diff --git a/src/core.ts b/src/core.ts
index 39c320149f..401e5da18b 100644
--- a/src/core.ts
+++ b/src/core.ts
@@ -938,7 +938,14 @@ export class ProcessOutput extends Error {
   }
 
   buffer(): Buffer {
-    return Buffer.from(this.stdall)
+    // Reconstruct from the raw store chunks. `this.stdall` is a UTF-8 decoded
+    // string, so building a Buffer from it corrupts any non-UTF-8 output
+    // (each invalid byte becomes U+FFFD). The original chunks are lossless.
+    return Buffer.concat(
+      [...this._dto.store.stdall].map((chunk) =>
+        isString(chunk) ? Buffer.from(chunk) : chunk
+      )
+    )
   }
 
   blob(type = 'text/plain'): Blob {
diff --git a/test/core.test.js b/test/core.test.js
index b80a2eda91..0ffcca45b0 100644
--- a/test/core.test.js
+++ b/test/core.test.js
@@ -1526,6 +1526,15 @@ describe('core', () => {
       assert.equal(o.buffer().compare(Buffer.from('foo\n', 'utf-8')), 0)
     })
 
+    test('buffer() preserves non-utf8 bytes', async () => {
+      const bytes = Buffer.from([0xff, 0xfe, 0x81, 0x82]) // invalid utf-8
+      const o = new ProcessOutput({
+        store: { stdall: [bytes], stdout: [bytes], stderr: [] },
+      })
+      assert.equal(o.buffer().toString('hex'), 'fffe8182')
+      assert.equal(o.text('hex'), 'fffe8182')
+    })
+
     test('blob()', async () => {
       const o = new ProcessOutput(null, null, '', '', 'foo\n')
       assert.equal(await o.blob().text(), 'foo\n')