diff --git a/src/app/(dashboard)/dashboard/providers/[id]/components/modals/AddApiKeyModal.tsx b/src/app/(dashboard)/dashboard/providers/[id]/components/modals/AddApiKeyModal.tsx
index 9802dafb35..43dc76981c 100644
--- a/src/app/(dashboard)/dashboard/providers/[id]/components/modals/AddApiKeyModal.tsx
+++ b/src/app/(dashboard)/dashboard/providers/[id]/components/modals/AddApiKeyModal.tsx
@@ -1,7 +1,7 @@
 "use client";
 import { useState, useEffect, useRef } from "react";
 import { useTranslations } from "next-intl";
-import { Button, Badge, Input, Modal, Toggle } from "@/shared/components";
+import { Button, Badge, Input, Modal, Toggle, TALL_MODAL_PROPS } from "@/shared/components";
 import {
   providerAllowsOptionalApiKey,
   supportsBulkApiKey,
@@ -445,7 +445,7 @@ export default function AddApiKeyModal({
       title={getAddCredentialModalTitle(t, providerDisplayName, webSessionCredential)}
       onClose={onClose}
       size="lg"
-      bodyClassName="p-6 max-h-[85vh] overflow-y-auto"
+      {...TALL_MODAL_PROPS}
     >
       <div className="flex flex-col gap-4">
         {webProviderHostLink && (
diff --git a/src/shared/components/Modal.tsx b/src/shared/components/Modal.tsx
index dfff5de2f1..5e385ebf8d 100644
--- a/src/shared/components/Modal.tsx
+++ b/src/shared/components/Modal.tsx
@@ -4,6 +4,14 @@ import { useEffect, useRef, useId } from "react";
 import { cn } from "@/shared/utils/cn";
 import Button from "./Button";
 
+// #6265 — preset for content-heavy modals: caps height on the OUTERMOST dialog
+// wrapper only (single scroll owner) and keeps the inner body plain (no
+// independent max-h/overflow), avoiding a double height cap that clips content.
+export const TALL_MODAL_PROPS = {
+  className: "max-h-[90vh] overflow-y-auto",
+  bodyClassName: "p-6",
+};
+
 interface ModalProps {
   isOpen: boolean;
   onClose: () => void;
diff --git a/src/shared/components/index.tsx b/src/shared/components/index.tsx
index a7ffdf4f36..774165b1ce 100644
--- a/src/shared/components/index.tsx
+++ b/src/shared/components/index.tsx
@@ -6,7 +6,7 @@ export { default as Checkbox } from "./Checkbox";
 export { default as Textarea } from "./Textarea";
 export { default as Card } from "./Card";
 export { default as Collapsible } from "./Collapsible";
-export { default as Modal, ConfirmModal } from "./Modal";
+export { default as Modal, ConfirmModal, TALL_MODAL_PROPS } from "./Modal";
 export { default as Loading, Spinner, PageLoading, Skeleton, CardSkeleton } from "./Loading";
 export { default as Avatar } from "./Avatar";
 export { default as Badge } from "./Badge";
diff --git a/tests/unit/ui/web-session-cookie-modal-size-6265.test.tsx b/tests/unit/ui/web-session-cookie-modal-size-6265.test.tsx
new file mode 100644
index 0000000000..6d569f9047
--- /dev/null
+++ b/tests/unit/ui/web-session-cookie-modal-size-6265.test.tsx
@@ -0,0 +1,81 @@
+// @vitest-environment jsdom
+// Issue #6265 — the "Add session cookie" modal (AddApiKeyModal, shared by every
+// `-web` cookie provider) was undersized on a 1920x1080 viewport: users had to
+// scroll *inside* the modal to reach Save, and the top of the cookie helper text
+// was clipped. Root cause: the height cap (`max-h-*` + `overflow-y-auto`) lived on
+// the INNER body div only, while the OUTERMOST dialog wrapper had no height bound
+// at all — so the whole box (header + body) could grow taller than the viewport
+// and get clipped by the centering flexbox, independent of the inner scrollbar.
+//
+// Fix: move the single height cap to the outermost dialog wrapper (`role="dialog"`)
+// and stop giving the inner body container its own independent `max-h-`/`overflow`
+// cap, so there is exactly one scroll owner for the whole modal.
+import React, { act } from "react";
+import { createRoot } from "react-dom/client";
+import { describe, it, expect, vi, afterEach } from "vitest";
+
+vi.mock("next-intl", () => ({
+  useTranslations: () => (key: string) => key,
+}));
+
+const { default: AddApiKeyModal } =
+  await import("../../../src/app/(dashboard)/dashboard/providers/[id]/components/modals/AddApiKeyModal");
+
+const containers: Array<{ root: ReturnType<typeof createRoot>; el: HTMLDivElement }> = [];
+
+function render(props: Record<string, unknown>) {
+  const el = document.createElement("div");
+  document.body.appendChild(el);
+  const root = createRoot(el);
+  act(() => {
+    root.render(
+      <AddApiKeyModal
+        isOpen
+        onSave={async () => undefined}
+        onClose={() => {}}
+        {...(props as any)}
+      />
+    );
+  });
+  containers.push({ root, el });
+  return el;
+}
+
+afterEach(() => {
+  for (const { root, el } of containers.splice(0)) {
+    act(() => root.unmount());
+    el.remove();
+  }
+});
+
+describe("AddApiKeyModal — cookie modal sizing (#6265)", () => {
+  it("caps height on the OUTERMOST dialog wrapper, not on an inner body div", () => {
+    // chatgpt-web is a `kind: "cookie"` web-session provider — same shared modal
+    // path lmarena/claude-web/gemini-web/kimi-web/z-ai all go through.
+    const el = render({ provider: "chatgpt-web", providerName: "ChatGPT (Web)" });
+
+    const dialog = el.querySelector<HTMLElement>('[role="dialog"]');
+    expect(dialog).toBeTruthy();
+
+    // The outermost wrapper must be the single owner of the height cap + scroll.
+    expect(dialog!.className).toMatch(/max-h-\[90vh\]/);
+    expect(dialog!.className).toMatch(/overflow-y-auto/);
+
+    // The body container (last child of the dialog — header is first, no footer
+    // prop is used by AddApiKeyModal) must NOT carry its own independent max-h/
+    // overflow cap — otherwise the outer cap and the inner cap fight (double cap),
+    // clipping content before the outer 90vh bound ever kicks in.
+    const bodyDiv = dialog!.children[dialog!.children.length - 1] as HTMLElement;
+    expect(bodyDiv).toBeTruthy();
+    expect(bodyDiv.className).not.toMatch(/max-h-/);
+    expect(bodyDiv.className).not.toMatch(/overflow-y-auto/);
+
+    // Sanity: the cookie helper text and Save button are both present in the DOM
+    // (this modal renders the full guide + form Save/Cancel inline in the body).
+    expect(el.textContent).toContain("How to get the session credential");
+    const saveBtn = Array.from(el.querySelectorAll("button")).find(
+      (b) => b.textContent?.trim() === "save"
+    );
+    expect(saveBtn).toBeTruthy();
+  });
+});