diff --git a/docs/tools/diffs.md b/docs/tools/diffs.md
index ed41ae690b9..31ccdfbfa86 100644
--- a/docs/tools/diffs.md
+++ b/docs/tools/diffs.md
@@ -237,6 +237,23 @@ Persistent viewer URL config:
   - Plugin-owned fallback for returned viewer links when a tool call does not pass `baseUrl`.
   - Must be `http` or `https`, no query/hash.
 
+Example:
+
+```json5
+{
+  plugins: {
+    entries: {
+      diffs: {
+        enabled: true,
+        config: {
+          viewerBaseUrl: "https://gateway.example.com/openclaw",
+        },
+      },
+    },
+  },
+}
+```
+
 ## Security config
 
 - `security.allowRemoteViewer` (`boolean`, default `false`)
diff --git a/extensions/diffs/README.md b/extensions/diffs/README.md
index 056818e8b28..31a5e82c93e 100644
--- a/extensions/diffs/README.md
+++ b/extensions/diffs/README.md
@@ -112,6 +112,23 @@ Security options:
 - `security.allowRemoteViewer` (default `false`): allows non-loopback access to `/plugins/diffs/view/...` token URLs
 - `viewerBaseUrl` (optional): persistent viewer-link origin/path fallback for shareable URLs
 
+Example:
+
+```json5
+{
+  plugins: {
+    entries: {
+      diffs: {
+        enabled: true,
+        config: {
+          viewerBaseUrl: "https://gateway.example.com/openclaw",
+        },
+      },
+    },
+  },
+}
+```
+
 ## Example Agent Prompts
 
 Open in canvas:
diff --git a/extensions/diffs/src/config.test.ts b/extensions/diffs/src/config.test.ts
index 8e0dc295f06..fbd740a6246 100644
--- a/extensions/diffs/src/config.test.ts
+++ b/extensions/diffs/src/config.test.ts
@@ -305,7 +305,7 @@ describe("diffs plugin schema surfaces", () => {
         issues: [
           {
             path: ["viewerBaseUrl"],
-            message: "baseUrl must use http or https: javascript:alert(1)",
+            message: "viewerBaseUrl must use http or https: javascript:alert(1)",
           },
         ],
       },
@@ -382,6 +382,12 @@ describe("diffs viewer URL helpers", () => {
       "baseUrl must not include query/hash",
     );
   });
+
+  it("uses the configured field name in viewerBaseUrl validation errors", () => {
+    expect(() => normalizeViewerBaseUrl("https://example.com?a=1", "viewerBaseUrl")).toThrow(
+      "viewerBaseUrl must not include query/hash",
+    );
+  });
 });
 
 describe("renderDiffDocument", () => {
diff --git a/extensions/diffs/src/config.ts b/extensions/diffs/src/config.ts
index a806e45e922..dd67e19afc4 100644
--- a/extensions/diffs/src/config.ts
+++ b/extensions/diffs/src/config.ts
@@ -100,7 +100,7 @@ const DiffsPluginJsonSchemaSource = z.strictObject({
     .string()
     .superRefine((value, ctx) => {
       try {
-        normalizeViewerBaseUrl(value);
+        normalizeViewerBaseUrl(value, "viewerBaseUrl");
       } catch (error) {
         ctx.addIssue({
           code: "custom",
diff --git a/extensions/diffs/src/url.ts b/extensions/diffs/src/url.ts
index e8c40e05753..5b3e29b20ff 100644
--- a/extensions/diffs/src/url.ts
+++ b/extensions/diffs/src/url.ts
@@ -1,6 +1,7 @@
 import type { OpenClawConfig } from "../api.js";
 
 const DEFAULT_GATEWAY_PORT = 18789;
+type ViewerBaseUrlFieldName = "baseUrl" | "viewerBaseUrl";
 
 export function buildViewerUrl(params: {
   config: OpenClawConfig;
@@ -20,18 +21,21 @@ export function buildViewerUrl(params: {
   return parsedBase.toString();
 }
 
-export function normalizeViewerBaseUrl(raw: string): string {
+export function normalizeViewerBaseUrl(
+  raw: string,
+  fieldName: ViewerBaseUrlFieldName = "baseUrl",
+): string {
   let parsed: URL;
   try {
     parsed = new URL(raw);
   } catch {
-    throw new Error(`Invalid baseUrl: ${raw}`);
+    throw new Error(`Invalid ${fieldName}: ${raw}`);
   }
   if (parsed.protocol !== "http:" && parsed.protocol !== "https:") {
-    throw new Error(`baseUrl must use http or https: ${raw}`);
+    throw new Error(`${fieldName} must use http or https: ${raw}`);
   }
   if (parsed.search || parsed.hash) {
-    throw new Error(`baseUrl must not include query/hash: ${raw}`);
+    throw new Error(`${fieldName} must not include query/hash: ${raw}`);
   }
   parsed.search = "";
   parsed.hash = "";