WebPerf Snippets

webperf-snippets CLI

Run curated WebPerf Snippets headlessly via Playwright. Diagnose Core Web Vitals beyond what Lighthouse exposes and gate CI on real performance budgets.

Status: v0.4. Core Web Vitals, loading, interaction and structural audit workflows, and every snippet of the collection by name. See the Roadmap.

Why

Lighthouse gives you a score. The DevTools snippets give you the diagnosis, such as TTFB / Resource Load Delay / Element Render Delay sub-parts, LoAF script attribution, and render-blocking resources. This CLI runs the same curated snippets in a headless browser so you can:

• Diagnose LCP regressions in CI without copy-pasting into DevTools.
• Gate pull requests on real performance budgets.
• Automate the snippets you already run by hand.

Install

Playwright is a peer dependency. Install both, plus the chromium browser:

npm install --save-dev webperf-snippets playwright
npx playwright install chromium

Usage

npx webperf-snippets <url> [options]

Examples

Run the default Core Web Vitals workflow (LCP + CLS, plus LCP-Subparts if LCP > 2.5s):

npx webperf-snippets https://example.com

Loading audit (11 steps: TTFB, FCP, render-blocking, scripts, resource timings, third-party impact, fonts, cache; the list is in the CLI page):

npx webperf-snippets https://example.com --workflow loading

The steps that tell first-party from third-party resources count as first party the root domain of the page and its subdomains (cdn.example.com for example.com). A site that serves its own assets from another domain, such as its CDN, lists it with --own-domains, comma-separated or repeated:

npx webperf-snippets https://example.com --workflow loading --own-domains cdn.example.net,static.example.org

Each domain counts with its root domain, so cdn.example.net makes every host under example.net first party. The CLI writes the list in the OWN_DOMAINS constant of each snippet, the same one you edit at the top of the snippet in the console. An empty domain or one with spaces exits with code 2.

Structural checks for CI (22 checks: render-blocking, compression, fonts, images, hints, DOM size and more; the list is in the CLI page):

npx webperf-snippets https://example.com --workflow audit

Markdown output for PR comments:

npx webperf-snippets https://example.com --markdown

JSON output (for piping into jq or CI):

npx webperf-snippets https://example.com --json

The JSON output records how the page was measured in conditions, so a later run can tell whether it measured the same way: the viewport, the workflow (or the path of the snippet with --snippet), the wait and the version of the CLI.

"conditions": { "viewport": "mobile", "workflow": "core-web-vitals", "waitMs": 3000, "cliVersion": "1.6.0" }

Single snippet:

npx webperf-snippets https://example.com --snippet LCP-Subparts

Synthetic INP measurement with an interaction script:

npx webperf-snippets https://example.com --snippet INP --interact-script interactions.json
npx webperf-snippets https://example.com --snippet Interaction/Interactions --interact-script interactions.json

The Interaction snippets (Interactions, Input-Latency-Breakdown, LongTask, Forced-Synchronous-Layout...) are installed before the first step and read after the last one, so their result holds what the steps caused.

The CLI waits for the browser to report the timing of the last interaction before it reads the snippets. A script that ends right after a click still reads the page before anything the click sets in motion later has happened, such as a request or an animation, so end it with a wait step when you want to measure that.

Interaction audit (long tasks, long animation frames, scroll, layout shifts, forced layouts, interaction latency). It scrolls the page, or runs your --interact-script; the steps that need a click are skipped when the script has none:

npx webperf-snippets https://example.com --workflow interaction

Pages behind a login, with a Playwright storage state (cookies and localStorage):

# Step 1: sign in once and save the session
npx playwright codegen --save-storage=auth.json https://example.com/login

# Step 2: measure an authenticated page
npx webperf-snippets https://example.com/dashboard --storage-state auth.json

The file holds live session cookies. Keep it out of version control (add it to .gitignore) and use a throwaway account when you run it in CI. The CLI exits with code 2 if the path does not exist.

Pages behind an access proxy that reads a token from every request, such as the service tokens of Cloudflare Access, take the token as request headers with --header, once per header:

npx webperf-snippets https://review.example.com \
  --header "CF-Access-Client-Id: $CF_ACCESS_CLIENT_ID" \
  --header "CF-Access-Client-Secret: $CF_ACCESS_CLIENT_SECRET"

The headers go with every request to the origin of the URL and never to another origin, so third parties do not receive the token and their cross-origin requests need no CORS preflight. Each request to that origin is paused in the browser to add them, which adds a few milliseconds to its timing; when the proxy accepts a session cookie, --storage-state measures without that cost. A header without a name or a colon exits with code 2. Read the values from environment variables, as above, to keep them out of the shell history.

Compare the measured values with CrUX field data (what real users experience at the 75th percentile):

CRUX_API_KEY=your-key npx webperf-snippets https://example.com

Field data (CrUX p75, mobile)
  🟡 LCP    2.80s      needs-improvement    (synthetic: 2.10s)
  🟢 CLS    0.0800     good                 (synthetic: 0.0500)
  🟡 INP    220ms      needs-improvement    (synthetic: n/a)

Get a key for the Chrome UX Report API and pass it with --crux-key or the CRUX_API_KEY environment variable; prefer the variable, because a flag ends up in shell history and CI logs. The section appears in the human and markdown output and under crux in the --json output.

• The population follows --viewport: desktop uses desktop data, mobile and tablet use phone data. The heading says which one.
• When the page itself has no data, the CLI looks the origin up instead and labels the section origin-level, so it is not mistaken for page-level data. If the origin has none either, the output says Field data: not available.
• A CrUX error (quota, network, no answer within 10 seconds) only skips the section. It never changes the exit code, and the key is never printed.
• INP shows n/a as the synthetic value unless you run an interaction script.

Send the results to an endpoint after the run with --report-to:

npx webperf-snippets https://example.com --workflow audit \
  --report-to https://reports.example.com/api/report \
  --api-key $REPORT_API_KEY

Without --report-to the CLI makes no external calls. With it, the results are POSTed as JSON once the snippets have finished:

{
  "url": "https://example.com",
  "workflow": "audit",
  "timestamp": "2026-05-06T10:00:00.000Z",
  "navMs": 1240,
  "results": [],
  "meta": { "viewport": "mobile", "waitMs": 3000, "cli_version": "1.6.0" }
}

For a --snippet run, workflow is null and a snippet field carries the name. With --cpu or --network, meta carries the conditions in throttling, and with --runs, the summary of the runs in runs. results is the same list the --json output contains, so it can include URLs and details of the measured page; send it only to an endpoint you trust.

• The URL must be https. http is accepted only for localhost, so a key is never sent in clear text.
• --api-key is sent as Authorization: Bearer <key>. Prefer the PERF_REVIEWS_API_KEY environment variable, because a flag ends up in shell history and CI logs. The key is never printed.
• The exit code never depends on the report. A failed POST (error status, unreachable endpoint, or no answer within 5 seconds) prints a warning and the exit code stays the one the budgets and snippets decide. If the endpoint answers { "ok": false, "regressions": [...] }, the regressions are printed as a warning, again without changing the exit code.

Measure under the conditions of a slower device and network with --cpu and --network. Without them the CLI runs at the speed of the machine, which on a laptop or a CI runner is faster than most phones:

npx webperf-snippets https://example.com --cpu 4 --network slow-4g

--cpu <factor> slows the main thread down by that factor. --network takes the presets of DevTools, with the same values:

Preset	Download	Upload	Latency
fast-4g	8.1 Mbps	1.35 Mbps	165 ms
slow-4g	1.44 Mbps	675 kbps	562.5 ms
3g	400 kbps	400 kbps	2000 ms

Chrome adds the latency to the body of each response, not to its headers. The values read from responseStart, such as the TTFB and its sub-parts, do not include it; the download of each resource, FCP, LCP and the load event do. The report names the conditions under its header, and the JSON output carries them in throttling, such as { "cpu": 4, "network": "slow-4g" }. A CPU factor below 1 or an unknown preset exits with code 2.

One page load is one sample, and lab values vary from one load to the next. --runs <n> measures the page n times, from 1 to 9, one after the other, and reports the median run, as WebPageTest does: every result comes from that run, so they agree with each other (the sub-parts of the LCP add up to its LCP):

npx webperf-snippets https://example.com --runs 5 --budget-lcp 2500

The median run is the one with the median LCP. A workflow without LCP uses the first of FCP, TTFB and the navigation time that it measures, and --median-by <metric> picks another one (LCP, FCP, TTFB, CLS, INP or navMs). With an even count it takes the lower of the two middle runs, so the report is always a run that happened. The report adds the median, min and max of each metric across the runs, and the JSON output carries them in runs:

"runs": {
  "count": 5,
  "failed": 0,
  "medianRun": 3,
  "medianBy": "LCP",
  "summary": {
    "LCP": { "median": 2140, "min": 1980, "max": 2610, "values": [2610, 2010, 2140, 1980, 2290], "unit": "ms" }
  }
}

medianRun counts from 1, and values keeps the order of the runs. The budgets compare the median of each metric, not one run. A run that fails, such as a navigation that does not finish, counts in failed and is left out of the median; when every run fails, the command exits with code 1. Each run is reported on stderr as it starts, so stdout keeps only the report. An invalid count or metric exits with code 2.

Measure several pages, on several devices and in several variants, in one command with --matrix <file>. The file lists them, and the CLI measures every URL on every device and in every variant, each combination runs times, one load after the other:

{
  "urls": {
    "home": "https://example.com/",
    "article": "https://example.com/news/some-article"
  },
  "devices": {
    "mobile": { "viewport": "mobile", "cpu": 4, "network": "slow-4g" },
    "high-end": { "viewport": "mobile", "cpu": 1, "network": { "downloadKbps": 50000, "uploadKbps": 10000, "latencyMs": 20 } },
    "desktop": { "viewport": "desktop" }
  },
  "variants": {
    "no-consent": {},
    "consent": { "storageState": "states/consent.json" }
  },
  "runs": 3
}

npx webperf-snippets --matrix matrix.json --budget-lcp 2500

• urls: names and URLs. An array of URLs also works, and names each one by its path and query (/, /news/), with the host when the URLs have more than one.
• devices: the viewport preset, the cpu factor and the network, a preset (fast-4g, slow-4g, 3g) or a network of its own in kilobits per second and milliseconds, for the ones DevTools has no preset for, such as 5G. An array of viewport presets (["mobile", "desktop"]) is a shortcut for devices without throttling. Without devices, the matrix measures a mid-range phone, with CPU 4x and Slow 4G, as Lighthouse does.
• variants: the conditions of each variant, storageState (relative to the file), headers and ownDomains. Without variants, the combinations are the pages on the devices: the report has no Variant column, and the id of a combination is its page and device (/news/ · mobile).
• runs (1 to 9) and medianBy, which --runs and --median-by replace.

The options of the command line apply to every combination: --workflow, --snippet, --wait, --interact-script, --header, --own-domains, --storage-state, --cpu, --network and the budgets. The file replaces cpu, network and storageState where it sets them, and its headers and ownDomains add to the ones of the command line, the variant winning on the same header. A matrix file is meant to be versioned, so pass secrets, such as the token of an access proxy, with --header and an environment variable; a header of the file whose name looks like a secret raises a warning. A key the CLI does not know is an error, so a typo does not measure without its condition.

The report is a table with one row per combination and the rated median of each metric, followed by the errors that decide the exit code; --verbose adds the report of the median run of each combination. --markdown gives the same table and folds the report of each combination into a <details> block, to keep a pull request comment readable. With --json, it is { "matrix", "runs", "combinations" }, each combination being the report of its median run with its id, name, device, variant (when the file defines variants), runs and conditions, with the viewport of its device. The budgets compare the median of each combination. The command exits with code 1 when a combination exceeds a budget, has an error or fails every run, and with code 2 for an invalid file, before any page load, naming the place of the problem in the JSON. --viewport does not apply, since each device sets its own.

With CrUX, the CLI asks once for each URL and form factor, so the variants of a device share their field data. With --report-to, it posts one report for each combination, the body of one page with meta.device, meta.variant and meta.runs; the headers of the command line and of the file are conditions of the load, and no report carries their values.

Compare a run with an earlier one with --baseline <file>, the --json output of that run. The comparison says which metrics got worse or better beyond the noise of the lab, which errors are new or fixed, and what differs in how both runs were measured:

# On main, once: the baseline
npx webperf-snippets https://example.com --runs 5 --json > baseline.json

# On the pull request: compare and report
npx webperf-snippets https://pr-123.example.com --runs 5 --baseline baseline.json --markdown

# Fail when the page gets worse
npx webperf-snippets https://pr-123.example.com --runs 5 --baseline baseline.json --fail-on-regression

LCP, FCP, TTFB, CLS and INP compare when both runs measured them, the median of each with --runs; a metric that only one side measured is not measured. A metric regresses when it gets worse by more than its threshold, the larger of a share of the baseline and a minimum, and, when the baseline has two runs or more, ends above the slowest of them: a median inside the range of the baseline is noise the baseline already had. It improves under the same rules the other way, and otherwise it is unchanged.

Metric	Share of the baseline	Minimum
LCP, FCP, TTFB	10%	100 ms
INP	10%	50 ms
CLS	none	0.05

--threshold changes one part of a threshold and keeps the other: --threshold LCP=15% the share, --threshold LCP=200ms the minimum, and --threshold CLS=0.1 the only part of CLS. Repeat it for more metrics.

The errors that compare are the ones that decide the exit code: the issues of severity error of the steps of the workflow and the snippets that failed. Two errors are the same when they share their code and the resource they name, whatever their messages say; a resource of the page itself compares by its path and query, so the review app of a pull request, on another host, matches production. A snippet without codes on one side, as in a baseline of an earlier version, compares by message with the figures left out, so "Render-blocking CSS (69.21 KB)…" and "Render-blocking CSS (71.80 KB)…" are one error. An error that only this run has is new, and one that only the baseline had is fixed. Warnings and information do not compare, since they change too easily.

The report adds the comparison after the usual one, and --markdown a table for the pull request comment. The JSON output carries it in baseline:

"baseline": {
  "file": "baseline.json",
  "url": "https://example.com/",
  "runs": { "count": 5, "medianBy": "LCP" },
  "metrics": {
    "LCP": { "baseline": 2100, "current": 2480, "delta": 380, "deltaPercent": 18.1, "threshold": 210, "verdict": "regressed" }
  },
  "errors": { "new": [{ "id": "render-blocking", "message": "1 render-blocking resource(s) delay rendering until 900ms" }], "fixed": [] },
  "warnings": []
}

When both runs were measured differently, with another workflow or snippet, viewport, wait, throttling, number of runs or version of the CLI, or on another path or query, the CLI compares anyway and warns on stderr, in the reports and in warnings: whoever reads the pull request comment does not see the stderr of the CI. Another host does not warn, since it is the review app of a pull request against production. A baseline of one run warns that only the thresholds apply.

The comparison only informs, and the exit code is the same as without it, unless --fail-on-regression, which exits with 1 when a metric regressed or an error is new, and names them on stderr. A baseline that does not exist or is not an output of the CLI, a baseline of a page for a matrix or of a matrix for a page, or a --threshold the CLI does not understand exits with code 2, before the page loads.

A matrix compares with the --json output of an earlier matrix, each combination with the one of the same id:

npx webperf-snippets --matrix matrix.json --runs 3 --baseline last-week.json

The table adds a change column after each metric: the change when it counts (▲ +380ms, ▼ −90ms), = when it is unchanged, and new for a combination the baseline did not have. Below the table come the warnings, once when every combination has them, the new and fixed errors of each combination, and the combinations that only one side has. --markdown gives the same, and the <details> block of each combination carries its own comparison. In the JSON output each combination carries its baseline, and the root baseline names the file and the added and missing combinations. --fail-on-regression exits with 1 when any combination regressed or has a new error.

CI gating:

npx webperf-snippets https://example.com --budget-lcp 2500 --budget-cls 0.1

Each budget compares the value measured, or the median with --runs, and fails when the value is above it. --budget-lcp and --budget-cls need the core-web-vitals workflow, --budget-fcp the loading workflow, --budget-ttfb the loading or audit workflow, and --budget-inp the interaction workflow with a click or a key press; --snippet with the snippet of the metric works too. A budget of a metric that the run does not measure exits with code 2 before the page loads, and one whose metric was not measured, such as INP without interactions, exits with code 1: a budget never passes unchecked. The report lists every budget with its value and verdict, passed, exceeded with by how much, or not measured with why: a Budgets section in the human and markdown reports, and budgets in the JSON output and in the body of --report-to. A matrix lists the failed budgets with their combination under its table. Each failed budget is also written to stderr, with --json too.

Options

Option	Description
--workflow <name>	Workflow to run. Default: core-web-vitals. Options: core-web-vitals, loading, audit, interaction.
--snippet <name>	Run a single snippet by name, alias or Category/Name path.
--json	Output JSON instead of formatted text.
--markdown	Output GitHub-renderable markdown (for PR comments).
--viewport <preset>	Viewport preset: mobile (default), tablet, desktop.
--wait <ms>	Post-load wait before evaluating snippets. Default: 3000.
--cpu <factor>	Slow the main thread down by this factor, such as 4. No throttling by default.
--network <preset>	Throttle the network with a preset of DevTools: fast-4g, slow-4g, 3g. No throttling by default.
--interact-script <path>	JSON file with interactions to run after the page loads (scroll, click, hover, type, wait). The Interaction snippets are installed first and read afterwards.
--storage-state <path>	Playwright storage state (cookies and localStorage) for pages that require authentication.
--header "Name: value"	Request header for the origin of the URL only, such as the token of an access proxy. Repeat it for more headers.
--own-domains <list>	Domains of the site to count as first party, such as its own CDN. Comma-separated or repeated.
--runs <n>	Measure the page n times, from 1 to 9, one after the other, and report the median run.
--median-by <metric>	Metric that picks the median run: LCP, FCP, TTFB, CLS, INP or navMs. Default: the first of LCP, FCP, TTFB and navMs measured.
--matrix <file>	Measure every URL of a JSON file on every device and in every variant it lists, and report a table of the medians.
--baseline <file>	Compare the page, or each combination of a matrix, with the --json output of an earlier run: which metrics got worse or better beyond the noise, and which errors are new or fixed.
--threshold <metric>=<v>	The threshold of a metric for --baseline, such as LCP=15%, LCP=200ms or CLS=0.1. Repeat it for more metrics.
--fail-on-regression	Exit 1 when a metric regressed or an error is new against --baseline.
--crux-key <key>	Add CrUX field data (p75) next to the measured values. Prefer CRUX_API_KEY.
--report-to <url>	POST the results to this https URL after the run. No external calls without it.
--api-key <key>	Sent as a Bearer token with --report-to. Prefer PERF_REVIEWS_API_KEY.
--budget-lcp <ms>	Exit 1 if LCP exceeds this value or was not measured.
--budget-fcp <ms>	Exit 1 if FCP exceeds this value or was not measured.
--budget-ttfb <ms>	Exit 1 if TTFB exceeds this value or was not measured.
--budget-cls <score>	Exit 1 if CLS exceeds this value or was not measured.
--budget-inp <ms>	Exit 1 if INP exceeds this value or was not measured.
--verbose	Show all items, including passing checks.
--headed	Show the browser window (debug).
-h, --help	Show help.

Snippet aliases

Alias	Snippet
LCP	CoreWebVitals/LCP
CLS	CoreWebVitals/CLS
INP	CoreWebVitals/INP
LCP-Subparts	CoreWebVitals/LCP-Subparts
fonts	Loading/Fonts-Preloaded-Loaded-and-used-above-the-fold
render-blocking	Loading/Find-render-blocking-resources
resource-hints	Loading/Resource-Hints-Validation
preload-scripts	Loading/Validate-Preload-Async-Defer-Scripts
priority-hints	Loading/Priority-Hints-Audit
critical-css	Loading/Critical-CSS-Detection
ttfb	Loading/TTFB-Sub-Parts
script-parties	Loading/First-And-Third-Party-Script-Info
script-loading	Loading/Script-Loading
lazy-atf	Loading/Find-Above-The-Fold-Lazy-Loaded-Images
lazy-conflict	Loading/Find-Images-With-Lazy-and-Fetchpriority
eager-below-fold	Loading/Find-non-Lazy-Loaded-Images-outside-of-the-viewport

Follow-up steps

The results of a run can add steps on the same page load: LCP over 2.5 s adds LCP-Subparts, over 4 s LCP-Trail; an image or video LCP adds its entropy, image or video audit; CLS over 0.1 adds Layout-Shift-Loading-and-Interaction, over 0.25 also the lazy images, fonts, critical CSS and images without dimensions; TTFB over 600 ms adds TTFB-Sub-Parts; FCP over 1.8 s adds the render-blocking check. The full table is in the CLI page. Each snippet runs once, and a follow-up informs: its error issues do not change the exit code.

Exit codes

Code	Meaning
0	All checks passed.
1	Budget violation, a snippet errored, a step of the workflow reported an error issue, or, with --fail-on-regression, a regression against the baseline.
2	Usage error (missing URL, unknown workflow, a baseline or threshold that is not valid).

MCP server

webperf-snippets mcp starts a Model Context Protocol server over stdio, so an AI agent can call the snippets by name instead of reading their source. Add it to a client that launches local servers, for example Claude Code:

claude mcp add webperf -- npx -y webperf-snippets mcp

It needs the Chromium of Playwright (npx playwright install chromium); without it, the tools answer with that command. The MCP server documentation has the configuration of other clients (Cursor, VS Code, Claude Desktop, Gemini CLI). The server is listed in the MCP Registry as io.github.nucliweb/webperf-snippets.

Tool	What it does
list_snippets	Lists the snippets with their category, title, short aliases and whether they track interactions. Takes an optional category.
run_snippet	Opens a URL in a headless Chromium, runs one snippet and returns the object it returns. Takes url, snippet (name, alias or Category/Name), and optional viewport and waitMs. The snippets that record interactions run on a page opened with open_page.
run_workflow	Runs a workflow of the CLI (core-web-vitals, audit, loading, interaction) and the follow-ups of its decision tree. Takes url, workflow, and optional viewport and waitMs.
get_crux	Returns the CrUX p75 of LCP, CLS and INP for a URL, or its origin. Takes url and an optional formFactor (phone, desktop, tablet). Needs CRUX_API_KEY in the environment of the server.
open_page	Opens a URL in a page that stays open between calls and installs tracking snippets (track, such as INP). Returns a sessionId.
interact	Runs steps on an open page: click, type, hover, scroll or wait.
read_results	Returns what the tracking snippets of an open page have recorded.
close_page	Closes an open page.

The server also offers resources, documents a client can attach to the context:

Resource	What it has
webperf://schema	The contract every snippet result follows: base shape, statuses, how to read a result, the rules of the issue codes.
webperf://schema/{snippet}	The fields one snippet returns, with an example, and the codes of its issues.
webperf://workflows/{category}	The workflows and the decision tree of a category.

And prompts, templates that chain the tools (in Claude Code, /mcp__webperf__audit_lcp): audit_lcp, debug_inp and audit_page, each with a url argument.

The CrUX key goes in the configuration of the client, never in a tool argument:

claude mcp add webperf -e CRUX_API_KEY=<key> -- npx -y webperf-snippets mcp

At most three pages are open at a time, a page closes after 10 minutes without use, and every page and Chromium close when the client disconnects.

The server answers clients of both eras of the protocol: the 2026-07-28 revision and the earlier ones that open with initialize. ADR 0002 explains why it is a subcommand and uses stdio.

CI example

GitHub Actions, fail the PR if LCP exceeds 2.5s:

- run: |
    npm install --no-save webperf-snippets playwright
    npx playwright install --with-deps chromium
    npx webperf-snippets https://staging.example.com --budget-lcp 2500 --budget-cls 0.1

GitHub Action

The repository ships a composite action that installs the CLI and Chromium, runs a workflow and exposes the result as markdown, so any repository can gate on performance budgets without extra setup:

jobs:
  webperf:
    runs-on: ubuntu-latest
    permissions:
      pull-requests: write
    steps:
      - uses: nucliweb/webperf-snippets/.github/actions/webperf-snippets@main
        id: webperf
        with:
          url: https://staging.example.com
          budget-lcp: 2500
          fail-on-budget: "false"

      - uses: actions/github-script@v7
        env:
          BODY: ${{ steps.webperf.outputs.markdown }}
        with:
          script: |
            await github.rest.issues.createComment({
              ...context.repo,
              issue_number: context.issue.number,
              body: process.env.BODY,
            });

      - if: steps.webperf.outputs.exit-code != '0'
        run: exit 1

Input	Default	Description
url	required	URL to measure.
workflow	core-web-vitals	Workflow to run: core-web-vitals, loading, audit or interaction.
budget-lcp	auto	LCP budget in ms. auto is 2500 with core-web-vitals and no budget with the other workflows. An empty value disables the check.
budget-fcp		FCP budget in ms, for loading.
budget-ttfb		TTFB budget in ms, for loading or audit.
budget-cls	auto	CLS budget. auto is 0.1 with core-web-vitals and no budget with the other workflows. An empty value disables the check.
budget-inp		INP budget in ms, for interaction with a click or a key press.
fail-on-budget	true	Fail the step when a budget is exceeded or a snippet errors. Use "false" to read exit-code and decide yourself.
cli-version	latest	Version of the webperf-snippets npm package to run.

Output	Description
markdown	The results as markdown. Also written to the job summary.
exit-code	Exit code of the CLI: 0 passed, 1 budget exceeded or a snippet errored, 2 usage error.

The markdown includes text taken from the measured page, so pass it to other steps through an environment variable (env: and process.env) as above, never by interpolating ${{ steps.webperf.outputs.markdown }} inside a script. With fail-on-budget: "false" the action step always succeeds, which lets later steps post the comment before the job fails. Without permissions: pull-requests: write the comment step cannot post on a pull request.

LCP, CLS and INP vary by 15 to 30% on shared runners; prefer the deterministic audit workflow for a gate, or generous budgets.

Publishing

The CLI package is published to npm via a tag-based workflow. Publishing is explicit and intentional; it only happens when a cli-v* tag is pushed.

Release steps

1. Bump the version in cli/package.json.
2. Commit the version change.
3. Tag and push:

   git tag cli-v0.2.0
   git push origin cli-v0.2.0
   

4. The publish-cli CI job runs, executes the full test suite, and publishes to npm.

Why tag-based and not path-based

An alternative is to publish automatically on every push to main that touches cli/, using a version check to skip republishes. Tag-based publishing was chosen instead because it keeps releases deliberate; a passing CI on main does not mean the package is ready to ship, and a tag communicates that intent explicitly.

Access control

Tag protection rules restrict who can push cli-v* tags. Configure them under Settings → Rules → New ruleset in the repository, targeting the cli-v* tag pattern and limiting push access to admins or maintainers. This ensures only authorized collaborators can trigger a publish.

Required secret

The NPM_TOKEN secret must be set in the repository settings with publish access to the webperf-snippets npm package.

Known limitations

• CLS in headless is conservative: layout shifts that only happen on scroll are missed unless you script the scroll.
• First navigation only: each webperf-snippets invocation runs one URL. SPAs need the post-route URL passed directly.
• Synthetic INP ≠ field INP: --interact-script measures handler latency for a single scripted event. Real INP reflects the worst interaction across all user sessions; use RUM for field data.

Roadmap

• v0.2: Loading workflow (TTFB, FCP, render-blocking, scripts, fonts), shared page session, synthetic interactions for INP, markdown reporter for PR comments. ✓ Released
• v0.3: GitHub Action wrapper, auth flows (login + measure logged-in pages), CrUX field-data enrichment, --report-to. ✓ Released
• v0.4: Every snippet by name, a 22-step audit, an 11-step loading workflow, the interaction workflow and decision-tree follow-ups for Core Web Vitals. ✓ Released

How it works

1. Launches headless chromium via Playwright.
2. Navigates to the URL, waits for the load event and then for --wait milliseconds (3000 by default), so late entries reach the page.
3. Evaluates each snippet's IIFE in the page context, capturing the returned object. The snippets read the entries Chrome only exposes to observers (LCP, layout shifts, long tasks, event timing) with a buffered PerformanceObserver, so they see the entries recorded before they ran.
4. With an interaction script, installs the tracking snippets first, runs the interactions, lets the page settle and then reads the tracking data and runs the other snippets.
5. Applies the workflow's decision tree to enqueue follow-up snippets.
6. Renders results (human or JSON) and exits with an appropriate code.

License

MIT, see LICENSE.