diff --git a/src/gateway/chat-attachments.test.ts b/src/gateway/chat-attachments.test.ts
new file mode 100644
index 000000000..e07116636
--- /dev/null
+++ b/src/gateway/chat-attachments.test.ts
@@ -0,0 +1,58 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  buildMessageWithAttachments,
+  type ChatAttachment,
+} from "./chat-attachments.js";
+
+const PNG_1x1 =
+  "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mP8/woAAn8B9FD5fHAAAAAASUVORK5CYII=";
+
+describe("buildMessageWithAttachments", () => {
+  it("embeds a single image as data URL", () => {
+    const msg = buildMessageWithAttachments("see this", [
+      {
+        type: "image",
+        mimeType: "image/png",
+        fileName: "dot.png",
+        content: PNG_1x1,
+      },
+    ]);
+    expect(msg).toContain("see this");
+    expect(msg).toContain(`data:image/png;base64,${PNG_1x1}`);
+    expect(msg).toContain("![dot.png]");
+  });
+
+  it("rejects non-image mime types", () => {
+    const bad: ChatAttachment = {
+      type: "file",
+      mimeType: "application/pdf",
+      fileName: "a.pdf",
+      content: "AAA",
+    };
+    expect(() => buildMessageWithAttachments("x", [bad])).toThrow(/image/);
+  });
+
+  it("rejects invalid base64 content", () => {
+    const bad: ChatAttachment = {
+      type: "image",
+      mimeType: "image/png",
+      fileName: "dot.png",
+      content: "%not-base64%",
+    };
+    expect(() => buildMessageWithAttachments("x", [bad])).toThrow(/base64/);
+  });
+
+  it("rejects images over limit", () => {
+    const big = Buffer.alloc(6_000_000, 0).toString("base64");
+    const att: ChatAttachment = {
+      type: "image",
+      mimeType: "image/png",
+      fileName: "big.png",
+      content: big,
+    };
+    expect(() =>
+      buildMessageWithAttachments("x", [att], { maxBytes: 5_000_000 }),
+    ).toThrow(/exceeds size limit/i);
+  });
+});
diff --git a/src/gateway/chat-attachments.ts b/src/gateway/chat-attachments.ts
index 7e7d97834..523da182d 100644
--- a/src/gateway/chat-attachments.ts
+++ b/src/gateway/chat-attachments.ts
@@ -29,8 +29,13 @@ export function buildMessageWithAttachments(
     }
 
     let sizeBytes = 0;
+    const b64 = content.trim();
+    // Basic base64 sanity: length multiple of 4 and charset check.
+    if (b64.length % 4 !== 0 || /[^A-Za-z0-9+/=]/.test(b64)) {
+      throw new Error(`attachment ${label}: invalid base64 content`);
+    }
     try {
-      sizeBytes = Buffer.from(content, "base64").byteLength;
+      sizeBytes = Buffer.from(b64, "base64").byteLength;
     } catch {
       throw new Error(`attachment ${label}: invalid base64 content`);
     }