diff --git a/packages/cli/src/capture/captureCompositionFrame.test.ts b/packages/cli/src/capture/captureCompositionFrame.test.ts
index a8954a6e5..37507bed2 100644
--- a/packages/cli/src/capture/captureCompositionFrame.test.ts
+++ b/packages/cli/src/capture/captureCompositionFrame.test.ts
@@ -339,3 +339,21 @@ describe("runFfmpegOnce", () => {
     }
   });
 });
+
+describe("installPageFunctionGuard", () => {
+  it("defines the keepNames __name shim in the page before any script runs", async () => {
+    const evaluateOnNewDocument = vi.fn(async () => undefined);
+
+    await installPageFunctionGuard({ evaluateOnNewDocument });
+
+    expect(evaluateOnNewDocument).toHaveBeenCalledOnce();
+    const source = evaluateOnNewDocument.mock.calls[0]?.[0] as string;
+    expect(source).toContain("self.__name");
+    // The shim must be a no-op passthrough so wrapped functions stay callable.
+    const shim = new Function(`const self = {}; ${source}; return self.__name;`)() as (
+      fn: unknown,
+    ) => unknown;
+    const marker = () => 42;
+    expect(shim(marker)).toBe(marker);
+  });
+});
diff --git a/packages/cli/src/capture/captureCompositionFrame.ts b/packages/cli/src/capture/captureCompositionFrame.ts
index b6350718d..2b0d16409 100644
--- a/packages/cli/src/capture/captureCompositionFrame.ts
+++ b/packages/cli/src/capture/captureCompositionFrame.ts
@@ -131,6 +131,19 @@ async function waitForCompositionSettle(
   return runtimeReady;
 }
 
+// tsx/esbuild-style dev transpilers run with keepNames, which rewrites named
+// inner functions in serialized page closures into __name(...) calls — a
+// helper that exists in the Node bundle but not in the browser realm, so any
+// page.evaluate/waitForFunction whose closure defines a named function throws
+// "__name is not defined" when the CLI runs from source. Defining a no-op in
+// the page before any script runs immunizes every serialized closure, current
+// and future, regardless of how the CLI was built.
+export async function installPageFunctionGuard(page: {
+  evaluateOnNewDocument(source: string): Promise<unknown>;
+}): Promise<void> {
+  await page.evaluateOnNewDocument("self.__name = self.__name || ((fn) => fn);");
+}
+
 export async function openSettledCompositionPage(
   html: string,
   url: string,
@@ -154,6 +167,7 @@ export async function openSettledCompositionPage(
     });
 
     const page = await chromeBrowser.newPage();
+    await installPageFunctionGuard(page);
     await page.setViewport(viewport);
     await options.beforeNavigate?.(page);
     await page.goto(url, { waitUntil: "domcontentloaded", timeout: 10000 });
diff --git a/packages/cli/src/commands/layout.ts b/packages/cli/src/commands/layout.ts
index 35202c874..a7025d787 100644
--- a/packages/cli/src/commands/layout.ts
+++ b/packages/cli/src/commands/layout.ts
@@ -28,6 +28,7 @@ import {
 import { findMotionSpec, readMotionSpec, type MotionSpec } from "../utils/motionSpec.js";
 import {
   AUDIT_SEEK_OPTIONS,
+  installPageFunctionGuard,
   seekCompositionTimeline,
   waitForCompositionFonts,
   type SeekCompositionTimelineOptions,
@@ -214,6 +215,7 @@ async function runLayoutAudit(
     });
 
     const page = await chromeBrowser.newPage();
+    await installPageFunctionGuard(page);
     await page.setViewport({ width: 1920, height: 1080 });
     await page.goto(server.url, { waitUntil: "domcontentloaded", timeout: 10000 });
     await alignViewportToComposition(page, server.url);
diff --git a/packages/cli/src/commands/validate.ts b/packages/cli/src/commands/validate.ts
index 8ce32879d..a3b99ae40 100644
--- a/packages/cli/src/commands/validate.ts
+++ b/packages/cli/src/commands/validate.ts
@@ -15,6 +15,7 @@ import { resolveCompositionViewportFromHtml } from "../utils/compositionViewport
 import { c } from "../ui/colors.js";
 import { printDeprecationNotice, withMeta } from "../utils/updateCheck.js";
 import {
+  installPageFunctionGuard,
   resolveCliChromeGpuMode,
   seekCompositionTimeline,
 } from "../capture/captureCompositionFrame.js";
@@ -421,6 +422,7 @@ async function validateInBrowser(
     });
 
     const page = await chromeBrowser.newPage();
+    await installPageFunctionGuard(page);
     await page.setViewport(viewport);
 
     page.on("console", (msg) => {
diff --git a/registry/examples/motion-blur/index.html b/registry/examples/motion-blur/index.html
index dacf6a0e5..034cc42bf 100644
--- a/registry/examples/motion-blur/index.html
+++ b/registry/examples/motion-blur/index.html
@@ -199,7 +199,9 @@
 
       <!-- Group 1: 3.0s slow — top: 80 -->
       <div class="group" style="top: 80px">
-        <div class="dur" data-layout-ignore><span class="dur-num">3.0</span><span class="dur-unit">seconds</span></div>
+        <div class="dur" data-layout-ignore>
+          <span class="dur-num">3.0</span><span class="dur-unit">seconds</span>
+        </div>
         <div class="row-hint" data-layout-ignore style="top: 0">shape</div>
         <div class="row-hint" data-layout-ignore style="top: 88px">text</div>
         <div class="track-a"></div>
@@ -225,7 +227,9 @@
 
       <!-- Group 2: 1.5s medium — top: 264 -->
       <div class="group" style="top: 264px">
-        <div class="dur" data-layout-ignore><span class="dur-num">1.5</span><span class="dur-unit">seconds</span></div>
+        <div class="dur" data-layout-ignore>
+          <span class="dur-num">1.5</span><span class="dur-unit">seconds</span>
+        </div>
         <div class="row-hint" data-layout-ignore style="top: 0">shape</div>
         <div class="row-hint" data-layout-ignore style="top: 88px">text</div>
         <div class="track-a"></div>
@@ -251,7 +255,9 @@
 
       <!-- Group 3: 0.7s fast — top: 448 -->
       <div class="group" style="top: 448px">
-        <div class="dur" data-layout-ignore><span class="dur-num">0.7</span><span class="dur-unit">seconds</span></div>
+        <div class="dur" data-layout-ignore>
+          <span class="dur-num">0.7</span><span class="dur-unit">seconds</span>
+        </div>
         <div class="row-hint" data-layout-ignore style="top: 0">shape</div>
         <div class="row-hint" data-layout-ignore style="top: 88px">text</div>
         <div class="track-a"></div>
@@ -305,7 +311,9 @@
 
       <!-- Group 5: 0.2s extreme — top: 816 -->
       <div class="group" style="top: 816px">
-        <div class="dur" data-layout-ignore><span class="dur-num">0.2</span><span class="dur-unit">seconds</span></div>
+        <div class="dur" data-layout-ignore>
+          <span class="dur-num">0.2</span><span class="dur-unit">seconds</span>
+        </div>
         <div class="row-hint" data-layout-ignore style="top: 0">shape</div>
         <div class="row-hint" data-layout-ignore style="top: 88px">text</div>
         <div class="track-a"></div>