diff --git a/.env.example b/.env.example
index 97c6c5cf7..33ac71f15 100644
--- a/.env.example
+++ b/.env.example
@@ -2,7 +2,6 @@ VITE_APP_NAME="MapRoulette"
 VITE_APP_DESCRIPTION="MapRoulette 4 Local Development Environment"
 VITE_API_BASE_URL="https://maproulette.org"
 VITE_MAP_ROULETTE_SERVER_WEBSOCKET_URL="wss://maproulette.org/ws"
-VITE_SERVER_OAUTH_URL="https://maproulette.org/auth/authenticate"
 VITE_SERVER_API_KEY=""
 # Base URL for short links embedded in OSM changeset comments
 # (e.g. https://mpr.lt/c/<challengeId>/t/<taskId>). If unset,
diff --git a/.env.test b/.env.test
index deb7e24e1..8701be4cc 100644
--- a/.env.test
+++ b/.env.test
@@ -4,7 +4,6 @@ VITE_APP_URL="http://localhost:3005"
 
 VITE_API_BASE_URL="http://localhost:9000"
 VITE_MAP_ROULETTE_SERVER_WEBSOCKET_URL="ws://localhost:9000/ws"
-VITE_SERVER_OAUTH_URL="http://localhost:9000/auth/authenticate"
 
 # Must match MR_SUPER_KEY in docker-compose.test.yaml.
 VITE_SERVER_API_KEY="super-secret-key"
diff --git a/src/api/index.ts b/src/api/index.ts
index ee6a8a6ab..6768bfc98 100644
--- a/src/api/index.ts
+++ b/src/api/index.ts
@@ -47,12 +47,6 @@ export const apiRequest = ky.extend({
   },
 })
 
-export const createApiWithBaseUrl = (baseUrl: string) => {
-  return apiRequest.extend({
-    prefixUrl: baseUrl,
-  })
-}
-
 export const convertParamsToSearchParams = (
   params: Record<
     string,
diff --git a/src/contexts/AuthContext.tsx b/src/contexts/AuthContext.tsx
index b8eaf8a43..a893ef10b 100644
--- a/src/contexts/AuthContext.tsx
+++ b/src/contexts/AuthContext.tsx
@@ -1,7 +1,7 @@
 import { useQueryClient } from '@tanstack/react-query'
 import { useLocation, useSearch } from '@tanstack/react-router'
 import { createContext, useCallback, useContext, useEffect, useMemo, useRef, useState } from 'react'
-import { api, createApiWithBaseUrl } from '@/api'
+import { api, apiRequest } from '@/api'
 import { Loader } from '@/components/ui/Loader'
 import { logger } from '@/lib/logger'
 import type { OAuthLoginResponse } from '@/types/Oauth'
@@ -175,13 +175,13 @@ export const AuthProvider = ({ children }: { children: React.ReactNode }) => {
     const currentUrl = location.pathname + location.searchStr
     setStoredRedirectUrl(currentUrl)
 
-    const frontendOrigin = window.env.VITE_APP_URL || window.location.origin
-    const oauthBaseUrl = window.env.VITE_SERVER_OAUTH_URL
-    const loginUrl = `?redirect=${encodeURIComponent(currentUrl)}&redirect_uri=${encodeURIComponent(frontendOrigin)}`
-
     try {
-      const oauthApi = createApiWithBaseUrl(oauthBaseUrl)
-      const response = await oauthApi.get(loginUrl)
+      // Hit the backend via the shared apiRequest client (same host as the
+      // callback) so the backend derives the OAuth redirect_uri from this
+      // request's Origin header, which resolves correctly across deployments
+      // (maproulette.org, beta.maproulette.org, 127.0.0.1). No redirect param:
+      // post-login navigation is handled client-side via the stored redirect URL.
+      const response = await apiRequest.get('auth/authenticate')
       const jsonData = (await response.json()) as OAuthLoginResponse
 
       if (jsonData.state) {
diff --git a/src/vite-env.d.ts b/src/vite-env.d.ts
index dfccc2098..d61fb398e 100644
--- a/src/vite-env.d.ts
+++ b/src/vite-env.d.ts
@@ -12,7 +12,6 @@ interface AppEnv {
   readonly VITE_APP_DESCRIPTION: string
   readonly VITE_API_BASE_URL: string | undefined
   readonly VITE_MAP_ROULETTE_SERVER_WEBSOCKET_URL: string
-  readonly VITE_SERVER_OAUTH_URL: string
   readonly VITE_SERVER_API_KEY: string | undefined
   readonly VITE_GITHUB_ISSUES_API_OWNER: string | undefined
   readonly VITE_GITHUB_ISSUES_API_REPO: string | undefined