diff --git a/CHANGELOG.md b/CHANGELOG.md
index 01213bd8c3..b2472823b8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,7 @@
 - Enhance: テーマをドラッグ&ドロップできるように
 - Enhance: 絵文字ピッカーのサイズをより大きくできるように
 - Enhance: 時刻計算のための基準値を一か所で管理するようにし、パフォーマンスを向上
+- Enhance: 「お問い合わせ」ページから、バグの調査等に役立つ情報（OSやブラウザのバージョン等）を取得・コピーできるように
 - Fix: iOSで、デバイスがダークモードだと初回読み込み時にエラーになる問題を修正
 - Fix: アクティビティウィジェットのグラフモードが動作しない問題を修正
 
diff --git a/locales/index.d.ts b/locales/index.d.ts
index fbe8c30c94..e2a239ae7b 100644
--- a/locales/index.d.ts
+++ b/locales/index.d.ts
@@ -5577,6 +5577,14 @@ export interface Locale extends ILocale {
      * 予約
      */
     "scheduled": string;
+    /**
+     * デバイス情報
+     */
+    "deviceInfo": string;
+    /**
+     * 技術的なお問い合わせの際に、以下の情報を併記すると問題の解決に役立つことがあります。
+     */
+    "deviceInfoDescription": string;
     "_compression": {
         "_quality": {
             /**
diff --git a/locales/ja-JP.yml b/locales/ja-JP.yml
index d21df5d5e1..ed8d8033d9 100644
--- a/locales/ja-JP.yml
+++ b/locales/ja-JP.yml
@@ -1389,6 +1389,8 @@ scheduleToPostOnX: "{x}に投稿を予約します"
 scheduledToPostOnX: "{x}に投稿が予約されています"
 schedule: "予約"
 scheduled: "予約"
+deviceInfo: "デバイス情報"
+deviceInfoDescription: "技術的なお問い合わせの際に、以下の情報を併記すると問題の解決に役立つことがあります。"
 
 _compression:
   _quality:
diff --git a/packages/frontend/src/components/MkCode.core.vue b/packages/frontend/src/components/MkCode.core.vue
index 8b39468d4c..f669e4b87a 100644
--- a/packages/frontend/src/components/MkCode.core.vue
+++ b/packages/frontend/src/components/MkCode.core.vue
@@ -5,7 +5,13 @@ SPDX-License-Identifier: AGPL-3.0-only
 
 <!-- eslint-disable vue/no-v-html -->
 <template>
-<div :class="[$style.codeBlockRoot, { [$style.codeEditor]: codeEditor }, (darkMode ? $style.dark : $style.light)]" v-html="html"></div>
+<div
+	:class="[$style.codeBlockRoot, {
+		[$style.codeEditor]: codeEditor,
+		[$style.outerStyle]: !codeEditor && withOuterStyle,
+		[$style.dark]: darkMode,
+		[$style.light]: !darkMode,
+	}]" v-html="html"></div>
 </template>
 
 <script lang="ts" setup>
@@ -15,11 +21,15 @@ import type { BundledLanguage } from 'shiki/langs';
 import { getHighlighter, getTheme } from '@/utility/code-highlighter.js';
 import { store } from '@/store.js';
 
-const props = defineProps<{
+const props = withDefaults(defineProps<{
 	code: string;
 	lang?: string;
 	codeEditor?: boolean;
-}>();
+	withOuterStyle?: boolean;
+}>(), {
+	codeEditor: false,
+	withOuterStyle: true,
+});
 
 const highlighter = await getHighlighter();
 const darkMode = store.r.darkMode;
@@ -73,17 +83,13 @@ watch(() => props.lang, (to) => {
 
 <style module lang="scss">
 .codeBlockRoot :global(.shiki) {
-	padding: 1em;
-	margin: 0;
 	overflow: auto;
 	font-family: Consolas, Monaco, Andale Mono, Ubuntu Mono, monospace;
 
 	color: var(--shiki-fallback);
-	background-color: var(--shiki-fallback-bg);
 
 	& span {
 		color: var(--shiki-fallback);
-		background-color: var(--shiki-fallback-bg);
 	}
 
 	& pre,
@@ -92,26 +98,40 @@ watch(() => props.lang, (to) => {
 	}
 }
 
+.outerStyle.codeBlockRoot :global(.shiki) {
+	padding: 1em;
+	margin: 0;
+	border-radius: 8px;
+	border: 1px solid var(--MI_THEME-divider);
+	background-color: var(--shiki-fallback-bg);
+}
+
 .light.codeBlockRoot :global(.shiki) {
 	color: var(--shiki-light);
-	background-color: var(--shiki-light-bg);
 
 	& span {
 		color: var(--shiki-light);
-		background-color: var(--shiki-light-bg);
 	}
 }
 
+.light.outerStyle.codeBlockRoot :global(.shiki),
+.light.codeEditor.codeBlockRoot :global(.shiki) {
+	background-color: var(--shiki-light-bg);
+}
+
 .dark.codeBlockRoot :global(.shiki) {
 	color: var(--shiki-dark);
-	background-color: var(--shiki-dark-bg);
 
 	& span {
 		color: var(--shiki-dark);
-		background-color: var(--shiki-dark-bg);
 	}
 }
 
+.dark.outerStyle.codeBlockRoot :global(.shiki),
+.dark.codeEditor.codeBlockRoot :global(.shiki) {
+	background-color: var(--shiki-dark-bg);
+}
+
 .codeBlockRoot.codeEditor {
 	min-width: 100%;
 	height: 100%;
diff --git a/packages/frontend/src/components/MkCode.vue b/packages/frontend/src/components/MkCode.vue
index f41cb0d00b..f43035f0e8 100644
--- a/packages/frontend/src/components/MkCode.vue
+++ b/packages/frontend/src/components/MkCode.vue
@@ -5,15 +5,32 @@ SPDX-License-Identifier: AGPL-3.0-only
 
 <template>
 <div :class="$style.codeBlockRoot">
-	<button v-if="copyButton" :class="$style.codeBlockCopyButton" class="_button" @click="copy">
+	<button v-if="copyButton" :class="[$style.codeBlockCopyButton, { [$style.withOuterStyle]: withOuterStyle }]" class="_button" @click="copy">
 		<i class="ti ti-copy"></i>
 	</button>
 	<Suspense>
 		<template #fallback>
-			<MkLoading/>
+			<pre
+				class="_selectable"
+				:class="[$style.codeBlockFallbackRoot, {
+					[$style.outerStyle]: withOuterStyle,
+				}]"
+			><code :class="$style.codeBlockFallbackCode">Loading...</code></pre>
 		</template>
-		<XCode v-if="show && lang" class="_selectable" :code="code" :lang="lang"/>
-		<pre v-else-if="show" class="_selectable" :class="$style.codeBlockFallbackRoot"><code :class="$style.codeBlockFallbackCode">{{ code }}</code></pre>
+		<XCode
+			v-if="show && lang"
+			class="_selectable"
+			:code="code"
+			:lang="lang"
+			:withOuterStyle="withOuterStyle"
+		/>
+		<pre
+			v-else-if="show"
+			class="_selectable"
+			:class="[$style.codeBlockFallbackRoot, {
+				[$style.outerStyle]: withOuterStyle,
+			}]"
+		><code :class="$style.codeBlockFallbackCode">{{ code }}</code></pre>
 		<button v-else :class="$style.codePlaceholderRoot" @click="show = true">
 			<div :class="$style.codePlaceholderContainer">
 				<div><i class="ti ti-code"></i> {{ i18n.ts.code }}</div>
@@ -26,8 +43,6 @@ SPDX-License-Identifier: AGPL-3.0-only
 
 <script lang="ts" setup>
 import { defineAsyncComponent, ref } from 'vue';
-import * as os from '@/os.js';
-import MkLoading from '@/components/global/MkLoading.vue';
 import { i18n } from '@/i18n.js';
 import { copyToClipboard } from '@/utility/copy-to-clipboard.js';
 import { prefer } from '@/preferences.js';
@@ -36,10 +51,12 @@ const props = withDefaults(defineProps<{
 	code: string;
 	forceShow?: boolean;
 	copyButton?: boolean;
+	withOuterStyle?: boolean;
 	lang?: string;
 }>(), {
 	copyButton: true,
 	forceShow: false,
+	withOuterStyle: true,
 });
 
 const show = ref(props.forceShow === true ? true : !prefer.s.dataSaver.code);
@@ -58,10 +75,16 @@ function copy() {
 
 .codeBlockCopyButton {
 	position: absolute;
-	top: 8px;
-	right: 8px;
 	opacity: 0.5;
 
+	top: 0;
+	right: 0;
+
+	&.withOuterStyle {
+		top: 8px;
+		right: 8px;
+	}
+
 	&:hover {
 		opacity: 0.8;
 	}
@@ -70,11 +93,17 @@ function copy() {
 .codeBlockFallbackRoot {
 	display: block;
 	overflow-wrap: anywhere;
-	padding: 1em;
-	margin: 0;
 	overflow: auto;
 }
 
+.outerStyle.codeBlockFallbackRoot {
+	background: var(--MI_THEME-bg);
+	padding: 1em;
+	margin: .5em 0;
+	border-radius: 8px;
+	border: 1px solid var(--MI_THEME-divider);
+}
+
 .codeBlockFallbackCode {
 	font-family: Consolas, Monaco, Andale Mono, Ubuntu Mono, monospace;
 }
diff --git a/packages/frontend/src/components/MkFolder.vue b/packages/frontend/src/components/MkFolder.vue
index 9f5bc8da6c..41134d7d7e 100644
--- a/packages/frontend/src/components/MkFolder.vue
+++ b/packages/frontend/src/components/MkFolder.vue
@@ -119,6 +119,10 @@ const props = withDefaults(defineProps<{
 	canPage: true,
 });
 
+const emit = defineEmits<{
+	(ev: 'headerClicked', ep: MouseEvent): void;
+}>();
+
 const rootEl = useTemplateRef('rootEl');
 const asPage = props.canPage && deviceKind === 'smartphone' && prefer.s['experimental.enableFolderPageView'];
 const bgSame = ref(false);
@@ -164,7 +168,9 @@ function afterLeave(el: Element) {
 let pageId = pageFolderTeleportCount.value;
 pageFolderTeleportCount.value += 1000;
 
-async function toggle() {
+async function toggle(ev: MouseEvent) {
+	emit('headerClicked', ev);
+
 	if (asPage && !opened.value) {
 		pageId++;
 		const { dispose } = await popup(MkFolderPage, {
diff --git a/packages/frontend/src/pages/contact.vue b/packages/frontend/src/pages/contact.vue
index eb94f23ac9..d225cb9e87 100644
--- a/packages/frontend/src/pages/contact.vue
+++ b/packages/frontend/src/pages/contact.vue
@@ -28,17 +28,37 @@ SPDX-License-Identifier: AGPL-3.0-only
 					<span v-else style="opacity: 0.7;">({{ i18n.ts.none }})</span>
 				</template>
 			</MkKeyValue>
+			<MkFolder @headerClicked="onHeaderClicked">
+				<template #icon><i class="ti ti-report-search"></i></template>
+				<template #label>{{ i18n.ts.deviceInfo }}</template>
+				<template #caption>{{ i18n.ts.deviceInfoDescription }}</template>
+				<MkLoading v-if="userEnv == null" />
+				<MkCode v-else lang="json" :code="JSON.stringify(userEnv, null, 2)" style="max-height: 300px; overflow: auto;"/>
+			</MkFolder>
 		</div>
 	</div>
 </PageWithHeader>
 </template>
 
 <script lang="ts" setup>
+import { ref } from 'vue';
 import { i18n } from '@/i18n.js';
 import { instance } from '@/instance.js';
 import { definePage } from '@/page.js';
+import { getUserEnvironment } from '@/utility/get-user-environment.js';
+import type { UserEnvironment } from '@/utility/get-user-environment.js';
 import MkKeyValue from '@/components/MkKeyValue.vue';
+import MkFolder from '@/components/MkFolder.vue';
 import MkLink from '@/components/MkLink.vue';
+import MkCode from '@/components/MkCode.vue';
+
+const userEnv = ref<UserEnvironment | null>(null);
+
+async function onHeaderClicked() {
+	if (userEnv.value == null) {
+		userEnv.value = await getUserEnvironment();
+	}
+}
 
 definePage(() => ({
 	title: i18n.ts.inquiry,
diff --git a/packages/frontend/src/utility/get-user-environment.ts b/packages/frontend/src/utility/get-user-environment.ts
new file mode 100644
index 0000000000..3b8d43fb2c
--- /dev/null
+++ b/packages/frontend/src/utility/get-user-environment.ts
@@ -0,0 +1,66 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+export type UserEnvironment = {
+	os: string;
+	browser: string;
+	userAgent: string;
+	screenWidth: number;
+	screenHeight: number;
+	viaGetHighEntropyValues: true;
+} | {
+	userAgent: string;
+	screenWidth: number;
+	screenHeight: number;
+	viaGetHighEntropyValues: false;
+};
+
+export async function getUserEnvironment(): Promise<UserEnvironment> {
+	if ('userAgentData' in navigator && navigator.userAgentData != null) {
+		try {
+			const uaData: any = await navigator.userAgentData.getHighEntropyValues([
+				'fullVersionList',
+				'platformVersion',
+			]);
+
+			let osVersion = 'v' + uaData.platformVersion;
+
+			if (uaData.platform === 'Windows' && uaData.platformVersion != null) {
+				// https://learn.microsoft.com/ja-jp/microsoft-edge/web-platform/how-to-detect-win11
+				const majorPlatformVersion = parseInt(uaData.platformVersion.split('.')[0]);
+				if (majorPlatformVersion >= 13) {
+					osVersion = '11 or later';
+				} else if (majorPlatformVersion > 0) {
+					osVersion = '10';
+				} else {
+					osVersion = '8.1 or earlier';
+				}
+			}
+
+			const browserData = uaData.fullVersionList.find((item) => !/^\s*not.+a.+brand\s*$/i.test(item.brand));
+			return {
+				os: `${uaData.platform} ${osVersion}`,
+				browser: browserData ? `${browserData.brand} v${browserData.version}` : 'Unknown',
+				userAgent: navigator.userAgent,
+				screenWidth: window.innerWidth,
+				screenHeight: window.innerHeight,
+				viaGetHighEntropyValues: true,
+			};
+		} catch {
+			return getViaUa();
+		}
+	} else {
+		return getViaUa();
+	}
+}
+
+function getViaUa(): UserEnvironment {
+	return {
+		userAgent: navigator.userAgent,
+		screenWidth: window.innerWidth,
+		screenHeight: window.innerHeight,
+		viaGetHighEntropyValues: false,
+	};
+}
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 8fa74ae74e..60efe3b63f 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -5552,8 +5552,8 @@ packages:
   boolbase@1.0.0:
     resolution: {integrity: sha512-JZOSA7Mo9sNGB8+UjSgzdLtokWAky1zbztM3WRLCbZ70/3cTANmQmOdR7y2g+J0e2WXywy1yS468tY+IruqEww==}
 
-  bowser@2.11.0:
-    resolution: {integrity: sha512-AlcaJBi/pqqJBIQ8U9Mcpc9i8Aqxn88Skv5d+xBX006BY5u8N3mGLHa5Lgppa7L/HfwgwLgZ6NYs+Ag6uUmJRA==}
+  bowser@2.12.1:
+    resolution: {integrity: sha512-z4rE2Gxh7tvshQ4hluIT7XcFrgLIQaw9X3A+kTTRdovCz5PMukm/0QC/BKSYPj3omF5Qfypn9O/c5kgpmvYUCw==}
 
   brace-expansion@1.1.11:
     resolution: {integrity: sha512-iCuPHDFgrHX7H2vEI/5xpz07zSHB00TpugqhmYtVmMO6518mCuRMoOYFldEBl0g187ufozdaHgWKcYFb61qGiA==}
@@ -12101,14 +12101,14 @@ snapshots:
     dependencies:
       '@aws-sdk/types': 3.862.0
       '@smithy/types': 4.4.0
-      bowser: 2.11.0
+      bowser: 2.12.1
       tslib: 2.8.1
 
   '@aws-sdk/util-user-agent-browser@3.893.0':
     dependencies:
       '@aws-sdk/types': 3.893.0
       '@smithy/types': 4.5.0
-      bowser: 2.11.0
+      bowser: 2.12.1
       tslib: 2.8.1
 
   '@aws-sdk/util-user-agent-node@3.873.0':
@@ -14668,7 +14668,7 @@ snapshots:
       '@smithy/property-provider': 4.1.0
       '@smithy/smithy-client': 4.6.0
       '@smithy/types': 4.4.0
-      bowser: 2.11.0
+      bowser: 2.12.1
       tslib: 2.8.1
 
   '@smithy/util-defaults-mode-browser@4.1.4':
@@ -14676,7 +14676,7 @@ snapshots:
       '@smithy/property-provider': 4.1.1
       '@smithy/smithy-client': 4.6.4
       '@smithy/types': 4.5.0
-      bowser: 2.11.0
+      bowser: 2.12.1
       tslib: 2.8.1
 
   '@smithy/util-defaults-mode-node@4.1.0':
@@ -15744,7 +15744,7 @@ snapshots:
       '@typescript-eslint/types': 8.44.1
       '@typescript-eslint/typescript-estree': 8.44.1(typescript@5.9.2)
       '@typescript-eslint/visitor-keys': 8.44.1
-      debug: 4.4.1(supports-color@10.2.0)
+      debug: 4.4.1(supports-color@5.5.0)
       eslint: 9.36.0
       typescript: 5.9.2
     transitivePeerDependencies:
@@ -15773,7 +15773,7 @@ snapshots:
       '@typescript-eslint/types': 8.44.1
       '@typescript-eslint/typescript-estree': 8.44.1(typescript@5.9.2)
       '@typescript-eslint/utils': 8.44.1(eslint@9.36.0)(typescript@5.9.2)
-      debug: 4.4.1(supports-color@10.2.0)
+      debug: 4.4.1(supports-color@5.5.0)
       eslint: 9.36.0
       ts-api-utils: 2.1.0(typescript@5.9.2)
       typescript: 5.9.2
@@ -15788,7 +15788,7 @@ snapshots:
       '@typescript-eslint/tsconfig-utils': 8.44.1(typescript@5.9.2)
       '@typescript-eslint/types': 8.44.1
       '@typescript-eslint/visitor-keys': 8.44.1
-      debug: 4.4.1(supports-color@10.2.0)
+      debug: 4.4.1(supports-color@5.5.0)
       fast-glob: 3.3.3
       is-glob: 4.0.3
       minimatch: 9.0.5
@@ -15827,7 +15827,7 @@ snapshots:
       '@ampproject/remapping': 2.3.0
       '@bcoe/v8-coverage': 1.0.2
       ast-v8-to-istanbul: 0.3.3
-      debug: 4.4.1(supports-color@10.2.0)
+      debug: 4.4.1(supports-color@5.5.0)
       istanbul-lib-coverage: 3.2.2
       istanbul-lib-report: 3.0.1
       istanbul-lib-source-maps: 5.0.6
@@ -16578,7 +16578,7 @@ snapshots:
 
   boolbase@1.0.0: {}
 
-  bowser@2.11.0: {}
+  bowser@2.12.1: {}
 
   brace-expansion@1.1.11:
     dependencies:
@@ -17657,7 +17657,7 @@ snapshots:
 
   esbuild-register@3.5.0(esbuild@0.25.10):
     dependencies:
-      debug: 4.4.1(supports-color@10.2.0)
+      debug: 4.4.1(supports-color@5.5.0)
       esbuild: 0.25.10
     transitivePeerDependencies:
       - supports-color
@@ -18633,7 +18633,7 @@ snapshots:
   http-proxy-agent@7.0.2:
     dependencies:
       agent-base: 7.1.3
-      debug: 4.4.1(supports-color@10.2.0)
+      debug: 4.4.3
     transitivePeerDependencies:
       - supports-color
 
@@ -18666,6 +18666,13 @@ snapshots:
       - supports-color
     optional: true
 
+  https-proxy-agent@7.0.6:
+    dependencies:
+      agent-base: 7.1.3
+      debug: 4.4.3
+    transitivePeerDependencies:
+      - supports-color
+
   https-proxy-agent@7.0.6(supports-color@10.2.0):
     dependencies:
       agent-base: 7.1.3
@@ -19008,7 +19015,7 @@ snapshots:
   istanbul-lib-source-maps@5.0.6:
     dependencies:
       '@jridgewell/trace-mapping': 0.3.29
-      debug: 4.4.1(supports-color@10.2.0)
+      debug: 4.4.1(supports-color@5.5.0)
       istanbul-lib-coverage: 3.2.2
     transitivePeerDependencies:
       - supports-color
@@ -19397,7 +19404,7 @@ snapshots:
       decimal.js: 10.5.0
       html-encoding-sniffer: 4.0.0
       http-proxy-agent: 7.0.2
-      https-proxy-agent: 7.0.6(supports-color@10.2.0)
+      https-proxy-agent: 7.0.6
       is-potential-custom-element-name: 1.0.1
       nwsapi: 2.2.16
       parse5: 7.3.0
@@ -22717,7 +22724,7 @@ snapshots:
   vite-node@3.2.4(@types/node@22.18.6)(sass@1.93.2)(terser@5.44.0)(tsx@4.20.6):
     dependencies:
       cac: 6.7.14
-      debug: 4.4.1(supports-color@10.2.0)
+      debug: 4.4.1(supports-color@5.5.0)
       es-module-lexer: 1.7.0
       pathe: 2.0.3
       vite: 7.1.7(@types/node@22.18.6)(sass@1.93.2)(terser@5.44.0)(tsx@4.20.6)
@@ -22773,7 +22780,7 @@ snapshots:
       '@vitest/spy': 3.2.4
       '@vitest/utils': 3.2.4
       chai: 5.2.0
-      debug: 4.4.1(supports-color@10.2.0)
+      debug: 4.4.1(supports-color@5.5.0)
       expect-type: 1.2.1
       magic-string: 0.30.19
       pathe: 2.0.3
@@ -22861,7 +22868,7 @@ snapshots:
 
   vue-eslint-parser@10.2.0(eslint@9.36.0):
     dependencies:
-      debug: 4.4.1(supports-color@10.2.0)
+      debug: 4.4.1(supports-color@5.5.0)
       eslint: 9.36.0
       eslint-scope: 8.4.0
       eslint-visitor-keys: 4.2.1