diff --git a/src/agents/tool-images.input-cap.test.ts b/src/agents/tool-images.input-cap.test.ts
new file mode 100644
index 000000000000..6a526933af47
--- /dev/null
+++ b/src/agents/tool-images.input-cap.test.ts
@@ -0,0 +1,25 @@
+// Tool image input-cap tests verify pathological oversized base64 input is
+// rejected before Buffer.from allocates a transient multi-MB buffer.
+import { describe, expect, it } from "vitest";
+import { sanitizeContentBlocksImages } from "./tool-images.js";
+
+const MAX_IMAGE_INPUT_BYTES = 10 * 1024 * 1024;
+
+describe("tool image sanitizer oversized input cap", () => {
+  it("rejects oversized estimated input before decode allocation", async () => {
+    const encodedLength = Math.ceil(((MAX_IMAGE_INPUT_BYTES + 1) * 4) / 3 / 4) * 4;
+    const oversizedBase64 = "A".repeat(encodedLength);
+
+    const out = await sanitizeContentBlocksImages(
+      [{ type: "image" as const, data: oversizedBase64, mimeType: "image/png" }],
+      "test",
+    );
+
+    expect(out).toStrictEqual([
+      {
+        type: "text",
+        text: "[test] omitted image payload: image exceeds input size limit (10.00MB)",
+      },
+    ]);
+  });
+});
diff --git a/src/agents/tool-images.ts b/src/agents/tool-images.ts
index 0550c3a55348..7fc219a495d1 100644
--- a/src/agents/tool-images.ts
+++ b/src/agents/tool-images.ts
@@ -3,7 +3,7 @@
  *
  * Downscales and recompresses oversized base64 image blocks before provider replay.
  */
-import { canonicalizeBase64 } from "@openclaw/media-core/base64";
+import { canonicalizeBase64, estimateBase64DecodedBytes } from "@openclaw/media-core/base64";
 import { resolveIntegerOption } from "@openclaw/normalization-core/number-coercion";
 import { toErrorObject } from "../infra/errors.js";
 import type { ImageContent } from "../llm/types.js";
@@ -33,6 +33,11 @@ type TextContentBlock = Extract<ToolContentBlock, { type: "text" }>;
 // tool outputs do not break later turns or silent channel replies.
 const MAX_IMAGE_DIMENSION_PX = DEFAULT_IMAGE_MAX_DIMENSION_PX;
 const MAX_IMAGE_BYTES = DEFAULT_IMAGE_MAX_BYTES;
+// Hard cap on decoded input bytes before Buffer.from/resizer allocation. A
+// conservative limit well below demonstrated OOM thresholds, leaving headroom
+// for canonicalization, decode, and image-processing allocations while still
+// permitting legitimate tool-output images.
+const MAX_IMAGE_INPUT_BYTES = 10 * 1024 * 1024;
 const log = createSubsystemLogger("agents/tool-images");
 
 function isImageTypeBlock(block: unknown): block is Record<string, unknown> & { type: "image" } {
@@ -327,6 +332,19 @@ export async function sanitizeContentBlocksImages(
       continue;
     }
 
+    // Estimate decoded bytes on the raw payload before trim/canonicalize/decode
+    // so pathological multi-GB base64 cannot force a transient large allocation.
+    // maxBytes is the post-decode resize target; MAX_IMAGE_INPUT_BYTES is a
+    // conservative pre-decode ceiling (10 MiB) far below the 25MP/100MB
+    // processing headroom, so legitimate tool images still decode and resize.
+    if (estimateBase64DecodedBytes(block.data) > MAX_IMAGE_INPUT_BYTES) {
+      out.push({
+        type: "text",
+        text: `[${label}] omitted image payload: image exceeds input size limit (${formatBytesShort(MAX_IMAGE_INPUT_BYTES)})`,
+      } satisfies TextContentBlock);
+      continue;
+    }
+
     const data = block.data.trim();
     if (!data) {
       out.push({