diff --git a/api/server/experimental.js b/api/server/experimental.js
index 2632c2d2bc..aa387616c4 100644
--- a/api/server/experimental.js
+++ b/api/server/experimental.js
@@ -24,6 +24,7 @@ const {
   maybeInjectQueryDevtoolsBootstrap,
   preAuthTenantMiddleware,
   configureServerTimeouts,
+  configureMessageFilterRegexValidator,
 } = require('@librechat/api');
 const { connectDb, indexSync } = require('~/db');
 const initializeOAuthReconnectManager = require('./services/initializeOAuthReconnectManager');
@@ -49,6 +50,9 @@ const optionalJwtAuth = require('./middleware/optionalJwtAuth');
 const noIndex = require('./middleware/noIndex');
 const routes = require('./routes');
 
+/** Reject messageFilter PII patterns the RE2 runtime engine cannot compile, at config load. */
+configureMessageFilterRegexValidator();
+
 const { PORT, HOST, ALLOW_SOCIAL_LOGIN, DISABLE_COMPRESSION, TRUST_PROXY } = process.env ?? {};
 
 /** Allow PORT=0 to be used for automatic free port assignment */
diff --git a/api/server/index.js b/api/server/index.js
index a27cedbb50..1401f8b9d6 100644
--- a/api/server/index.js
+++ b/api/server/index.js
@@ -32,6 +32,7 @@ const {
   configureServerTimeouts,
   setupGracefulShutdown,
   updateInterfacePermissions,
+  configureMessageFilterRegexValidator,
 } = require('@librechat/api');
 const { connectDb, indexSync } = require('~/db');
 const {
@@ -56,6 +57,9 @@ const staticCache = require('./utils/staticCache');
 const noIndex = require('./middleware/noIndex');
 const routes = require('./routes');
 
+/** Reject messageFilter PII patterns the RE2 runtime engine cannot compile, at config load. */
+configureMessageFilterRegexValidator();
+
 const { PORT, HOST, ALLOW_SOCIAL_LOGIN, DISABLE_COMPRESSION, TRUST_PROXY } = process.env ?? {};
 
 // Allow PORT=0 to be used for automatic free port assignment
diff --git a/packages/api/src/middleware/messageFilterPii.spec.ts b/packages/api/src/middleware/messageFilterPii.spec.ts
index c01fbe6f89..9eb27416fc 100644
--- a/packages/api/src/middleware/messageFilterPii.spec.ts
+++ b/packages/api/src/middleware/messageFilterPii.spec.ts
@@ -1,10 +1,15 @@
+import { messageFilterPiiSchema, setMessageFilterRegexValidator } from 'librechat-data-provider';
 import type { MessageFilterPiiConfig } from 'librechat-data-provider';
 import type { Request, Response, NextFunction } from 'express';
 
 jest.mock('@librechat/data-schemas', () => ({
   logger: { warn: jest.fn(), error: jest.fn(), info: jest.fn(), debug: jest.fn() },
 }));
-import { createMessageFilterPii, findPiiMatchInMessages } from './messageFilterPii';
+import {
+  createMessageFilterPii,
+  findPiiMatchInMessages,
+  configureMessageFilterRegexValidator,
+} from './messageFilterPii';
 
 type CapturedResponse = { status?: number; body?: unknown };
 
@@ -366,3 +371,30 @@ describe('findPiiMatchInMessages', () => {
     expect(hit).toEqual({ id: 'org', label: 'Org token' });
   });
 });
+
+describe('configureMessageFilterRegexValidator (RE2 config-load validation)', () => {
+  afterAll(() => {
+    setMessageFilterRegexValidator((value) => {
+      try {
+        new RegExp(value, 'g');
+        return true;
+      } catch {
+        return false;
+      }
+    });
+  });
+
+  it('rejects RE2-incompatible custom patterns at config parse once wired', () => {
+    configureMessageFilterRegexValidator();
+    const reject = (regex: string) =>
+      messageFilterPiiSchema.safeParse({ customPatterns: [{ id: 'a', label: 'A', regex }] })
+        .success;
+    // lookahead, numeric + named backreference, control escape: all valid JS, unsupported by RE2
+    expect(reject('(?=x)y')).toBe(false);
+    expect(reject('(a)\\1')).toBe(false);
+    expect(reject('(?<n>x)\\k<n>')).toBe(false);
+    expect(reject('token-\\cA+')).toBe(false);
+    // a normal RE2-compatible pattern still passes
+    expect(reject('\\bORG-[A-Z0-9]{6,}')).toBe(true);
+  });
+});
diff --git a/packages/api/src/middleware/messageFilterPii.ts b/packages/api/src/middleware/messageFilterPii.ts
index bce3d7d42b..810800cb64 100644
--- a/packages/api/src/middleware/messageFilterPii.ts
+++ b/packages/api/src/middleware/messageFilterPii.ts
@@ -1,5 +1,6 @@
 import { RE2JS } from 're2js';
 import { logger } from '@librechat/data-schemas';
+import { setMessageFilterRegexValidator } from 'librechat-data-provider';
 import type {
   NextFunction,
   RequestHandler,
@@ -9,6 +10,23 @@ import type {
 import type { MessageFilterPiiConfig } from 'librechat-data-provider';
 import { getReferencedQuotes, mergeQuotedText } from '../utils/quotes';
 
+/**
+ * Wire the messageFilter PII config validator to the linear-time engine (RE2) so a custom
+ * pattern the runtime cannot compile is rejected at config load rather than silently dropped at
+ * request time. Call once at server startup, before config is parsed; browser builds keep the
+ * native default and pull in no engine.
+ */
+export function configureMessageFilterRegexValidator(): void {
+  setMessageFilterRegexValidator((pattern) => {
+    try {
+      RE2JS.compile(pattern);
+      return true;
+    } catch {
+      return false;
+    }
+  });
+}
+
 type CompiledPattern = { id: string; label: string; pattern: RE2JS };
 
 const STARTER_PATTERNS: CompiledPattern[] = [
diff --git a/packages/data-provider/src/config.ts b/packages/data-provider/src/config.ts
index ea717d3733..e18e01f3fc 100644
--- a/packages/data-provider/src/config.ts
+++ b/packages/data-provider/src/config.ts
@@ -1874,29 +1874,36 @@ export type SummarizationConfig = z.infer<typeof summarizationConfigSchema>;
 
 const customEndpointsSchema = z.array(endpointSchema.partial()).optional();
 
+/**
+ * Validates a messageFilter PII regex at config load. Defaults to native RegExp so browser
+ * builds add no extra engine; the server injects a check backed by the linear-time runtime
+ * engine (RE2) via setMessageFilterRegexValidator, so a pattern the runtime cannot compile
+ * (backreferences, lookaround, control escapes, and so on) is rejected at load rather than
+ * silently dropped at request time.
+ */
+let messageFilterRegexValidator: (pattern: string) => boolean = (value) => {
+  try {
+    new RegExp(value, 'g');
+    return true;
+  } catch {
+    return false;
+  }
+};
+
+export const setMessageFilterRegexValidator = (validate: (pattern: string) => boolean): void => {
+  messageFilterRegexValidator = validate;
+};
+
 const messageFilterPiiCustomPatternSchema = z.object({
   id: z.string().min(1),
   label: z.string().min(1),
   regex: z
     .string()
     .min(1)
-    .refine(
-      (value) => {
-        // The server runs these patterns through a linear-time engine (RE2), which does not
-        // support backreferences (numeric `\1` or named `\k<name>`) or lookaround. Reject those
-        // at config load with actionable feedback instead of silently dropping them at runtime.
-        if (/\\[1-9]/.test(value) || /\\k[<']/.test(value) || /\(\?<?[=!]/.test(value)) {
-          return false;
-        }
-        try {
-          new RegExp(value, 'g');
-          return true;
-        } catch {
-          return false;
-        }
-      },
-      { message: 'Unsupported regex: backreferences and lookaround are not supported' },
-    ),
+    .refine((value) => messageFilterRegexValidator(value), {
+      message:
+        'Unsupported regex: not compatible with the RE2 engine (no backreferences, lookaround, or control escapes)',
+    }),
 });
 
 export const messageFilterPiiSchema = z.object({