diff --git a/src/backend/auth/login-pipeline.ts b/src/backend/auth/login-pipeline.ts
index 2d29a1351..7ed3dffe7 100644
--- a/src/backend/auth/login-pipeline.ts
+++ b/src/backend/auth/login-pipeline.ts
@@ -171,6 +171,27 @@ async function isTrustedDevice(req: Request, userId: string): Promise<boolean> {
 
 type DeviceType = ReturnType<typeof parseUserAgent>["type"];
 
+/**
+ * An account that only signs in through SSO or LDAP must keep doing so. A
+ * passkey or any other local method would skip the identity provider, so a
+ * user disabled there could still get in.
+ */
+export function assertMethodAllowedFor(
+  user: Pick<UserRecord, "isExternal" | "passwordHash">,
+  identity: VerifiedIdentity,
+  methodId: string,
+): void {
+  if (identity.kind === "external" || !user.isExternal) return;
+  if (user.passwordHash && user.passwordHash.trim() !== "") return;
+  ensureCoreLoginProviders();
+  if (getLoginMethod(methodId)?.external) return;
+  throw new LoginMethodError(
+    "This account signs in through its identity provider",
+    403,
+    "external_login_required",
+  );
+}
+
 async function resolveUser(
   identity: VerifiedIdentity,
   deviceType: DeviceType,
@@ -253,6 +274,7 @@ export async function runLogin(
 ): Promise<LoginResult> {
   const deviceType = parseUserAgent(req).type;
   const user = await resolveUser(identity, deviceType);
+  assertMethodAllowedFor(user, identity, context.methodId);
 
   await unlockUser(user, identity, deviceType);
   if ("password" in identity && identity.password) {
diff --git a/src/backend/tests/auth/login-pipeline.test.ts b/src/backend/tests/auth/login-pipeline.test.ts
index 55b1742cb..c74588791 100644
--- a/src/backend/tests/auth/login-pipeline.test.ts
+++ b/src/backend/tests/auth/login-pipeline.test.ts
@@ -527,6 +527,46 @@ describe("external identities", () => {
   });
   afterEach(() => disposeOidc());
 
+  function addSsoUser(passwordHash = "") {
+    addUser({ passwordHash });
+    h.state.identities.push({
+      userId: "u1",
+      providerId: "3",
+      subject: "sub-1",
+    } as never);
+  }
+
+  it("refuses a passkey sign in for an account that only uses SSO", async () => {
+    addSsoUser();
+    await expect(
+      runLogin(
+        fakeRequest() as never,
+        { kind: "user", userId: "u1", mfaSatisfied: true },
+        { methodId: "passkey", rememberMe: false },
+      ),
+    ).rejects.toMatchObject({ status: 403, code: "external_login_required" });
+  });
+
+  it("still lets an SSO account with a password use a passkey", async () => {
+    addSsoUser(bcrypt.hashSync(PASSWORD, 4));
+    const result = await runLogin(
+      fakeRequest() as never,
+      { kind: "user", userId: "u1", mfaSatisfied: true },
+      { methodId: "passkey", rememberMe: false },
+    );
+    expect(result.kind).toBe("session");
+  });
+
+  it("lets an external method sign an SSO account in by user id", async () => {
+    addSsoUser();
+    const result = await runLogin(
+      fakeRequest() as never,
+      { kind: "user", userId: "u1" },
+      { methodId: "oidc", rememberMe: false },
+    );
+    expect(result.kind).toBe("session");
+  });
+
   it("provisions the first user as admin and links the identity", async () => {
     const result = await runLogin(fakeRequest() as never, external, {
       methodId: "oidc",
diff --git a/src/ui/auth/Auth.tsx b/src/ui/auth/Auth.tsx
index 4774a6010..7600cc496 100644
--- a/src/ui/auth/Auth.tsx
+++ b/src/ui/auth/Auth.tsx
@@ -725,6 +725,10 @@ export function Auth({ onLogin }: AuthProps) {
       toast.error(t("auth.secondFactorUnavailable"));
       return;
     }
+    if (error?.response?.data?.code === "external_login_required") {
+      toast.error(t("auth.externalLoginRequired"));
+      return;
+    }
     toast.error(
       error?.response?.data?.error || error?.message || t(fallbackKey),
     );
diff --git a/src/ui/locales/en.json b/src/ui/locales/en.json
index 697ca244a..897aff74e 100644
--- a/src/ui/locales/en.json
+++ b/src/ui/locales/en.json
@@ -1027,6 +1027,7 @@
     "loadingServer": "Loading server...",
     "password": "Password",
     "secondFactorUnavailable": "Your account uses a second factor that is not available right now. Contact an admin to reset it.",
+    "externalLoginRequired": "This account signs in through single sign-on. Use that button to sign in.",
     "secondFactorNoUI": "This second factor cannot be shown on this screen. Contact an admin.",
     "serverAuthentication": "Server sign-in",
     "minChars": "min. {{min}} characters",