diff --git a/docs/process-hardening.md b/docs/process-hardening.md
index d0746fbf8..97ea39824 100644
--- a/docs/process-hardening.md
+++ b/docs/process-hardening.md
@@ -179,7 +179,8 @@ All approved render-surface modules are extracted. `ClinicalDashboard.tsx` went
 - **P0 carry-over (PR #278 / `cursor/audit-p2-p3-hardening-b54f`):** RAG cache owner/indexing-version guards, synopsis parity in ranking/detectors, DELETE TOCTOU re-check, worker cache invalidation on job completion, and `RAG_QUERY_HASH_SECRET` required in production-like readiness checks.
 - **P2 M13:** `20260702000000_commit_generation_preserve_legacy_artifacts.sql` must be applied to live Supabase before reindex commits can safely purge legacy NULL-generation rows. After apply, run `npm run check:m13-migration`, `npm run reindex:health`, and `npm run check:indexing`. `search_schema_health()` now reports `commit_document_index_generation.preserve_legacy_artifacts_migration` when the live function body is stale.
 - **P2 upload hardening:** `/api/upload` consumes the `document_upload` rate-limit bucket (12/min owner, 3/min anonymous).
-- **P3 dispositioned (no code change):** L9 searchable-only `image_count` (documented in `worker/main.ts`); L11 triple `readFile` peak-memory trade-off (documented at the ingestion site); L18 duplicate `audit_logs` policy in an already-applied migration (do not edit applied migrations — consolidate only if migrations are ever squashed); L19 CSP `script-src 'unsafe-inline'` deferred (no active XSS sink today; nonce migration needs dedicated UI verification).
+- **P3 dispositioned (no code change):** L9 searchable-only `image_count` (documented in `worker/main.ts`); L11 triple `readFile` peak-memory trade-off (documented at the ingestion site); L18 duplicate `audit_logs` policy in an already-applied migration (do not edit applied migrations — consolidate only if migrations are ever squashed).
+- **L19 RESOLVED — CSP `script-src` nonce migration (branch `claude/csp-nonce-migration`).** Production `script-src` is now `'self' 'nonce-<per-request>' 'strict-dynamic'` (no `'unsafe-inline'`, no `'unsafe-eval'`); the nonce is generated per request in `src/proxy.ts`, threaded into SSR via the `x-nonce` + CSP request headers, and applied to Next's own bootstrap/bundle/flight scripts automatically plus the one hand-authored inline script (theme-flash guard in `src/app/layout.tsx`). Every other CSP directive and security header is unchanged; the static (non-CSP) headers still come from `next.config.ts` → `buildSecurityHeaders`. **Development keeps the pre-migration `'self' 'unsafe-inline' 'unsafe-eval'` (no `'strict-dynamic'`)** because the Turbopack dev server injects non-nonced HMR/route-chunk `<script>` tags that `'strict-dynamic'` would block. **Dedicated UI verification (why L19 was deferred):** `npm run build`, full-Chromium `npm run verify:ui`, and a scripted prod-server (`next start`) console sweep of answer / search / document-viewer / auth flows confirming **zero** `securitypolicyviolation`. That sweep surfaced a pre-existing (already blocked under the old prod policy) benign eval violation from Zod 4's JIT probe (`new Function("")` in a swallowed try/catch); it is silenced by setting Zod `jitless` on the client in `src/instrumentation-client.ts` (server keeps JIT — no CSP there).
 
 ## Design convergence & type-scale ratchet (2026-07-06)
 
diff --git a/next.config.ts b/next.config.ts
index 264a2da58..f732fd9dd 100644
--- a/next.config.ts
+++ b/next.config.ts
@@ -1,13 +1,13 @@
 import type { NextConfig } from "next";
 import path from "node:path";
 import { fileURLToPath } from "node:url";
-import { buildSecurityHeaders } from "./src/lib/security-headers";
+import { buildSecurityHeaders, resolveRuntimeFlags } from "./src/lib/security-headers";
 
-const isDevelopment = process.env.NODE_ENV === "development";
-const isLocalHttpRuntime = isDevelopment || process.env.PLAYWRIGHT_BASE_URL?.startsWith("http://localhost:");
 const projectRoot = path.dirname(fileURLToPath(import.meta.url));
 
-const securityHeaders = buildSecurityHeaders({ isDevelopment, isLocalHttpRuntime: Boolean(isLocalHttpRuntime) });
+// Static (non-CSP) headers for every route. The nonce'd CSP is emitted per
+// request from src/proxy.ts; both derive their runtime flags from the same helper.
+const securityHeaders = buildSecurityHeaders(resolveRuntimeFlags());
 
 const nextConfig: NextConfig = {
   devIndicators: false,
diff --git a/src/app/layout.tsx b/src/app/layout.tsx
index bdccb0e9b..a90eeef86 100644
--- a/src/app/layout.tsx
+++ b/src/app/layout.tsx
@@ -1,5 +1,6 @@
 import type { Metadata, Viewport } from "next";
 import { Geist, Geist_Mono } from "next/font/google";
+import { headers } from "next/headers";
 import { AuthProvider } from "@/lib/supabase/client";
 import { WebVitalsReporter } from "@/components/web-vitals-reporter";
 import "./globals.css";
@@ -36,11 +37,17 @@ export const viewport: Viewport = {
   ],
 };
 
-export default function RootLayout({
+export default async function RootLayout({
   children,
 }: Readonly<{
   children: React.ReactNode;
 }>) {
+  // Per-request CSP nonce set by src/proxy.ts. Next.js stamps its own scripts
+  // automatically, but the hand-authored theme-flash <script> below is ours, so
+  // it must carry the nonce explicitly or the strict script-src blocks it (a
+  // silent runtime failure: theme flash returns). Reading headers() opts the app
+  // into dynamic rendering — inherent to nonce-based CSP.
+  const nonce = (await headers()).get("x-nonce") ?? undefined;
   return (
     <html
       lang="en"
@@ -53,6 +60,10 @@ export default function RootLayout({
             Mirrors resolveThemePreference in src/lib/theme.ts: stored choice wins,
             otherwise the OS preference. Key must match use-theme.ts. */}
         <script
+          nonce={nonce}
+          // Next.js strips the nonce from the client payload (so scripts can't
+          // read it), which reads as a hydration mismatch on this attribute.
+          suppressHydrationWarning
           dangerouslySetInnerHTML={{
             __html: `(function(){try{var t=localStorage.getItem("clinical-kb-theme");var d=t==="dark"||(t!=="light"&&window.matchMedia("(prefers-color-scheme: dark)").matches);document.documentElement.classList.toggle("dark",d);}catch(e){}})();`,
           }}
diff --git a/src/instrumentation-client.ts b/src/instrumentation-client.ts
new file mode 100644
index 000000000..42c22f74f
--- /dev/null
+++ b/src/instrumentation-client.ts
@@ -0,0 +1,14 @@
+// Runs on the client after the HTML loads but before React hydration (see
+// node_modules/next/dist/docs/.../instrumentation-client.md), so this preempts
+// the first client-side Zod schema compile.
+//
+// Why: the production CSP (src/lib/security-headers.ts) has no 'unsafe-eval'.
+// Zod 4's JIT compiler probes for eval with `new Function("")` inside a try/catch
+// (node_modules/zod/src/v4/core/util.ts) — the throw is swallowed and validation
+// still works, but the browser reports the caught eval as a
+// `securitypolicyviolation` on every page. Disabling JIT skips the probe entirely
+// (validation stays correct, just interpreted rather than compiled). The server
+// has no CSP, so it keeps the faster JIT path — this is client-only by design.
+import { config } from "zod";
+
+config({ jitless: true });
diff --git a/src/lib/security-headers.ts b/src/lib/security-headers.ts
index 6822cf8ea..acff2f410 100644
--- a/src/lib/security-headers.ts
+++ b/src/lib/security-headers.ts
@@ -21,8 +21,46 @@ export type SecurityHeaderFlags = {
   isLocalHttpRuntime: boolean;
 };
 
-export function buildContentSecurityPolicy({ isDevelopment, isLocalHttpRuntime }: SecurityHeaderFlags): string {
-  const scriptSrc = `script-src 'self' 'unsafe-inline'${isDevelopment ? " 'unsafe-eval'" : ""}; `;
+// Single source of truth for the runtime flags that shape the policy. Both the
+// static headers (next.config.ts) and the per-request nonce CSP (src/proxy.ts)
+// derive from this, so the HSTS/upgrade-insecure-requests gating stays in lockstep
+// with the script-src gating instead of drifting between two hand-copied copies.
+export function resolveRuntimeFlags(): SecurityHeaderFlags {
+  const isDevelopment = process.env.NODE_ENV === "development";
+  return {
+    isDevelopment,
+    isLocalHttpRuntime: isDevelopment || process.env.PLAYWRIGHT_BASE_URL?.startsWith("http://localhost:") === true,
+  };
+}
+
+export type ContentSecurityPolicyOptions = SecurityHeaderFlags & {
+  // Per-request nonce (base64) generated in src/proxy.ts. script-src allow-lists
+  // this nonce instead of 'unsafe-inline', so only scripts carrying it execute.
+  nonce: string;
+};
+
+export function buildContentSecurityPolicy({
+  isDevelopment,
+  isLocalHttpRuntime,
+  nonce,
+}: ContentSecurityPolicyOptions): string {
+  // Production: nonce + 'strict-dynamic' is the modern strict-CSP shape. CSP3
+  // browsers ignore host allow-lists AND 'unsafe-inline' for scripts, running
+  // only the nonce'd bootstrap (and scripts it loads). Next.js reads the nonce
+  // from the request CSP header and stamps its own framework/bundle/flight
+  // scripts automatically; the one hand-authored inline script (theme-flash guard
+  // in app/layout.tsx) carries the nonce explicitly via the `x-nonce` header.
+  //
+  // Development: keep 'unsafe-inline' + 'unsafe-eval' and NO 'strict-dynamic'.
+  // The Turbopack dev server injects HMR/runtime and route-chunk <script src>
+  // tags that are not nonce-tagged; 'strict-dynamic' would disable the 'self'
+  // allow-list and block every one of them (blank page + broken interactions).
+  // Dev is not the shipped security boundary, so it retains the pre-migration
+  // policy. style-src keeps 'unsafe-inline' in both (Next's font + inline styles
+  // are not nonce-tagged); only production script-src is nonce-gated.
+  const scriptSrc = isDevelopment
+    ? "script-src 'self' 'unsafe-inline' 'unsafe-eval'; "
+    : `script-src 'self' 'nonce-${nonce}' 'strict-dynamic'; `;
   const upgradeInsecureRequests = isLocalHttpRuntime ? "" : "upgrade-insecure-requests; ";
 
   return (
@@ -43,6 +81,9 @@ export function buildContentSecurityPolicy({ isDevelopment, isLocalHttpRuntime }
   );
 }
 
+// Static security headers applied to every route via next.config.ts. NOTE:
+// Content-Security-Policy is intentionally NOT here — it carries a per-request
+// nonce and is emitted from src/proxy.ts (see buildContentSecurityPolicy).
 export function buildSecurityHeaders(flags: SecurityHeaderFlags): SecurityHeader[] {
   return [
     { key: "X-Content-Type-Options", value: "nosniff" },
@@ -53,7 +94,6 @@ export function buildSecurityHeaders(flags: SecurityHeaderFlags): SecurityHeader
     // No Cross-Origin-Embedder-Policy — see module header note.
     { key: "Permissions-Policy", value: "camera=(), microphone=(), geolocation=(), payment=()" },
     { key: "Origin-Agent-Cluster", value: "?1" },
-    { key: "Content-Security-Policy", value: buildContentSecurityPolicy(flags) },
     { key: "X-Permitted-Cross-Domain-Policies", value: "none" },
     ...(flags.isLocalHttpRuntime
       ? []
diff --git a/src/proxy.ts b/src/proxy.ts
index 474347845..4a3bf84d0 100644
--- a/src/proxy.ts
+++ b/src/proxy.ts
@@ -2,15 +2,24 @@ import { createServerClient } from "@supabase/ssr";
 import { NextResponse, type NextRequest } from "next/server";
 
 import { env } from "@/lib/env";
+import { buildContentSecurityPolicy, resolveRuntimeFlags } from "@/lib/security-headers";
 
 // Next 16 renamed the `middleware` file convention to `proxy` (see
 // node_modules/next/dist/docs/.../file-conventions/proxy.md). Proxy defaults to
 // the Node.js runtime, which the Supabase client requires.
 //
-// Purpose: keep the user's @supabase/ssr session cookie fresh on navigation and
-// API calls so persistent logins survive refreshes. It is a no-op unless the
-// public Supabase env is configured AND an `sb-` auth cookie is present, so
-// demo / local-no-auth traffic is untouched and adds no auth round-trip.
+// Two jobs:
+//   1. Content-Security-Policy nonce. A fresh per-request nonce is generated and
+//      threaded into the SSR request (`x-nonce` + the CSP header, which Next.js
+//      parses to stamp its framework/bundle scripts) and onto the response so the
+//      browser enforces it. This is why the CSP header lives here and not in
+//      next.config.ts: a nonce cannot be a build-time constant. Using a nonce
+//      opts pages into dynamic rendering (app/layout.tsx reads the nonce), which
+//      is inherent to nonce-based CSP.
+//   2. Session refresh. Keep the user's @supabase/ssr session cookie fresh on
+//      navigation and API calls so persistent logins survive refreshes. It is a
+//      no-op unless the public Supabase env is configured AND an `sb-` auth
+//      cookie is present, so demo / local-no-auth traffic is untouched.
 
 const documentFlowRedirects: Record<string, string> = {
   "/mockups/document-search-command": "/documents/search",
@@ -18,28 +27,54 @@ const documentFlowRedirects: Record<string, string> = {
   "/mockups/document-search/source/evidence": "/documents/source/evidence",
 };
 
+// Same runtime flags next.config.ts uses for the static headers, so the nonce'd
+// CSP matches the rest of the policy (unsafe-eval in dev, HTTPS upgrade off local
+// http). Evaluated once at module load.
+const { isDevelopment, isLocalHttpRuntime } = resolveRuntimeFlags();
+
 export async function proxy(request: NextRequest) {
+  // A fresh, unguessable nonce per request (see Next.js CSP guide). Buffer+base64
+  // matches the documented pattern and keeps the value header-safe.
+  const nonce = Buffer.from(crypto.randomUUID()).toString("base64");
+  const csp = buildContentSecurityPolicy({ isDevelopment, isLocalHttpRuntime, nonce });
+
+  // Request headers Next.js reads during SSR: `x-nonce` for our own inline
+  // <script>, and the CSP header from which Next extracts the nonce for its
+  // scripts. Rebuilt from the *current* request each call so session-cookie
+  // mutations below still propagate to the render.
+  const requestHeadersWithNonce = () => {
+    const headers = new Headers(request.headers);
+    headers.set("x-nonce", nonce);
+    headers.set("content-security-policy", csp);
+    return headers;
+  };
+  // Every response the browser sees must carry the enforced CSP header.
+  const withCsp = (response: NextResponse) => {
+    response.headers.set("content-security-policy", csp);
+    return response;
+  };
+
   const { pathname } = request.nextUrl;
   const redirectTarget = documentFlowRedirects[pathname];
 
   if (redirectTarget) {
     const url = request.nextUrl.clone();
     url.pathname = redirectTarget;
-    return NextResponse.redirect(url);
+    return withCsp(NextResponse.redirect(url));
   }
 
   if (pathname.startsWith("/mockups") && process.env.NODE_ENV === "production") {
-    return new NextResponse(null, { status: 404 });
+    return withCsp(new NextResponse(null, { status: 404 }));
   }
 
   const url = env.NEXT_PUBLIC_SUPABASE_URL;
   const key = env.NEXT_PUBLIC_SUPABASE_PUBLISHABLE_KEY;
   const hasAuthCookie = request.cookies.getAll().some((cookie) => cookie.name.startsWith("sb-"));
   if (!url || !key || !hasAuthCookie) {
-    return NextResponse.next({ request });
+    return withCsp(NextResponse.next({ request: { headers: requestHeadersWithNonce() } }));
   }
 
-  let response = NextResponse.next({ request });
+  let response = NextResponse.next({ request: { headers: requestHeadersWithNonce() } });
   const supabase = createServerClient(url, key, {
     cookies: {
       getAll() {
@@ -47,7 +82,7 @@ export async function proxy(request: NextRequest) {
       },
       setAll(cookiesToSet) {
         for (const { name, value } of cookiesToSet) request.cookies.set(name, value);
-        response = NextResponse.next({ request });
+        response = NextResponse.next({ request: { headers: requestHeadersWithNonce() } });
         for (const { name, value, options } of cookiesToSet) response.cookies.set(name, value, options);
       },
     },
@@ -57,7 +92,7 @@ export async function proxy(request: NextRequest) {
   // between createServerClient and getUser — a stale token here would sign the
   // user out on the next request.
   await supabase.auth.getUser();
-  return response;
+  return withCsp(response);
 }
 
 export const config = {
diff --git a/tests/proxy.test.ts b/tests/proxy.test.ts
new file mode 100644
index 000000000..da57ea13e
--- /dev/null
+++ b/tests/proxy.test.ts
@@ -0,0 +1,67 @@
+import { describe, expect, it } from "vitest";
+import { NextRequest } from "next/server";
+import { proxy } from "../src/proxy";
+
+// The proxy owns the per-request nonce CSP (see src/proxy.ts). CI's verify:ui
+// only exercises the *dev* CSP path (Turbopack keeps 'unsafe-inline'); these
+// unit tests run under NODE_ENV=test, so buildContentSecurityPolicy takes its
+// production branch — this is the only automated coverage of the strict,
+// shipped nonce policy. With no Supabase env configured the proxy short-circuits
+// on the "no auth cookie" path, which is exactly where the nonce/CSP wiring runs.
+
+function requestFor(path = "/"): NextRequest {
+  return new NextRequest(new URL(`http://localhost${path}`));
+}
+
+function scriptSrcOf(csp: string): string {
+  const directive = csp.split(";").find((d) => d.trim().startsWith("script-src"));
+  if (!directive) throw new Error(`no script-src in CSP: ${csp}`);
+  return directive.trim();
+}
+
+describe("proxy content-security-policy", () => {
+  it("emits a per-request nonce with strict-dynamic and no unsafe-inline (production shape)", async () => {
+    const res = await proxy(requestFor("/"));
+    const csp = res.headers.get("content-security-policy");
+    expect(csp).toBeTruthy();
+
+    const scriptSrc = scriptSrcOf(csp!);
+    expect(scriptSrc).toMatch(/'nonce-[A-Za-z0-9+/=_-]+'/);
+    expect(scriptSrc).toContain("'strict-dynamic'");
+    expect(scriptSrc).not.toContain("'unsafe-inline'");
+    expect(scriptSrc).not.toContain("'unsafe-eval'");
+  });
+
+  it("preserves the other CSP directives unchanged", async () => {
+    const csp = (await proxy(requestFor("/"))).headers.get("content-security-policy")!;
+    expect(csp).toContain("default-src 'self'");
+    expect(csp).toContain("object-src 'none'");
+    expect(csp).toContain("frame-ancestors 'none'");
+    expect(csp).toContain("img-src 'self' data: blob: https:");
+    expect(csp).toContain("connect-src 'self' https://*.supabase.co https://api.openai.com");
+    expect(csp).toContain("style-src 'self' 'unsafe-inline'");
+  });
+
+  it("generates a fresh, unguessable nonce on every request", async () => {
+    const nonces = new Set<string>();
+    for (let i = 0; i < 5; i += 1) {
+      const csp = (await proxy(requestFor("/"))).headers.get("content-security-policy")!;
+      const nonce = csp.match(/'nonce-([^']+)'/)![1];
+      expect(nonce.length).toBeGreaterThanOrEqual(16);
+      nonces.add(nonce);
+    }
+    expect(nonces.size).toBe(5);
+  });
+
+  it("threads the same nonce into the SSR request headers (x-nonce)", async () => {
+    const res = await proxy(requestFor("/"));
+    const cspNonce = res.headers.get("content-security-policy")!.match(/'nonce-([^']+)'/)![1];
+
+    // NextResponse.next({ request: { headers } }) forwards overridden request
+    // headers back through the response via x-middleware-request-* so the SSR
+    // render sees x-nonce. Assert the forwarded nonce matches the enforced CSP.
+    const overridden = res.headers.get("x-middleware-override-headers") ?? "";
+    expect(overridden).toContain("x-nonce");
+    expect(res.headers.get("x-middleware-request-x-nonce")).toBe(cspNonce);
+  });
+});
diff --git a/tests/security-headers.test.ts b/tests/security-headers.test.ts
index 976ed6987..22ba595bb 100644
--- a/tests/security-headers.test.ts
+++ b/tests/security-headers.test.ts
@@ -1,5 +1,5 @@
-import { describe, expect, it } from "vitest";
-import { buildContentSecurityPolicy, buildSecurityHeaders } from "../src/lib/security-headers";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { buildContentSecurityPolicy, buildSecurityHeaders, resolveRuntimeFlags } from "../src/lib/security-headers";
 
 // Regression guard for the "all images fail to render" incident. Document page
 // images and the PDF embed load cross-origin from Supabase Storage signed URLs
@@ -16,12 +16,14 @@ const flagVariants = [
   { name: "playwright local http", isDevelopment: false, isLocalHttpRuntime: true },
 ] as const;
 
+const NONCE = "dGVzdC1ub25jZQ==";
+
 describe("security headers", () => {
   for (const flags of flagVariants) {
     describe(flags.name, () => {
       const headers = buildSecurityHeaders(flags);
       const byKey = new Map(headers.map((header) => [header.key, header.value]));
-      const csp = buildContentSecurityPolicy(flags);
+      const csp = buildContentSecurityPolicy({ ...flags, nonce: NONCE });
 
       it("never sets Cross-Origin-Embedder-Policy (would block cross-origin Supabase images)", () => {
         expect(byKey.has("Cross-Origin-Embedder-Policy")).toBe(false);
@@ -49,8 +51,8 @@ describe("security headers", () => {
   }
 
   it("only emits unsafe-eval in development and HSTS/upgrade-insecure-requests off local http", () => {
-    const prod = buildContentSecurityPolicy({ isDevelopment: false, isLocalHttpRuntime: false });
-    const dev = buildContentSecurityPolicy({ isDevelopment: true, isLocalHttpRuntime: true });
+    const prod = buildContentSecurityPolicy({ isDevelopment: false, isLocalHttpRuntime: false, nonce: NONCE });
+    const dev = buildContentSecurityPolicy({ isDevelopment: true, isLocalHttpRuntime: true, nonce: NONCE });
 
     expect(dev).toContain("'unsafe-eval'");
     expect(prod).not.toContain("'unsafe-eval'");
@@ -62,4 +64,73 @@ describe("security headers", () => {
     expect(prodHeaders.some((header) => header.key === "Strict-Transport-Security")).toBe(true);
     expect(localHeaders.some((header) => header.key === "Strict-Transport-Security")).toBe(false);
   });
+
+  // Nonce migration (L19): in production script-src is nonce + strict-dynamic and
+  // never 'unsafe-inline'. A missed inline script fails silently at runtime, so
+  // this guards the policy shape that makes the nonce the only way scripts run.
+  it("gates production script-src on the nonce with strict-dynamic and no unsafe-inline", () => {
+    const csp = buildContentSecurityPolicy({ isDevelopment: false, isLocalHttpRuntime: false, nonce: NONCE });
+    const scriptSrc = csp.split(";").find((directive) => directive.trim().startsWith("script-src"));
+    expect(scriptSrc).toBeDefined();
+    expect(scriptSrc).toContain(`'nonce-${NONCE}'`);
+    expect(scriptSrc).toContain("'strict-dynamic'");
+    expect(scriptSrc).not.toContain("'unsafe-inline'");
+    expect(scriptSrc).not.toContain("'unsafe-eval'");
+  });
+
+  // Development keeps the pre-migration script-src: 'unsafe-inline' + 'unsafe-eval'
+  // and NO 'strict-dynamic'. The Turbopack dev server injects HMR/runtime and
+  // route-chunk <script src> tags that carry no nonce; 'strict-dynamic' would
+  // disable the 'self' allow-list and block them all. See buildContentSecurityPolicy.
+  it("keeps the permissive, nonce-free script-src in development", () => {
+    const csp = buildContentSecurityPolicy({ isDevelopment: true, isLocalHttpRuntime: true, nonce: NONCE });
+    const scriptSrc = csp.split(";").find((directive) => directive.trim().startsWith("script-src"));
+    expect(scriptSrc).toContain("'unsafe-inline'");
+    expect(scriptSrc).toContain("'unsafe-eval'");
+    expect(scriptSrc).not.toContain("'strict-dynamic'");
+    expect(scriptSrc).not.toContain("'nonce-");
+  });
+
+  // style-src still relies on 'unsafe-inline' (Next font + inline styles are not
+  // nonce-tagged); the migration only hardened script-src.
+  it("keeps style-src 'unsafe-inline'", () => {
+    const csp = buildContentSecurityPolicy({ isDevelopment: false, isLocalHttpRuntime: false, nonce: NONCE });
+    const styleSrc = csp.split(";").find((directive) => directive.trim().startsWith("style-src"));
+    expect(styleSrc).toContain("'unsafe-inline'");
+  });
+
+  // CSP is emitted per-request from proxy.ts (it carries a nonce), so the static
+  // header set must not also emit it — two CSP headers would be enforced as their
+  // intersection and break the app.
+  it("does not emit Content-Security-Policy in the static header set", () => {
+    for (const flags of flagVariants) {
+      const headers = buildSecurityHeaders(flags);
+      expect(headers.some((header) => header.key === "Content-Security-Policy")).toBe(false);
+    }
+  });
+
+  // Single source of truth shared by next.config.ts and proxy.ts. The
+  // isLocalHttpRuntime flag gates HTTPS-only hardening (HSTS,
+  // upgrade-insecure-requests), so its derivation is security-relevant.
+  describe("resolveRuntimeFlags", () => {
+    afterEach(() => vi.unstubAllEnvs());
+
+    it("treats development as local http", () => {
+      vi.stubEnv("NODE_ENV", "development");
+      vi.stubEnv("PLAYWRIGHT_BASE_URL", "");
+      expect(resolveRuntimeFlags()).toEqual({ isDevelopment: true, isLocalHttpRuntime: true });
+    });
+
+    it("keeps production HTTPS-hardened unless Playwright targets local http", () => {
+      vi.stubEnv("NODE_ENV", "production");
+      vi.stubEnv("PLAYWRIGHT_BASE_URL", "");
+      expect(resolveRuntimeFlags()).toEqual({ isDevelopment: false, isLocalHttpRuntime: false });
+
+      vi.stubEnv("PLAYWRIGHT_BASE_URL", "http://localhost:4788");
+      expect(resolveRuntimeFlags()).toEqual({ isDevelopment: false, isLocalHttpRuntime: true });
+
+      vi.stubEnv("PLAYWRIGHT_BASE_URL", "https://staging.example.com");
+      expect(resolveRuntimeFlags()).toEqual({ isDevelopment: false, isLocalHttpRuntime: false });
+    });
+  });
 });