fix: accept empty rendered diff output

CHANGELOG.md

@@ -25,6 +25,7 @@ Docs: https://docs.openclaw.ai
 - Flows/tasks: add a minimal SQLite-backed flow registry plus task-to-flow linkage scaffolding, so orchestrated work can start gaining a first-class parent record without changing current task delivery behavior.
 - Flows/tasks: route one-task ACP and subagent updates through a parent flow owner context, so detached work can emerge back through the intended parent thread/session instead of speaking only as a raw child task.
 - Matrix/history: add optional room history context for Matrix group triggers via `channels.matrix.historyLimit`, with per-agent watermarks and retry-safe snapshots so failed trigger retries do not drift into newer room messages. (#57022) thanks @chain710.
+- Diffs: skip unused viewer-versus-file SSR preload work so `diffs` view-only and file-only runs do less render work while keeping mode outputs aligned. (#57909) thanks @gumadeiras.
 
 ### Fixes
 

extensions/diffs/src/tool-render-output.test.ts

@@ -0,0 +1,99 @@
+import fs from "node:fs/promises";
+import path from "node:path";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { createTestPluginApi } from "../../../test/helpers/plugins/plugin-api.js";
+import type { OpenClawPluginApi } from "../api.js";
+import type { DiffScreenshotter } from "./browser.js";
+import { DEFAULT_DIFFS_TOOL_DEFAULTS } from "./config.js";
+import { createDiffStoreHarness } from "./test-helpers.js";
+
+const { renderDiffDocumentMock } = vi.hoisted(() => ({
+  renderDiffDocumentMock: vi.fn(),
+}));
+
+vi.mock("./render.js", () => ({
+  renderDiffDocument: renderDiffDocumentMock,
+}));
+
+describe("diffs tool rendered output guards", () => {
+  let cleanupRootDir: () => Promise<void>;
+  let store: Awaited<ReturnType<typeof createDiffStoreHarness>>["store"];
+
+  beforeEach(async () => {
+    vi.resetModules();
+    renderDiffDocumentMock.mockReset();
+    ({ store, cleanup: cleanupRootDir } = await createDiffStoreHarness(
+      "openclaw-diffs-tool-render-output-",
+    ));
+  });
+
+  afterEach(async () => {
+    await cleanupRootDir();
+  });
+
+  it("accepts empty string image html for file output", async () => {
+    renderDiffDocumentMock.mockResolvedValue({
+      title: "Text diff",
+      fileCount: 1,
+      inputKind: "before_after",
+      imageHtml: "",
+    });
+
+    const { createDiffsTool } = await import("./tool.js");
+    const screenshotter = createPngScreenshotter({
+      assertHtml: (html) => {
+        expect(html).toBe("");
+      },
+    });
+
+    const tool = createDiffsTool({
+      api: createApi(),
+      store,
+      defaults: DEFAULT_DIFFS_TOOL_DEFAULTS,
+      screenshotter,
+    });
+
+    const result = await tool.execute?.("tool-empty-image-html", {
+      before: "one\n",
+      after: "two\n",
+      mode: "file",
+    });
+
+    expect(screenshotter.screenshotHtml).toHaveBeenCalledTimes(1);
+    expect((result?.details as Record<string, unknown>).filePath).toEqual(expect.any(String));
+  });
+});
+
+function createApi(): OpenClawPluginApi {
+  return createTestPluginApi({
+    id: "diffs",
+    name: "Diffs",
+    description: "Diffs",
+    source: "test",
+    config: {
+      gateway: {
+        port: 18789,
+        bind: "loopback",
+      },
+    },
+    runtime: {} as OpenClawPluginApi["runtime"],
+  }) as OpenClawPluginApi;
+}
+
+function createPngScreenshotter(
+  params: {
+    assertHtml?: (html: string) => void;
+  } = {},
+): DiffScreenshotter {
+  const screenshotHtml: DiffScreenshotter["screenshotHtml"] = vi.fn(
+    async ({ html, outputPath }: { html: string; outputPath: string }) => {
+      params.assertHtml?.(html);
+      await fs.mkdir(path.dirname(outputPath), { recursive: true });
+      await fs.writeFile(outputPath, Buffer.from("png"));
+      return outputPath;
+    },
+  );
+  return {
+    screenshotHtml,
+  };
+}

extensions/diffs/src/tool.ts

@@ -341,7 +341,7 @@ function resolveRenderTarget(mode: DiffMode): DiffRenderTarget {
 }
 
 function requireRenderedHtml(html: string | undefined, target: DiffRenderTarget): string {
-  if (html) {
+  if (html !== undefined) {
     return html;
   }
   throw new Error(`Missing ${target} render output.`);