Fix Windows .bat/.cmd shim spawn failure in copilotSdk.ts by adding early-exit to external server mode

[nicolehaugen]

Summary

##Repro:
1.) Run: agentrc instructions on Windows machine when underlying call to copilot sdk is resolved from VS Code global Storage as copilot.bat
2.) Error:
Generating instructions...
[agentrc:copilot] trying override AGENTRC_COPILOT_CLI_PATH=C:\Users\nicolela\AppData\Roaming\Code\User\globalStorage\github.copilot-chat\copilotCli\copilot.bat
[agentrc:copilot] override CLI is compatible
[agentrc:copilot] validating CLI compatibility with C:\Users\nicolela\AppData\Roaming\Code\User\globalStorage\github.copilot-chat\copilotCli\copilot.bat
[agentrc:copilot] creating SDK client with cliPath=C:\Users\nicolela\AppData\Roaming\Code\User\globalStorage\github.copilot-chat\copilotCli\copilot.bat useStdio=false
Error: Failed to generate instructions with Copilot SDK. Ensure the Copilot CLI is installed (copilot --version) and logged in. spawn EINVAL

Problem

On Windows, when @github/copilot is not installed globally via npm, the Copilot CLI discovery chain in findCopilotCliConfig() (in copilot.ts) resolves to a .bat shim at:

%APPDATA%\Code\User\globalStorage\github.copilot-chat\copilotCli\copilot.bat

This is step 3 of the 6-step discovery chain:

1. AGENTRC_COPILOT_CLI_PATH env var override
2. npm global install (node.exe + npm-loader.js) — no .bat issue
3. VS Code globalStorage → copilot.bat on Windows ← this is where the problem occurs
4. where copilot (PATH lookup)
5. npx
6. VS Code extensions glob

When a user has Copilot only through the VS Code extension (not npm-installed), discovery lands on step 3 and returns { cliPath: "...\\copilot.bat" }.

Node.js child_process.spawn() calls the Windows CreateProcess API, which can only execute PE binaries (.exe, .dll). A .bat file is a script that needs cmd.exe as its interpreter. Spawning it directly produces:

Error: spawn EINVAL

The SDK-managed primary attempt always fails on this path. Before this fix, it would:

1. Try SDK-managed mode → fail with spawn EINVAL or Copilot CLI not found at cmd
2. Fall back to external server mode (which wraps the .bat in cmd /c)

This wasteful "sacrificial" primary attempt adds latency on every invocation.

How this situation occurs

A developer installs VS Code with the GitHub Copilot Chat extension. The extension downloads the Copilot CLI to its globalStorage as copilot.bat. If the developer also runs npm install -g @github/copilot, discovery resolves at step 2 to node.exe + npm-loader.js (a real PE binary), and there's no issue. But if they only have the VS Code extension, step 2 fails and step 3 returns the .bat shim.

Solution

Two changes in packages/core/src/services/copilotSdk.ts:

1. Early-exit for .bat/.cmd shims (alongside existing npx early-exit) - In createCopilotClient(), detect .bat/.cmd paths on Windows and skip the SDK-managed primary attempt entirely — go straight to external server mode, just like the existing isNpx check.
2. Safety-net in shouldFallbackToExternalServer() - Add "spawn einval" as a recognized fallback trigger so that if the early-exit is somehow bypassed (e.g. a future code path), the error is still caught.

Checklist

• [NA ] Tests added or updated
• [X ] Lint/typecheck pass
• [NA ] Docs updated if needed

[Copilot]

Pull request overview

Improves Windows compatibility for Copilot SDK startup by avoiding SDK-managed spawning when the discovered Copilot CLI path is a Windows .bat/.cmd shim (which can fail with spawn EINVAL), and instead going directly to the existing external server mode.

Changes:

• Add .bat/.cmd shim detection (Windows-only) to skip the SDK-managed startup path and immediately use external server mode.
• Treat spawn EINVAL as a recognized trigger for falling back to external server mode (as a safety net).