From 69e853d843297c025a1a9205e03c44535b3fb09c Mon Sep 17 00:00:00 2001
From: Emerson Lopes <24904209+emerson-d-lopes@users.noreply.github.com>
Date: Wed, 22 Jul 2026 18:45:20 -0300
Subject: [PATCH] Don't pass duplicate /d flag in Windows cmd use-on-cd hook

The generated cd.cmd always prepended /d, so a user-supplied /d was
passed twice and `cd /d D:\` failed with "The syntax of the command
is incorrect". Only add /d when the first argument isn't already /d.

The two guarded single-line ifs are deliberate: parenthesized blocks
break on paths containing ")" (e.g. "C:\Program Files (x86)"), goto
labels misbehave with LF line endings (the file is stored with LF and
embedded via include_bytes!), and setlocal would revert the directory
change on endlocal.

Fixes #1556

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U5TQ1y3yBEDx73tYTHoUBj
---
 .changeset/witty-drives-turn.md | 10 ++++++++++
 e2e/repros/issue-1556.test.ts   | 22 ++++++++++++++++++++++
 src/shell/windows_cmd/cd.cmd    |  6 +++++-
 3 files changed, 37 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/witty-drives-turn.md
 create mode 100644 e2e/repros/issue-1556.test.ts

diff --git a/.changeset/witty-drives-turn.md b/.changeset/witty-drives-turn.md
new file mode 100644
index 000000000..48e746168
--- /dev/null
+++ b/.changeset/witty-drives-turn.md
@@ -0,0 +1,10 @@
+---
+"fnm": patch
+---
+
+fix `cd /d <drive>` failing in Windows Command Prompt when use-on-cd is enabled: the generated `cd.cmd` always prepended `/d`, so a user-supplied `/d` was passed twice and `cd` failed with "The syntax of the command is incorrect". Fixes #1556
+
+```sh-session
+> cd /d D:\
+The syntax of the command is incorrect.
+```
diff --git a/e2e/repros/issue-1556.test.ts b/e2e/repros/issue-1556.test.ts
new file mode 100644
index 000000000..edbf81532
--- /dev/null
+++ b/e2e/repros/issue-1556.test.ts
@@ -0,0 +1,22 @@
+import { writeFile, mkdir } from "node:fs/promises"
+import { join } from "node:path"
+import { script } from "../shellcode/script.js"
+import { WinCmd } from "../shellcode/shells.js"
+import testCwd from "../shellcode/test-cwd.js"
+import testNodeVersion from "../shellcode/test-node-version.js"
+import describe from "../describe.js"
+
+describe(WinCmd, () => {
+  test(`issue #1556: cd /d works with use-on-cd`, async () => {
+    const subdir = join(testCwd(), "subdir")
+    await mkdir(subdir, { recursive: true })
+    await writeFile(join(subdir, ".node-version"), "v12.22.12")
+    await script(WinCmd)
+      .then(WinCmd.env({ useOnCd: true }))
+      .then(WinCmd.call("fnm", ["install", "v8.11.3"]))
+      .then(WinCmd.call("fnm", ["install", "v12.22.12"]))
+      .then(WinCmd.call("cd", ["/d", subdir]))
+      .then(testNodeVersion(WinCmd, "v12.22.12"))
+      .execute(WinCmd)
+  })
+})
diff --git a/src/shell/windows_cmd/cd.cmd b/src/shell/windows_cmd/cd.cmd
index ef7a9fc73..0adc7b14b 100644
--- a/src/shell/windows_cmd/cd.cmd
+++ b/src/shell/windows_cmd/cd.cmd
@@ -1,5 +1,9 @@
 @echo off
-cd /d %*
+rem Add /d only when the user didn't pass it themselves, so `cd /d D:\` works (#1556).
+rem No if/else parentheses or setlocal here: %* may contain ")" (e.g. "C:\Program Files (x86)"),
+rem and endlocal would revert the directory change.
+if /i "%~1" == "/d" cd %*
+if /i not "%~1" == "/d" cd /d %*
 if "%FNM_VERSION_FILE_STRATEGY%" == "recursive" (
   fnm use --silent-if-unchanged
 ) else (