diff --git a/e2e/test-app/src/pages/_app.tsx b/e2e/test-app/src/pages/_app.tsx
index f53ea50..4bc14a9 100644
--- a/e2e/test-app/src/pages/_app.tsx
+++ b/e2e/test-app/src/pages/_app.tsx
@@ -18,17 +18,14 @@ function MyApp({ Component, pageProps, nextlyticsCtx }: MyAppProps) {
 MyApp.getInitialProps = async (appContext: AppContext) => {
   const { ctx } = appContext;
 
-  // Only get nextlytics props on server-side (when req is available)
-  let nextlyticsCtx: NextlyticsContext = { requestId: "" };
-  if (ctx.req) {
-    nextlyticsCtx = getNextlyticsProps({ req: { headers: ctx.req.headers } });
-  }
-
+  // getInitialProps re-runs in the browser on client-side navigation, where
+  // there is no `ctx.req`. getNextlyticsProps handles that and returns an empty
+  // context; the client keeps the templates and scripts it already has.
   return {
     pageProps: appContext.Component.getInitialProps
       ? await appContext.Component.getInitialProps(ctx)
       : {},
-    nextlyticsCtx,
+    nextlyticsCtx: getNextlyticsProps(ctx),
   };
 };
 
diff --git a/e2e/tests/analytics.test.ts b/e2e/tests/analytics.test.ts
index b603b36..0e6352e 100644
--- a/e2e/tests/analytics.test.ts
+++ b/e2e/tests/analytics.test.ts
@@ -194,6 +194,33 @@ describe.each(versions)("%s", (version) => {
       await page.close();
     });
 
+    it("compiles and runs client-side script templates on initial load", async () => {
+      // Regression guard for template delivery. App Router gets templates from
+      // NextlyticsServer; Pages Router can't read config in getNextlyticsProps,
+      // so it receives them in the /api/event response. Either way the scripts
+      // must compile and run — without the template, __nextlyticsTestInit stays
+      // undefined and this times out.
+      const page = await testApp.newPage();
+
+      await testApp.visitHome(page);
+
+      await page.waitForFunction(() => window.__nextlyticsTestInit !== undefined, undefined, {
+        timeout: 5000,
+      });
+
+      const counters = await page.evaluate(() => ({
+        init: window.__nextlyticsTestInit,
+        config: window.__nextlyticsTestConfig,
+        event: window.__nextlyticsTestEvent,
+      }));
+
+      expect(counters.init).toBeGreaterThanOrEqual(1);
+      expect(counters.config).toBeGreaterThanOrEqual(1);
+      expect(counters.event).toBeGreaterThanOrEqual(1);
+
+      await page.close();
+    });
+
     it("script modes work correctly during soft navigation", async () => {
       // This test only applies to App Router (soft navigation with <Link>)
       if (routerType !== "app") return;
@@ -264,11 +291,11 @@ describe.each(versions)("%s", (version) => {
   });
 });
 
-// Type augmentation for test globals
+// Type augmentation for test globals (set by the console-test backend scripts)
 declare global {
   interface Window {
-    __nextlyticsTestOnce?: number;
-    __nextlyticsTestParamsChange?: number;
-    __nextlyticsTestEveryRender?: number;
+    __nextlyticsTestInit?: number;
+    __nextlyticsTestConfig?: number;
+    __nextlyticsTestEvent?: number;
   }
 }
diff --git a/packages/core/src/api-handler.ts b/packages/core/src/api-handler.ts
index f51f91d..3a285e2 100644
--- a/packages/core/src/api-handler.ts
+++ b/packages/core/src/api-handler.ts
@@ -8,6 +8,7 @@ import type {
   ClientContext,
   ClientRequest,
   DispatchResult,
+  JavascriptTemplate,
   PageViewDelivery,
   NextlyticsEvent,
   RequestContext,
@@ -30,6 +31,9 @@ export type UpdateEvent = (
   ctx: RequestContext
 ) => Promise<void>;
 
+/** Collect the client-side templates from the configured backends. */
+export type CollectTemplates = (ctx: RequestContext) => Record<string, JavascriptTemplate>;
+
 type HandlerContext = {
   pageRenderId: string;
   isSoftNavigation: boolean;
@@ -39,8 +43,26 @@ type HandlerContext = {
   config: NextlyticsConfigWithDefaults;
   dispatchEvent: DispatchEvent;
   updateEvent: UpdateEvent;
+  collectTemplates: CollectTemplates;
+  /** Template ids the client already holds (from the known-templates header). */
+  knownTemplateIds: Set<string>;
 };
 
+/**
+ * Collect the templates for this request and drop the ones the client already
+ * has. App Router clients already hold the full set (from the ctx prop), so they
+ * get nothing back; a Pages Router client receives each template once. Returns
+ * undefined when there is nothing new, to keep it out of the JSON response.
+ */
+function newTemplatesFor(hctx: HandlerContext): Record<string, JavascriptTemplate> | undefined {
+  const all = hctx.collectTemplates(hctx.ctx);
+  const missing: Record<string, JavascriptTemplate> = {};
+  for (const [id, template] of Object.entries(all)) {
+    if (!hctx.knownTemplateIds.has(id)) missing[id] = template;
+  }
+  return Object.keys(missing).length > 0 ? missing : undefined;
+}
+
 function createRequestContext(request: NextRequest): RequestContext {
   return {
     headers: request.headers,
@@ -158,6 +180,7 @@ async function handleClientInit(
     return Response.json({
       ok: true,
       items: filterScripts(actions),
+      templates: newTemplatesFor(hctx),
     });
   }
 
@@ -167,7 +190,7 @@ async function handleClientInit(
   after(() => completion);
   after(() => updateEvent(pageRenderId, { clientContext, userContext, anonymousUserId }, ctx));
 
-  return Response.json({ ok: true });
+  return Response.json({ ok: true, templates: newTemplatesFor(hctx) });
 }
 
 async function handleClientEvent(
@@ -208,14 +231,19 @@ async function handleClientEvent(
   const actions = await clientActions;
   after(() => completion);
 
-  return Response.json({ ok: true, items: filterScripts(actions) });
+  return Response.json({
+    ok: true,
+    items: filterScripts(actions),
+    templates: newTemplatesFor(hctx),
+  });
 }
 
 export async function handleEventPost(
   request: NextRequest,
   config: NextlyticsConfigWithDefaults,
   dispatchEvent: DispatchEvent,
-  updateEvent: UpdateEvent
+  updateEvent: UpdateEvent,
+  collectTemplates: CollectTemplates
 ): Promise<Response> {
   const softNavHeader = request.headers.get(analyticsHeaders.isSoftNavigation);
   const isSoftNavigation = softNavHeader === "1";
@@ -235,6 +263,16 @@ export async function handleEventPost(
   const apiCallServerContext = createServerContext(request);
   const userContext = await getUserContext(config, ctx);
 
+  const knownTemplatesHeader = request.headers.get(analyticsHeaders.knownTemplates);
+  const knownTemplateIds = new Set(
+    knownTemplatesHeader
+      ? knownTemplatesHeader
+          .split(",")
+          .map((id) => id.trim())
+          .filter(Boolean)
+      : []
+  );
+
   const cookiePageRenderId = request.cookies.get(LAST_PAGE_RENDER_ID_COOKIE)?.value;
   const pageRenderId = isSoftNavigation ? (cookiePageRenderId ?? generateId()) : pageRenderIdHeader;
   if (isSoftNavigation && !cookiePageRenderId && config.debug) {
@@ -252,6 +290,8 @@ export async function handleEventPost(
     config,
     dispatchEvent,
     updateEvent,
+    collectTemplates,
+    knownTemplateIds,
   };
 
   const bodyType = body.type;
diff --git a/packages/core/src/client.tsx b/packages/core/src/client.tsx
index da963f3..e5f1321 100644
--- a/packages/core/src/client.tsx
+++ b/packages/core/src/client.tsx
@@ -9,6 +9,7 @@ import {
   useMemo,
   useReducer,
   useRef,
+  useState,
 } from "react";
 import { useNavigation, debug, InjectScript, type InjectScriptProps } from "./client-utils";
 import type {
@@ -41,6 +42,8 @@ type NextlyticsContextValue = {
   addScripts: (scripts: TemplatizedScriptInsertion<unknown>[]) => void;
   scriptsRef: React.MutableRefObject<TemplatizedScriptInsertion<unknown>[]>;
   subscribersRef: React.MutableRefObject<Set<() => void>>;
+  mergeTemplates: (incoming?: Record<string, JavascriptTemplate>) => void;
+  knownTemplateIdsRef: React.MutableRefObject<string[]>;
 };
 
 const NextlyticsContext = createContext<NextlyticsContextValue | null>(null);
@@ -227,7 +230,11 @@ function NextlyticsScripts({
 async function sendEventToServer(
   requestId: string,
   request: ClientRequest,
-  { signal, isSoftNavigation }: { signal?: AbortSignal; isSoftNavigation?: boolean } = {}
+  {
+    signal,
+    isSoftNavigation,
+    knownTemplateIds,
+  }: { signal?: AbortSignal; isSoftNavigation?: boolean; knownTemplateIds?: string[] } = {}
 ): Promise<ClientRequestResult> {
   try {
     const headers: Record<string, string> = {
@@ -237,6 +244,11 @@ async function sendEventToServer(
     if (isSoftNavigation) {
       headers[headerNames.isSoftNavigation] = "1";
     }
+    // Tell the server which templates we already have so it only sends new ones.
+    // App Router clients already hold the full set, so nothing comes back.
+    if (knownTemplateIds?.length) {
+      headers[headerNames.knownTemplates] = knownTemplateIds.join(",");
+    }
     const response = await fetch("/api/event", {
       method: "POST",
       headers,
@@ -251,9 +263,9 @@ async function sendEventToServer(
       return { ok: false };
     }
 
-    // Parse response to get scripts
+    // Parse response to get scripts (and templates, for Pages Router clients)
     const data = await response.json().catch(() => ({ ok: response.ok }));
-    return { ok: data.ok ?? response.ok, items: data.items };
+    return { ok: data.ok ?? response.ok, items: data.items, templates: data.templates };
   } catch (error) {
     if (error instanceof Error && error.name === "AbortError") {
       return { ok: false };
@@ -264,12 +276,43 @@ async function sendEventToServer(
 }
 
 export function NextlyticsClient(props: { ctx: NextlyticsContext; children?: ReactNode }) {
-  const { requestId, scripts: initialScripts = [], templates = {} } = props.ctx;
+  const { requestId, scripts: initialScripts = [] } = props.ctx;
 
   // Refs for dynamic scripts (from sendEvent calls) - stable, no re-renders
   const scriptsRef = useRef<TemplatizedScriptInsertion<unknown>[]>([]);
   const subscribersRef = useRef<Set<() => void>>(new Set());
 
+  // Templates can arrive from two places: the ctx prop (App Router's
+  // NextlyticsServer collects them from config) and the /api/event response
+  // (Pages Router, where getNextlyticsProps has no access to config). Hold them
+  // in state and merge from whichever source supplies them, so they survive
+  // client-side navigations regardless of router.
+  const [templates, setTemplates] = useState<Record<string, JavascriptTemplate>>(
+    () => props.ctx.templates ?? {}
+  );
+  const mergeTemplates = useCallback((incoming?: Record<string, JavascriptTemplate>) => {
+    if (!incoming) return;
+    const keys = Object.keys(incoming);
+    if (keys.length === 0) return;
+    setTemplates((prev) => {
+      const hasNew = keys.some((k) => prev[k] !== incoming[k]);
+      return hasNew ? { ...prev, ...incoming } : prev;
+    });
+  }, []);
+
+  // Merge templates supplied via the ctx prop (App Router).
+  useEffect(() => {
+    mergeTemplates(props.ctx.templates);
+  }, [props.ctx.templates, mergeTemplates]);
+
+  // Mirror the template ids into a ref so sendEventToServer can tell the server
+  // which templates we already have (sent as a header), without re-creating the
+  // request callbacks on every merge.
+  const knownTemplateIdsRef = useRef<string[]>(Object.keys(props.ctx.templates ?? {}));
+  useEffect(() => {
+    knownTemplateIdsRef.current = Object.keys(templates);
+  }, [templates]);
+
   const addScripts = useCallback((newScripts: TemplatizedScriptInsertion<unknown>[]) => {
     debug("Adding scripts", {
       newCount: newScripts.length,
@@ -281,8 +324,16 @@ export function NextlyticsClient(props: { ctx: NextlyticsContext; children?: Rea
 
   // Context value is stable - refs don't change identity
   const contextValue = useMemo<NextlyticsContextValue>(
-    () => ({ requestId, templates, addScripts, scriptsRef, subscribersRef }),
-    [requestId, templates, addScripts]
+    () => ({
+      requestId,
+      templates,
+      addScripts,
+      scriptsRef,
+      subscribersRef,
+      mergeTemplates,
+      knownTemplateIdsRef,
+    }),
+    [requestId, templates, addScripts, mergeTemplates]
   );
 
   // Send page-view on mount and soft navigations
@@ -292,9 +343,10 @@ export function NextlyticsClient(props: { ctx: NextlyticsContext; children?: Rea
     sendEventToServer(
       requestId,
       { type: "page-view", clientContext, softNavigation: softNavigation || undefined },
-      { signal, isSoftNavigation: softNavigation }
-    ).then(({ items }) => {
+      { signal, isSoftNavigation: softNavigation, knownTemplateIds: knownTemplateIdsRef.current }
+    ).then(({ items, templates: responseTemplates }) => {
       debug("page-view response", { scriptsCount: items?.length ?? 0 });
+      mergeTemplates(responseTemplates);
       if (items?.length) addScripts(items);
     });
   });
@@ -324,28 +376,33 @@ export function useNextlytics(): NextlyticsClientApi {
     );
   }
 
-  const { requestId, addScripts } = context;
+  const { requestId, addScripts, mergeTemplates, knownTemplateIdsRef } = context;
 
   const sendEvent = useCallback(
     async (
       eventName: string,
       opts?: { props?: Record<string, unknown> }
     ): Promise<{ ok: boolean }> => {
-      const result = await sendEventToServer(requestId, {
-        type: "custom-event",
-        name: eventName,
-        props: opts?.props,
-        collectedAt: new Date().toISOString(),
-        clientContext: createClientContext(),
-      });
+      const result = await sendEventToServer(
+        requestId,
+        {
+          type: "custom-event",
+          name: eventName,
+          props: opts?.props,
+          collectedAt: new Date().toISOString(),
+          clientContext: createClientContext(),
+        },
+        { knownTemplateIds: knownTemplateIdsRef.current }
+      );
 
+      mergeTemplates(result.templates);
       if (result.items && result.items.length > 0) {
         addScripts(result.items);
       }
 
       return { ok: result.ok };
     },
-    [requestId, addScripts]
+    [requestId, addScripts, mergeTemplates, knownTemplateIdsRef]
   );
 
   return { sendEvent };
diff --git a/packages/core/src/middleware.ts b/packages/core/src/middleware.ts
index 0ba2604..6a9a9a2 100644
--- a/packages/core/src/middleware.ts
+++ b/packages/core/src/middleware.ts
@@ -19,6 +19,7 @@ import {
   handleEventPost,
   getUserContext,
   getEventProps,
+  type CollectTemplates,
   type DispatchEvent,
   type UpdateEvent,
 } from "./api-handler";
@@ -34,7 +35,8 @@ function createRequestContext(request: NextRequest): RequestContext {
 export function createNextlyticsMiddleware(
   config: NextlyticsConfigWithDefaults,
   dispatchEvent: DispatchEvent,
-  updateEvent: UpdateEvent
+  updateEvent: UpdateEvent,
+  collectTemplates: CollectTemplates
 ): NextMiddleware {
   const { eventEndpoint } = config;
 
@@ -74,7 +76,7 @@ export function createNextlyticsMiddleware(
     // Handle event endpoint directly in middleware
     if (pathname === eventEndpoint) {
       if (request.method === "POST") {
-        return handleEventPost(request, config, dispatchEvent, updateEvent);
+        return handleEventPost(request, config, dispatchEvent, updateEvent, collectTemplates);
       }
       return Response.json({ error: "Method not allowed" }, { status: 405 });
     }
diff --git a/packages/core/src/pages-router.ts b/packages/core/src/pages-router.ts
index c259bb5..b598fd5 100644
--- a/packages/core/src/pages-router.ts
+++ b/packages/core/src/pages-router.ts
@@ -2,16 +2,29 @@ import type { NextlyticsContext } from "./client";
 import { restoreServerComponentContext } from "./server-component-context";
 
 export type PagesRouterContext = {
-  req: { headers: Record<string, string | string[] | undefined>; cookies?: Record<string, string> };
+  req?: {
+    headers: Record<string, string | string[] | undefined>;
+    cookies?: Record<string, string>;
+  };
 };
 
 /**
  * Get Nextlytics props for Pages Router _app.tsx.
  * Reads context from headers set by middleware.
+ *
+ * `_app`'s getInitialProps re-runs on every client-side navigation, where there
+ * is no `req`. Return an empty context in that case rather than throwing — the
+ * client already has its scripts and templates from the initial render and the
+ * /api/event round-trip.
  */
 export function getNextlyticsProps(ctx: PagesRouterContext): NextlyticsContext {
+  const reqHeaders = ctx?.req?.headers;
+  if (!reqHeaders) {
+    return { requestId: "" };
+  }
+
   const headersList = new Headers();
-  for (const [key, value] of Object.entries(ctx.req.headers)) {
+  for (const [key, value] of Object.entries(reqHeaders)) {
     if (value) {
       headersList.set(key, Array.isArray(value) ? value[0] : value);
     }
diff --git a/packages/core/src/server-component-context.ts b/packages/core/src/server-component-context.ts
index 8b8257a..f637c33 100644
--- a/packages/core/src/server-component-context.ts
+++ b/packages/core/src/server-component-context.ts
@@ -10,6 +10,9 @@ const headerNames = {
   isSoftNavigation: `${HEADER_PREFIX}is-soft-nav`,
   active: `${HEADER_PREFIX}active`,
   scripts: `${HEADER_PREFIX}scripts`,
+  /** Comma-separated template ids the client already holds, so the server only
+   * returns the ones it's missing (see api-handler / client templates merge). */
+  knownTemplates: `${HEADER_PREFIX}known-templates`,
 } as const;
 
 export const LAST_PAGE_RENDER_ID_COOKIE = "last-page-render-id";
diff --git a/packages/core/src/server.tsx b/packages/core/src/server.tsx
index f6a2e33..5fe93c9 100644
--- a/packages/core/src/server.tsx
+++ b/packages/core/src/server.tsx
@@ -235,7 +235,12 @@ export function Nextlytics(userConfig: NextlyticsConfig): NextlyticsResult {
     return updateEventInternal(eventId, patch, ctx);
   };
 
-  const middleware = createNextlyticsMiddleware(config, dispatchEventInternal, updateEventInternal);
+  const middleware = createNextlyticsMiddleware(
+    config,
+    dispatchEventInternal,
+    updateEventInternal,
+    (ctx) => collectTemplates(config, ctx)
+  );
 
   /** Server component that provides analytics context to the app */
   async function Server({ children }: { children: ReactNode }) {
@@ -245,7 +250,9 @@ export function Nextlytics(userConfig: NextlyticsConfig): NextlyticsResult {
     if (!ctx) {
       // x-nl-page-render-id absent → check if middleware is at least active
       if (!headersList.get(headerNames.active)) {
-        console.warn("[Nextlytics] nextlyticsMiddleware should be added in order for Server to work");
+        console.warn(
+          "[Nextlytics] nextlyticsMiddleware should be added in order for Server to work"
+        );
       }
       return <>{children}</>;
     }
diff --git a/packages/core/src/types.ts b/packages/core/src/types.ts
index 0632fa3..e3260c7 100644
--- a/packages/core/src/types.ts
+++ b/packages/core/src/types.ts
@@ -292,6 +292,9 @@ export type ClientRequest =
 export type ClientRequestResult = {
   ok: boolean;
   items?: ClientActionItem[];
+  /** Client-side templates from the backends, so Pages Router clients (which
+   * can't read them from config) can compile the script insertions. */
+  templates?: Record<string, JavascriptTemplate>;
 };
 
 /** Return value from Nextlytics() */