diff --git a/docs/tools/diffs.md b/docs/tools/diffs.md
index 3f19fae586a..90da207e234 100644
--- a/docs/tools/diffs.md
+++ b/docs/tools/diffs.md
@@ -145,6 +145,7 @@ Compatibility note: `defaults.format` is accepted as an alias for `defaults.file
 - `mode: "file"` uses a faster file-only render path and does not create a viewer URL.
 - File quality presets include hard pixel caps to prevent runaway renders on very large diffs.
 - PNG or PDF rendering requires a Chromium-compatible browser. If auto-detection is not enough, set `browser.executablePath`.
+- If your channel compresses images aggressively (for example Telegram or WhatsApp), prefer `fileFormat: "pdf"` to preserve diff readability.
 - Diff rendering is powered by [Diffs](https://diffs.com).
 
 ## Related docs
diff --git a/extensions/diffs/README.md b/extensions/diffs/README.md
index 62bec10017e..3a385ee5c2c 100644
--- a/extensions/diffs/README.md
+++ b/extensions/diffs/README.md
@@ -117,7 +117,7 @@ After:
 This is version two.
 ```
 
-Render a PNG:
+Render a file (PNG or PDF):
 
 ```text
 Use the `diffs` tool in `file` mode for this before and after input. After it returns `details.filePath`, use the `message` tool with `path` or `filePath` to send me the rendered diff image.
@@ -165,4 +165,5 @@ diff --git a/src/example.ts b/src/example.ts
 - The viewer is hosted locally through the gateway under `/plugins/diffs/...`.
 - Artifacts are ephemeral and stored in the local temp directory.
 - PNG/PDF rendering requires a Chromium-compatible browser. Set `browser.executablePath` if auto-detection is not enough.
+- If your delivery channel compresses images heavily (for example Telegram or WhatsApp), prefer `fileFormat: "pdf"` to preserve readability.
 - Diff rendering is powered by [Diffs](https://diffs.com).
diff --git a/extensions/diffs/src/browser.ts b/extensions/diffs/src/browser.ts
index 96402fdf2ef..a4956de99fe 100644
--- a/extensions/diffs/src/browser.ts
+++ b/extensions/diffs/src/browser.ts
@@ -8,6 +8,7 @@ import { VIEWER_ASSET_PREFIX, getServedViewerAsset } from "./viewer-assets.js";
 
 const DEFAULT_BROWSER_IDLE_MS = 30_000;
 const SHARED_BROWSER_KEY = "__default__";
+const IMAGE_SIZE_LIMIT_ERROR = "Diff frame did not render within image size limits.";
 
 export type DiffScreenshotter = {
   screenshotHtml(params: {
@@ -237,8 +238,11 @@ export class PlaywrightDiffScreenshotter implements DiffScreenshotter {
         });
         return params.outputPath;
       }
-      throw new Error("Diff frame did not render within image size limits.");
+      throw new Error(IMAGE_SIZE_LIMIT_ERROR);
     } catch (error) {
+      if (error instanceof Error && error.message === IMAGE_SIZE_LIMIT_ERROR) {
+        throw error;
+      }
       const reason = error instanceof Error ? error.message : String(error);
       throw new Error(
         `Diff PNG/PDF rendering requires a Chromium-compatible browser. Set browser.executablePath or install Chrome/Chromium. ${reason}`,
diff --git a/extensions/diffs/src/tool.test.ts b/extensions/diffs/src/tool.test.ts
index 2e55ba38493..7e0de7d5318 100644
--- a/extensions/diffs/src/tool.test.ts
+++ b/extensions/diffs/src/tool.test.ts
@@ -206,6 +206,42 @@ describe("diffs tool", () => {
     expect((result?.details as Record<string, unknown>).fileMaxWidth).toBe(1100);
   });
 
+  it("accepts deprecated format alias for fileFormat", async () => {
+    const screenshotter = {
+      screenshotHtml: vi.fn(
+        async ({
+          outputPath,
+          image,
+        }: {
+          outputPath: string;
+          image: { format: string; qualityPreset: string; scale: number; maxWidth: number };
+        }) => {
+          expect(image.format).toBe("pdf");
+          await fs.mkdir(path.dirname(outputPath), { recursive: true });
+          await fs.writeFile(outputPath, Buffer.from("%PDF-1.7"));
+          return outputPath;
+        },
+      ),
+    };
+
+    const tool = createDiffsTool({
+      api: createApi(),
+      store,
+      defaults: DEFAULT_DIFFS_TOOL_DEFAULTS,
+      screenshotter,
+    });
+
+    const result = await tool.execute?.("tool-2format", {
+      before: "one\n",
+      after: "two\n",
+      mode: "file",
+      format: "pdf",
+    });
+
+    expect((result?.details as Record<string, unknown>).fileFormat).toBe("pdf");
+    expect((result?.details as Record<string, unknown>).filePath).toMatch(/preview\.pdf$/);
+  });
+
   it("honors defaults.mode=file when mode is omitted", async () => {
     const screenshotter = {
       screenshotHtml: vi.fn(async ({ outputPath }: { outputPath: string }) => {