diff --git a/.github/scripts/chrome.mts b/.github/scripts/chrome.mts
index 5d07ac6efe..2be10bd261 100644
--- a/.github/scripts/chrome.mts
+++ b/.github/scripts/chrome.mts
@@ -7,6 +7,7 @@ import { createRequire } from 'node:module';
 import { writeFile } from 'node:fs/promises';
 import type { Browser, BrowserContext, CDPSession, Page } from 'playwright';
 import type { HeapSnapshotData } from './heap-snapshot-util.mts';
+import * as util from './utility.mts';
 
 export type NetworkRequest = {
 	requestId: string;
@@ -89,11 +90,65 @@ export type TabMemory = {
 	totalBytes: number;
 };
 
+export type BrowserConsoleMetrics = {
+	log: number;
+	warn: number;
+	error: number;
+	info: number;
+};
+
+export type BrowserDiagnostics = {
+	pageErrorCount: number;
+	console: BrowserConsoleMetrics;
+};
+
+export function createBrowserDiagnostics(): BrowserDiagnostics {
+	return {
+		pageErrorCount: 0,
+		console: {
+			log: 0,
+			warn: 0,
+			error: 0,
+			info: 0,
+		},
+	};
+}
+
+export function recordPageError(diagnostics: BrowserDiagnostics) {
+	diagnostics.pageErrorCount += 1;
+}
+
+function isBrowserConsoleMetricKey(value: string): value is keyof BrowserConsoleMetrics {
+	return value === 'log' || value === 'warn' || value === 'error' || value === 'info';
+}
+
+export function recordConsoleMessageType(diagnostics: BrowserDiagnostics, messageType: string) {
+	const metricKey = messageType === 'warning' ? 'warn' : messageType;
+	if (!isBrowserConsoleMetricKey(metricKey)) return;
+	diagnostics.console[metricKey] += 1;
+}
+
+export function summarizeBrowserDiagnostics(samples: BrowserDiagnostics[]): BrowserDiagnostics {
+	if (samples.length === 0) throw new Error('No browser diagnostic samples');
+	const median = (select: (sample: BrowserDiagnostics) => number) => util.median(samples.map(select));
+
+	return {
+		pageErrorCount: median(sample => sample.pageErrorCount),
+		console: {
+			log: median(sample => sample.console.log),
+			warn: median(sample => sample.console.warn),
+			error: median(sample => sample.console.error),
+			info: median(sample => sample.console.info),
+		},
+	};
+}
+
 export type BrowserMeasurement = {
 	label: string;
 	timestamp: string;
 	url: string;
 	scenario: string;
+	diagnostics: BrowserDiagnostics;
 	durationMs: number;
 	network: NetworkSummary;
 	performance: {
@@ -149,6 +204,7 @@ type PlaywrightBrowserOptions = {
 export class HeadlessChromeController {
 	public networkRequests: NetworkRequest[] = [];
 	public webSocketConnections: WebSocketConnection[] = [];
+	private readonly diagnostics = createBrowserDiagnostics();
 	private readonly browser: Browser;
 	private readonly context: BrowserContext;
 	public readonly page: Page;
@@ -168,6 +224,12 @@ export class HeadlessChromeController {
 		this.cdp = cdp;
 		this.page.setDefaultTimeout(options.scenarioTimeoutMs);
 		this.page.setDefaultNavigationTimeout(options.scenarioTimeoutMs);
+		this.page.on('pageerror', () => {
+			recordPageError(this.diagnostics);
+		});
+		this.page.on('console', message => {
+			recordConsoleMessageType(this.diagnostics, message.type());
+		});
 	}
 
 	static async create(label: string, options: PlaywrightBrowserOptions): Promise<HeadlessChromeController> {
@@ -376,6 +438,13 @@ export class HeadlessChromeController {
 		]) as T;
 	}
 
+	public collectDiagnostics(): BrowserDiagnostics {
+		return {
+			pageErrorCount: this.diagnostics.pageErrorCount,
+			console: { ...this.diagnostics.console },
+		};
+	}
+
 	public async collectPerformance(): Promise<BrowserMeasurement['performance']> {
 		const cdpMetricsResult = await this.cdp.send<{ metrics: { name: string; value: number }[] }>('Performance.getMetrics');
 		const cdpMetrics = Object.fromEntries(cdpMetricsResult.metrics.map(metric => [metric.name, metric.value]));
diff --git a/.github/scripts/frontend-browser-report.mts b/.github/scripts/frontend-browser-report.mts
index 755b29bc88..829041f1d0 100644
--- a/.github/scripts/frontend-browser-report.mts
+++ b/.github/scripts/frontend-browser-report.mts
@@ -8,13 +8,14 @@ import { pathToFileURL } from 'node:url';
 import * as util from './utility.mts';
 import * as heapSnapshotUtil from './heap-snapshot-util.mts';
 import type { HeapSnapshotData, HeapSnapshotReport } from './heap-snapshot-util.mts';
-import type { NetworkRequest } from './chrome.mts';
+import type { BrowserDiagnostics, NetworkRequest } from './chrome.mts';
 
 export type BrowserMeasurement = {
 	label: string;
 	timestamp: string;
 	url: string;
 	scenario: string;
+	diagnostics: BrowserDiagnostics;
 	durationMs: number;
 	network: {
 		requestCount: number;
@@ -171,8 +172,19 @@ function getMetric(report: BrowserMeasurement, key: string) {
 	return report.performance.cdpMetrics[key];
 }
 
+export function renderBrowserDiagnosticsRows(base: BrowserMetricsReport, head: BrowserMetricsReport, all = false) {
+	return [
+		metricRow('Page errors', base, head, summary => summary.diagnostics.pageErrorCount, sample => sample.diagnostics.pageErrorCount, util.formatNumber, 1, !all),
+		metricRow('Console log', base, head, summary => summary.diagnostics.console.log, sample => sample.diagnostics.console.log, util.formatNumber, 1, !all),
+		metricRow('Console warnings', base, head, summary => summary.diagnostics.console.warn, sample => sample.diagnostics.console.warn, util.formatNumber, 1, !all),
+		metricRow('Console errors', base, head, summary => summary.diagnostics.console.error, sample => sample.diagnostics.console.error, util.formatNumber, 1, !all),
+		metricRow('Console info', base, head, summary => summary.diagnostics.console.info, sample => sample.diagnostics.console.info, util.formatNumber, 1, !all),
+	].filter(row => row != null);
+}
+
 function renderSummaryTable(base: BrowserMetricsReport, head: BrowserMetricsReport, all = false) {
 	const rows = [
+		...renderBrowserDiagnosticsRows(base, head, all),
 		//metricRow('Scenario duration', base, head, summary => summary.durationMs, sample => sample.durationMs, formatMs),
 		metricRow('Requests', base, head, summary => summary.network.requestCount, sample => sample.network.requestCount, util.formatNumber, 1, !all),
 		//metricRow('Failed requests', base, head, summary => summary.network.failedRequestCount, sample => sample.network.failedRequestCount, util.formatNumber),
diff --git a/.github/scripts/frontend-browser-report.test.mts b/.github/scripts/frontend-browser-report.test.mts
new file mode 100644
index 0000000000..4d4f130504
--- /dev/null
+++ b/.github/scripts/frontend-browser-report.test.mts
@@ -0,0 +1,98 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import {
+	createBrowserDiagnostics,
+	recordConsoleMessageType,
+	recordPageError,
+	summarizeBrowserDiagnostics,
+	type BrowserDiagnostics,
+} from './chrome.mts';
+import {
+	renderBrowserDiagnosticsRows,
+	type BrowserMeasurement,
+	type BrowserMeasurementSample,
+	type BrowserMetricsReport,
+} from './frontend-browser-report.mts';
+
+function diagnostics(pageErrorCount: number, log: number, warn: number, error: number, info: number): BrowserDiagnostics {
+	return {
+		pageErrorCount,
+		console: { log, warn, error, info },
+	};
+}
+
+function browserReport(label: 'base' | 'head', values: BrowserDiagnostics[]): BrowserMetricsReport {
+	const samples = values.map((value, index) => ({
+		round: index + 1,
+		diagnostics: value,
+	} as BrowserMeasurementSample));
+
+	return {
+		label,
+		timestamp: '2026-07-16T00:00:00.000Z',
+		url: 'http://127.0.0.1:61812',
+		scenario: 'test',
+		sampleCount: samples.length,
+		aggregation: 'median',
+		summary: {
+			diagnostics: summarizeBrowserDiagnostics(values),
+		} as BrowserMeasurement,
+		samples,
+	};
+}
+
+test('records page errors and only the requested console message types', () => {
+	const result = createBrowserDiagnostics();
+	recordPageError(result);
+	recordPageError(result);
+	for (const type of ['log', 'warning', 'error', 'info', 'debug', 'trace']) {
+		recordConsoleMessageType(result, type);
+	}
+
+	assert.deepEqual(result, diagnostics(2, 1, 1, 1, 1));
+});
+
+test('summarizes browser diagnostics with a median for every counter', () => {
+	const result = summarizeBrowserDiagnostics([
+		diagnostics(0, 10, 20, 30, 40),
+		diagnostics(4, 14, 24, 34, 44),
+		diagnostics(2, 12, 22, 32, 42),
+		diagnostics(3, 13, 23, 33, 43),
+		diagnostics(1, 11, 21, 31, 41),
+	]);
+
+	assert.deepEqual(result, diagnostics(2, 12, 22, 32, 42));
+});
+
+test('renders each changed browser diagnostics metric as a separate row', () => {
+	const base = browserReport('base', Array.from({ length: 5 }, () => diagnostics(0, 0, 0, 0, 0)));
+	const head = browserReport('head', Array.from({ length: 5 }, () => diagnostics(1, 1, 1, 1, 1)));
+	const rows = renderBrowserDiagnosticsRows(base, head).join('\n');
+
+	for (const label of ['Page errors', 'Console log', 'Console warnings', 'Console errors', 'Console info']) {
+		assert.match(rows, new RegExp(`\\*\\*${label}\\*\\*`));
+	}
+});
+
+test('renders browser diagnostics deltas from paired rounds instead of summary medians', () => {
+	const base = browserReport('base', [0, 9, 10, 100, 101].map(value => diagnostics(value, 0, 0, 0, 0)));
+	const head = browserReport('head', [10, 100, 101, 0, 9].map(value => diagnostics(value, 0, 0, 0, 0)));
+	const rows = renderBrowserDiagnosticsRows(base, head);
+
+	assert.equal(rows.length, 1);
+	assert.match(rows[0], /\*\*Page errors\*\*/);
+	assert.match(rows[0], /\\text\{\+10\}/);
+});
+
+test('omits browser diagnostics rows whose paired median does not change', () => {
+	const values = Array.from({ length: 5 }, () => diagnostics(2, 3, 4, 5, 6));
+	const base = browserReport('base', values);
+	const head = browserReport('head', values);
+
+	assert.deepEqual(renderBrowserDiagnosticsRows(base, head), []);
+});
diff --git a/.github/scripts/measure-frontend-browser-comparison.mts b/.github/scripts/measure-frontend-browser-comparison.mts
index 0825133b1d..3e357c3396 100644
--- a/.github/scripts/measure-frontend-browser-comparison.mts
+++ b/.github/scripts/measure-frontend-browser-comparison.mts
@@ -7,7 +7,7 @@ import { copyFile, mkdir, rm, writeFile } from 'node:fs/promises';
 import { join, resolve } from 'node:path';
 import * as util from './utility.mts';
 import * as heapSnapshotUtil from './heap-snapshot-util.mts';
-import { HeadlessChromeController, summarizeNetwork } from './chrome.mts';
+import { HeadlessChromeController, summarizeBrowserDiagnostics, summarizeNetwork } from './chrome.mts';
 import type { BrowserMeasurement, NetworkRequest, NetworkSummary } from './chrome.mts';
 import { closeUserSetupDialog, postNote, signupThroughUi, visitHome } from '../../packages/frontend/test/e2e/shared.ts';
 
@@ -173,6 +173,7 @@ function summarizeSamples(label: 'base' | 'head', samples: BrowserMeasurementSam
 		timestamp: new Date().toISOString(),
 		url: baseUrl,
 		scenario: representative.scenario,
+		diagnostics: summarizeBrowserDiagnostics(samples.map(sample => sample.diagnostics)),
 		durationMs: finiteMedian(samples.map(sample => sample.durationMs)),
 		network: summarizeNetworkSamples(samples),
 		performance: summarizePerformanceSamples(samples),
@@ -210,6 +211,7 @@ async function measureSample(label: 'base' | 'head', round: number, heapSnapshot
 			timestamp: new Date().toISOString(),
 			url: baseUrl,
 			scenario: 'fresh browser signup, first timeline note, after the note becomes visible',
+			diagnostics: chrome.collectDiagnostics(),
 			durationMs,
 			network: summarizeNetwork(chrome.networkRequests, baseUrl, chrome.webSocketConnections),
 			networkRequests: chrome.networkRequests,
diff --git a/.github/workflows/lint.yml b/.github/workflows/lint.yml
index 63f8684c33..c1a3aeda6e 100644
--- a/.github/workflows/lint.yml
+++ b/.github/workflows/lint.yml
@@ -152,3 +152,4 @@ jobs:
         node-version-file: '.node-version'
     - run: node --test .github/scripts/frontend-js-size.test.mts
     - run: node --test .github/scripts/memory-stability-util.test.mts
+    - run: node --test .github/scripts/frontend-browser-report.test.mts