diff --git a/lib/services/extension-upload-service.js b/lib/services/extension-upload-service.js
index d7ca9fc..e8db5b9 100644
--- a/lib/services/extension-upload-service.js
+++ b/lib/services/extension-upload-service.js
@@ -1,6 +1,27 @@
 import fs from 'node:fs';
 
 import { parseSpaceIdFromInput } from '../utils/space-id.js';
+import { readZipEntryNames } from '../utils/zip-entries.js';
+
+/**
+ * The zip format requires "/" in entry names. Archives built with a "\"
+ * separator (for example by .NET's ZipFile in Windows PowerShell 5.1) are
+ * rejected by the API with only "Invalid extension package", so they are
+ * caught here with the cause and the fix.
+ */
+async function assertForwardSlashEntries(file) {
+  const names = await readZipEntryNames(file);
+  if (!names) return;
+  const offending = names.filter((name) => name.includes('\\'));
+  if (offending.length === 0) return;
+  const shown = offending.slice(0, 3).join(', ');
+  const more = offending.length > 3 ? ` and ${offending.length - 3} more` : '';
+  throw new Error(
+    `Extension package has entries with backslash paths (${shown}${more}). `
+    + 'Mantis rejects these as "Invalid extension package". '
+    + 'Re-create the archive with forward slashes ("/") in entry names, as the zip format requires.',
+  );
+}
 
 /** Uploads an extension package through the developer API. */
 export class ExtensionUploadService {
@@ -20,6 +41,7 @@ export class ExtensionUploadService {
     if (!fs.existsSync(file) || !fs.statSync(file).isFile()) {
       throw new Error(`Extension package not found: ${file}`);
     }
+    await assertForwardSlashEntries(file);
 
     const result = await this.client.installExtensionInSpace(resolvedSpaceId, file);
     const bundle = result.bundle;
diff --git a/lib/utils/zip-entries.js b/lib/utils/zip-entries.js
new file mode 100644
index 0000000..5f06a38
--- /dev/null
+++ b/lib/utils/zip-entries.js
@@ -0,0 +1,108 @@
+import fs from 'node:fs/promises';
+
+const EOCD_SIGNATURE = 0x06054b50;
+const CENTRAL_HEADER_SIGNATURE = 0x02014b50;
+const EOCD_SIZE = 22;
+const CENTRAL_HEADER_SIZE = 46;
+const MAX_COMMENT_SIZE = 0xffff;
+// The EOCD record and its trailing comment always fit in this many final bytes.
+const MAX_TAIL_SIZE = EOCD_SIZE + MAX_COMMENT_SIZE;
+
+/**
+ * Finds the end-of-central-directory record in `tail`, the last bytes of an
+ * archive of `archiveSize` bytes, and returns where its central directory is.
+ *
+ * A candidate signature only counts when its comment length reaches exactly
+ * to the end of the archive, so a "PK\x05\x06" inside a comment is skipped.
+ * Returns null for anything this reader does not handle: no EOCD, split
+ * archives, zip64 markers, or a directory that is not wholly before the EOCD.
+ */
+function locateCentralDirectory(tail, archiveSize) {
+  const tailStart = archiveSize - tail.length;
+  for (let i = tail.length - EOCD_SIZE; i >= 0; i--) {
+    if (tail.readUInt32LE(i) !== EOCD_SIGNATURE) continue;
+    const commentLength = tail.readUInt16LE(i + 20);
+    if (i + EOCD_SIZE + commentLength !== tail.length) continue;
+
+    const eocdOffset = tailStart + i;
+    const diskNumber = tail.readUInt16LE(i + 4);
+    const directoryDisk = tail.readUInt16LE(i + 6);
+    const entriesOnDisk = tail.readUInt16LE(i + 8);
+    const entryCount = tail.readUInt16LE(i + 10);
+    const size = tail.readUInt32LE(i + 12);
+    const offset = tail.readUInt32LE(i + 16);
+
+    if (diskNumber !== 0 || directoryDisk !== 0 || entriesOnDisk !== entryCount) return null;
+    if (entryCount === 0xffff || size === 0xffffffff || offset === 0xffffffff) return null;
+    if (offset + size > eocdOffset) return null;
+    return { offset, size, entryCount };
+  }
+  return null;
+}
+
+/**
+ * Reads entry names from a central directory buffer that holds exactly
+ * `entryCount` records. Returns null when any record, name, extra field or
+ * comment would run past the directory, or when bytes are left over.
+ */
+function parseCentralDirectory(directory, entryCount) {
+  const names = [];
+  let offset = 0;
+  for (let i = 0; i < entryCount; i++) {
+    if (offset + CENTRAL_HEADER_SIZE > directory.length) return null;
+    if (directory.readUInt32LE(offset) !== CENTRAL_HEADER_SIGNATURE) return null;
+    const nameLength = directory.readUInt16LE(offset + 28);
+    const extraLength = directory.readUInt16LE(offset + 30);
+    const commentLength = directory.readUInt16LE(offset + 32);
+    const nameStart = offset + CENTRAL_HEADER_SIZE;
+    const recordEnd = nameStart + nameLength + extraLength + commentLength;
+    if (recordEnd > directory.length) return null;
+    names.push(directory.toString('utf8', nameStart, nameStart + nameLength));
+    offset = recordEnd;
+  }
+  return offset === directory.length ? names : null;
+}
+
+/**
+ * Lists entry names from a complete zip archive held in memory.
+ *
+ * Returns null when the buffer is not a zip this reader handles (for example
+ * a JSON package, a zip64 or split archive, or a malformed directory), so
+ * callers can leave validation of those files to the server.
+ */
+export function listZipEntryNames(buffer) {
+  if (buffer.length < EOCD_SIZE) return null;
+  const tail = buffer.subarray(Math.max(0, buffer.length - MAX_TAIL_SIZE));
+  const directory = locateCentralDirectory(tail, buffer.length);
+  if (!directory) return null;
+  return parseCentralDirectory(
+    buffer.subarray(directory.offset, directory.offset + directory.size),
+    directory.entryCount,
+  );
+}
+
+/**
+ * Lists entry names from a zip archive on disk, reading only the final
+ * bytes that can hold the EOCD record and then the central directory itself.
+ * Returns null under the same conditions as listZipEntryNames.
+ */
+export async function readZipEntryNames(file) {
+  const handle = await fs.open(file, 'r');
+  try {
+    const { size } = await handle.stat();
+    if (size < EOCD_SIZE) return null;
+
+    const tailLength = Math.min(size, MAX_TAIL_SIZE);
+    const tail = Buffer.alloc(tailLength);
+    await handle.read(tail, 0, tailLength, size - tailLength);
+    const directory = locateCentralDirectory(tail, size);
+    if (!directory) return null;
+
+    const region = Buffer.alloc(directory.size);
+    const { bytesRead } = await handle.read(region, 0, directory.size, directory.offset);
+    if (bytesRead !== directory.size) return null;
+    return parseCentralDirectory(region, directory.entryCount);
+  } finally {
+    await handle.close();
+  }
+}
diff --git a/test/extension-upload-service.test.js b/test/extension-upload-service.test.js
index 3620aab..d9d7847 100644
--- a/test/extension-upload-service.test.js
+++ b/test/extension-upload-service.test.js
@@ -9,6 +9,49 @@ import { ToolService } from '../lib/services/tool-service.js';
 
 const SPACE_ID = '4c9beaf7-85db-4648-b5f6-bb2acdea48dd';
 
+// Minimal stored (uncompressed) zip with the given entry names. The CRC fields
+// are left at zero because only the entry names are read.
+function makeZip(names) {
+  const locals = [];
+  const centrals = [];
+  let offset = 0;
+  for (const name of names) {
+    const nameBuf = Buffer.from(name, 'utf8');
+    const data = Buffer.from('x');
+    const local = Buffer.alloc(30);
+    local.writeUInt32LE(0x04034b50, 0);
+    local.writeUInt32LE(data.length, 18);
+    local.writeUInt32LE(data.length, 22);
+    local.writeUInt16LE(nameBuf.length, 26);
+    const localBlock = Buffer.concat([local, nameBuf, data]);
+    const central = Buffer.alloc(46);
+    central.writeUInt32LE(0x02014b50, 0);
+    central.writeUInt32LE(data.length, 20);
+    central.writeUInt32LE(data.length, 24);
+    central.writeUInt16LE(nameBuf.length, 28);
+    central.writeUInt32LE(offset, 42);
+    locals.push(localBlock);
+    centrals.push(Buffer.concat([central, nameBuf]));
+    offset += localBlock.length;
+  }
+  const directory = Buffer.concat(centrals);
+  const eocd = Buffer.alloc(22);
+  eocd.writeUInt32LE(0x06054b50, 0);
+  eocd.writeUInt16LE(names.length, 8);
+  eocd.writeUInt16LE(names.length, 10);
+  eocd.writeUInt32LE(directory.length, 12);
+  eocd.writeUInt32LE(offset, 16);
+  return Buffer.concat([...locals, directory, eocd]);
+}
+
+function writeTempPackage(t, name, contents) {
+  const tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'mantis-extension-test-'));
+  t.after(() => fs.rmSync(tempDir, { recursive: true, force: true }));
+  const file = path.join(tempDir, name);
+  fs.writeFileSync(file, contents);
+  return file;
+}
+
 test('installs an extension package into the requested space', async (t) => {
   const tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'mantis-extension-test-'));
   t.after(() => fs.rmSync(tempDir, { recursive: true, force: true }));
@@ -86,3 +129,48 @@ test('rejects a missing package before making an API request', async () => {
     /Extension package not found/,
   );
 });
+
+test('rejects a zip with backslash entry names before making an API request', async (t) => {
+  const file = writeTempPackage(
+    t,
+    'windows.mantisx',
+    makeZip(['mantis.extension.json', 'extension.js', 'panel\\main.js']),
+  );
+  const service = new ExtensionUploadService({
+    configStore: { requireAuth: () => ({ apiKey: 'test' }) },
+    client: {
+      installExtensionInSpace: async () => assert.fail('client should not be called'),
+    },
+  });
+
+  await assert.rejects(
+    service.install(file, { spaceId: SPACE_ID }),
+    (err) => {
+      assert.match(err.message, /backslash paths \(panel\\main\.js\)/);
+      assert.match(err.message, /forward slashes/);
+      return true;
+    },
+  );
+});
+
+test('uploads a zip whose entry names use forward slashes', async (t) => {
+  const file = writeTempPackage(
+    t,
+    'portable.mantisx',
+    makeZip(['mantis.extension.json', 'extension.js', 'panel/main.js']),
+  );
+  let called = false;
+  const service = new ExtensionUploadService({
+    configStore: { requireAuth: () => ({ apiKey: 'test' }) },
+    client: {
+      installExtensionInSpace: async (spaceId) => {
+        called = true;
+        return { extension_id: 'demo.extension', version: '1.0.0', space_id: spaceId, scope: 'user' };
+      },
+    },
+  });
+
+  const result = await service.install(file, { spaceId: SPACE_ID });
+  assert.equal(called, true);
+  assert.equal(result.extension_id, 'demo.extension');
+});
diff --git a/test/zip-entries.test.js b/test/zip-entries.test.js
new file mode 100644
index 0000000..4725ca0
--- /dev/null
+++ b/test/zip-entries.test.js
@@ -0,0 +1,145 @@
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import test from 'node:test';
+
+import { listZipEntryNames, readZipEntryNames } from '../lib/utils/zip-entries.js';
+
+const EOCD_SIGNATURE = 0x06054b50;
+
+// Builds a stored (uncompressed) zip with the given entry names and optional
+// archive comment. Returns the bytes plus the offsets tests need to corrupt
+// specific fields. CRCs are zero because only entry names are read.
+function buildZip(names, { comment = Buffer.alloc(0) } = {}) {
+  const locals = [];
+  const centrals = [];
+  let offset = 0;
+  for (const name of names) {
+    const nameBuf = Buffer.from(name, 'utf8');
+    const data = Buffer.from('x');
+    const local = Buffer.alloc(30);
+    local.writeUInt32LE(0x04034b50, 0);
+    local.writeUInt32LE(data.length, 18);
+    local.writeUInt32LE(data.length, 22);
+    local.writeUInt16LE(nameBuf.length, 26);
+    const localBlock = Buffer.concat([local, nameBuf, data]);
+    const central = Buffer.alloc(46);
+    central.writeUInt32LE(0x02014b50, 0);
+    central.writeUInt32LE(data.length, 20);
+    central.writeUInt32LE(data.length, 24);
+    central.writeUInt16LE(nameBuf.length, 28);
+    central.writeUInt32LE(offset, 42);
+    locals.push(localBlock);
+    centrals.push(Buffer.concat([central, nameBuf]));
+    offset += localBlock.length;
+  }
+  const directory = Buffer.concat(centrals);
+  const eocd = Buffer.alloc(22);
+  eocd.writeUInt32LE(EOCD_SIGNATURE, 0);
+  eocd.writeUInt16LE(names.length, 8);
+  eocd.writeUInt16LE(names.length, 10);
+  eocd.writeUInt32LE(directory.length, 12);
+  eocd.writeUInt32LE(offset, 16);
+  eocd.writeUInt16LE(comment.length, 20);
+  return {
+    buffer: Buffer.concat([...locals, directory, eocd, comment]),
+    directoryOffset: offset,
+    directorySize: directory.length,
+    eocdOffset: offset + directory.length,
+  };
+}
+
+// A comment that contains a complete-looking EOCD record followed by text.
+function commentWithFakeEocd() {
+  const fake = Buffer.alloc(22);
+  fake.writeUInt32LE(EOCD_SIGNATURE, 0);
+  return Buffer.concat([Buffer.from('built by '), fake, Buffer.from(' on Windows')]);
+}
+
+function writeTemp(t, contents) {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'mantis-zip-test-'));
+  t.after(() => fs.rmSync(dir, { recursive: true, force: true }));
+  const file = path.join(dir, 'package.mantisx');
+  fs.writeFileSync(file, contents);
+  return file;
+}
+
+test('lists entry names of a valid archive', () => {
+  const { buffer } = buildZip(['mantis.extension.json', 'extension.js', 'panel/main.js']);
+  assert.deepEqual(listZipEntryNames(buffer), ['mantis.extension.json', 'extension.js', 'panel/main.js']);
+});
+
+test('returns an empty list for an empty zip archive', () => {
+  assert.deepEqual(listZipEntryNames(buildZip([]).buffer), []);
+});
+
+test('returns null for data that is not a zip archive', () => {
+  assert.equal(listZipEntryNames(Buffer.from('{"manifest":{}}')), null);
+  assert.equal(listZipEntryNames(Buffer.alloc(0)), null);
+});
+
+test('ignores a fake EOCD signature inside the archive comment', () => {
+  const { buffer } = buildZip(['extension.js', 'panel\\main.js'], { comment: commentWithFakeEocd() });
+  assert.deepEqual(listZipEntryNames(buffer), ['extension.js', 'panel\\main.js']);
+});
+
+test('returns null when a filename runs past the central directory', () => {
+  const { buffer, directoryOffset } = buildZip(['panel/main.js']);
+  buffer.writeUInt16LE(200, directoryOffset + 28);
+  assert.equal(listZipEntryNames(buffer), null);
+});
+
+test('returns null when the extra field or comment runs past the central directory', () => {
+  const extra = buildZip(['panel/main.js']);
+  extra.buffer.writeUInt16LE(50, extra.directoryOffset + 30);
+  assert.equal(listZipEntryNames(extra.buffer), null);
+
+  const comment = buildZip(['panel/main.js']);
+  comment.buffer.writeUInt16LE(50, comment.directoryOffset + 32);
+  assert.equal(listZipEntryNames(comment.buffer), null);
+});
+
+test('returns null when a central directory record is truncated', () => {
+  const { buffer, eocdOffset } = buildZip(['panel/main.js']);
+  buffer.writeUInt32LE(40, eocdOffset + 12);
+  assert.equal(listZipEntryNames(buffer), null);
+});
+
+test('returns null when the entry count exceeds the records present', () => {
+  const { buffer, eocdOffset } = buildZip(['panel/main.js']);
+  buffer.writeUInt16LE(2, eocdOffset + 8);
+  buffer.writeUInt16LE(2, eocdOffset + 10);
+  assert.equal(listZipEntryNames(buffer), null);
+});
+
+test('returns null when the central directory size is wrong', () => {
+  const tooLarge = buildZip(['panel/main.js']);
+  tooLarge.buffer.writeUInt32LE(tooLarge.directorySize + 10, tooLarge.eocdOffset + 12);
+  assert.equal(listZipEntryNames(tooLarge.buffer), null);
+
+  const leftover = buildZip(['panel/main.js']);
+  leftover.buffer.writeUInt32LE(leftover.directorySize + 4, leftover.eocdOffset + 12);
+  leftover.buffer.writeUInt32LE(leftover.directoryOffset - 4, leftover.eocdOffset + 16);
+  assert.equal(listZipEntryNames(leftover.buffer), null);
+});
+
+test('returns null when the central directory offset is out of bounds', () => {
+  const { buffer, eocdOffset } = buildZip(['panel/main.js']);
+  buffer.writeUInt32LE(1000, eocdOffset + 16);
+  assert.equal(listZipEntryNames(buffer), null);
+});
+
+test('reads entry names from a file without loading the whole archive', async (t) => {
+  // A maximum-length comment puts the central directory outside the tail read.
+  const comment = Buffer.alloc(0xffff, 'x');
+  const { buffer } = buildZip(['extension.js', 'panel\\main.js'], { comment });
+  const file = writeTemp(t, buffer);
+  assert.deepEqual(await readZipEntryNames(file), ['extension.js', 'panel\\main.js']);
+});
+
+test('file reader matches the buffer reader for fake signatures and non-zip files', async (t) => {
+  const { buffer } = buildZip(['panel\\main.js'], { comment: commentWithFakeEocd() });
+  assert.deepEqual(await readZipEntryNames(writeTemp(t, buffer)), ['panel\\main.js']);
+  assert.equal(await readZipEntryNames(writeTemp(t, '{"manifest":{}}')), null);
+});