diff --git a/.superpowers/sdd/container-task-4-report.md b/.superpowers/sdd/container-task-4-report.md
index 3f3df636..3ea43714 100644
--- a/.superpowers/sdd/container-task-4-report.md
+++ b/.superpowers/sdd/container-task-4-report.md
@@ -55,3 +55,28 @@ Implemented and verified runtime-configured frontend packaging.
   this task.
 - `.superpowers/sdd/progress.md` was already modified by the orchestrator and was intentionally
   excluded from this task's commit.
+
+## P1 review fixes
+
+Follow-up commit work addressed both review findings:
+
+- Runtime configuration is now produced with `jq -cn --arg`, so `BACKEND_BASE_URL` is encoded
+  by a real JSON serializer rather than interpolated into JavaScript by `sed`.
+- The image includes `frontend-config-smoke`, which strips only the fixed assignment wrapper,
+  parses the remaining JSON with `jq`, requires exactly the `backendBaseUrl` key, and compares
+  the decoded value to the environment input.
+- The hostile smoke passed with quotes, backslashes, a literal newline, ampersand, pipe, and
+  `"; globalThis.PWNED=true; //` in the value. A breakout would leave non-JSON trailing input
+  and fail parsing.
+- Added `joinBackendPath`, shared by API fetch and EventSource creation. It removes duplicate
+  boundary slashes for relative and absolute bases while keeping empty and `/` bases rooted.
+
+Follow-up verification:
+
+- RED: six join cases failed with `joinBackendPath is not a function` before implementation.
+- Targeted: runtime config, API client, and EventSource suites — 14 tests passed.
+- Full frontend gate — exit 0 (40 test files, 191 tests, TypeScript, Vite build).
+- Rebuilt `thothii-frontend:test` successfully.
+- Hostile config image smoke — `frontend runtime config smoke: ok`.
+- Rebuilt two-container smoke — default `/api` config, proxied `/api/health`, SPA fallback,
+  and SSE-safe nginx directives all passed.
diff --git a/docker/frontend-entrypoint.sh b/docker/frontend-entrypoint.sh
index 0c418cdc..fdc6fbab 100644
--- a/docker/frontend-entrypoint.sh
+++ b/docker/frontend-entrypoint.sh
@@ -2,8 +2,13 @@
 set -eu
 
 backend_base_url=${BACKEND_BASE_URL:-/api}
-escaped_backend_base_url=$(printf '%s' "$backend_base_url" | sed 's/[&|\\]/\\&/g')
-sed "s|__BACKEND_BASE_URL__|${escaped_backend_base_url}|g" \
-  /usr/share/nginx/html/config.template.js > /usr/share/nginx/html/config.js
+runtime_config=$(jq -cn --arg backend_base_url "$backend_base_url" \
+  '{backendBaseUrl: $backend_base_url}')
+printf 'window.__THOTHII_CONFIG__ = %s;\n' "$runtime_config" \
+  > /usr/share/nginx/html/config.js
+
+if [ "$#" -gt 0 ]; then
+  exec "$@"
+fi
 
 exec nginx -g 'daemon off;'
diff --git a/docker/frontend.Dockerfile b/docker/frontend.Dockerfile
index a1c5beba..c6ae417d 100644
--- a/docker/frontend.Dockerfile
+++ b/docker/frontend.Dockerfile
@@ -8,12 +8,12 @@ RUN npm run build
 
 FROM nginxinc/nginx-unprivileged:1.27-alpine
 USER root
+RUN apk add --no-cache jq
 COPY --from=build /src/frontend/dist /usr/share/nginx/html
 COPY docker/nginx.conf.template /etc/nginx/conf.d/default.conf
 COPY docker/frontend-entrypoint.sh /usr/local/bin/frontend-entrypoint
-RUN mv /usr/share/nginx/html/config.js /usr/share/nginx/html/config.template.js \
-    && sed -i 's|{}|{ backendBaseUrl: "__BACKEND_BASE_URL__" }|' /usr/share/nginx/html/config.template.js \
-    && chmod 0555 /usr/local/bin/frontend-entrypoint \
+COPY docker/smoke/frontend-smoke.sh /usr/local/bin/frontend-config-smoke
+RUN chmod 0555 /usr/local/bin/frontend-entrypoint /usr/local/bin/frontend-config-smoke \
     && chown -R 101:101 /usr/share/nginx/html
 
 EXPOSE 8080
diff --git a/docker/smoke/frontend-smoke.sh b/docker/smoke/frontend-smoke.sh
new file mode 100644
index 00000000..30bf47cb
--- /dev/null
+++ b/docker/smoke/frontend-smoke.sh
@@ -0,0 +1,14 @@
+#!/bin/sh
+set -eu
+
+assignment=$(sed \
+  -e 's/^window\.__THOTHII_CONFIG__ = //' \
+  -e 's/;$//' \
+  /usr/share/nginx/html/config.js)
+
+printf '%s\n' "$assignment" \
+  | jq -e --arg expected "${BACKEND_BASE_URL:-/api}" \
+      'type == "object" and keys == ["backendBaseUrl"] and .backendBaseUrl == $expected' \
+  >/dev/null
+
+printf '%s\n' "frontend runtime config smoke: ok"
diff --git a/frontend/src/api/client.ts b/frontend/src/api/client.ts
index 5fb75b8d..d602ba13 100644
--- a/frontend/src/api/client.ts
+++ b/frontend/src/api/client.ts
@@ -1,4 +1,4 @@
-import { backendBaseUrl as BASE } from "./runtime-config";
+import { backendBaseUrl as BASE, joinBackendPath } from "./runtime-config";
 
 export async function apiFetch<T>(path: string, init?: RequestInit): Promise<T> {
   // Only declare a JSON content-type when we actually send a body. Body-less
@@ -10,7 +10,7 @@ export async function apiFetch<T>(path: string, init?: RequestInit): Promise<T>
   if (init?.body != null && !("content-type" in headers) && !("Content-Type" in headers)) {
     headers["content-type"] = "application/json";
   }
-  const res = await fetch(`${BASE}${path}`, { ...init, headers });
+  const res = await fetch(joinBackendPath(BASE, path), { ...init, headers });
   if (!res.ok) throw new Error(`${res.status} ${await res.text().catch(() => "")}`);
   return res.status === 204 ? (undefined as T) : ((await res.json()) as T);
 }
diff --git a/frontend/src/api/runtime-config.test.ts b/frontend/src/api/runtime-config.test.ts
index bede385b..7d6a5fa5 100644
--- a/frontend/src/api/runtime-config.test.ts
+++ b/frontend/src/api/runtime-config.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from "vitest";
 
-import { backendBaseUrl, resolveBackendUrl } from "./runtime-config";
+import { backendBaseUrl, joinBackendPath, resolveBackendUrl } from "./runtime-config";
 
 describe("resolveBackendUrl", () => {
   it("uses the runtime-injected backend URL", () => {
@@ -15,3 +15,16 @@ describe("resolveBackendUrl", () => {
     expect(backendBaseUrl).toBe(import.meta.env.VITE_BACKEND_URL ?? "http://localhost:8787");
   });
 });
+
+describe("joinBackendPath", () => {
+  it.each([
+    ["", "/sessions/s1", "/sessions/s1"],
+    ["/", "/sessions/s1", "/sessions/s1"],
+    ["/api", "/sessions/s1", "/api/sessions/s1"],
+    ["/api/", "/sessions/s1", "/api/sessions/s1"],
+    ["https://example.test/api", "/sessions/s1", "https://example.test/api/sessions/s1"],
+    ["https://example.test/api/", "sessions/s1", "https://example.test/api/sessions/s1"],
+  ])("joins base %j and path %j", (base, path, expected) => {
+    expect(joinBackendPath(base, path)).toBe(expected);
+  });
+});
diff --git a/frontend/src/api/runtime-config.ts b/frontend/src/api/runtime-config.ts
index 0f045159..0d0e7706 100644
--- a/frontend/src/api/runtime-config.ts
+++ b/frontend/src/api/runtime-config.ts
@@ -12,6 +12,12 @@ export function resolveBackendUrl(config: RuntimeConfig | undefined): string {
   return config?.backendBaseUrl ?? import.meta.env.VITE_BACKEND_URL ?? "";
 }
 
+export function joinBackendPath(base: string, path: string): string {
+  const normalizedBase = base === "/" ? "" : base.replace(/\/+$/, "");
+  const normalizedPath = path.replace(/^\/+/, "");
+  return `${normalizedBase}/${normalizedPath}`;
+}
+
 export const backendBaseUrl =
   resolveBackendUrl(typeof window === "undefined" ? undefined : window.__THOTHII_CONFIG__) ||
   "http://localhost:8787";
diff --git a/frontend/src/stream/useSessionStream.test.tsx b/frontend/src/stream/useSessionStream.test.tsx
index c77d8fcd..a5da11f5 100644
--- a/frontend/src/stream/useSessionStream.test.tsx
+++ b/frontend/src/stream/useSessionStream.test.tsx
@@ -13,7 +13,7 @@ beforeEach(() => {
 test("opens an EventSource and feeds NAMED events to the store", () => {
   renderHook(() => useSessionStream("s1"));
   const es = FakeEventSource.instances[0];
-  expect(es.url).toContain("/sessions/s1/events");
+  expect(es.url).toBe("http://localhost:8787/sessions/s1/events");
   // Backend sends `event: ui_request` (named) — drive the addEventListener path
   // that production relies on, not the unnamed onmessage fallback.
   act(() =>
diff --git a/frontend/src/stream/useSessionStream.ts b/frontend/src/stream/useSessionStream.ts
index 982cdf98..5289cdda 100644
--- a/frontend/src/stream/useSessionStream.ts
+++ b/frontend/src/stream/useSessionStream.ts
@@ -1,5 +1,6 @@
 import { useEffect, useState } from "react";
 import { BASE } from "../api/client";
+import { joinBackendPath } from "../api/runtime-config";
 import { useSessionStore } from "../store/sessionStore";
 import type { StreamEvent } from "../api/types";
 
@@ -10,7 +11,7 @@ export function useSessionStream(sessionId: string | null) {
   useEffect(() => {
     if (!sessionId) return;
 
-    const es = new EventSource(`${BASE}/sessions/${sessionId}/events`);
+    const es = new EventSource(joinBackendPath(BASE, `/sessions/${sessionId}/events`));
     es.onopen = () => setConnected(true);
     es.onerror = () => setConnected(false);