Chris Tam · Last updated 14 October 2026
Bug reports rarely include the state that caused them. Session replay records what each user of your React Router app saw and did, so "the button didn't work" becomes a session you can watch.
In this tutorial, you'll turn on session replay, set a sample rate per environment, mask sensitive components and routes, and confirm a replay lands. It builds on How to add product analytics to a React Router app.
What you'll build
A React Router app that records every session in development and 1% of sessions in production, masks text inputs by default, masks anything wrapped in an AmpMask component and every page under /billing, replaces blocked elements with a placeholder, and tags the session's events with [Amplitude] Session Replay ID. By the end you'll play back your own test session from your event stream in Amplitude.
Prerequisites
- The setup from How to add product analytics to a React Router app:
@amplitude/unifiedinstalled,VITE_AMPLITUDE_API_KEYin.env,initAllinapp/entry.client.tsx, and the root identity effect, so Verify can find your session by user ID. - An Amplitude project API key. In Amplitude, go to Settings > Projects > your project > General.
- To change sampling or masking in Amplitude's settings, an Admin or Manager role. The code on this page doesn't need it.
No Amplitude account yet? Session replay for web is available on all plans, so a free account covers this tutorial. Sign up for free to get your project API key.
1. Install the SDK
The Quickstart installs @amplitude/unified for browser apps that use npm. It bundles analytics, session replay, Feature Experiment, and guides and surveys behind one initAll call, and session replay records once sessionReplay.sampleRate is above 0. If you followed the React Router product analytics page, it's already installed and .env holds VITE_AMPLITUDE_API_KEY; skip to step 2. Otherwise install it, then add the key to .env as that page's step 1 shows.
npm install @amplitude/unifiedAlready on @amplitude/analytics-browser to keep bundle size down? The session replay plugin takes the same options as the sessionReplay block below. Add it to your existing init rather than installing a second SDK.
2. Set a sample rate per environment
Initialize once, in the client entry, and choose the sample rate from Vite's import.meta.env.DEV flag. This is the complete app/entry.client.tsx. The first comment names the lines other React Router pages add here; if you followed any of them, keep those lines and change only the sessionReplay block.
// Keep lines other React Router pages added here: pageViews: { trackHistoryChanges:
// "pathOnly" } (/build/react-router-web-analytics); the amplitude-cookies import and
// handoff lines, the deviceId spread, and the experiment block
// (/build/react-router-feature-flags); the setAmplitudeReady import, the
// setAmplitudeReady(...) wrapper, and setAmplitudeReady(null)
// (/build/react-router-feature-flags, /build/react-router-guides-surveys); the
// amplitude.add(webExperiment.plugin()) lines (/build/react-router-web-experiment).
// This page changes only sessionReplay.
import { startTransition, StrictMode } from "react";
import { hydrateRoot } from "react-dom/client";
import { HydratedRouter } from "react-router/dom";
import * as amplitude from "@amplitude/unified";
const apiKey = import.meta.env.VITE_AMPLITUDE_API_KEY;
if (apiKey) {
void amplitude.initAll(apiKey, {
analytics: {
autocapture: {
attribution: true,
pageViews: true,
// Tracks [Amplitude] Start Session and End Session events.
sessions: true,
formInteractions: true,
fileDownloads: true,
elementInteractions: true,
},
},
sessionReplay: {
// Fraction of sessions to record: every session in `npm run dev`, 1% in a production build.
sampleRate: import.meta.env.DEV ? 1 : 0.01,
// Ignores the focus check so replays record with DevTools open. Setup only.
debugMode: import.meta.env.DEV,
// Selectors for the data attributes in steps 3 and 4.
privacyConfig: {
maskSelector: ["[data-amp-mask]"],
blockSelector: ["[data-amp-block]"],
unmaskSelector: ["[data-amp-unmask]"],
},
},
});
} else {
console.warn("VITE_AMPLITUDE_API_KEY is not set; analytics is off.");
}
startTransition(() => {
hydrateRoot(
document,
<StrictMode>
<HydratedRouter />
</StrictMode>
);
});Why. Session replay takes a full snapshot of the page when it initializes, and nothing that happens before that point appears in the replay. The client entry runs once, in the browser, before React hydrates the server-rendered HTML, so the masking attributes are already in the DOM, straight from the server HTML, when the first snapshot is taken. It also matters for matching: initializing the SDK more than once can leave the replay and the session's events with different device or session IDs, and a replay that doesn't match its events is one you can't open from the event stream. An initAll in a route component's useEffect runs again on every visit to that route, and under StrictMode in development it runs twice on mount.
The plugin's own default sample rate is 0, so nothing records until you set one. Amplitude recommends starting production low, around 0.01, and raising it over a few days; capture stops for the month once your org's quota runs out. To stop recording, set the rate to 0. Session Replay Settings in Amplitude can also set a project's rate remotely, so check Settings > Organizational Settings > Session Replay Settings when the rate you see doesn't match your code. For every option, see the session replay plugin configuration; the Unified SDK accepts all of them inside sessionReplay except serverZone and instanceName, which go at the top level of the initAll options.
Unified also initializes guides and surveys by default. If you won't use them, the Unified SDK docs describe an engagement: { skip: true } option that stops that SDK from loading.
3. Mask sensitive elements in React components
Session replay masks every text input by default: typed characters arrive as asterisks. The project's default masking level, Medium, masks all form fields and text inputs and captures other text as-is. Everything else is your call, element by element.
The SDK recognizes three classes out of the box, amp-mask, amp-block, and amp-unmask, and they work as className on any element. The data attributes registered in step 2 keep privacy separate from styling, and they're what the wrapper component below sets. Put one small component in charge of masking, so every route says it the same way:
import type { HTMLAttributes, ReactNode } from "react";
type Props = HTMLAttributes<HTMLElement> & {
// Replace the content with a same-size placeholder instead of asterisks.
block?: boolean;
// Use "span" inside inline content.
as?: "div" | "span" | "section";
children: ReactNode;
};
export function AmpMask({ block = false, as: Tag = "div", children, ...rest }: Props) {
return (
<Tag
{...rest}
data-amp-mask={block ? undefined : ""}
data-amp-block={block ? "" : undefined}
>
{children}
</Tag>
);
}Use it in any route or component, next to plain data attributes where a wrapper would be overkill:
import { AmpMask } from "../components/AmpMask";
type Member = { id: string; name: string; email: string; avatarUrl: string };
// Replace with data from your loader.
const members: Member[] = [
{ id: "1", name: "Test User", email: "test@example.com", avatarUrl: "https://placehold.co/32" },
];
export default function Team() {
return (
<main>
<h1>Team</h1>
{/* Inputs are masked by default. This one is safe to watch. */}
<input data-amp-unmask="" name="filter" placeholder="Filter by name" />
<ul>
{members.map((member) => (
<li key={member.id} style={{ display: "flex", gap: "0.5rem" }}>
{/* Replaced by a same-size placeholder */}
<AmpMask block as="span">
<img src={member.avatarUrl} alt="" />
</AmpMask>
<span>{member.name}</span>
{/* Captured as asterisks */}
<AmpMask as="span">{member.email}</AmpMask>
</li>
))}
</ul>
</main>
);
}React renders data-amp-unmask="" and the component's "" values as empty attributes, which the attribute selectors match, and leaves an attribute off when its value is undefined. All of it renders on the server, so the attributes are in the HTML before the SDK starts.
Why. React components don't forward attributes to an element on their own. Writing data-amp-mask on a component tag, such as <MemberEmail data-amp-mask="" />, passes a prop the component ignores unless it spreads its props onto an element, and nothing in the replay tells you. The wrapper puts the attribute on an element you can see in the markup, and its ...rest spread lets you pass id, role, or className through as well. Masking also follows the DOM, not the React tree: a dialog, popover, or toast that createPortal or a component library renders into document.body leaves its masked parent, so wrap the content inside the portal itself. And masking covers text and form inputs, not HTML attributes. alt, title, placeholder, aria-label, value, and data-* attributes stay visible in replays even inside a masked element, so a tooltip written as title={member.email} leaks the address no matter what you mask.
Don't use generated class names, such as CSS Modules hashes like _row_1x2y3, as masking selectors, in code or in Session Replay Settings. They're produced at build time and change when the stylesheet changes, at which point the rule stops matching without any error.
When the masking rules in Session Replay Settings and the SDK disagree about an element, the settings page wins. You can change the project's level (Light, Medium, Conservative) or add selector overrides there, per project; see Manage privacy settings for session replay.
4. Mask whole routes with a layout route
Some pages shouldn't show any text in a replay, such as billing, invoices, or a patient record. Masking each element there is how one gets missed. In React Router, a layout route wraps every child route through its <Outlet />, so one file covers a whole section:
import { Outlet } from "react-router";
import { AmpMask } from "../components/AmpMask";
// Everything rendered under /billing is masked, including routes added later.
export default function BillingLayout() {
return (
<AmpMask>
<Outlet />
</AmpMask>
);
}Add two pages for the section, so there's text to check in the replay. If you already have billing routes, nest those instead:
export default function BillingHome() {
return (
<>
<h1>Billing</h1>
<p>Card ending 4242, billed to Test User</p>
</>
);
}export default function Invoices() {
return <h1>Invoices</h1>;
}Register them as children of the layout in app/routes.ts, next to your existing entries:
// Keep lines other React Router pages added here: their routes
// (/build/react-router-product-analytics, /build/react-router-web-analytics,
// /build/react-router-heatmaps). Merge new entries into the array.
import { type RouteConfig, index, route } from "@react-router/dev/routes";
export default [
// ... your existing routes
route("team", "routes/team.tsx"),
route("billing", "routes/billing.tsx", [
index("routes/billing-home.tsx"),
route("invoices", "routes/invoices.tsx"),
]),
] satisfies RouteConfig;Using flatRoutes() instead? app/routes/billing.tsx becomes the layout for app/routes/billing._index.tsx and app/routes/billing.invoices.tsx on its own. Your app/root.tsx with the identity effect from the React Router product analytics page stays as it is, and this layout renders inside it.
To mask several sections that don't share a URL prefix, such as /billing and /account, nest them under a pathless layout: layout("routes/masked.tsx", [route("billing", ...), route("account", ...)]) with layout imported from @react-router/dev/routes. A pathless layout doesn't change the URL. If you added the account guard from the web analytics page, move its whole route("account", ...) entry, children included, inside the pathless layout.
Why. Masking an element through a selector masks all of its children, and a child inside it can't be unmasked again. That's what you want here: a developer who adds a payment-methods child route next year gets masking without knowing this file exists. It also means data-amp-unmask does nothing under /billing, so put content you want visible outside the section. React Router keeps the layout mounted while someone moves between /billing and /billing/invoices, so the wrapper never disappears mid-session.
The layout masks text, not structure. Clicks, scrolling, and layout still record, so you can follow what the user did on /billing without reading it. To hide the region entirely, use <AmpMask block> in the layout instead.
The layout doesn't cover the page <title>, which a route's meta export sets; keep personal data out of titles under /billing.
MCP. Working in Cursor or Claude Code? Amplitude's MCP server includes get_session_replays, and the debug-replay skill in Amplitude's MCP marketplace plugin turns a bug report into numbered reproduction steps from the replay's interaction timeline, so you don't have to scrub through sessions by hand.
5. Allow Amplitude in your CSP
React Router sends no Content-Security-Policy unless you add one, from a route's headers export, from app/entry.server.tsx, or at your host. Skip this step unless you did. If you did, add Amplitude's hosts to the connect-src directive you already have:
connect-src 'self' https://api2.amplitude.com https://api-sr.amplitude.com https://sr-client-cfg.amplitude.comapi-sr.amplitude.com receives replay data and sr-client-cfg.amplitude.com serves the session replay remote configuration. api2.amplitude.com is where the analytics SDK sends events. EU projects use api-sr.eu.amplitude.com and sr-client-cfg.eu.amplitude.com and set serverZone: 'EU' at the top level of the initAll options. Add worker-src blob: only if you turn on useWebWorker. Unified also loads guides and surveys by default, which needs https://*.amplitude.com in script-src, connect-src, img-src, media-src, and style-src. Allow those, or turn guides and surveys off with engagement: { skip: true }.
Why. If remote configuration fails to load, session replay captures nothing, on purpose, so it never records without the privacy settings you defined in Amplitude. A blocked config request looks like "replay does nothing" rather than like an error, until you open the console and find a Content Security Policy message.
Verify it worked
Run npm run dev, open http://localhost:5173 in a fresh private window, visit /team, type into the filter, open /billing, then check in this order:
- Console and Network tab. No Content Security Policy errors in the console. In the Network tab, filter for
amplitude: you should see event uploads toapi2.amplitude.comreturning 200, a request tosr-client-cfg.amplitude.com, and replay uploads toapi-sr.amplitude.com. None of them start until the client entry runs in the browser; the server render sends nothing. - Ingestion Debugger. In Amplitude, click Data, select Sources, then open the Ingestion Debugger tab. The successful requests chart should show your event uploads. It shows counts, not individual events.
- User lookup. Go to Users > User Profiles and search your device ID (the Instrumentation Explorer shows it as
device_id) or youruser-prefixed user ID if you're signed in. Within ten seconds to a minute, the session's events, such as[Amplitude] Start Session, carry[Amplitude] Session Replay ID, and the event stream shows a Play Session button next to that session. Replays are generally ready about five minutes after the session begins. - Session Replay debug panel. Install the Instrumentation Explorer Chrome extension and turn on its Session Replay debug panel on your page. It floats over the page and shows the current replay ID, the session replay SDK and plugin versions, the sample rate, and the default and effective masking settings. A sample rate of 1 in development confirms step 2 took effect, and the effective masking settings show whether Session Replay Settings overrode your code.
- The replay itself. Play it. On
/team, the filter text is visible, emails are asterisks, and avatars are grey placeholders. On/billing, every line of text is masked while clicks and scrolling still show.
If the replay won't load, open the Ingestion Monitor from the top right of the Session Replay section of your project's Users & Sessions tab. It shows replay request statuses, including 429 throttling and 402 quota errors.
To force a fresh session while testing, delete the cookies that start with AMP_ in DevTools and reload.
Replays recorded against localhost can play back unstyled, because Amplitude can't fetch files from your machine. Check styling against a deployed preview.
Troubleshooting
No session replay requests at all
The effective sample rate is 0. Check sessionReplay.sampleRate, then the rate in Session Replay Settings; the debug panel shows the rate the SDK is using. A production build (npm run build, then npm start) has import.meta.env.DEV set to false and records only about one session in a hundred at 0.01, so test with npm run dev or raise the rate temporarily.
Replays record in production but not while you debug
Session replay only captures the tab that has focus, and an open DevTools panel can take it. Keep debugMode: import.meta.env.DEV from step 2, which ignores the focus check in development.
Events arrive but the session has no Play Session button
No event in that session carries [Amplitude] Session Replay ID. The usual cause is a second initAll or init call, for example in a useEffect in root.tsx or a route, so the replay and the events disagree on device or session ID. Search the codebase and keep the one in app/entry.client.tsx.
Masked text shows up in a replay
Check four things in order. The text sits in an attribute such as title or aria-label, which masking never covers. The data-amp-mask sits on a component tag and never reached the DOM; inspect the element, then wrap the content in AmpMask. The text renders through a portal into document.body, outside the masked wrapper. Or the element was unmasked in Session Replay Settings, which overrides the SDK; the debug panel's effective masking settings show it.
Old replays lose their styling after a deploy
Vite writes CSS to hashed file names under build/client/assets/, and a deploy that replaces the build output removes the old ones. Keep previous build assets available on your host, allow app.amplitude.com (app.eu.amplitude.com for EU projects) in your asset server's CORS policy, and add crossOrigin="anonymous" to any external stylesheet link you return from a links export.
Requests fail with ERR_BLOCKED_BY_CLIENT
An ad blocker is dropping them. Session replay isn't compatible with ad blocking software, so test in a clean profile. For production traffic, route requests through your own domain and point trackServerUrl and configServerUrl at it, and set serverUrl (events) and remoteConfig.serverUrl (remote configuration) in the analytics block.
No requests to Amplitude at all, and the console says the key is not set
VITE_AMPLITUDE_API_KEY is empty in the browser bundle. The name must start with VITE_, and Vite reads .env only at startup and inlines the value at build time, so restart the dev server, and set it in your build environment before you build for production.
Full code reference
Every file this tutorial adds or changes, in one block, the version to hand to an agent. 7 files, 155 lines. Shared files start with a comment naming the lines other React Router pages add there; merge into your file and keep those lines. Unchanged from earlier pages; keep your versions: .env, app/root.tsx, app/auth.server.ts, app/components/SignOutButton.tsx.
// -- app/entry.client.tsx --
// Keep lines other React Router pages added here: pageViews: { trackHistoryChanges:
// "pathOnly" } (/build/react-router-web-analytics); the amplitude-cookies import and
// handoff lines, the deviceId spread, and the experiment block
// (/build/react-router-feature-flags); the setAmplitudeReady import, the
// setAmplitudeReady(...) wrapper, and setAmplitudeReady(null)
// (/build/react-router-feature-flags, /build/react-router-guides-surveys); the
// amplitude.add(webExperiment.plugin()) lines (/build/react-router-web-experiment).
// This page changes only sessionReplay.
import { startTransition, StrictMode } from "react";
import { hydrateRoot } from "react-dom/client";
import { HydratedRouter } from "react-router/dom";
import * as amplitude from "@amplitude/unified";
const apiKey = import.meta.env.VITE_AMPLITUDE_API_KEY;
if (apiKey) {
void amplitude.initAll(apiKey, {
analytics: {
autocapture: {
attribution: true,
pageViews: true,
// Tracks [Amplitude] Start Session and End Session events.
sessions: true,
formInteractions: true,
fileDownloads: true,
elementInteractions: true,
},
},
sessionReplay: {
// Fraction of sessions to record: every session in `npm run dev`, 1% in a production build.
sampleRate: import.meta.env.DEV ? 1 : 0.01,
// Ignores the focus check so replays record with DevTools open. Setup only.
debugMode: import.meta.env.DEV,
// Selectors for the data attributes in steps 3 and 4.
privacyConfig: {
maskSelector: ["[data-amp-mask]"],
blockSelector: ["[data-amp-block]"],
unmaskSelector: ["[data-amp-unmask]"],
},
},
});
} else {
console.warn("VITE_AMPLITUDE_API_KEY is not set; analytics is off.");
}
startTransition(() => {
hydrateRoot(
document,
<StrictMode>
<HydratedRouter />
</StrictMode>
);
});
// -- app/components/AmpMask.tsx --
import type { HTMLAttributes, ReactNode } from "react";
type Props = HTMLAttributes<HTMLElement> & {
// Replace the content with a same-size placeholder instead of asterisks.
block?: boolean;
// Use "span" inside inline content.
as?: "div" | "span" | "section";
children: ReactNode;
};
export function AmpMask({ block = false, as: Tag = "div", children, ...rest }: Props) {
return (
<Tag
{...rest}
data-amp-mask={block ? undefined : ""}
data-amp-block={block ? "" : undefined}
>
{children}
</Tag>
);
}
// -- app/routes/team.tsx --
import { AmpMask } from "../components/AmpMask";
type Member = { id: string; name: string; email: string; avatarUrl: string };
// Replace with data from your loader.
const members: Member[] = [
{ id: "1", name: "Test User", email: "test@example.com", avatarUrl: "https://placehold.co/32" },
];
export default function Team() {
return (
<main>
<h1>Team</h1>
{/* Inputs are masked by default. This one is safe to watch. */}
<input data-amp-unmask="" name="filter" placeholder="Filter by name" />
<ul>
{members.map((member) => (
<li key={member.id} style={{ display: "flex", gap: "0.5rem" }}>
{/* Replaced by a same-size placeholder */}
<AmpMask block as="span">
<img src={member.avatarUrl} alt="" />
</AmpMask>
<span>{member.name}</span>
{/* Captured as asterisks */}
<AmpMask as="span">{member.email}</AmpMask>
</li>
))}
</ul>
</main>
);
}
// -- app/routes/billing.tsx --
import { Outlet } from "react-router";
import { AmpMask } from "../components/AmpMask";
// Everything rendered under /billing is masked, including routes added later.
export default function BillingLayout() {
return (
<AmpMask>
<Outlet />
</AmpMask>
);
}
// -- app/routes/billing-home.tsx --
export default function BillingHome() {
return (
<>
<h1>Billing</h1>
<p>Card ending 4242, billed to Test User</p>
</>
);
}
// -- app/routes/invoices.tsx --
export default function Invoices() {
return <h1>Invoices</h1>;
}
// -- app/routes.ts --
// Keep lines other React Router pages added here: their routes
// (/build/react-router-product-analytics, /build/react-router-web-analytics,
// /build/react-router-heatmaps). Merge new entries into the array.
import { type RouteConfig, index, route } from "@react-router/dev/routes";
export default [
// ... your existing routes
route("team", "routes/team.tsx"),
route("billing", "routes/billing.tsx", [
index("routes/billing-home.tsx"),
route("invoices", "routes/invoices.tsx"),
]),
] satisfies RouteConfig;Hand it to your agent
Prefer to have Cursor, Claude Code, or Codex CLI do the wiring? This prompt points the agent at this page and pins the decisions that matter.
Complete https://amplitude.com/build/react-router-product-analytics first, or
confirm this React Router framework mode app already has its setup;
create its app/auth.server.ts stub and app/routes/new-project.tsx route
at projects/new if they're missing. Then add Amplitude session replay by following
https://amplitude.com/build/react-router-session-replay exactly. If the app
already uses @amplitude/analytics-browser, add
@amplitude/plugin-session-replay-browser to the existing init instead of
installing a second SDK, pass the same options to
sessionReplayPlugin({...}), and call amplitude.add() with it before
amplitude.init(). Otherwise use @amplitude/unified, with initAll called
once in app/entry.client.tsx, before hydrateRoot. Merge into existing
shared files (app/entry.client.tsx, app/routes.ts) and never delete lines
another Amplitude Build page added; each file's first comment names them
(experiment block, deviceId handoff, setAmplitudeReady import and calls,
webExperiment plugin, pageViews options). In app/entry.client.tsx change
only sessionReplay: sampleRate import.meta.env.DEV ? 1 : 0.01, debugMode
import.meta.env.DEV, and privacyConfig with [data-amp-mask],
[data-amp-block], and [data-amp-unmask]. Never initialize in root.tsx, a
route module, a loader, or a useEffect. Create app/components/AmpMask.tsx,
a wrapper with block and as props that renders data-amp-mask (or
data-amp-block when block is set) on the element around its children and
spreads the rest of its props onto it. Wrap elements that render emails,
billing names, or account data in it, and wrap images of people or
documents in <AmpMask block>. Put billing and any other sensitive section
under a layout route that renders <AmpMask><Outlet /></AmpMask>, using a
pathless layout if the sections don't share a URL prefix. If the app has
no /team or /billing routes, create the page's demo routes and register
them in app/routes.ts so the Verify steps run: routes/team.tsx, the
routes/billing.tsx layout with routes/billing-home.tsx as its index and
routes/invoices.tsx at invoices. Don't put masking attributes on
component tags, wrap portal content separately, and never use generated
CSS Modules class names as masking selectors. Flag any title, aria-label,
alt, or placeholder that contains personal data. Don't hardcode the key.
Report every file you changed and any Content-Security-Policy you found.What this unlocks
- Session replay sits on the same events as your charts: open a funnel step that ends at
/billingand watch the sessions where users dropped, text masked. - Heatmaps are built from session replays, so once heatmap capture is on for the project in Session Replay Settings, clicks and scrolls on
/teamaggregate without new code. - Feature Experiment, bundled in
@amplitude/unified, lets you filter replays by variant and see how people use a new version of/team. - Guides and surveys, also bundled, can ask the users whose replays show a stall on
/billingwhat they were looking for.
One integration; everything else reads from the same events.
Next steps
Reference docs
Amplitude is the leading all-in-one analytics platform for building better products. Product and engineering teams use Amplitude to connect product analytics and web analytics with session replay, heatmaps, experimentation, feature flags, and guides and surveys in one unified workflow, with AI agents working across all of it. Learn how Amplitude is building self-improving products with Wave, helping teams turn product data into PRs.