diff --git a/CHANGES.md b/CHANGES.md
index 7473a95..9a0b83e 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -9,6 +9,8 @@ but this file may sometimes contain later improvements (e.g. typo fixes).
 
 - Updated the link to the [User-Agent policy][]
   after it was moved from metawiki to foundationwiki.
+- Optimized the internal encoding of POST requests;
+  this also fixes [m3api-oauth2][] when targeting wikis running MediaWiki 1.43 or later.
 
 ## v0.8.2 (2024-04-09)
 
diff --git a/fetch.js b/fetch.js
index 2b2a1e2..5248116 100644
--- a/fetch.js
+++ b/fetch.js
@@ -44,14 +44,23 @@ class FetchSession extends Session {
 	async internalPost( apiUrl, urlParams, bodyParams, headers ) {
 		const url = new URL( apiUrl );
 		url.search = new URLSearchParams( urlParams );
-		const body = new FormData();
+		// try to send the body as application/x-www-form-urlencoded (URLSearchParams),
+		// as this is required for OAuth 2.0 and also shorter;
+		// fall back to multipart/form-data (FormData) if needed for e.g. file params
+		let body1 = new URLSearchParams();
+		const body2 = new FormData();
 		for ( const [ paramName, paramValue ] of Object.entries( bodyParams ) ) {
-			body.append( paramName, paramValue );
+			if ( body1 !== null && typeof paramValue === 'string' ) {
+				body1.append( paramName, paramValue );
+			} else {
+				body1 = null;
+			}
+			body2.append( paramName, paramValue );
 		}
 		const response = await fetch( url, {
 			...this.getFetchOptions( headers ),
 			method: 'POST',
-			body,
+			body: body1 !== null ? body1 : body2,
 		} );
 		return transformResponse( response );
 	}
diff --git a/test/unit/fetch.test.js b/test/unit/fetch.test.js
new file mode 100644
index 0000000..18350ce
--- /dev/null
+++ b/test/unit/fetch.test.js
@@ -0,0 +1,78 @@
+/* eslint-env mocha */
+
+import { set } from '../../core.js';
+import { FetchSession } from '../../fetch.js';
+import { expect } from 'chai';
+import { File } from 'buffer'; // only available globally since Node 20
+
+describe( 'FetchSession', () => {
+
+	let realFetch, mockFetchArguments;
+
+	before( function storeFetch() {
+		realFetch = global.fetch;
+	} );
+
+	beforeEach( function mockFetch() {
+		global.fetch = async function ( ...args ) {
+			mockFetchArguments = args;
+			return new Response( '{}' );
+		};
+	} );
+
+	afterEach( function unmockFetch() {
+		global.fetch = realFetch;
+		mockFetchArguments = undefined;
+	} );
+
+	const session = new FetchSession(
+		'en.wikipedia.org',
+		{},
+		{ userAgent: 'm3api-unit-test' },
+	);
+
+	describe( 'internalPost', () => {
+
+		it( 'uses URLSearchParams if possible', async () => {
+			await session.request( {
+				action: 'query',
+				meta: set( 'userinfo', 'siteinfo' ),
+				siprop: [ 'general', 'namespaces' ], // array instead of set just for coverage
+				curtimestamp: true,
+				formatversion: 2,
+			}, { method: 'POST' } );
+			expect( mockFetchArguments ).to.have.lengthOf( 2 );
+			expect( mockFetchArguments[ 0 ] ).to.be.an.instanceof( URL );
+			expect( mockFetchArguments[ 0 ].toString() ).to.equal(
+				'https://en.wikipedia.org/w/api.php?action=query',
+			);
+			expect( mockFetchArguments[ 1 ] ).to.have.deep.property( 'headers', {
+				'user-agent': session.getUserAgent( {} ),
+			} );
+			expect( mockFetchArguments[ 1 ] ).to.have.property( 'method', 'POST' );
+			expect( mockFetchArguments[ 1 ].body ).to.be.an.instanceof( URLSearchParams );
+			expect( mockFetchArguments[ 1 ].body.toString() ).to.equal(
+				new URLSearchParams( {
+					meta: 'userinfo|siteinfo',
+					siprop: 'general|namespaces',
+					curtimestamp: '',
+					formatversion: '2',
+					format: 'json',
+				} ).toString(),
+			);
+		} );
+
+		it( 'uses FormData if necessary', async () => {
+			const file = new File( [ '1' ], { type: 'text/plain' } );
+			await session.request( {
+				file,
+			}, { method: 'POST' } );
+			expect( mockFetchArguments[ 1 ].body ).to.be.an.instanceof( FormData );
+			expect( [ ...mockFetchArguments[ 1 ].body.keys() ] ).to.eql( [ 'file', 'format' ] );
+			expect( mockFetchArguments[ 1 ].body.getAll( 'format' ) ).to.eql( [ 'json' ] );
+			expect( mockFetchArguments[ 1 ].body.getAll( 'file' ) ).to.eql( [ file ] );
+		} );
+
+	} );
+
+} );