diff --git a/frontend/editor/.env.saas b/frontend/editor/.env.saas
index 2de6a435f..ca6c4b0e1 100644
--- a/frontend/editor/.env.saas
+++ b/frontend/editor/.env.saas
@@ -8,8 +8,7 @@
 # Userback feedback widget — leave blank to disable
 VITE_USERBACK_TOKEN=
 
-# URL subpath prefix for SaaS deployments (e.g. "app" if serving at /app/) — leave blank for root
-VITE_RUN_SUBPATH=
-
 # Development-only auth bypass — allows unauthenticated access on localhost in dev mode
+# The URL subpath prefix is read from BASE_PATH at runtime, sourced from the
+# top-level RUN_SUBPATH env var which Vite consumes at build time.
 VITE_DEV_BYPASS_AUTH=false
diff --git a/frontend/editor/src/core/workers/pixelCompareWorker.ts b/frontend/editor/src/core/workers/pixelCompareWorker.ts
index bb57bb75f..987857821 100644
--- a/frontend/editor/src/core/workers/pixelCompareWorker.ts
+++ b/frontend/editor/src/core/workers/pixelCompareWorker.ts
@@ -395,7 +395,11 @@ self.addEventListener(
       // the default DOMFilterFactory crashes in a worker calling document.createElementNS.
       // Resolve CMap/standard-font URLs against the worker's own origin. Vite copies these
       // directories from pdfjs-dist at build time via viteStaticCopy (see vite.config.ts).
-      const assetsBase = new URL("/pdfjs/", self.location.origin).toString();
+      // import.meta.env.BASE_URL ends with a slash and includes any subpath prefix.
+      const assetsBase = new URL(
+        `${import.meta.env.BASE_URL}pdfjs/`,
+        self.location.origin,
+      ).toString();
       const loaderOpts = (data: ArrayBuffer) =>
         ({
           data,
diff --git a/frontend/editor/src/proprietary/components/chat/ChatContext.tsx b/frontend/editor/src/proprietary/components/chat/ChatContext.tsx
index 6cd473eda..65c2e9dd3 100644
--- a/frontend/editor/src/proprietary/components/chat/ChatContext.tsx
+++ b/frontend/editor/src/proprietary/components/chat/ChatContext.tsx
@@ -10,6 +10,7 @@ import { useTranslation } from "react-i18next";
 import { generateId } from "@app/utils/generateId";
 import { useAllFiles, useFileActions } from "@app/contexts/FileContext";
 import apiClient from "@app/services/apiClient";
+import { getApiBaseUrl } from "@app/services/apiClientConfig";
 import { getAuthHeaders } from "@app/services/apiClientSetup";
 import { createChildStub } from "@app/contexts/file/fileActions";
 import {
@@ -461,13 +462,16 @@ export function ChatProvider({ children }: { children: ReactNode }) {
           formData.append(`conversationHistory[${i}].content`, message.content);
         });
 
-        const response = await fetch("/api/v1/ai/orchestrate/stream", {
-          method: "POST",
-          body: formData,
-          headers: getAuthHeaders(),
-          credentials: "include",
-          signal: controller.signal,
-        });
+        const response = await fetch(
+          `${getApiBaseUrl()}/api/v1/ai/orchestrate/stream`,
+          {
+            method: "POST",
+            body: formData,
+            headers: getAuthHeaders(),
+            credentials: "include",
+            signal: controller.signal,
+          },
+        );
 
         if (!response.ok) {
           let detail: string | undefined;
diff --git a/frontend/editor/src/prototypes/components/chat/ChatContext.tsx b/frontend/editor/src/prototypes/components/chat/ChatContext.tsx
index 796422bc6..a6c2123ba 100644
--- a/frontend/editor/src/prototypes/components/chat/ChatContext.tsx
+++ b/frontend/editor/src/prototypes/components/chat/ChatContext.tsx
@@ -9,6 +9,7 @@ import {
 import { useAllFiles, useFileActions } from "@app/contexts/FileContext";
 import { generateId } from "@app/utils/generateId";
 import apiClient from "@app/services/apiClient";
+import { getApiBaseUrl } from "@app/services/apiClientConfig";
 import { getAuthHeaders } from "@app/services/apiClientSetup";
 import { createChildStub } from "@app/contexts/file/fileActions";
 import {
@@ -440,13 +441,16 @@ export function ChatProvider({ children }: { children: ReactNode }) {
           formData.append(`conversationHistory[${i}].content`, message.content);
         });
 
-        const response = await fetch("/api/v1/ai/orchestrate/stream", {
-          method: "POST",
-          body: formData,
-          headers: getAuthHeaders(),
-          credentials: "include",
-          signal: controller.signal,
-        });
+        const response = await fetch(
+          `${getApiBaseUrl()}/api/v1/ai/orchestrate/stream`,
+          {
+            method: "POST",
+            body: formData,
+            headers: getAuthHeaders(),
+            credentials: "include",
+            signal: controller.signal,
+          },
+        );
 
         if (!response.ok) {
           throw new Error(`AI engine request failed: ${response.status}`);
diff --git a/frontend/editor/src/saas/utils/pathUtils.ts b/frontend/editor/src/saas/utils/pathUtils.ts
index d2fa188e2..d746fff5b 100644
--- a/frontend/editor/src/saas/utils/pathUtils.ts
+++ b/frontend/editor/src/saas/utils/pathUtils.ts
@@ -1,6 +1,9 @@
 import { URL_TO_TOOL_MAP } from "@app/utils/urlMapping";
+import { BASE_PATH } from "@app/constants/app";
 
-const SUBPATH = import.meta.env.VITE_RUN_SUBPATH.replace(/^\/|\/$/g, ""); // "app" or ""
+// BASE_PATH is "/bpp" or "" (no trailing slash). Strip the leading slash
+// to match the legacy SUBPATH shape used below ("bpp" or "").
+const SUBPATH = BASE_PATH.replace(/^\//, "");
 
 /**
  * Normalize pathname by stripping subpath prefix and trailing slashes
diff --git a/frontend/editor/vite-env.d.ts b/frontend/editor/vite-env.d.ts
index 2edad0618..d26c3bceb 100644
--- a/frontend/editor/vite-env.d.ts
+++ b/frontend/editor/vite-env.d.ts
@@ -14,7 +14,6 @@ interface ImportMetaEnv {
 
   // SaaS only (.env.saas)
   readonly VITE_USERBACK_TOKEN: string;
-  readonly VITE_RUN_SUBPATH: string;
   readonly VITE_DEV_BYPASS_AUTH: string;
 
   // Desktop only (.env.desktop)