diff --git a/src/server/routes/proxy/endpointFlow.test.ts b/src/server/routes/proxy/endpointFlow.test.ts
index b029566..968763e 100644
--- a/src/server/routes/proxy/endpointFlow.test.ts
+++ b/src/server/routes/proxy/endpointFlow.test.ts
@@ -1,11 +1,15 @@
 import { beforeEach, describe, expect, it, vi } from 'vitest';
 import { fetch } from 'undici';
-import { executeEndpointFlow, type BuiltEndpointRequest } from './endpointFlow.js';
+import type { BuiltEndpointRequest } from './endpointFlow.js';
 
 vi.mock('undici', () => ({
   fetch: vi.fn(),
 }));
 
+vi.mock('../../services/siteProxy.js', () => ({
+  withSiteProxyRequestInit: async (_targetUrl: string, init: RequestInit) => init,
+}));
+
 const fetchMock = vi.mocked(fetch);
 
 function requestFor(path: string): BuiltEndpointRequest {
@@ -22,6 +26,14 @@ function toUndiciResponse(response: Response): Awaited<ReturnType<typeof fetch>>
 }
 
 describe('executeEndpointFlow', () => {
+  let executeEndpointFlow: (input: any) => Promise<any>;
+
+  beforeEach(async () => {
+    if (!executeEndpointFlow) {
+      ({ executeEndpointFlow } = await import('./endpointFlow.js'));
+    }
+  });
+
   beforeEach(() => {
     fetchMock.mockReset();
   });
@@ -42,9 +54,40 @@ describe('executeEndpointFlow', () => {
     if (result.ok) {
       expect(result.upstreamPath).toBe('/v1/responses');
     }
+    expect(fetchMock.mock.calls[0]?.[0]).toBe('https://example.com/v1/responses');
     expect(fetchMock).toHaveBeenCalledTimes(1);
   });
 
+  it('avoids duplicated /v1 when base url already ends with /v1', async () => {
+    fetchMock.mockResolvedValueOnce(toUndiciResponse(new Response(JSON.stringify({ ok: true }), {
+      status: 200,
+      headers: { 'content-type': 'application/json' },
+    })));
+
+    await executeEndpointFlow({
+      siteUrl: 'https://api.example.com/v1',
+      endpointCandidates: ['chat'],
+      buildRequest: () => ({ ...requestFor('/v1/chat/completions'), endpoint: 'chat' }),
+    });
+
+    expect(fetchMock.mock.calls[0]?.[0]).toBe('https://api.example.com/v1/chat/completions');
+  });
+
+  it('avoids duplicated /v1 when base url already ends with /api/v1', async () => {
+    fetchMock.mockResolvedValueOnce(toUndiciResponse(new Response(JSON.stringify({ ok: true }), {
+      status: 200,
+      headers: { 'content-type': 'application/json' },
+    })));
+
+    await executeEndpointFlow({
+      siteUrl: 'https://openrouter.ai/api/v1',
+      endpointCandidates: ['chat'],
+      buildRequest: () => ({ ...requestFor('/v1/chat/completions'), endpoint: 'chat' }),
+    });
+
+    expect(fetchMock.mock.calls[0]?.[0]).toBe('https://openrouter.ai/api/v1/chat/completions');
+  });
+
   it('downgrades to next endpoint when policy allows', async () => {
     fetchMock
       .mockResolvedValueOnce(toUndiciResponse(new Response(JSON.stringify({
diff --git a/src/server/routes/proxy/endpointFlow.ts b/src/server/routes/proxy/endpointFlow.ts
index 97115f2..b017b3c 100644
--- a/src/server/routes/proxy/endpointFlow.ts
+++ b/src/server/routes/proxy/endpointFlow.ts
@@ -3,6 +3,28 @@ import { withSiteProxyRequestInit } from '../../services/siteProxy.js';
 import { summarizeUpstreamError } from './upstreamError.js';
 import type { UpstreamEndpoint } from './upstreamEndpoint.js';
 
+function normalizeUpstreamTargetUrl(siteUrl: string, requestPath: string): string {
+  const baseRaw = typeof siteUrl === 'string' ? siteUrl.trim() : '';
+  const pathRaw = typeof requestPath === 'string' ? requestPath.trim() : '';
+  const base = baseRaw.replace(/\/+$/, '');
+  let path = pathRaw.startsWith('/') ? pathRaw : `/${pathRaw}`;
+
+  try {
+    const parsed = new URL(base);
+    const basePath = parsed.pathname.replace(/\/+$/, '');
+    const baseHasVersionSuffix = /\/(?:api\/)?v1$/i.test(basePath);
+    if (baseHasVersionSuffix && path.startsWith('/v1/')) {
+      path = path.slice('/v1'.length) || '/';
+    }
+  } catch {
+    // Ignore URL parsing errors and fall back to naive join.
+  }
+
+  if (!base) return path || '/';
+  if (!path || path === '/') return base;
+  return `${base}${path}`;
+}
+
 export type BuiltEndpointRequest = {
   endpoint: UpstreamEndpoint;
   path: string;
@@ -66,7 +88,7 @@ export async function executeEndpointFlow(input: ExecuteEndpointFlowInput): Prom
   for (let endpointIndex = 0; endpointIndex < endpointCount; endpointIndex += 1) {
     const endpoint = input.endpointCandidates[endpointIndex] as UpstreamEndpoint;
     const request = input.buildRequest(endpoint, endpointIndex);
-    const targetUrl = `${input.siteUrl}${request.path}`;
+    const targetUrl = normalizeUpstreamTargetUrl(input.siteUrl, request.path);
 
     let response = await fetch(targetUrl, await withSiteProxyRequestInit(targetUrl, {
       method: 'POST',