diff --git a/CHANGELOG.md b/CHANGELOG.md
index 670e47926262..c7587e99e9ac 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,6 +17,7 @@ Docs: https://docs.openclaw.ai
 
 ### Fixes
 
+- **Diffs rendering:** render viewer and image output from one SSR preload, preserve language-pack highlighting through hydration, normalize language hints case-insensitively, skip identical before/after inputs with an explicit `changed` result, report truthful file-render and input errors, cache hash-pinned viewer runtimes, and prefer canonical file settings over stale aliases. (#100487)
 - **Remote browser reliability:** bound persistent Playwright tab enumeration by the existing remote CDP timeout budget and retire timed-out connection attempts so late completions cannot restore a stuck connection. (#80147, #58968) Thanks @HemantSudarshan and @KeaneYan.
 - **Control UI approval prompts:** keep stale resolve failures and busy-state cleanup from leaking across newer approvals or Gateway reconnects. (#98394) Thanks @haruaiclone-droid.
 - **Agent empty replies:** surface a visible failure when a completed interactive turn has no deliverable reply, including queued follow-ups, while preserving explicit silence, pending continuations, and committed side effects. (#100456) Thanks @mushuiyu886.
diff --git a/docs/tools/diffs.md b/docs/tools/diffs.md
index 66ecaca624ad..5f12b0a84f80 100644
--- a/docs/tools/diffs.md
+++ b/docs/tools/diffs.md
@@ -171,8 +171,11 @@ Without the pack, unsupported languages still render as readable plain text. See
 
 ## Output details contract
 
+All successful results include `changed`: identical before/after input returns `false` without creating an artifact; rendered results return `true`.
+
 <AccordionGroup>
   <Accordion title="Viewer fields (view and both modes)">
+    - `changed`
     - `artifactId`
     - `viewerUrl`
     - `viewerPath`
@@ -185,6 +188,7 @@ Without the pack, unsupported languages still render as readable plain text. See
 
   </Accordion>
   <Accordion title="File fields (file and both modes)">
+    - `changed`
     - `artifactId`
     - `expiresAt`
     - `filePath`
diff --git a/extensions/diffs-language-pack/src/plugin.ts b/extensions/diffs-language-pack/src/plugin.ts
index c3405b54640b..e9c295974039 100644
--- a/extensions/diffs-language-pack/src/plugin.ts
+++ b/extensions/diffs-language-pack/src/plugin.ts
@@ -1,7 +1,9 @@
 // Diffs Language Pack plugin module implements plugin behavior.
 import type { IncomingMessage, ServerResponse } from "node:http";
 import type { OpenClawPluginApi } from "../api.js";
-import { VIEWER_ASSET_PREFIX, getServedViewerAsset } from "./viewer-assets.js";
+import { VIEWER_ASSET_PREFIX, VIEWER_RUNTIME_PATH, getServedViewerAsset } from "./viewer-assets.js";
+
+const IMMUTABLE_ASSET_CACHE_CONTROL = "public, max-age=31536000, immutable";
 
 export function registerDiffsLanguagePackPlugin(api: OpenClawPluginApi): void {
   api.registerHttpRoute({
@@ -30,7 +32,11 @@ function createDiffsLanguagePackHttpHandler() {
     }
 
     res.statusCode = 200;
-    setSharedHeaders(res, asset.contentType);
+    setSharedHeaders(
+      res,
+      asset.contentType,
+      parsed.pathname === VIEWER_RUNTIME_PATH ? IMMUTABLE_ASSET_CACHE_CONTROL : undefined,
+    );
     if (req.method === "HEAD") {
       res.end();
     } else {
@@ -57,8 +63,12 @@ function respondText(res: ServerResponse, statusCode: number, body: string): voi
   res.end(body);
 }
 
-function setSharedHeaders(res: ServerResponse, contentType: string): void {
-  res.setHeader("cache-control", "no-store, max-age=0");
+function setSharedHeaders(
+  res: ServerResponse,
+  contentType: string,
+  cacheControl = "no-store, max-age=0",
+): void {
+  res.setHeader("cache-control", cacheControl);
   res.setHeader("content-type", contentType);
   res.setHeader("x-content-type-options", "nosniff");
   res.setHeader("referrer-policy", "no-referrer");
diff --git a/extensions/diffs/README.md b/extensions/diffs/README.md
index 84d76f7d3bbd..fdd80b32ff3e 100644
--- a/extensions/diffs/README.md
+++ b/extensions/diffs/README.md
@@ -20,6 +20,7 @@ It gives agents one tool, `diffs`, that can:
 
 The tool can return:
 
+- `details.changed`: `false` when before/after inputs are identical and no artifact was rendered; `true` for rendered results
 - `details.viewerUrl`: a gateway URL that can be opened in the canvas
 - `details.filePath`: a local rendered artifact path when file rendering is requested
 - `details.fileFormat`: the rendered file format (`png` or `pdf`)
diff --git a/extensions/diffs/openclaw.plugin.json b/extensions/diffs/openclaw.plugin.json
index 277b2585cc40..a8b6eb4582af 100644
--- a/extensions/diffs/openclaw.plugin.json
+++ b/extensions/diffs/openclaw.plugin.json
@@ -145,8 +145,7 @@
           },
           "fileFormat": {
             "type": "string",
-            "enum": ["png", "pdf"],
-            "default": "png"
+            "enum": ["png", "pdf"]
           },
           "format": {
             "type": "string",
@@ -155,8 +154,7 @@
           },
           "fileQuality": {
             "type": "string",
-            "enum": ["standard", "hq", "print"],
-            "default": "standard"
+            "enum": ["standard", "hq", "print"]
           },
           "fileScale": {
             "type": "number",
diff --git a/extensions/diffs/skills/diffs/SKILL.md b/extensions/diffs/skills/diffs/SKILL.md
index b049bd901c55..abf4802d98db 100644
--- a/extensions/diffs/skills/diffs/SKILL.md
+++ b/extensions/diffs/skills/diffs/SKILL.md
@@ -7,6 +7,8 @@ When you need to show edits as a real diff, prefer the `diffs` tool instead of w
 
 The `diffs` tool accepts either `before` + `after` text, or a unified `patch` string.
 
+Check `details.changed`: identical before/after input returns `false` without creating an artifact; rendered results return `true`.
+
 Use `mode=view` when you want an interactive gateway-hosted viewer. After the tool returns, use `details.viewerUrl` with the canvas tool via `canvas present` or `canvas navigate`.
 If the deployment uses a loopback trusted proxy (for example Tailscale Serve with `gateway.trustedProxies` including `127.0.0.1`), raw loopback viewer requests can fail closed without forwarded client-IP headers. In that topology, prefer `mode=file` / `mode=both`, or use a configured `viewerBaseUrl` / explicit proxy/public `baseUrl` when you need a shareable viewer URL.
 
diff --git a/extensions/diffs/src/browser.test.ts b/extensions/diffs/src/browser.test.ts
index ba1d9534728f..2b33dc54e9da 100644
--- a/extensions/diffs/src/browser.test.ts
+++ b/extensions/diffs/src/browser.test.ts
@@ -208,6 +208,81 @@ describe("PlaywrightDiffScreenshotter", () => {
     expect(pages).toHaveLength(1);
     expect(pages[0]?.screenshot).toHaveBeenCalledTimes(0);
   });
+
+  it("wraps browser launch failures with Chromium installation guidance", async () => {
+    launchMock.mockRejectedValue(new Error("launch failed"));
+    const screenshotter = new PlaywrightDiffScreenshotter({
+      config: createConfig(),
+      browserIdleMs: 1_000,
+    });
+
+    await expect(
+      screenshotter.screenshotHtml({
+        html: '<html><head></head><body><main class="oc-frame"></main></body></html>',
+        outputPath,
+        theme: "dark",
+        image: {
+          format: "png",
+          qualityPreset: "standard",
+          scale: 2,
+          maxWidth: 960,
+          maxPixels: 8_000_000,
+        },
+      }),
+    ).rejects.toThrow("requires a Chromium-compatible browser");
+  });
+
+  it("wraps new-page failures with Chromium installation guidance", async () => {
+    const browser = createMockBrowser([]);
+    browser.newPage.mockRejectedValue(new Error("page creation failed"));
+    launchMock.mockResolvedValue(browser);
+    const screenshotter = new PlaywrightDiffScreenshotter({
+      config: createConfig(),
+      browserIdleMs: 1_000,
+    });
+
+    await expect(
+      screenshotter.screenshotHtml({
+        html: '<html><head></head><body><main class="oc-frame"></main></body></html>',
+        outputPath,
+        theme: "dark",
+        image: {
+          format: "png",
+          qualityPreset: "standard",
+          scale: 2,
+          maxWidth: 960,
+          maxPixels: 8_000_000,
+        },
+      }),
+    ).rejects.toThrow("requires a Chromium-compatible browser");
+  });
+
+  it("preserves render errors after a browser page has opened", async () => {
+    const browser = createMockBrowser([]);
+    const page = createMockPage();
+    page.waitForFunction.mockRejectedValue(new Error("hydration timeout"));
+    browser.newPage.mockResolvedValue(page);
+    launchMock.mockResolvedValue(browser);
+    const screenshotter = new PlaywrightDiffScreenshotter({
+      config: createConfig(),
+      browserIdleMs: 1_000,
+    });
+
+    await expect(
+      screenshotter.screenshotHtml({
+        html: '<html><head></head><body><main class="oc-frame"></main></body></html>',
+        outputPath,
+        theme: "dark",
+        image: {
+          format: "png",
+          qualityPreset: "standard",
+          scale: 2,
+          maxWidth: 960,
+          maxPixels: 8_000_000,
+        },
+      }),
+    ).rejects.toThrow("hydration timeout");
+  });
 });
 
 describe("diffs plugin registration", () => {
@@ -428,6 +503,7 @@ describe("diffs plugin registration", () => {
       [
         "When you need to show edits as a real diff, prefer the `diffs` tool instead of writing a manual summary.",
         "It accepts either `before` + `after` text or a unified `patch`.",
+        "Check `details.changed`: identical before/after input returns `false` without creating an artifact; rendered results return `true`.",
         "`mode=view` returns `details.viewerUrl` for canvas use; `mode=file` returns `details.filePath`; `mode=both` returns both.",
         "If you need to send the rendered file, use the `message` tool with `path` or `filePath`.",
         "Include `path` when you know the filename, and omit presentation overrides unless needed.",
diff --git a/extensions/diffs/src/browser.ts b/extensions/diffs/src/browser.ts
index b04a75d5724a..0891afd76d41 100644
--- a/extensions/diffs/src/browser.ts
+++ b/extensions/diffs/src/browser.ts
@@ -68,24 +68,33 @@ export class PlaywrightDiffScreenshotter implements DiffScreenshotter {
     theme: DiffTheme;
     image: DiffRenderOptions["image"];
   }): Promise<string> {
-    const lease = await acquireSharedBrowser({
-      config: this.config,
-      idleMs: this.browserIdleMs,
-    });
+    let lease: BrowserLease;
+    try {
+      lease = await acquireSharedBrowser({
+        config: this.config,
+        idleMs: this.browserIdleMs,
+      });
+    } catch (error) {
+      throw buildBrowserUnavailableError(error);
+    }
     let page: Awaited<ReturnType<BrowserInstance["newPage"]>> | undefined;
     let currentScale = params.image.scale;
     const maxRetries = 2;
 
     try {
       for (let attempt = 0; attempt <= maxRetries; attempt += 1) {
-        page = await lease.browser.newPage({
-          viewport: {
-            width: Math.max(Math.ceil(params.image.maxWidth + 240), 1200),
-            height: 900,
-          },
-          deviceScaleFactor: currentScale,
-          colorScheme: params.theme,
-        });
+        try {
+          page = await lease.browser.newPage({
+            viewport: {
+              width: Math.max(Math.ceil(params.image.maxWidth + 240), 1200),
+              height: 900,
+            },
+            deviceScaleFactor: currentScale,
+            colorScheme: params.theme,
+          });
+        } catch (error) {
+          throw buildBrowserUnavailableError(error);
+        }
         await page.route("**/*", async (route) => {
           const requestUrl = route.request().url();
           if (requestUrl === "about:blank" || requestUrl.startsWith("data:")) {
@@ -276,15 +285,6 @@ export class PlaywrightDiffScreenshotter implements DiffScreenshotter {
         return params.outputPath;
       }
       throw new Error(IMAGE_SIZE_LIMIT_ERROR);
-    } catch (error) {
-      if (error instanceof Error && error.message === IMAGE_SIZE_LIMIT_ERROR) {
-        throw error;
-      }
-      const reason = formatErrorMessage(error);
-      throw new Error(
-        `Diff PNG/PDF rendering requires a Chromium-compatible browser. Set browser.executablePath or install Chrome/Chromium. ${reason}`,
-        { cause: error },
-      );
     } finally {
       await page?.close().catch(() => {});
       await lease.release();
@@ -292,6 +292,14 @@ export class PlaywrightDiffScreenshotter implements DiffScreenshotter {
   }
 }
 
+function buildBrowserUnavailableError(error: unknown): Error {
+  const reason = formatErrorMessage(error);
+  return new Error(
+    `Diff PNG/PDF rendering requires a Chromium-compatible browser. Set browser.executablePath or install Chrome/Chromium. ${reason}`,
+    { cause: error },
+  );
+}
+
 async function writeExternalArtifactFile(params: {
   outputPath: string;
   write: (tempPath: string) => Promise<void>;
@@ -449,6 +457,7 @@ function scheduleIdleBrowserClose(state: SharedBrowserState, idleMs: number): vo
       void closeSharedBrowser();
     }
   }, idleMs);
+  state.idleTimer.unref();
 }
 
 function clearIdleTimer(state: SharedBrowserState): void {
diff --git a/extensions/diffs/src/config.test.ts b/extensions/diffs/src/config.test.ts
index a9a756a5ed94..3dbaf619067a 100644
--- a/extensions/diffs/src/config.test.ts
+++ b/extensions/diffs/src/config.test.ts
@@ -62,7 +62,7 @@ function compileManifestConfigSchema() {
       schema: manifest.configSchema,
       value,
       applyDefaults: true,
-    }).ok;
+    });
 }
 
 function requireRecord(value: unknown, label: string): Record<string, unknown> {
@@ -216,6 +216,20 @@ describe("resolveDiffsPluginDefaults", () => {
     );
   });
 
+  it("prefers an explicit canonical default value over a deprecated alias", () => {
+    expectFields(
+      resolveDiffsPluginDefaults({
+        defaults: {
+          fileFormat: "png",
+          imageFormat: "pdf",
+        },
+      }),
+      {
+        fileFormat: "png",
+      },
+    );
+  });
+
   it("accepts plugin-wide artifact TTL defaults", () => {
     expectFields(
       resolveDiffsPluginDefaults({
@@ -240,7 +254,7 @@ describe("resolveDiffsPluginDefaults", () => {
     );
   });
 
-  it("keeps loader-applied schema defaults from shadowing aliases and quality-derived defaults", () => {
+  it("keeps alias-only config values after manifest validation", () => {
     const validate = compileManifestConfigSchema();
 
     const aliasOnly = {
@@ -249,8 +263,11 @@ describe("resolveDiffsPluginDefaults", () => {
         imageQuality: "hq",
       },
     };
-    expect(validate(aliasOnly)).toBe(true);
-    expectFields(resolveDiffsPluginDefaults(aliasOnly), {
+    const validatedAliasOnly = validate(aliasOnly);
+    if (!validatedAliasOnly.ok) {
+      throw new Error("Expected alias-only config to pass manifest validation.");
+    }
+    expectFields(resolveDiffsPluginDefaults(validatedAliasOnly.value), {
       fileFormat: "pdf",
       fileQuality: "hq",
       fileScale: 2.5,
@@ -262,8 +279,11 @@ describe("resolveDiffsPluginDefaults", () => {
         fileQuality: "hq",
       },
     };
-    expect(validate(qualityOnly)).toBe(true);
-    expectFields(resolveDiffsPluginDefaults(qualityOnly), {
+    const validatedQualityOnly = validate(qualityOnly);
+    if (!validatedQualityOnly.ok) {
+      throw new Error("Expected quality-only config to pass manifest validation.");
+    }
+    expectFields(resolveDiffsPluginDefaults(validatedQualityOnly.value), {
       fileQuality: "hq",
       fileScale: 2.5,
       fileMaxWidth: 1200,
@@ -301,10 +321,10 @@ describe("diffs plugin schema surfaces", () => {
   it("rejects invalid viewerBaseUrl values at manifest-validation time too", () => {
     const validate = compileManifestConfigSchema();
 
-    expect(validate({ viewerBaseUrl: "javascript:alert(1)" })).toBe(false);
-    expect(validate({ viewerBaseUrl: "https://example.com/openclaw?x=1" })).toBe(false);
-    expect(validate({ viewerBaseUrl: "https://example.com/openclaw#frag" })).toBe(false);
-    expect(validate({ viewerBaseUrl: "https://example.com/openclaw/" })).toBe(true);
+    expect(validate({ viewerBaseUrl: "javascript:alert(1)" }).ok).toBe(false);
+    expect(validate({ viewerBaseUrl: "https://example.com/openclaw?x=1" }).ok).toBe(false);
+    expect(validate({ viewerBaseUrl: "https://example.com/openclaw#frag" }).ok).toBe(false);
+    expect(validate({ viewerBaseUrl: "https://example.com/openclaw/" }).ok).toBe(true);
   });
 
   it("preserves defaults and security for direct safeParse callers", () => {
@@ -344,7 +364,7 @@ describe("diffs plugin schema surfaces", () => {
     expectFields(data.security, { allowRemoteViewer: true });
   });
 
-  it("canonicalizes alias-driven defaults for direct safeParse callers", () => {
+  it("resolves deprecated aliases before safeParse applies runtime defaults", () => {
     const parsed = requireRecord(
       diffsPluginConfigSchema.safeParse?.({
         defaults: {
diff --git a/extensions/diffs/src/config.ts b/extensions/diffs/src/config.ts
index 0b0efec768a3..2e8b481257f8 100644
--- a/extensions/diffs/src/config.ts
+++ b/extensions/diffs/src/config.ts
@@ -144,15 +144,9 @@ const DiffsPluginJsonSchemaSource = z.strictObject({
       wordWrap: z.boolean().default(DEFAULT_DIFFS_TOOL_DEFAULTS.wordWrap).optional(),
       background: z.boolean().default(DEFAULT_DIFFS_TOOL_DEFAULTS.background).optional(),
       theme: z.enum(DIFF_THEMES).default(DEFAULT_DIFFS_TOOL_DEFAULTS.theme).optional(),
-      fileFormat: z
-        .enum(DIFF_OUTPUT_FORMATS)
-        .default(DEFAULT_DIFFS_TOOL_DEFAULTS.fileFormat)
-        .optional(),
+      fileFormat: z.enum(DIFF_OUTPUT_FORMATS).optional(),
       format: z.enum(DIFF_OUTPUT_FORMATS).optional().describe("Deprecated alias for fileFormat."),
-      fileQuality: z
-        .enum(DIFF_IMAGE_QUALITY_PRESETS)
-        .default(DEFAULT_DIFFS_TOOL_DEFAULTS.fileQuality)
-        .optional(),
+      fileQuality: z.enum(DIFF_IMAGE_QUALITY_PRESETS).optional(),
       fileScale: z.number().min(1).max(4).optional(),
       fileMaxWidth: z.number().min(640).max(2400).optional(),
       imageFormat: z
@@ -225,12 +219,8 @@ export const diffsPluginConfigSchema: OpenClawPluginConfigSchema = {
 function resolveConfiguredValue<T>(options: {
   primary: T | undefined;
   aliases: Array<T | undefined>;
-  schemaDefault?: T;
 }): T | undefined {
   const alias = options.aliases.find((value): value is T => value !== undefined);
-  if (alias !== undefined && options.primary === options.schemaDefault) {
-    return alias;
-  }
   return options.primary ?? alias;
 }
 
@@ -257,14 +247,12 @@ export function resolveDiffsPluginDefaults(config: unknown): DiffToolDefaults {
     resolveConfiguredValue({
       primary: defaults.fileQuality,
       aliases: [defaults.imageQuality],
-      schemaDefault: DEFAULT_DIFFS_TOOL_DEFAULTS.fileQuality,
     }),
   );
   const profile = DEFAULT_IMAGE_QUALITY_PROFILES[fileQuality];
   const fileFormat = resolveConfiguredValue({
     primary: defaults.fileFormat,
     aliases: [defaults.imageFormat, defaults.format],
-    schemaDefault: DEFAULT_DIFFS_TOOL_DEFAULTS.fileFormat,
   });
   const fileScale = resolveConfiguredValue({
     primary: defaults.fileScale,
diff --git a/extensions/diffs/src/http.ts b/extensions/diffs/src/http.ts
index 620a8d05702a..38ba90a1c761 100644
--- a/extensions/diffs/src/http.ts
+++ b/extensions/diffs/src/http.ts
@@ -5,7 +5,7 @@ import type { PluginLogger } from "../api.js";
 import { resolveRequestClientIp } from "../runtime-api.js";
 import type { DiffArtifactStore } from "./store.js";
 import { DIFF_ARTIFACT_ID_PATTERN, DIFF_ARTIFACT_TOKEN_PATTERN } from "./types.js";
-import { VIEWER_ASSET_PREFIX, getServedViewerAsset } from "./viewer-assets.js";
+import { VIEWER_ASSET_PREFIX, VIEWER_RUNTIME_PATH, getServedViewerAsset } from "./viewer-assets.js";
 
 const VIEW_PREFIX = "/plugins/diffs/view/";
 const VIEWER_MAX_FAILURES_PER_WINDOW = 40;
@@ -23,6 +23,7 @@ const VIEWER_CONTENT_SECURITY_POLICY = [
   "frame-ancestors 'self'",
   "object-src 'none'",
 ].join("; ");
+const IMMUTABLE_ASSET_CACHE_CONTROL = "public, max-age=31536000, immutable";
 
 export function createDiffsHttpHandler(params: {
   store: DiffArtifactStore;
@@ -154,7 +155,11 @@ async function serveAsset(
     }
 
     res.statusCode = 200;
-    setSharedHeaders(res, asset.contentType);
+    setSharedHeaders(
+      res,
+      asset.contentType,
+      pathname === VIEWER_RUNTIME_PATH ? IMMUTABLE_ASSET_CACHE_CONTROL : undefined,
+    );
     if (req.method === "HEAD") {
       res.end();
     } else {
@@ -174,8 +179,12 @@ function respondText(res: ServerResponse, statusCode: number, body: string): voi
   res.end(body);
 }
 
-function setSharedHeaders(res: ServerResponse, contentType: string): void {
-  res.setHeader("cache-control", "no-store, max-age=0");
+function setSharedHeaders(
+  res: ServerResponse,
+  contentType: string,
+  cacheControl = "no-store, max-age=0",
+): void {
+  res.setHeader("cache-control", cacheControl);
   res.setHeader("content-type", contentType);
   res.setHeader("x-content-type-options", "nosniff");
   res.setHeader("referrer-policy", "no-referrer");
diff --git a/extensions/diffs/src/language-hints.test.ts b/extensions/diffs/src/language-hints.test.ts
index 982f142da5a4..d26662355b30 100644
--- a/extensions/diffs/src/language-hints.test.ts
+++ b/extensions/diffs/src/language-hints.test.ts
@@ -19,6 +19,16 @@ describe("normalizeSupportedLanguageHint", () => {
     ]);
   });
 
+  it("normalizes language hint casing", async () => {
+    await expect(normalizeHints(["Python", "TypeScript"])).resolves.toEqual([
+      "python",
+      "typescript",
+    ]);
+    await expect(
+      normalizeSupportedLanguageHint("AbAp", { languagePackAvailable: true }),
+    ).resolves.toBe("abap");
+  });
+
   it("normalizes common aliases to base viewer languages", async () => {
     await expect(
       normalizeHints(["ts", "c++", "c#", "bash", "dockerfile", "rb", "kt", "ps1"]),
@@ -36,17 +46,7 @@ describe("normalizeSupportedLanguageHint", () => {
 
   it("keeps mainstream languages in the base viewer without the language pack", async () => {
     await expect(
-      normalizeHints([
-        "ruby",
-        "swift",
-        "kotlin",
-        "r",
-        "dart",
-        "lua",
-        "powershell",
-        "xml",
-        "toml",
-      ]),
+      normalizeHints(["ruby", "swift", "kotlin", "r", "dart", "lua", "powershell", "xml", "toml"]),
     ).resolves.toEqual([
       "ruby",
       "swift",
diff --git a/extensions/diffs/src/language-hints.ts b/extensions/diffs/src/language-hints.ts
index 05bd7676af49..1c31ee37767b 100644
--- a/extensions/diffs/src/language-hints.ts
+++ b/extensions/diffs/src/language-hints.ts
@@ -26,8 +26,8 @@ function normalizeOptionalString(value: unknown): string | undefined {
   if (typeof value !== "string") {
     return undefined;
   }
-  const trimmed = value.trim();
-  return trimmed ? trimmed : undefined;
+  const normalized = value.trim().toLowerCase();
+  return normalized ? normalized : undefined;
 }
 
 export async function normalizeSupportedLanguageHint(
diff --git a/extensions/diffs/src/prompt-guidance.ts b/extensions/diffs/src/prompt-guidance.ts
index 6e9e3415da50..fa962218e80b 100644
--- a/extensions/diffs/src/prompt-guidance.ts
+++ b/extensions/diffs/src/prompt-guidance.ts
@@ -2,6 +2,7 @@
 export const DIFFS_AGENT_GUIDANCE = [
   "When you need to show edits as a real diff, prefer the `diffs` tool instead of writing a manual summary.",
   "It accepts either `before` + `after` text or a unified `patch`.",
+  "Check `details.changed`: identical before/after input returns `false` without creating an artifact; rendered results return `true`.",
   "`mode=view` returns `details.viewerUrl` for canvas use; `mode=file` returns `details.filePath`; `mode=both` returns both.",
   "If you need to send the rendered file, use the `message` tool with `path` or `filePath`.",
   "Include `path` when you know the filename, and omit presentation overrides unless needed.",
diff --git a/extensions/diffs/src/render-target.test.ts b/extensions/diffs/src/render-target.test.ts
index 4e13fc7774db..5dab3c7e4291 100644
--- a/extensions/diffs/src/render-target.test.ts
+++ b/extensions/diffs/src/render-target.test.ts
@@ -72,7 +72,7 @@ describe("renderDiffDocument render targets", () => {
 
     expect(rendered.html).toContain("mock diff");
     expect(rendered.imageHtml).toContain("mock diff");
-    expect(preloadMultiFileDiffMock).toHaveBeenCalledTimes(2);
+    expect(preloadMultiFileDiffMock).toHaveBeenCalledTimes(1);
   });
 
   it("renders only the image variant for patch image mode", async () => {
diff --git a/extensions/diffs/src/render.preload.test.ts b/extensions/diffs/src/render.preload.test.ts
new file mode 100644
index 000000000000..a7b22c3ded94
--- /dev/null
+++ b/extensions/diffs/src/render.preload.test.ts
@@ -0,0 +1,70 @@
+// Diffs tests cover shared SSR preload behavior.
+import { disposeHighlighter } from "@pierre/diffs";
+import * as diffsSsr from "@pierre/diffs/ssr";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { DEFAULT_DIFFS_TOOL_DEFAULTS, resolveDiffImageRenderOptions } from "./config.js";
+import { renderDiffDocument } from "./render.js";
+
+vi.mock("@pierre/diffs/ssr", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("@pierre/diffs/ssr")>();
+  return {
+    ...actual,
+    preloadFileDiff: vi.fn(actual.preloadFileDiff),
+    preloadMultiFileDiff: vi.fn(actual.preloadMultiFileDiff),
+  };
+});
+
+describe("renderDiffDocument SSR preloads", () => {
+  afterEach(async () => {
+    vi.clearAllMocks();
+    await disposeHighlighter();
+  });
+
+  it("preloads a before/after diff once for viewer and image output", async () => {
+    await renderDiffDocument(
+      {
+        kind: "before_after",
+        before: "const value = 1;\n",
+        after: "const value = 2;\n",
+        path: "src/example.ts",
+      },
+      {
+        presentation: DEFAULT_DIFFS_TOOL_DEFAULTS,
+        image: resolveDiffImageRenderOptions({ defaults: DEFAULT_DIFFS_TOOL_DEFAULTS }),
+        expandUnchanged: false,
+      },
+      "both",
+    );
+
+    expect(diffsSsr.preloadMultiFileDiff).toHaveBeenCalledTimes(1);
+  });
+
+  it("preloads each patch file once for viewer and image output", async () => {
+    const patch = [
+      "diff --git a/a.ts b/a.ts",
+      "--- a/a.ts",
+      "+++ b/a.ts",
+      "@@ -1 +1 @@",
+      "-const a = 1;",
+      "+const a = 2;",
+      "diff --git a/b.ts b/b.ts",
+      "--- a/b.ts",
+      "+++ b/b.ts",
+      "@@ -1 +1 @@",
+      "-const b = 1;",
+      "+const b = 2;",
+    ].join("\n");
+
+    await renderDiffDocument(
+      { kind: "patch", patch },
+      {
+        presentation: DEFAULT_DIFFS_TOOL_DEFAULTS,
+        image: resolveDiffImageRenderOptions({ defaults: DEFAULT_DIFFS_TOOL_DEFAULTS }),
+        expandUnchanged: false,
+      },
+      "both",
+    );
+
+    expect(diffsSsr.preloadFileDiff).toHaveBeenCalledTimes(2);
+  });
+});
diff --git a/extensions/diffs/src/render.ts b/extensions/diffs/src/render.ts
index 09fe4a4d0052..6bf3b5dd8628 100644
--- a/extensions/diffs/src/render.ts
+++ b/extensions/diffs/src/render.ts
@@ -24,6 +24,13 @@ const MAX_PATCH_TOTAL_LINES = 120_000;
 const VIEWER_LOADER_DOCUMENT_PATH = "../../assets/viewer.js";
 const LANGUAGE_PACK_VIEWER_LOADER_DOCUMENT_PATH = "../../../diffs-language-pack/assets/viewer.js";
 
+export class DiffRenderInputError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "DiffRenderInputError";
+  }
+}
+
 function escapeCssString(value: string): string {
   return value.replaceAll("\\", "\\\\").replaceAll('"', '\\"');
 }
@@ -65,11 +72,19 @@ function resolveBeforeAfterFileName(params: {
   return DEFAULT_FILE_NAME;
 }
 
+function resolveDiffTypography(presentation: DiffRenderOptions["presentation"]): {
+  fontSize: number;
+  lineHeight: number;
+} {
+  const fontSize = normalizeDiffFontSize(presentation.fontSize);
+  const lineSpacing = normalizeDiffLineSpacing(presentation.lineSpacing);
+  const lineHeight = Math.max(20, Math.round(fontSize * lineSpacing));
+  return { fontSize, lineHeight };
+}
+
 function buildDiffOptions(options: DiffRenderOptions): DiffViewerOptions {
   const fontFamily = escapeCssString(options.presentation.fontFamily);
-  const fontSize = normalizeDiffFontSize(options.presentation.fontSize);
-  const lineSpacing = normalizeDiffLineSpacing(options.presentation.lineSpacing);
-  const lineHeight = Math.max(20, Math.round(fontSize * lineSpacing));
+  const { fontSize, lineHeight } = resolveDiffTypography(options.presentation);
   return {
     theme: {
       light: "pierre-light",
@@ -201,6 +216,10 @@ function buildHtmlDocument(params: {
   bodyHtml: string;
   theme: DiffRenderOptions["presentation"]["theme"];
   imageMaxWidth: number;
+  imageTypography: {
+    fontSize: number;
+    lineHeight: number;
+  };
   runtimeMode: "viewer" | "image";
   viewerRuntime: "base" | "language-pack";
 }): string {
@@ -208,6 +227,15 @@ function buildHtmlDocument(params: {
     params.viewerRuntime === "language-pack"
       ? LANGUAGE_PACK_VIEWER_LOADER_DOCUMENT_PATH
       : VIEWER_LOADER_DOCUMENT_PATH;
+  const imageTypographyCss =
+    params.runtimeMode === "image"
+      ? `
+      .oc-frame[data-render-mode="image"] .oc-diff-host {
+        --diffs-font-size: ${params.imageTypography.fontSize}px;
+        --diffs-line-height: ${params.imageTypography.lineHeight}px;
+      }
+`
+      : "";
   return `<!doctype html>
 <html lang="en">
   <head>
@@ -256,6 +284,7 @@ function buildHtmlDocument(params: {
       .oc-frame[data-render-mode="image"] {
         max-width: ${Math.max(640, Math.round(params.imageMaxWidth))}px;
       }
+${imageTypographyCss}
 
       [data-openclaw-diff-root] {
         display: grid;
@@ -364,54 +393,32 @@ async function renderBeforeAfterDiff(
     ...(lang ? { lang } : {}),
   };
   const { viewerOptions, imageOptions } = buildRenderVariants({ options, target });
-  const [viewerResult, imageResult] = await Promise.all([
-    viewerOptions
-      ? preloadMultiFileDiffWithFallback({
-          oldFile,
-          newFile,
-          options: viewerOptions,
-        })
-      : Promise.resolve(undefined),
-    imageOptions
-      ? preloadMultiFileDiffWithFallback({
-          oldFile,
-          newFile,
-          options: imageOptions,
-        })
-      : Promise.resolve(undefined),
-  ]);
-  const [viewerPayload, imagePayload] = await Promise.all([
-    viewerResult && viewerOptions
-      ? normalizeDiffViewerPayloadLanguages(
-          {
-            prerenderedHTML: viewerResult.prerenderedHTML,
-            oldFile: viewerResult.oldFile,
-            newFile: viewerResult.newFile,
-            options: viewerOptions,
-            langs: collectDiffPayloadLanguageHints({
-              oldFile: viewerResult.oldFile,
-              newFile: viewerResult.newFile,
-            }),
-          },
-          { languagePackAvailable },
-        )
-      : Promise.resolve(undefined),
-    imageResult && imageOptions
-      ? normalizeDiffViewerPayloadLanguages(
-          {
-            prerenderedHTML: imageResult.prerenderedHTML,
-            oldFile: imageResult.oldFile,
-            newFile: imageResult.newFile,
-            options: imageOptions,
-            langs: collectDiffPayloadLanguageHints({
-              oldFile: imageResult.oldFile,
-              newFile: imageResult.newFile,
-            }),
-          },
-          { languagePackAvailable },
-        )
-      : Promise.resolve(undefined),
-  ]);
+  const preloadOptions = viewerOptions ?? imageOptions;
+  if (!preloadOptions) {
+    throw new Error(`Unsupported diff render target: ${target}`);
+  }
+  const preloadResult = await preloadMultiFileDiffWithFallback({
+    oldFile,
+    newFile,
+    options: preloadOptions,
+  });
+  const normalizedPayload = await normalizeDiffViewerPayloadLanguages(
+    {
+      prerenderedHTML: preloadResult.prerenderedHTML,
+      oldFile: preloadResult.oldFile,
+      newFile: preloadResult.newFile,
+      options: preloadOptions,
+      langs: collectDiffPayloadLanguageHints({
+        oldFile: preloadResult.oldFile,
+        newFile: preloadResult.newFile,
+      }),
+    },
+    { languagePackAvailable },
+  );
+  const viewerPayload = viewerOptions
+    ? { ...normalizedPayload, options: viewerOptions }
+    : undefined;
+  const imagePayload = imageOptions ? { ...normalizedPayload, options: imageOptions } : undefined;
   const section = buildRenderedSection({
     ...(viewerPayload ? { viewerPayload } : {}),
     ...(imagePayload ? { imagePayload } : {}),
@@ -441,10 +448,12 @@ async function renderPatchDiff(
       .map((fileDiff) => normalizePatchFileLanguage(fileDiff, { languagePackAvailable })),
   );
   if (files.length === 0) {
-    throw new Error("Patch input did not contain any file diffs.");
+    throw new DiffRenderInputError("Patch input did not contain any file diffs.");
   }
   if (files.length > MAX_PATCH_FILE_COUNT) {
-    throw new Error(`Patch input contains too many files (max ${MAX_PATCH_FILE_COUNT}).`);
+    throw new DiffRenderInputError(
+      `Patch input contains too many files (max ${MAX_PATCH_FILE_COUNT}).`,
+    );
   }
   const totalLines = files.reduce((sum, fileDiff) => {
     const splitLines = Number.isFinite(fileDiff.splitLineCount) ? fileDiff.splitLineCount : 0;
@@ -452,51 +461,37 @@ async function renderPatchDiff(
     return sum + Math.max(splitLines, unifiedLines, 0);
   }, 0);
   if (totalLines > MAX_PATCH_TOTAL_LINES) {
-    throw new Error(`Patch input is too large to render (max ${MAX_PATCH_TOTAL_LINES} lines).`);
+    throw new DiffRenderInputError(
+      `Patch input is too large to render (max ${MAX_PATCH_TOTAL_LINES} lines).`,
+    );
   }
 
   const { viewerOptions, imageOptions } = buildRenderVariants({ options, target });
+  const preloadOptions = viewerOptions ?? imageOptions;
+  if (!preloadOptions) {
+    throw new Error(`Unsupported diff render target: ${target}`);
+  }
   const sections = await Promise.all(
     files.map(async (fileDiff) => {
-      const [viewerResult, imageResult] = await Promise.all([
-        viewerOptions
-          ? preloadFileDiffWithFallback({
-              fileDiff,
-              options: viewerOptions,
-            })
-          : Promise.resolve(undefined),
-        imageOptions
-          ? preloadFileDiffWithFallback({
-              fileDiff,
-              options: imageOptions,
-            })
-          : Promise.resolve(undefined),
-      ]);
-
-      const [viewerPayload, imagePayload] = await Promise.all([
-        viewerResult && viewerOptions
-          ? normalizeDiffViewerPayloadLanguages(
-              {
-                prerenderedHTML: viewerResult.prerenderedHTML,
-                fileDiff: viewerResult.fileDiff,
-                options: viewerOptions,
-                langs: collectDiffPayloadLanguageHints({ fileDiff: viewerResult.fileDiff }),
-              },
-              { languagePackAvailable },
-            )
-          : Promise.resolve(undefined),
-        imageResult && imageOptions
-          ? normalizeDiffViewerPayloadLanguages(
-              {
-                prerenderedHTML: imageResult.prerenderedHTML,
-                fileDiff: imageResult.fileDiff,
-                options: imageOptions,
-                langs: collectDiffPayloadLanguageHints({ fileDiff: imageResult.fileDiff }),
-              },
-              { languagePackAvailable },
-            )
-          : Promise.resolve(undefined),
-      ]);
+      const preloadResult = await preloadFileDiffWithFallback({
+        fileDiff,
+        options: preloadOptions,
+      });
+      const normalizedPayload = await normalizeDiffViewerPayloadLanguages(
+        {
+          prerenderedHTML: preloadResult.prerenderedHTML,
+          fileDiff: preloadResult.fileDiff,
+          options: preloadOptions,
+          langs: collectDiffPayloadLanguageHints({ fileDiff: preloadResult.fileDiff }),
+        },
+        { languagePackAvailable },
+      );
+      const viewerPayload = viewerOptions
+        ? { ...normalizedPayload, options: viewerOptions }
+        : undefined;
+      const imagePayload = imageOptions
+        ? { ...normalizedPayload, options: imageOptions }
+        : undefined;
 
       return buildRenderedSection({
         ...(viewerPayload ? { viewerPayload } : {}),
@@ -537,6 +532,7 @@ export async function renderDiffDocument(
       ? await renderBeforeAfterDiff(input, options, target)
       : await renderPatchDiff(input, options, target);
   const viewerRuntime = rendered.usesLanguagePack ? "language-pack" : "base";
+  const imageTypography = resolveDiffTypography(buildImageRenderOptions(options).presentation);
 
   return {
     ...(rendered.viewerBodyHtml
@@ -546,6 +542,7 @@ export async function renderDiffDocument(
             bodyHtml: rendered.viewerBodyHtml,
             theme: options.presentation.theme,
             imageMaxWidth: options.image.maxWidth,
+            imageTypography,
             runtimeMode: "viewer",
             viewerRuntime,
           }),
@@ -558,6 +555,7 @@ export async function renderDiffDocument(
             bodyHtml: rendered.imageBodyHtml,
             theme: options.presentation.theme,
             imageMaxWidth: options.image.maxWidth,
+            imageTypography,
             runtimeMode: "image",
             viewerRuntime,
           }),
diff --git a/extensions/diffs/src/store.test.ts b/extensions/diffs/src/store.test.ts
index 87ee6478cec5..a5ddc0b2b2e5 100644
--- a/extensions/diffs/src/store.test.ts
+++ b/extensions/diffs/src/store.test.ts
@@ -53,6 +53,8 @@ describe("DiffArtifactStore", () => {
       agentAccountId: "default",
     });
     expect(await store.readHtml(artifact.id)).toBe("<html>demo</html>");
+    await expect(store.getArtifact(artifact.id, "0".repeat(48))).resolves.toBeNull();
+    await expect(store.getArtifact(artifact.id, "short")).resolves.toBeNull();
   });
 
   it("caps artifact expiry instead of throwing near the Date boundary", async () => {
@@ -280,6 +282,7 @@ describe("createDiffsHttpHandler", () => {
     expect(res.statusCode).toBe(200);
     expect(res.body).toBe("<html>viewer</html>");
     expect(res.getHeader("content-security-policy")).toContain("default-src 'none'");
+    expect(res.getHeader("cache-control")).toBe("no-store, max-age=0");
   });
 
   it("rejects invalid tokens", async () => {
@@ -321,6 +324,7 @@ describe("createDiffsHttpHandler", () => {
     expect(handled).toBe(true);
     expect(res.statusCode).toBe(200);
     expect(String(res.body)).toContain("./viewer-runtime.js?v=");
+    expect(res.getHeader("cache-control")).toBe("no-store, max-age=0");
   });
 
   it("serves the shared viewer runtime asset", async () => {
@@ -337,6 +341,7 @@ describe("createDiffsHttpHandler", () => {
     expect(handled).toBe(true);
     expect(res.statusCode).toBe(200);
     expect(String(res.body)).toContain("openclawDiffsReady");
+    expect(res.getHeader("cache-control")).toBe("public, max-age=31536000, immutable");
   });
 
   it.each([
diff --git a/extensions/diffs/src/store.ts b/extensions/diffs/src/store.ts
index b6e7b95760fa..b33138d5cffd 100644
--- a/extensions/diffs/src/store.ts
+++ b/extensions/diffs/src/store.ts
@@ -3,7 +3,7 @@ import crypto from "node:crypto";
 import fs from "node:fs/promises";
 import path from "node:path";
 import { MAX_DATE_TIMESTAMP_MS, timestampMsToIsoString } from "openclaw/plugin-sdk/number-runtime";
-import { root as fsRoot } from "openclaw/plugin-sdk/security-runtime";
+import { root as fsRoot, safeEqualSecret } from "openclaw/plugin-sdk/security-runtime";
 import { normalizeOptionalString } from "openclaw/plugin-sdk/string-coerce-runtime";
 import type { PluginLogger } from "../api.js";
 import type { DiffArtifactContext, DiffArtifactMeta, DiffOutputFormat } from "./types.js";
@@ -94,7 +94,7 @@ export class DiffArtifactStore {
     if (!meta) {
       return null;
     }
-    if (meta.token !== token) {
+    if (!safeEqualSecret(token, meta.token)) {
       return null;
     }
     if (isExpired(meta)) {
diff --git a/extensions/diffs/src/tool.test.ts b/extensions/diffs/src/tool.test.ts
index b8805776173d..9bab645743d7 100644
--- a/extensions/diffs/src/tool.test.ts
+++ b/extensions/diffs/src/tool.test.ts
@@ -12,11 +12,16 @@ import { createDiffsTool } from "./tool.js";
 import type { DiffRenderOptions } from "./types.js";
 
 describe("diffs tool", () => {
+  let rootDir: string;
   let store: DiffArtifactStore;
   let cleanupRootDir: () => Promise<void>;
 
   beforeEach(async () => {
-    ({ store, cleanup: cleanupRootDir } = await createDiffStoreHarness("openclaw-diffs-tool-"));
+    ({
+      rootDir,
+      store,
+      cleanup: cleanupRootDir,
+    } = await createDiffStoreHarness("openclaw-diffs-tool-"));
   });
 
   afterEach(async () => {
@@ -42,6 +47,32 @@ describe("diffs tool", () => {
     expect(String(readDetails(result).viewerUrl)).toContain(
       "http://127.0.0.1:18789/plugins/diffs/view/",
     );
+    expect(readDetails(result).changed).toBe(true);
+  });
+
+  it("short-circuits identical before/after input without creating an artifact", async () => {
+    const screenshotHtml = vi.fn<DiffScreenshotter["screenshotHtml"]>();
+    const tool = createToolWithScreenshotter(store, { screenshotHtml });
+
+    const result = await tool.execute?.("tool-identical", {
+      before: "same\n",
+      after: "same\n",
+    });
+
+    expect(readTextContent(result, 0)).toBe(
+      "Before and after are identical — no changes to render.",
+    );
+    expect(readDetails(result)).toEqual({
+      changed: false,
+      context: {
+        agentId: "main",
+        sessionId: "session-123",
+        messageChannel: "discord",
+        agentAccountId: "default",
+      },
+    });
+    expect(screenshotHtml).not.toHaveBeenCalled();
+    await expect(fs.readdir(rootDir)).resolves.toEqual([]);
   });
 
   it("uses configured viewerBaseUrl when tool input omits baseUrl", async () => {
@@ -388,6 +419,29 @@ describe("diffs tool", () => {
     ).rejects.toThrow("patch exceeds maximum size");
   });
 
+  it("classifies patch render validation failures as tool input errors", async () => {
+    const tool = createDiffsTool({
+      api: createApi(),
+      store,
+      defaults: DEFAULT_DIFFS_TOOL_DEFAULTS,
+    });
+
+    const error = await tool
+      .execute?.("tool-invalid-patch", {
+        patch: "not a unified patch",
+        mode: "view",
+      })
+      .then(
+        () => undefined,
+        (caught: unknown) => caught,
+      );
+
+    expect(error).toMatchObject({
+      name: "ToolInputError",
+      message: "Patch input did not contain any file diffs.",
+    });
+  });
+
   it("rejects oversized before/after payloads", async () => {
     const tool = createDiffsTool({
       api: createApi(),
diff --git a/extensions/diffs/src/tool.ts b/extensions/diffs/src/tool.ts
index 42918a3f4145..593ed28ab9a0 100644
--- a/extensions/diffs/src/tool.ts
+++ b/extensions/diffs/src/tool.ts
@@ -9,7 +9,7 @@ import type { Static } from "typebox";
 import type { AnyAgentTool, OpenClawPluginApi, OpenClawPluginToolContext } from "../api.js";
 import { PlaywrightDiffScreenshotter, type DiffScreenshotter } from "./browser.js";
 import { resolveDiffImageRenderOptions } from "./config.js";
-import { renderDiffDocument } from "./render.js";
+import { DiffRenderInputError, renderDiffDocument } from "./render.js";
 import type { DiffArtifactStore } from "./store.js";
 import type {
   DiffArtifactContext,
@@ -167,6 +167,20 @@ export function createDiffsTool(params: {
       const rawRecord = rawParams as Record<string, unknown>;
       const artifactContext = buildArtifactContext(params.context);
       const input = normalizeDiffInput(toolParams);
+      if (input.kind === "before_after" && input.before === input.after) {
+        return {
+          content: [
+            {
+              type: "text",
+              text: "Before and after are identical — no changes to render.",
+            },
+          ],
+          details: {
+            changed: false,
+            ...(artifactContext ? { context: artifactContext } : {}),
+          },
+        };
+      }
       const mode = normalizeMode(toolParams.mode, params.defaults.mode);
       const theme = normalizeTheme(toolParams.theme, params.defaults.theme);
       const layout = normalizeLayout(toolParams.layout, params.defaults.layout);
@@ -204,7 +218,12 @@ export function createDiffsTool(params: {
           languagePackAvailable: params.languagePackAvailable,
         },
         renderTarget,
-      );
+      ).catch((error: unknown) => {
+        if (error instanceof DiffRenderInputError) {
+          throw new PluginToolInputError(error.message);
+        }
+        throw error;
+      });
 
       const screenshotter =
         params.screenshotter ?? new PlaywrightDiffScreenshotter({ config: params.api.config });
@@ -232,6 +251,7 @@ export function createDiffsTool(params: {
           ],
           details: buildArtifactDetails({
             baseDetails: {
+              changed: true,
               ...(artifactFile.artifactId ? { artifactId: artifactFile.artifactId } : {}),
               ...(artifactFile.expiresAt ? { expiresAt: artifactFile.expiresAt } : {}),
               title: rendered.title,
@@ -262,6 +282,7 @@ export function createDiffsTool(params: {
       });
 
       const baseDetails = {
+        changed: true,
         artifactId: artifact.id,
         viewerUrl,
         viewerPath: artifact.viewerPath,
diff --git a/extensions/diffs/src/viewer-client.test.ts b/extensions/diffs/src/viewer-client.test.ts
index 55a847070fdb..1e2b34439263 100644
--- a/extensions/diffs/src/viewer-client.test.ts
+++ b/extensions/diffs/src/viewer-client.test.ts
@@ -192,6 +192,16 @@ describe("hydrateViewer", () => {
   });
 });
 
+describe("resolveViewerLanguagePackAvailability", () => {
+  it("resolves defined and undefined build flags", async () => {
+    const { resolveViewerLanguagePackAvailability } = await import("./viewer-client.js");
+
+    expect(resolveViewerLanguagePackAvailability(true)).toBe(true);
+    expect(resolveViewerLanguagePackAvailability(false)).toBe(false);
+    expect(resolveViewerLanguagePackAvailability(undefined)).toBe(false);
+  });
+});
+
 describe("viewerState initialization", () => {
   beforeEach(() => {
     document.body.innerHTML = "";
diff --git a/extensions/diffs/src/viewer-client.ts b/extensions/diffs/src/viewer-client.ts
index cd6810a08f54..a207aae392d1 100644
--- a/extensions/diffs/src/viewer-client.ts
+++ b/extensions/diffs/src/viewer-client.ts
@@ -10,6 +10,23 @@ import { normalizeDiffViewerPayloadLanguages } from "./language-hints.js";
 import type { DiffViewerPayload, DiffLayout, DiffTheme } from "./types.js";
 import { parseViewerPayloadJson } from "./viewer-payload.js";
 
+// oxlint-disable-next-line eslint/no-underscore-dangle -- Bundled builds replace this compile-time define identifier.
+declare const __OPENCLAW_DIFFS_LANGUAGE_PACK__: boolean | undefined;
+
+// Build-time esbuild define; typeof guard keeps the module loadable where the
+// define is absent (vitest/node), matching the __OPENCLAW_VERSION__ pattern.
+function readInjectedLanguagePackFlag(): boolean | undefined {
+  return typeof __OPENCLAW_DIFFS_LANGUAGE_PACK__ === "boolean"
+    ? __OPENCLAW_DIFFS_LANGUAGE_PACK__
+    : undefined;
+}
+
+export function resolveViewerLanguagePackAvailability(
+  buildFlag: boolean | undefined = readInjectedLanguagePackFlag(),
+): boolean {
+  return buildFlag === true;
+}
+
 type ViewerState = {
   theme: DiffTheme;
   layout: DiffLayout;
@@ -290,10 +307,11 @@ export async function hydrateViewer(): Promise<void> {
   // Rehydration replaces the current DOM card set; do not retain controllers
   // from a previous render because they can keep stale DOM references alive.
   controllers.length = 0;
+  const languagePackAvailable = resolveViewerLanguagePackAvailability();
   const cards = await Promise.all(
     getCards().map(async ({ host, payload }) => ({
       host,
-      payload: await normalizeDiffViewerPayloadLanguages(payload),
+      payload: await normalizeDiffViewerPayloadLanguages(payload, { languagePackAvailable }),
     })),
   );
   const langs = new Set<SupportedLanguages>();
diff --git a/scripts/build-diffs-viewer-runtime.mjs b/scripts/build-diffs-viewer-runtime.mjs
index ba701dc05091..720d50e498e0 100644
--- a/scripts/build-diffs-viewer-runtime.mjs
+++ b/scripts/build-diffs-viewer-runtime.mjs
@@ -16,10 +16,12 @@ const targets = {
     entry: "extensions/diffs/src/viewer-client.ts",
     output: "extensions/diffs/assets/viewer-runtime.js",
     shikiAlias: "scripts/diffs-shiki-curated.ts",
+    languagePackAvailable: false,
   },
   full: {
     entry: "extensions/diffs/src/viewer-client.ts",
     output: "extensions/diffs-language-pack/assets/viewer-runtime.js",
+    languagePackAvailable: true,
   },
 };
 
@@ -81,6 +83,7 @@ export async function buildDiffsViewerRuntime(targetName) {
     format: "esm",
     minify: true,
     define: {
+      __OPENCLAW_DIFFS_LANGUAGE_PACK__: String(target.languagePackAvailable),
       NaN: "Number.NaN",
     },
     legalComments: "none",
diff --git a/test/scripts/lint-suppressions.test.ts b/test/scripts/lint-suppressions.test.ts
index fa9e7f050823..53cecdd37dd0 100644
--- a/test/scripts/lint-suppressions.test.ts
+++ b/test/scripts/lint-suppressions.test.ts
@@ -188,6 +188,7 @@ describe("production lint suppressions", () => {
         "extensions/browser/src/browser/pw-tools-core.interactions.ts|@typescript-eslint/no-implied-eval|2",
         "extensions/browser/src/cli/browser-cli-actions-input/register.files-downloads.ts|typescript/no-unnecessary-type-parameters|1",
         "extensions/browser/src/node-host/invoke-browser.ts|typescript/no-unnecessary-type-parameters|1",
+        "extensions/diffs/src/viewer-client.ts|eslint/no-underscore-dangle|1",
         "extensions/discord/src/outbound-adapter.test-harness.ts|typescript/no-unnecessary-type-parameters|1",
         "extensions/discord/src/test-support/provider.test-support.ts|typescript/no-unnecessary-type-parameters|1",
         "extensions/feishu/src/bitable.ts|typescript/no-unnecessary-type-parameters|1",