diff --git a/packages/backend/test-federation/test/api/ap/show.test.ts b/packages/backend/test-federation/test/api/ap/show.test.ts
new file mode 100644
index 0000000000..eb949754d7
--- /dev/null
+++ b/packages/backend/test-federation/test/api/ap/show.test.ts
@@ -0,0 +1,55 @@
+import { strictEqual, rejects } from 'node:assert';
+import * as Misskey from 'misskey-js';
+import { createAccount, resolveRemoteUser, sleep, type LoginUser } from '../../utils.js';
+
+describe('API ap/show', () => {
+	let alice: LoginUser, bob: LoginUser;
+
+	beforeAll(async () => {
+		[alice, bob] = await Promise.all([
+			createAccount('a.test'),
+			createAccount('b.test'),
+		]);
+	});
+
+	describe('User resolution', () => {
+		test('resolve by canonical user URL (https://b.test/users/:id)', async () => {
+			const res = await alice.client.request('ap/show', { uri: `https://b.test/users/${bob.id}` });
+			strictEqual(res.type, 'User');
+			strictEqual(res.object.uri, `https://b.test/users/${bob.id}`);
+		});
+
+		test('resolve by user profile URL (https://b.test/@bob)', async () => {
+			const res = await alice.client.request('ap/show', { uri: `https://b.test/@${bob.username}` });
+			strictEqual(res.type, 'User');
+			strictEqual(res.object.uri, `https://b.test/users/${bob.id}`);
+		});
+
+		test('resolve local user by local profile url', async () => {
+			const res = await alice.client.request('ap/show', { uri: `https://a.test/@${alice.username}` });
+			strictEqual(res.type, 'User');
+			strictEqual(res.object.id, alice.id);
+		});
+
+		test('resolve remote user by local profile URL (https://a.test/@bob@b.test)', async () => {
+			const res = await alice.client.request('ap/show', { uri: `https://a.test/@${bob.username}@b.test` });
+			strictEqual(res.type, 'User');
+			strictEqual(res.object.uri, `https://b.test/users/${bob.id}`);
+		});
+	});
+
+	describe('Note resolution', () => {
+		test('resolve by note URL (https://b.test/notes/:id)', async () => {
+			const note = (await bob.client.request('notes/create', { text: 'hello from Bob' })).createdNote;
+			// 伝搬待ち
+			await sleep();
+
+			const res = await alice.client.request('ap/show', { uri: `https://b.test/notes/${note.id}` });
+			strictEqual(res.type, 'Note');
+			strictEqual(res.object.uri, `https://b.test/notes/${note.id}`);
+			// 投稿者が a.test 側で解決済みの Bob になること
+			strictEqual(res.object.user.username, bob.username);
+			strictEqual(res.object.user.host, 'b.test');
+		});
+	});
+});
diff --git a/packages/backend/test-federation/test/user.test.ts b/packages/backend/test-federation/test/user.test.ts
index ebbe9ff5ba..3b9722975d 100644
--- a/packages/backend/test-federation/test/user.test.ts
+++ b/packages/backend/test-federation/test/user.test.ts
@@ -516,7 +516,7 @@ describe('User', () => {
 				await rejects(
 					async () => await resolveRemoteUser('a.test', alice.id, bob),
 					(err: any) => {
-						strictEqual(err.code, 'INTERNAL_ERROR');
+						strictEqual(err.code, 'SOMETHING_HAPPENED_IN_FETCHING_URI');
 						return true;
 					},
 				);
@@ -551,7 +551,7 @@ describe('User', () => {
 				await rejects(
 					async () => await resolveRemoteUser('a.test', alice.id, bob),
 					(err: any) => {
-						strictEqual(err.code, 'INTERNAL_ERROR');
+						strictEqual(err.code, 'SOMETHING_HAPPENED_IN_FETCHING_URI');
 						return true;
 					},
 				);
diff --git a/packages/misskey-js/etc/misskey-js.api.md b/packages/misskey-js/etc/misskey-js.api.md
index 3388a3d9a9..423b3be916 100644
--- a/packages/misskey-js/etc/misskey-js.api.md
+++ b/packages/misskey-js/etc/misskey-js.api.md
@@ -23,9 +23,13 @@ export type Acct = {
 
 declare namespace acct {
     export {
+        correctAcct,
         parse,
+        parseUrl,
+        parseAcctOrUrl,
         toString_2 as toString,
-        Acct
+        Acct,
+        UrlIsNotAcctLikeError
     }
 }
 export { acct }
@@ -1229,6 +1233,9 @@ type ClipsUpdateRequest = operations['clips___update']['requestBody']['content']
 // @public (undocumented)
 type ClipsUpdateResponse = operations['clips___update']['responses']['200']['content']['application/json'];
 
+// @public (undocumented)
+function correctAcct(acct: Acct, localHostname?: string): Acct;
+
 // @public (undocumented)
 type DateString = string;
 
@@ -3261,7 +3268,13 @@ type PagesUnlikeRequest = operations['pages___unlike']['requestBody']['content']
 type PagesUpdateRequest = operations['pages___update']['requestBody']['content']['application/json'];
 
 // @public (undocumented)
-function parse(_acct: string): Acct;
+function parse(acct: string): Acct;
+
+// @public (undocumented)
+function parseAcctOrUrl(acct: string, localHostname?: string): Acct;
+
+// @public (undocumented)
+function parseUrl(str: string): Acct;
 
 // Warning: (ae-forgotten-export) The symbol "Values" needs to be exported by the entry point index.d.ts
 //
@@ -3628,6 +3641,11 @@ type TestResponse = operations['test']['responses']['200']['content']['applicati
 // @public (undocumented)
 function toString_2(acct: Acct): string;
 
+// @public (undocumented)
+class UrlIsNotAcctLikeError extends Error {
+    constructor();
+}
+
 // @public (undocumented)
 type User = components['schemas']['User'];
 
diff --git a/packages/misskey-js/src/acct.ts b/packages/misskey-js/src/acct.ts
index aa8658cdbd..06dcd6fa2c 100644
--- a/packages/misskey-js/src/acct.ts
+++ b/packages/misskey-js/src/acct.ts
@@ -3,13 +3,77 @@ export type Acct = {
 	host: string | null;
 };
 
-export function parse(_acct: string): Acct {
-	let acct = _acct;
-	if (acct.startsWith('@')) acct = acct.substring(1);
-	const split = acct.split('@', 2);
+export function correctAcct(acct: Acct, localHostname?: string): Acct {
+	const result = { ...acct };
+	if (!acct.host) {
+		result.host = null;
+	} else if (localHostname && acct.host === localHostname) {
+		result.host = null;
+	}
+	return result;
+}
+
+export function parse(acct: string): Acct {
+	let acctWithNoPrefix = acct;
+
+	if (acct.startsWith('@')) {
+		acctWithNoPrefix = acct.substring(1);
+	} else if (acct.startsWith('acct:')) {
+		acctWithNoPrefix = acct.substring(5);
+	}
+
+	const split = acctWithNoPrefix.split('@', 2);
+
 	return { username: split[0], host: split[1] || null };
 }
 
+export class UrlIsNotAcctLikeError extends Error {
+	constructor() {
+		super('This url is not acct like.');
+	}
+}
+
+/**
+ * Only supports `https?://example.com/@username` or `https?://example.com/@username@other.example.com`
+ */
+export function parseUrl(str: string): Acct {
+	const url = new URL(str);
+
+	if (url.hash.length > 0) throw new UrlIsNotAcctLikeError();
+	if (url.search.length > 0) throw new UrlIsNotAcctLikeError();
+
+	const splited = url.pathname.split('/');
+	const path = splited[1];
+
+	if (!path) throw new UrlIsNotAcctLikeError();
+	if (!path.startsWith('@')) throw new UrlIsNotAcctLikeError();
+	if (path.length <= 1) throw new UrlIsNotAcctLikeError();
+
+	const split = path.split('@', 3); // ['', 'username', 'other.example.com']
+
+	return { username: split[1], host: split[2] || url.hostname };
+}
+
+/**
+ * Parses a string and returns an Acct object.
+ * @param acct String to parse
+ *   The string should be in one of the following formats:
+ *   * At-mark style: `@username@example.com`
+ *   * Starts with `acct:`: `acct:username@example.com`
+ *   * URL style: `https://example.com/@username`, `https://self.example.com/@username@other.example.com`
+ * @param localHostname If provided and matches the host found in acct, the returned `host` will be set to `null`.
+ * @returns Acct object
+ */
+export function parseAcctOrUrl(acct: string, localHostname?: string): Acct {
+	if (acct.startsWith('https://') || acct.startsWith('http://')) {
+		// url style
+		return correctAcct(parseUrl(acct), localHostname);
+	}
+
+	// acct style
+	return correctAcct(parse(acct), localHostname);
+}
+
 export function toString(acct: Acct): string {
 	return acct.host == null ? acct.username : `${acct.username}@${acct.host}`;
 }
diff --git a/packages/misskey-js/src/autogen/types.ts b/packages/misskey-js/src/autogen/types.ts
index d92ef599f3..da04e975d6 100644
--- a/packages/misskey-js/src/autogen/types.ts
+++ b/packages/misskey-js/src/autogen/types.ts
@@ -13554,6 +13554,7 @@ export interface operations {
             content: {
                 'application/json': {
                     uri: string;
+                    onlyUriFetch?: boolean;
                 };
             };
         };
diff --git a/packages/misskey-js/test/acct.ts b/packages/misskey-js/test/acct.ts
new file mode 100644
index 0000000000..4200e593de
--- /dev/null
+++ b/packages/misskey-js/test/acct.ts
@@ -0,0 +1,130 @@
+import { describe, it, expect } from 'vitest';
+import * as acct from '../src/acct.js';
+
+function testParseAcct(fn: (acct: string) => acct.Acct) {
+	it('parses plain username', () => {
+		const res = fn('alice');
+		expect(res).toEqual({ username: 'alice', host: null });
+	});
+
+	it('parses at-mark style without host', () => {
+		const res = fn('@alice');
+		expect(res).toEqual({ username: 'alice', host: null });
+	});
+
+	it('parses at-mark style with host', () => {
+		const res = fn('@alice@example.com');
+		expect(res).toEqual({ username: 'alice', host: 'example.com' });
+	});
+
+	it('parses acct: style', () => {
+		const res = fn('acct:alice@example.com');
+		expect(res).toEqual({ username: 'alice', host: 'example.com' });
+	});
+
+	it('parse Mr.http', () => {
+		const res = fn('http');
+		expect(res).toEqual({ username: 'http', host: null });
+	});
+}
+
+function testParseUrl(fn: (acct: string) => acct.Acct) {
+	it('parses url style https with same host -> host kept when localHostname not provided', () => {
+		const res = fn('https://example.com/@alice');
+		expect(res).toEqual({ username: 'alice', host: 'example.com' });
+	});
+
+	it('parses url style with remote host contained in path', () => {
+		const res = fn('https://self.example.com/@alice@other.example.com');
+		expect(res).toEqual({ username: 'alice', host: 'other.example.com' });
+	});
+
+	it('throws on non-acct-like url path (root)', () => {
+		expect(() => fn('https://example.com')).toThrowError();
+	});
+
+	it('throws on non-acct-like url path (users/alice)', () => {
+		expect(() => fn('https://example.com/users/alice')).toThrowError();
+	});
+
+	it('throws ended with @', () => {
+		expect(() => fn('https://example.com/@')).toThrowError();
+	});
+
+	it('parses url ended with /', () => {
+		const res = fn('https://example.com/@alice/');
+		expect(res).toEqual({ username: 'alice', host: 'example.com' });
+	});
+
+	it('throws url have @username path but ended with sub directory', () => {
+		expect(() => fn('https://example.com/@alice/subdir')).toThrowError();
+	});
+
+	it('throws url with search params', () => {
+		expect(() => fn('https://example.com/@alice?foo=bar')).toThrowError();
+	});
+
+	it('throws url with hash', () => {
+		expect(() => fn('https://example.com/@alice#fragment')).toThrowError();
+	});
+}
+
+describe('acct.parse', () => {
+	testParseAcct(acct.parse);
+});
+
+describe('acct.parseUrl', () => {
+	testParseUrl(acct.parseUrl);
+});
+
+describe('acct.parseAcctOrUrl', () => {
+	testParseAcct(acct.parseAcctOrUrl);
+	testParseUrl(acct.parseAcctOrUrl);
+
+	it('parse url with localHostname', () => {
+		const res = acct.parseAcctOrUrl('https://example.com/@alice', 'example.com');
+		expect(res).toEqual({ username: 'alice', host: null });
+	});
+
+	it('parse @username with localHostname', () => {
+		const res = acct.parseAcctOrUrl('@alice', 'example.com');
+		expect(res).toEqual({ username: 'alice', host: null });
+	});
+});
+
+describe('acct.correctAcct', () => {
+	it('returns host=null when acct.host is null', () => {
+		const input: acct.Acct = { username: 'alice', host: null };
+		const out = acct.correctAcct(input);
+		expect(out).toEqual({ username: 'alice', host: null });
+		expect(out).not.toBe(input); // immutability
+	});
+
+	it('keeps host when localHostname not provided', () => {
+		const input: acct.Acct = { username: 'bob', host: 'example.com' };
+		const out = acct.correctAcct(input);
+		expect(out).toEqual({ username: 'bob', host: 'example.com' });
+	});
+
+	it('nulls host when it matches localHostname', () => {
+		const input: acct.Acct = { username: 'carol', host: 'example.com' };
+		const out = acct.correctAcct(input, 'example.com');
+		expect(out).toEqual({ username: 'carol', host: null });
+	});
+
+	it('keeps host when it differs from localHostname', () => {
+		const input: acct.Acct = { username: 'dave', host: 'other.example.com' };
+		const out = acct.correctAcct(input, 'example.com');
+		expect(out).toEqual({ username: 'dave', host: 'other.example.com' });
+	});
+});
+
+describe('acct.toString', () => {
+	it('returns username when host is null', () => {
+		expect(acct.toString({ username: 'alice', host: null })).toBe('alice');
+	});
+
+	it('returns username@host when host exists', () => {
+		expect(acct.toString({ username: 'alice', host: 'example.com' })).toBe('alice@example.com');
+	});
+});