    "SESSION_COOKIE_SECURE=false. If a reverse proxy terminates TLS in front " +
+    "of this container, make sure it forwards `X-Forwarded-Proto: https`."
+  );
+}