fix: prevent undeletable nul file on Windows

packages/opencode/src/session/prompt.ts

@@ -1574,12 +1574,14 @@ NOTE: At any point in time through this workflow you should feel free to ask the
       process.platform === "win32" ? path.win32.basename(shell, ".exe") : path.basename(shell)
     ).toLowerCase()
 
+    const sanitizedCommand = Shell.sanitizeNullRedirect(input.command, shell)
+
     const invocations: Record<string, { args: string[] }> = {
       nu: {
-        args: ["-c", input.command],
+        args: ["-c", sanitizedCommand],
       },
       fish: {
-        args: ["-c", input.command],
+        args: ["-c", sanitizedCommand],
       },
       zsh: {
         args: [
@@ -1588,7 +1590,7 @@ NOTE: At any point in time through this workflow you should feel free to ask the
           `
             [[ -f ~/.zshenv ]] && source ~/.zshenv >/dev/null 2>&1 || true
             [[ -f "\${ZDOTDIR:-$HOME}/.zshrc" ]] && source "\${ZDOTDIR:-$HOME}/.zshrc" >/dev/null 2>&1 || true
-            eval ${JSON.stringify(input.command)}
+            eval ${JSON.stringify(sanitizedCommand)}
           `,
         ],
       },
@@ -1599,25 +1601,25 @@ NOTE: At any point in time through this workflow you should feel free to ask the
           `
             shopt -s expand_aliases
             [[ -f ~/.bashrc ]] && source ~/.bashrc >/dev/null 2>&1 || true
-            eval ${JSON.stringify(input.command)}
+            eval ${JSON.stringify(sanitizedCommand)}
           `,
         ],
       },
       // Windows cmd
       cmd: {
-        args: ["/c", input.command],
+        args: ["/c", sanitizedCommand],
       },
       // Windows PowerShell
       powershell: {
-        args: ["-NoProfile", "-Command", input.command],
+        args: ["-NoProfile", "-Command", sanitizedCommand],
       },
       pwsh: {
-        args: ["-NoProfile", "-Command", input.command],
+        args: ["-NoProfile", "-Command", sanitizedCommand],
       },
       // Fallback: any shell that doesn't match those above
       //  - No -l, for max compatibility
       "": {
-        args: ["-c", `${input.command}`],
+        args: ["-c", `${sanitizedCommand}`],
       },
     }
 

packages/opencode/src/shell/shell.ts

@@ -64,4 +64,29 @@ export namespace Shell {
     if (s && !BLACKLIST.has(process.platform === "win32" ? path.win32.basename(s) : path.basename(s))) return s
     return fallback()
   })
+
+  const UNIX_SHELLS = new Set(["bash", "sh", "zsh", "fish", "nu"])
+
+  export function isUnixLike(shell: string): boolean {
+    const base = path.basename(shell).toLowerCase().replace(/\.exe$/, "")
+    return UNIX_SHELLS.has(base)
+  }
+
+  /**
+   * On Windows, when using Git Bash the AI model may generate Windows-style
+   * `> nul` / `2>nul` redirections.  Git Bash interprets these literally and
+   * creates a file called "nul" instead of discarding output.
+   *
+   * This helper rewrites null-device redirections so they match the shell
+   * that will actually execute the command.
+   */
+  export function sanitizeNullRedirect(command: string, shell: string): string {
+    if (process.platform !== "win32") return command
+    if (isUnixLike(shell)) {
+      // Git Bash / Unix shell: replace Windows NUL with /dev/null
+      return command.replace(/(\d?>)\s*(?:nul|NUL)\b/g, "$1/dev/null")
+    }
+    // cmd.exe / PowerShell: replace /dev/null with NUL
+    return command.replace(/(\d?>)\s*\/dev\/null\b/g, "$1NUL")
+  }
 }

packages/opencode/src/tool/bash.ts

@@ -164,7 +164,8 @@ export const BashTool = Tool.define("bash", async () => {
       }
 
       const shellEnv = await Plugin.trigger("shell.env", { cwd }, { env: {} })
-      const proc = spawn(params.command, {
+      const command = Shell.sanitizeNullRedirect(params.command, shell)
+      const proc = spawn(command, {
         shell,
         cwd,
         env: {

packages/opencode/src/tool/bash.txt

@@ -38,6 +38,7 @@ Usage notes:
     - If the commands depend on each other and must run sequentially, use a single Bash call with '&&' to chain them together (e.g., `git add . && git commit -m "message" && git push`). For instance, if one operation must complete before another starts (like mkdir before cp, Write before Bash for git operations, or git add before git commit), run these operations sequentially instead.
     - Use ';' only when you need to run commands sequentially but don't care if earlier commands fail
     - DO NOT use newlines to separate commands (newlines are ok in quoted strings)
+  - IMPORTANT: On Windows, the bash tool uses Git Bash. Always use /dev/null for output redirection (e.g., `command 2>/dev/null`), NOT the Windows `nul` device. Using `> nul` or `2>nul` will create a literal file named "nul" instead of discarding output.
   - AVOID using `cd <directory> && <command>`. Use the `workdir` parameter to change directories instead.
     <good-example>
     Use workdir="/foo/bar" with command: pytest tests