diff --git a/apps/server/src/provider/agentContextBudget.json b/apps/server/src/provider/agentContextBudget.json
index d5496ea8cfc57..1711e148d4ca4 100644
--- a/apps/server/src/provider/agentContextBudget.json
+++ b/apps/server/src/provider/agentContextBudget.json
@@ -1,5 +1,5 @@
 {
   "claudeSystemAppendBytes": 5135,
   "codexAdditionalContextBytes": 6865,
-  "mcpToolListJsonBytes": 45907
+  "mcpToolListJsonBytes": 46007
 }
diff --git a/apps/web/src/components/files/BrowserDocumentFrame.browser.test.tsx b/apps/web/src/components/files/BrowserDocumentFrame.browser.test.tsx
index 9785a6764b2b6..3915ecce9a4ca 100644
--- a/apps/web/src/components/files/BrowserDocumentFrame.browser.test.tsx
+++ b/apps/web/src/components/files/BrowserDocumentFrame.browser.test.tsx
@@ -1,3 +1,4 @@
+import { injectHtmlRenderBootstrap } from "@t3tools/shared/htmlRender";
 import { describe, expect, it, vi } from "vite-plus/test";
 
 import "~/index.css";
@@ -34,3 +35,109 @@ describe("HtmlRenderDocument", () => {
     expect(onContentHeight).toHaveBeenCalledTimes(1);
   });
 });
+
+it("keeps the first and last selected card rings inside the fitted renderer frame", async () => {
+  const html = injectHtmlRenderBootstrap(`<!doctype html><html><head><style>
+    main { display: grid; gap: 12px; }
+    button { box-sizing: border-box; width: 100%; height: 72px; border: 0; }
+    button[aria-pressed="true"] { outline: 3px solid blue; outline-offset: 2px; }
+  </style></head><body><main>
+    <button id="first" aria-pressed="false">Meadow</button>
+    <button id="middle" aria-pressed="false">Harbor</button>
+    <button id="last" aria-pressed="false">Orchid</button>
+  </main><script>
+    document.querySelectorAll("button").forEach(function(button) {
+      button.addEventListener("click", function() {
+        document.querySelectorAll("button").forEach(function(card) {
+          card.setAttribute("aria-pressed", String(card === button));
+        });
+      });
+    });
+    window.addEventListener("message", function(event) {
+      if (event.source !== window.parent || event.data.type !== "select-card") return;
+      var button = document.getElementById(event.data.id);
+      button.click();
+      var rect = button.getBoundingClientRect();
+      var style = getComputedStyle(button);
+      var ring = parseFloat(style.outlineWidth) + parseFloat(style.outlineOffset);
+      window.parent.postMessage({
+        type: "selected-card-bounds",
+        id: button.id,
+        selected: button.getAttribute("aria-pressed") === "true",
+        ring: ring,
+        left: rect.left - ring,
+        right: rect.right + ring,
+        top: rect.top - ring,
+        bottom: rect.bottom + ring,
+        pageHeight: document.documentElement.getBoundingClientRect().height,
+        viewportHeight: window.innerHeight,
+        viewportWidth: window.innerWidth,
+        scrollWidth: document.documentElement.scrollWidth
+      }, "*");
+    });
+  </script></body></html>`);
+  const src = URL.createObjectURL(new Blob([html], { type: "text/html" }));
+  using _url = { [Symbol.dispose]: () => URL.revokeObjectURL(src) };
+  const container = document.createElement("div");
+  container.style.width = "728px";
+  const reports = new Map<
+    string,
+    {
+      selected: boolean;
+      ring: number;
+      left: number;
+      right: number;
+      top: number;
+      bottom: number;
+      pageHeight: number;
+      viewportHeight: number;
+      viewportWidth: number;
+      scrollWidth: number;
+    }
+  >();
+  const messages = new AbortController();
+  using _messages = { [Symbol.dispose]: () => messages.abort() };
+  window.addEventListener(
+    "message",
+    (event) => {
+      const frame = container.querySelector("iframe");
+      if (event.source === frame?.contentWindow && event.data?.type === "selected-card-bounds") {
+        reports.set(event.data.id, event.data);
+      }
+    },
+    { signal: messages.signal },
+  );
+  const contentHeight = vi.fn((height: number) => {
+    container.style.setProperty("--content-height", `${height}px`);
+  });
+  await using screen = await render(
+    <>
+      <style>{`iframe[title="Edge cards"] {
+        display: block; width: 100%; height: var(--content-height, 120px); border: 0;
+      }`}</style>
+      <HtmlRenderDocument src={src} title="Edge cards" onContentHeight={contentHeight} />
+    </>,
+    { container },
+  );
+
+  await vi.waitFor(() => expect(contentHeight).toHaveBeenCalled());
+  const frame = screen.getByTitle("Edge cards").element() as HTMLIFrameElement;
+  await vi.waitFor(() => {
+    expect(frame.getBoundingClientRect().height).toBe(contentHeight.mock.lastCall?.[0]);
+  });
+
+  for (const id of ["first", "last"]) {
+    frame.contentWindow?.postMessage({ type: "select-card", id }, "*");
+    await vi.waitFor(() => expect(reports.get(id)?.selected).toBe(true));
+    const bounds = reports.get(id)!;
+    expect(bounds.ring).toBe(5);
+    expect(bounds.viewportWidth).toBe(728);
+    expect(bounds.pageHeight).toBe(contentHeight.mock.lastCall?.[0]);
+    expect(bounds.viewportHeight).toBe(bounds.pageHeight);
+    expect(bounds.left).toBeGreaterThanOrEqual(0);
+    expect(bounds.top).toBeGreaterThanOrEqual(0);
+    expect(bounds.right).toBeLessThanOrEqual(bounds.viewportWidth);
+    expect(bounds.bottom).toBeLessThanOrEqual(bounds.viewportHeight);
+    expect(bounds.scrollWidth).toBeLessThanOrEqual(bounds.viewportWidth);
+  }
+});
diff --git a/packages/shared/src/htmlRender.ts b/packages/shared/src/htmlRender.ts
index 572441ffe5366..12af0f39336b2 100644
--- a/packages/shared/src/htmlRender.ts
+++ b/packages/shared/src/htmlRender.ts
@@ -232,6 +232,7 @@ export const HTML_RENDER_THEME_GUIDE = [
 export const HTML_RENDER_LAYOUT_GUIDE = [
   `Frame width is reply width (${HTML_RENDER_COLUMN_WIDTH}px desktop, about 360px phones), on the thread background.`,
   "Leave html, body, and the outer element with no background color, no outer card, border, banner title, or side padding. This wins over any rule that sets a page background.",
+  "T3 adds an overridable 8px html gutter for selection rings and shadows. Use fluid widths within it.",
   "A box that needs its own background gets 16px padding and var(--radius) corners. Give charts fixed pixel heights.",
   "Let content set the height. Do not use vh units or height:100% on html or body. Both are rejected or loop.",
 ].join(" ");
@@ -353,7 +354,7 @@ export function htmlRenderThemeMessage(theme: HtmlRenderTheme) {
 }
 
 const BASE_CSS =
-  "html{background:var(--background);color:var(--foreground);font-family:var(--font-sans);font-size:14px;line-height:1.5;-webkit-font-smoothing:antialiased;-webkit-text-size-adjust:100%;scrollbar-width:none}" +
+  "html{box-sizing:border-box;padding:8px;background:var(--background);color:var(--foreground);font-family:var(--font-sans);font-size:14px;line-height:1.5;-webkit-font-smoothing:antialiased;-webkit-text-size-adjust:100%;scrollbar-width:none}" +
   "html::-webkit-scrollbar{display:none}body{margin:0}code,kbd,pre,samp{font-family:var(--font-mono)}" +
   // Focus outlines paint inside the element, so the frame edge never clips them.
   ":where(:focus-visible){outline:2px solid var(--ring)}:focus-visible{outline-width:2px!important;outline-offset:-2px!important}" +