diff --git a/README.md b/README.md
index d16df97..2611b66 100644
--- a/README.md
+++ b/README.md
@@ -116,7 +116,9 @@ cd frontend && npm run preview
 Main runtime variables are defined in `docker-compose.yml`:
 
 - API and worker: `DATABASE_URL`, `REDIS_URL`, `REDIS_SECURITY_MODE`, `REDIS_TLS_MODE`, `STORAGE_ROOT`, `PUBLIC_BASE_URL`, `CORS_ORIGINS`, `ALLOW_DEVELOPMENT_ANONYMOUS_USER_ACCESS`, `TYPESENSE_*`, `APP_SETTINGS_ENCRYPTION_KEY`
-- Frontend: `VITE_API_BASE`, optional `VITE_API_TOKEN` compatibility fallback
+- Frontend: optional `VITE_API_BASE`, optional `VITE_API_TOKEN` compatibility fallback
+
+When `VITE_API_BASE` is unset, the frontend defaults to `http://<current-hostname>:8000/api/v1`.
 
 Application settings saved from the UI persist at:
 
diff --git a/backend/app/main.py b/backend/app/main.py
index f8c18ce..1539c0b 100644
--- a/backend/app/main.py
+++ b/backend/app/main.py
@@ -18,6 +18,16 @@ from app.services.typesense_index import ensure_typesense_collection
 settings = get_settings()
 UPLOAD_ENDPOINT_PATH = "/api/v1/documents/upload"
 UPLOAD_ENDPOINT_METHOD = "POST"
+DEVELOPMENT_CORS_PRIVATE_ORIGIN_REGEX = (
+    r"^https?://("
+    r"localhost"
+    r"|127(?:\.\d{1,3}){3}"
+    r"|0\.0\.0\.0"
+    r"|10(?:\.\d{1,3}){3}"
+    r"|192\.168(?:\.\d{1,3}){2}"
+    r"|172\.(?:1[6-9]|2\d|3[0-1])(?:\.\d{1,3}){2}"
+    r")(?::\d+)?$"
+)
 
 
 def _is_upload_size_guard_target(request: Request) -> bool:
@@ -34,9 +44,14 @@ def create_app() -> FastAPI:
     """Builds and configures the FastAPI application instance."""
 
     app = FastAPI(title="DCM DMS API", version="0.1.0")
+    app_env = settings.app_env.strip().lower()
+    allow_origin_regex = (
+        DEVELOPMENT_CORS_PRIVATE_ORIGIN_REGEX if app_env in {"development", "dev"} else None
+    )
     app.add_middleware(
         CORSMiddleware,
         allow_origins=settings.cors_origins,
+        allow_origin_regex=allow_origin_regex,
         allow_credentials=True,
         allow_methods=["*"],
         allow_headers=["*"],
diff --git a/backend/tests/test_upload_request_size_middleware.py b/backend/tests/test_upload_request_size_middleware.py
index 43efa21..33deec7 100644
--- a/backend/tests/test_upload_request_size_middleware.py
+++ b/backend/tests/test_upload_request_size_middleware.py
@@ -115,6 +115,7 @@ def _install_main_import_stubs() -> dict[str, ModuleType | None]:
         """Returns minimal settings consumed by app.main during test import."""
 
         return SimpleNamespace(
+            app_env="development",
             cors_origins=["http://localhost:5173"],
             max_upload_request_size_bytes=1024,
         )
diff --git a/doc/operations-and-configuration.md b/doc/operations-and-configuration.md
index 44feb77..a6bcd36 100644
--- a/doc/operations-and-configuration.md
+++ b/doc/operations-and-configuration.md
@@ -113,9 +113,14 @@ Selected defaults from `Settings` (`backend/app/core/config.py`):
 ## Frontend Configuration
 
 Frontend runtime API target:
-- `VITE_API_BASE` in `docker-compose.yml` frontend service
+- `VITE_API_BASE` in `docker-compose.yml` frontend service (optional override)
 - `VITE_API_TOKEN` in `docker-compose.yml` frontend service (optional compatibility fallback only)
 
+When `VITE_API_BASE` is unset, frontend API helpers resolve the backend URL dynamically as:
+- `http://<current-frontend-hostname>:8000/api/v1`
+
+This keeps development access working when the UI is opened through a LAN IP instead of `localhost`.
+
 Frontend API authentication behavior:
 - `frontend/src/lib/api.ts` resolves bearer tokens at request time in this order:
   - custom runtime resolver (`setApiTokenResolver`)
@@ -163,6 +168,7 @@ Retention settings are used by worker cleanup and by `POST /api/v1/processing/lo
   - `documents` endpoints: user token or admin token
   - `settings` and `processing/logs` endpoints: admin token only
 - Development environments can allow tokenless user-role access for document/search routes via `ALLOW_DEVELOPMENT_ANONYMOUS_USER_ACCESS=true`; production remains token-enforced.
+- Development CORS allows localhost and RFC1918 private-network origins via regex in addition to explicit `CORS_ORIGINS`, so LAN-hosted frontend access remains functional.
 - Authentication fails closed when `ADMIN_API_TOKEN` is not configured and admin access is requested.
 - Document preview endpoint blocks inline rendering for script-capable MIME types and forces attachment responses for active content.
 - Provider base URLs are validated on settings updates and before outbound model calls:
diff --git a/docker-compose.yml b/docker-compose.yml
index a3b8d40..ef2424a 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -119,7 +119,7 @@ services:
     build:
       context: ./frontend
     environment:
-      VITE_API_BASE: ${VITE_API_BASE:-http://localhost:8000/api/v1}
+      VITE_API_BASE: ${VITE_API_BASE:-}
       VITE_API_TOKEN: ${VITE_API_TOKEN:-}
     ports:
       - "${HOST_BIND_IP:-127.0.0.1}:5173:5173"
diff --git a/frontend/src/lib/api.ts b/frontend/src/lib/api.ts
index 14b3070..7fa3ddb 100644
--- a/frontend/src/lib/api.ts
+++ b/frontend/src/lib/api.ts
@@ -14,9 +14,25 @@ import type {
 } from '../types';
 
 /**
- * Resolves backend base URL from environment with localhost fallback.
+ * Resolves backend base URL from environment with current-host HTTP fallback.
  */
-const API_BASE = import.meta.env?.VITE_API_BASE ?? 'http://localhost:8000/api/v1';
+function resolveApiBase(): string {
+  const envValue = import.meta.env?.VITE_API_BASE;
+  if (typeof envValue === 'string') {
+    const trimmed = envValue.trim().replace(/\/+$/, '');
+    if (trimmed) {
+      return trimmed;
+    }
+  }
+
+  if (typeof window !== 'undefined' && window.location?.hostname) {
+    return `http://${window.location.hostname}:8000/api/v1`;
+  }
+
+  return 'http://localhost:8000/api/v1';
+}
+
+const API_BASE = resolveApiBase();
 
 /**
  * Legacy environment token fallback used only when no runtime token source is available.