diff --git a/packages/data-provider/specs/createPayload.spec.ts b/packages/data-provider/specs/createPayload.spec.ts
new file mode 100644
index 0000000000..5ac03a7009
--- /dev/null
+++ b/packages/data-provider/specs/createPayload.spec.ts
@@ -0,0 +1,55 @@
+import type { TSubmission } from '../src/types';
+import { EModelEndpoint } from '../src/schemas';
+import { EndpointURLs } from '../src/config';
+import createPayload from '../src/createPayload';
+
+const agentsChatRoot = EndpointURLs[EModelEndpoint.agents];
+
+const makeSubmission = (endpoint: string): TSubmission =>
+  ({
+    conversation: { conversationId: 'convo-1', endpoint },
+    userMessage: { text: 'hello' },
+    endpointOption: { endpoint },
+  }) as unknown as TSubmission;
+
+describe('createPayload server URL', () => {
+  it('builds the chat URL from a plain custom endpoint name', () => {
+    const { server } = createPayload(makeSubmission('OpenRouter'));
+
+    expect(server).toBe(`${agentsChatRoot}/OpenRouter`);
+  });
+
+  it('percent-encodes a slash so the name stays a single path segment', () => {
+    /** A raw `/` would split into an extra path segment and miss the server's
+     * `/:endpoint` route, which answers with a generic 404 "Endpoint not found"
+     * (danny-avila/LibreChat#15270). */
+    const { server } = createPayload(makeSubmission('Company/API'));
+
+    expect(server).toBe(`${agentsChatRoot}/Company%2FAPI`);
+    expect(server.slice(agentsChatRoot.length + 1)).not.toContain('/');
+  });
+
+  it('round-trips the encoded name back to the configured value', () => {
+    const { server } = createPayload(makeSubmission('Company/API'));
+
+    expect(decodeURIComponent(server.slice(agentsChatRoot.length + 1))).toBe('Company/API');
+  });
+
+  it('encodes other characters that are unsafe in a path segment', () => {
+    const { server } = createPayload(makeSubmission('Team A?v=1#x'));
+
+    expect(server).toBe(`${agentsChatRoot}/Team%20A%3Fv%3D1%23x`);
+  });
+
+  it('leaves the endpoint name itself untouched in the payload body', () => {
+    const { payload } = createPayload(makeSubmission('Company/API'));
+
+    expect(payload.endpoint).toBe('Company/API');
+  });
+
+  it('does not touch the assistants URL, which carries no endpoint segment', () => {
+    const { server } = createPayload(makeSubmission(EModelEndpoint.assistants));
+
+    expect(server).toBe(EndpointURLs[EModelEndpoint.assistants]);
+  });
+});
diff --git a/packages/data-provider/src/createPayload.ts b/packages/data-provider/src/createPayload.ts
index 62271ac032..d52aeab974 100644
--- a/packages/data-provider/src/createPayload.ts
+++ b/packages/data-provider/src/createPayload.ts
@@ -35,7 +35,9 @@ export default function createPayload(submission: t.TSubmission) {
   };
 
   const endpoint = _e as s.EModelEndpoint;
-  let server = `${EndpointURLs[s.EModelEndpoint.agents]}/${endpoint}`;
+  /** Custom endpoint names are user-defined and may contain `/`, which would
+   * otherwise split into extra path segments and miss the `/:endpoint` route. */
+  let server = `${EndpointURLs[s.EModelEndpoint.agents]}/${encodeURIComponent(endpoint)}`;
   if (s.isAssistantsEndpoint(endpoint)) {
     server =
       EndpointURLs[(endpointType ?? endpoint) as 'assistants' | 'azureAssistants'] +