Copilot agent session worktree creation silently fails when repository is in detached HEAD state

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#323,751 1 comment 0 reactions 1 assignee View on GitHub

@lszomoru is already working on this.

Since Jun 30, 2026.

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Description

Bug Report

VS Code version: 1.123.0
Copilot Chat version: 0.51.0
OS: Windows

Describe the bug

When the workspace repository is in detached HEAD state, starting a new Copilot CLI agent session fails to create an isolated worktree. The session silently falls back to isolationEnabled: false with no visible error to the user.

Steps to reproduce
  1. Have a git repository where the main working directory is in detached HEAD (e.g. git checkout <commit-sha>)
  2. Start a new Copilot agent session that triggers worktree creation
Expected behavior

The worktree is created successfully (the underlying git worktree add command succeeds), and the session runs with isolation enabled.

Actual behavior

The Copilot Chat extension log shows:

[ChatSessionWorktreeService][_createWorktree] Failed to create worktree for isolation.

…and the session starts with isolationEnabled: false, worktreePath: undefined.

Notably, the git worktree add command itself succeeds — the worktree directory is created on disk — but the result is discarded.

Root cause

In ChatSessionWorktreeService._createWorktree, after gitService.createWorktree() succeeds, the result is only returned if activeRepository.headBranchName is truthy:

if (worktreePath && activeRepository.headCommitHash && activeRepository.headBranchName) {
    // build and return properties
}
// falls through → logs error, returns undefined

In detached HEAD, headBranchName is undefined, so the condition fails even though the worktree was created successfully. However, headBranchName is only actually used as a fallback when baseBranch is not provided:

const baseBranchName = baseBranch ?? activeRepository.headBranchName;

The guard is overly strict — it should not require headBranchName when baseBranch is already supplied by the caller, or when the worktree was successfully created regardless.

Suggested fix
if (worktreePath && activeRepository.headCommitHash && (activeRepository.headBranchName || baseBranch)) {

Or more robustly, separate the guard from the fallback so that a missing headBranchName only affects baseBranchName computation rather than discarding the entire result.

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