What we collect

The client sends one JSON payload per batch. This page lists every field. The collector validates payloads against exactly this shape and rejects anything else.

Privacy guarantees

• No user identifiers. No user IDs, account IDs, emails, cookies, or local storage values. The client reads none of them.
• No IP addresses in stored data. The collector uses the client IP only to rate limit, keeps it in memory for the length of the rate-limit window, and never writes it to storage.
• No page content. No text, no input values, no element names, no props or state. Areas are identified only by the id strings you choose.
• No query strings or hashes. Routes come from location.pathname (or your getRoute), never from location.search or location.hash.
• The session id can't follow a user. It's random per page load (crypto.randomUUID() where available), is never stored on the device, and isn't derived from anything about the user. Reloading the page starts a new session.
• Unknown fields are rejected. Every object in the collector's schema is strict: a payload with a field that isn't listed here gets a 400 response and isn't stored. This stops a modified client from smuggling extra data into your database.

What you control:

• Area ids are strings you write. Keep them fixed ("orders-table"), not built from data.
• Routes: the default getRoute replaces numeric segments with :id, but other dynamic segments (usernames, slugs, emails in paths) pass through. If your paths can contain personal data, pass a getRoute that returns route templates. See Route grouping.
• appVersion is whatever string you pass.

Example

{
  "v": 1,
  "session": {
    "id": "6f1c2a7e-4b8d-4f7e-9a51-2d3c8b0e7f14",
    "seq": 1,
    "mode": "lightweight",
    "route": "/orders/:id",
    "appVersion": "2.4.0",
    "startedAt": 1761552000000,
    "sentAt": 1761552184000
  },
  "device": { "memoryGb": 4, "cores": 8, "connection": "4g", "viewport": "390x844" },
  "summary": {
    "renders": 42,
    "slowRenders": 5,
    "longTasks": 2,
    "longTaskMs": 263.4,
    "longFrames": 1,
    "longFrameMs": 212
  },
  "areas": [
    {
      "id": "orders-table",
      "route": "/orders/:id",
      "renders": 42,
      "mounts": 1,
      "slow": 5,
      "totalMs": 391.6,
      "min": 0.6,
      "max": 61.2,
      "p50": 5.7,
      "p75": 10.8,
      "p95": 32.1,
      "hist": [3, 4, 9, 12, 5, 4, 2, 1, 1, 1, 0, 0, 0, 0, 0, 0],
      "worst": [{ "ms": 61.2, "phase": "mount", "at": 412 }, { "ms": 38.9, "phase": "update", "at": 9120 }]
    }
  ],
  "longTasks": [{ "area": "orders-table", "route": "/orders/:id", "count": 2, "totalMs": 263.4, "maxMs": 212 }],
  "longFrames": [{ "area": "orders-table", "route": "/orders/:id", "count": 1, "totalMs": 212, "maxMs": 212 }],
  "dropped": { "areas": 0 }
}

All durations are in milliseconds, rounded to 0.1 ms.

Fields

Top level

Field	Type	Description
v	1	Payload version.
session	object	About this page load and batch.
device	object	Coarse device details.
summary	object	Totals for the batch.
areas	array	Render timings per area and route.
longTasks	array	Long tasks per area and route.
longFrames	array	Long animation frames per area and route.
dropped.areas	number	Area entries left out to respect maxAreas or maxPayloadBytes.

session

Field	Type	Description
id	string	Random per page load, never stored on the device. Lets the collector group batches from one page load.
seq	number	Batch number within the session, starting at 1.
mode	"profiling" | "lightweight"	The session's measurement mode when the batch was built. In a profiling session, useRenderMonitor timings are still marks measurements; see Modes.
route	string	The route when the batch was built.
appVersion	string | null	The appVersion option, or null.
startedAt	number	Epoch milliseconds when the session started.
sentAt	number	Epoch milliseconds when this batch was built.

device

Values come from browser APIs. Fields the browser doesn't expose are null; memoryGb and connection are Chromium-only.

Field	Type	Description
memoryGb	number | null	navigator.deviceMemory, which browsers round to a few coarse values.
cores	number | null	navigator.hardwareConcurrency.
connection	string | null	navigator.connection.effectiveType, e.g. "4g".
viewport	string	`${innerWidth}x${innerHeight}` in CSS pixels.

summary

Field	Type	Description
renders	number	Renders measured across all areas.
slowRenders	number	Renders slower than slowRenderMs.
longTasks	number	Long tasks observed.
longTaskMs	number	Their total duration.
longFrames	number	Long animation frames observed.
longFrameMs	number	Their total duration.

areas[]

One entry per area id and route.

Field	Type	Description
id	string	The id given to <RenderMonitor> or useRenderMonitor.
route	string	The route the renders happened on.
renders	number	Measured renders (mounts and updates).
mounts	number	How many of those were mounts.
slow	number	Renders slower than slowRenderMs (default 16).
totalMs	number	Sum of all render durations.
min	number	Fastest render.
max	number	Slowest render.
p50, p75, p95	number	Percentiles, estimated from the histogram and kept within the observed min and max.
hist	number[]	Render counts per bucket of HISTOGRAM_BOUNDS, plus a final overflow bucket (16 numbers).
worst	array	The slowest renders in this batch (up to worstCases, default 5): ms, phase ("mount", "update" or "nested-update"), and at, milliseconds since the session started.

HISTOGRAM_BOUNDS is [1, 2, 4, 8, 12, 16, 24, 33, 50, 75, 100, 150, 250, 500, 1000]: each bucket counts renders up to and including its bound. Histograms from many batches can be added bucket by bucket, which is how the viewer computes percentiles across sessions.

longTasks[] and longFrames[]

Main-thread blocks of at least longTaskMs (default 50), from the longtask and long-animation-frame performance entries. Browsers that don't support an entry type send an empty list.

Field	Type	Description
area	string | null	The monitored area that was rendering during the block, or null when none was. When several overlap (nested areas), the innermost one that accounts for at least half of the longest overlapping render.
route	string	The route at the time.
count	number	Number of blocks.
totalMs	number	Their total duration.
maxMs	number	The longest one.

Limits the collector enforces

Besides rejecting unknown fields, the collector bounds every string and list, so a hostile client can't store unbounded data:

Field	Limit
Area ids	1 to 200 characters
Routes	Up to 500 characters
session.id	1 to 64 characters
appVersion	Up to 100 characters
device.connection	Up to 20 characters
device.viewport	<width>x<height>, up to 5 digits each
areas, longTasks, longFrames	Up to 1000 entries each
worst	Up to 50 entries
hist	Exactly 16 entries
Durations (min, max, percentiles, worst-case ms, maxMs)	0 to 3,600,000 ms
Request body	65,536 bytes by default (maxBodyBytes)

The client keeps its batches within these limits on its own: it clamps maxPayloadBytes to 1,000–65,536, maxAreas to 1–1000 and worstCases to 0–50, truncates area ids to 200 characters (an empty id is reported as "(unnamed)"), routes to 500 and appVersion to 100. If you lower the collector's maxBodyBytes below the client's maxPayloadBytes, larger batches get a 413.