diff --git a/extensions/canvas/src/host/a2ui.ts b/extensions/canvas/src/host/a2ui.ts
index aa555ebd40a1..21779707a2cc 100644
--- a/extensions/canvas/src/host/a2ui.ts
+++ b/extensions/canvas/src/host/a2ui.ts
@@ -136,20 +136,25 @@ async function handleA2uiHttpRequestWithRootResolver(
         : ((await detectMime({ filePath: result.realPath })) ?? "application/octet-stream");
     res.setHeader("Cache-Control", "no-store");
 
-    if (req.method === "HEAD") {
-      res.setHeader("Content-Type", mime === "text/html" ? "text/html; charset=utf-8" : mime);
-      res.end();
-      return true;
-    }
-
     if (mime === "text/html") {
       const buf = await result.handle.readFile({ encoding: "utf8" });
+      const body = injectCanvasRuntime(buf, options);
       res.setHeader("Content-Type", "text/html; charset=utf-8");
-      res.end(injectCanvasRuntime(buf, options));
+      if (req.method === "HEAD") {
+        res.setHeader("Content-Length", String(Buffer.byteLength(body)));
+        res.end();
+        return true;
+      }
+      res.end(body);
       return true;
     }
 
     res.setHeader("Content-Type", mime);
+    if (req.method === "HEAD") {
+      res.setHeader("Content-Length", String(result.stat.size));
+      res.end();
+      return true;
+    }
     res.end(await result.handle.readFile());
     return true;
   } finally {
diff --git a/extensions/canvas/src/host/server.test.ts b/extensions/canvas/src/host/server.test.ts
index 6ec1ce481307..ace8919395fd 100644
--- a/extensions/canvas/src/host/server.test.ts
+++ b/extensions/canvas/src/host/server.test.ts
@@ -672,4 +672,43 @@ describe("canvas host", () => {
       await fs.rm(linkPath, { force: true });
     }
   });
+
+  it("serves Content-Length on A2UI HEAD responses", async () => {
+    const fixtureEntryDir = await createCaseDir();
+    const a2uiRoot = path.join(fixtureEntryDir, "a2ui");
+    const { setA2uiRootRealForTest } = await import("../../test-api.js");
+
+    try {
+      await fs.mkdir(a2uiRoot, { recursive: true });
+      await fs.writeFile(
+        path.join(a2uiRoot, "index.html"),
+        `<openclaw-a2ui-host></openclaw-a2ui-host>
+<script>openclawCanvasA2UIAction</script>`,
+        "utf8",
+      );
+      await fs.writeFile(
+        path.join(a2uiRoot, "a2ui.bundle.js"),
+        "window.openclawA2UI = {};",
+        "utf8",
+      );
+      setA2uiRootRealForTest(await fs.realpath(a2uiRoot));
+
+      // HTML: HEAD Content-Length must match the injected GET body bytes.
+      const getHtml = await captureA2uiFixtureResponse(`${A2UI_PATH}/`);
+      const headHtml = await captureA2uiFixtureResponse(`${A2UI_PATH}/`, "HEAD");
+      expect(headHtml.status).toBe(200);
+      expect(headHtml.headers["content-length"]).toBe(String(getHtml.bodyBytes.byteLength));
+      expect(headHtml.bodyBytes.byteLength).toBe(0);
+      expect(headHtml.headers["content-type"]).toBe("text/html; charset=utf-8");
+
+      // Static asset: HEAD Content-Length must match the file bytes.
+      const getBundle = await captureA2uiFixtureResponse(`${A2UI_PATH}/a2ui.bundle.js`);
+      const headBundle = await captureA2uiFixtureResponse(`${A2UI_PATH}/a2ui.bundle.js`, "HEAD");
+      expect(headBundle.status).toBe(200);
+      expect(headBundle.headers["content-length"]).toBe(String(getBundle.bodyBytes.byteLength));
+      expect(headBundle.bodyBytes.byteLength).toBe(0);
+    } finally {
+      setA2uiRootRealForTest(undefined);
+    }
+  });
 });