diff --git a/src/ClaudeDo.Data/Git/GitService.cs b/src/ClaudeDo.Data/Git/GitService.cs
index 5cb0fb30..eb3febbc 100644
--- a/src/ClaudeDo.Data/Git/GitService.cs
+++ b/src/ClaudeDo.Data/Git/GitService.cs
@@ -25,7 +25,16 @@ public sealed class GitService
     {
         var (exitCode, stdout, stderr) = await RunGitAsync(dir, ["rev-parse", "HEAD"], ct);
         if (exitCode != 0)
+        {
+            // Unborn HEAD (fresh `git init`, no commits) — the raw git stderr ("ambiguous
+            // argument 'HEAD'") is useless to a user; call sites should have run
+            // EnsureHeadCommitAsync, this message is the safety net for any that didn't.
+            if (stderr.Contains("ambiguous argument 'HEAD'", StringComparison.Ordinal) ||
+                stderr.Contains("unknown revision", StringComparison.Ordinal))
+                throw new InvalidOperationException(
+                    $"The repository at {dir} has no commits yet (unborn or broken HEAD) — ClaudeDo needs at least one commit to work in it.");
             throw new InvalidOperationException($"git rev-parse HEAD failed (exit {exitCode}): {stderr}");
+        }
         return stdout.Trim();
     }
 
@@ -55,6 +64,67 @@ public sealed class GitService
         return exitCode == 0 ? stdout.Trim() : null;
     }
 
+    /// <summary>HEAD resolves to a commit (false for a fresh `git init` repo or a broken HEAD).</summary>
+    public async Task<bool> HasHeadCommitAsync(string dir, CancellationToken ct = default)
+    {
+        // ^{commit} forces the object to actually exist and be a commit — a bare
+        // `--verify HEAD` happily accepts a detached HEAD pointing at a missing object.
+        var (exitCode, _, _) = await RunGitAsync(dir, ["rev-parse", "--verify", "--quiet", "HEAD^{commit}"], ct);
+        return exitCode == 0;
+    }
+
+    private const string FallbackUserName = "ClaudeDo";
+    private const string FallbackUserEmail = "claudedo@local";
+
+    // Two parallel task starts on the same empty repo must create exactly one initial commit.
+    private static readonly SemaphoreSlim BootstrapGate = new(1, 1);
+
+    /// <summary>
+    /// Guarantees HEAD resolves to a commit so worktrees can be created. A repo with an
+    /// unborn HEAD (fresh `git init`) gets an empty initial commit built from plumbing only
+    /// (hash-object/commit-tree/update-ref) — the user's index and working tree are never
+    /// touched, so staged and untracked files stay exactly as they were. Identity is passed
+    /// inline, so this also works on machines with no git identity configured. A repo whose
+    /// HEAD is broken (not a symbolic ref and not resolvable) throws instead of guessing.
+    /// </summary>
+    /// <returns>true if a bootstrap commit was created; false if HEAD already resolved.</returns>
+    public async Task<bool> EnsureHeadCommitAsync(string dir, CancellationToken ct = default)
+    {
+        if (await HasHeadCommitAsync(dir, ct)) return false;
+
+        var (symExit, symRef, _) = await RunGitAsync(dir, ["symbolic-ref", "--quiet", "HEAD"], ct);
+        if (symExit != 0)
+            throw new InvalidOperationException(
+                $"The repository at {dir} has a broken HEAD (detached onto a missing commit, or a corrupt .git/HEAD). Repair the repository manually.");
+
+        await BootstrapGate.WaitAsync(ct);
+        try
+        {
+            if (await HasHeadCommitAsync(dir, ct)) return false;
+
+            var (treeExit, treeSha, treeErr) = await RunGitAsync(dir, ["mktree"], ct, stdinData: "");
+            if (treeExit != 0)
+                throw new InvalidOperationException($"git mktree failed (exit {treeExit}): {treeErr}");
+
+            var (commitExit, commitSha, commitErr) = await RunGitAsync(dir,
+                ["-c", $"user.name={FallbackUserName}", "-c", $"user.email={FallbackUserEmail}",
+                 "commit-tree", treeSha.Trim(), "-m", "chore: initialize repository (ClaudeDo)"], ct);
+            if (commitExit != 0)
+                throw new InvalidOperationException($"git commit-tree failed (exit {commitExit}): {commitErr}");
+
+            var (refExit, _, refErr) = await RunGitAsync(dir,
+                ["update-ref", symRef.Trim(), commitSha.Trim()], ct);
+            if (refExit != 0)
+                throw new InvalidOperationException($"git update-ref failed (exit {refExit}): {refErr}");
+
+            return true;
+        }
+        finally
+        {
+            BootstrapGate.Release();
+        }
+    }
+
     public async Task WorktreeAddAsync(string repoDir, string branchName, string worktreePath, string baseCommit, CancellationToken ct = default)
     {
         await WorktreeAddGate.WaitAsync(ct);
@@ -135,10 +205,24 @@ public sealed class GitService
     {
         // Use -F - (read message from stdin) to handle multi-line messages safely.
         var (exitCode, _, stderr) = await RunGitAsync(worktreePath, ["commit", "-F", "-"], ct, stdinData: message);
+        if (exitCode != 0 && IsMissingIdentityError(stderr))
+        {
+            // A machine without git identity configured must not fail every auto-commit —
+            // retry once as ClaudeDo. The happy path pays no preflight cost.
+            (exitCode, _, stderr) = await RunGitAsync(worktreePath,
+                ["-c", $"user.name={FallbackUserName}", "-c", $"user.email={FallbackUserEmail}",
+                 "commit", "-F", "-"], ct, stdinData: message);
+        }
         if (exitCode != 0)
             throw new InvalidOperationException($"git commit failed (exit {exitCode}): {stderr}");
     }
 
+    private static bool IsMissingIdentityError(string stderr) =>
+        stderr.Contains("tell me who you are", StringComparison.OrdinalIgnoreCase) ||
+        stderr.Contains("empty ident", StringComparison.OrdinalIgnoreCase) ||
+        stderr.Contains("no email was given", StringComparison.OrdinalIgnoreCase) ||
+        stderr.Contains("unable to auto-detect email address", StringComparison.OrdinalIgnoreCase);
+
     public async Task<string> GetDiffAsync(
         string worktreePath, IReadOnlyList<string>? paths = null, CancellationToken ct = default)
     {
@@ -584,7 +668,9 @@ public sealed class GitService
             CreateNoWindow = true,
             StandardOutputEncoding = Encoding.UTF8,
             StandardErrorEncoding = Encoding.UTF8,
-            StandardInputEncoding = stdinData is not null ? Encoding.UTF8 : null,
+            // BOM-less: Encoding.UTF8 writes a BOM preamble, which git rejects as input
+            // (mktree "input format error") and would prepend to stdin commit messages.
+            StandardInputEncoding = stdinData is not null ? new UTF8Encoding(false) : null,
         };
         psi.ArgumentList.Add("-C");
         psi.ArgumentList.Add(workDir);
diff --git a/tests/ClaudeDo.Worker.Tests/Runner/GitServiceBootstrapTests.cs b/tests/ClaudeDo.Worker.Tests/Runner/GitServiceBootstrapTests.cs
new file mode 100644
index 00000000..14924db9
--- /dev/null
+++ b/tests/ClaudeDo.Worker.Tests/Runner/GitServiceBootstrapTests.cs
@@ -0,0 +1,139 @@
+using ClaudeDo.Data.Git;
+using ClaudeDo.Worker.Tests.Infrastructure;
+
+namespace ClaudeDo.Worker.Tests.Runner;
+
+// Covers the empty-repo (unborn HEAD) hardening: HasHeadCommitAsync, EnsureHeadCommitAsync
+// (plumbing-only bootstrap commit), the friendly RevParseHeadAsync unborn message, and the
+// CommitAsync git-identity fallback. All tests run real git against throwaway repos.
+public class GitServiceBootstrapTests : IDisposable
+{
+    private readonly List<string> _tempDirs = new();
+    private readonly GitService _git = new();
+
+    private static bool GitAvailable => GitRepoFixture.IsGitAvailable();
+
+    // A repo straight out of `git init` — no commits, and deliberately NO identity
+    // configured: the bootstrap must not depend on machine-level git config.
+    private string InitEmptyRepo()
+    {
+        var dir = Path.Combine(Path.GetTempPath(), $"claudedo_emptyrepo_{Guid.NewGuid():N}");
+        Directory.CreateDirectory(dir);
+        GitRepoFixture.RunGit(dir, "init", "-b", "main");
+        _tempDirs.Add(dir);
+        return dir;
+    }
+
+    [Fact]
+    public async Task HasHeadCommit_FalseOnEmptyRepo_TrueAfterCommit()
+    {
+        if (!GitAvailable) { Assert.True(true, "git not available -- skipping"); return; }
+
+        var dir = InitEmptyRepo();
+        Assert.False(await _git.HasHeadCommitAsync(dir));
+
+        GitRepoFixture.RunGit(dir, "-c", "user.name=t", "-c", "user.email=t@t", "commit", "--allow-empty", "-m", "x");
+        Assert.True(await _git.HasHeadCommitAsync(dir));
+    }
+
+    [Fact]
+    public async Task EnsureHeadCommit_BootstrapsEmptyRepo_Once()
+    {
+        if (!GitAvailable) { Assert.True(true, "git not available -- skipping"); return; }
+
+        var dir = InitEmptyRepo();
+
+        Assert.True(await _git.EnsureHeadCommitAsync(dir));
+
+        // HEAD now resolves and RevParseHeadAsync works — the original failure mode is gone.
+        var head = await _git.RevParseHeadAsync(dir);
+        Assert.Matches("^[0-9a-f]{40,64}$", head);
+
+        // The bootstrap commit is empty: nothing of the user's ends up in it.
+        var tree = GitRepoFixture.RunGit(dir, "ls-tree", "HEAD").Trim();
+        Assert.Equal(string.Empty, tree);
+
+        // Idempotent: a repo with a HEAD commit is left alone.
+        Assert.False(await _git.EnsureHeadCommitAsync(dir));
+        Assert.Equal(head, await _git.RevParseHeadAsync(dir));
+    }
+
+    [Fact]
+    public async Task EnsureHeadCommit_LeavesStagedFilesStaged()
+    {
+        if (!GitAvailable) { Assert.True(true, "git not available -- skipping"); return; }
+
+        var dir = InitEmptyRepo();
+        File.WriteAllText(Path.Combine(dir, "mine.txt"), "user work\n");
+        GitRepoFixture.RunGit(dir, "add", "mine.txt");
+
+        Assert.True(await _git.EnsureHeadCommitAsync(dir));
+
+        // Still staged (index vs HEAD shows it as pending), NOT committed.
+        var staged = GitRepoFixture.RunGit(dir, "diff", "--cached", "--name-only").Trim();
+        Assert.Equal("mine.txt", staged);
+        var tree = GitRepoFixture.RunGit(dir, "ls-tree", "HEAD").Trim();
+        Assert.Equal(string.Empty, tree);
+    }
+
+    [Fact]
+    public async Task EnsureHeadCommit_CorruptHead_ThrowsClearError()
+    {
+        if (!GitAvailable) { Assert.True(true, "git not available -- skipping"); return; }
+
+        var dir = InitEmptyRepo();
+        // Detach HEAD onto a commit that doesn't exist — not an unborn branch, a broken repo.
+        File.WriteAllText(Path.Combine(dir, ".git", "HEAD"), new string('f', 40) + "\n");
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => _git.EnsureHeadCommitAsync(dir));
+        Assert.Contains(dir, ex.Message);
+    }
+
+    [Fact]
+    public async Task RevParseHead_EmptyRepo_ReportsNoCommitsInsteadOfRawGitError()
+    {
+        if (!GitAvailable) { Assert.True(true, "git not available -- skipping"); return; }
+
+        var dir = InitEmptyRepo();
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => _git.RevParseHeadAsync(dir));
+        Assert.Contains("no commits yet", ex.Message);
+        Assert.Contains(dir, ex.Message);
+        Assert.DoesNotContain("ambiguous argument", ex.Message);
+    }
+
+    [Fact]
+    public async Task CommitAsync_MissingIdentity_FallsBackToClaudeDoIdentity()
+    {
+        if (!GitAvailable) { Assert.True(true, "git not available -- skipping"); return; }
+
+        var dir = InitEmptyRepo();
+        GitRepoFixture.RunGit(dir, "-c", "user.name=t", "-c", "user.email=t@t", "commit", "--allow-empty", "-m", "seed");
+        // Empty local identity overrides any machine-level config, so the first commit
+        // attempt fails with git's identity error no matter where this test runs.
+        GitRepoFixture.RunGit(dir, "config", "user.name", "");
+        GitRepoFixture.RunGit(dir, "config", "user.email", "");
+
+        File.WriteAllText(Path.Combine(dir, "work.txt"), "content\n");
+        await _git.AddAllAsync(dir);
+
+        await _git.CommitAsync(dir, "feat(test): identity fallback");
+
+        var author = GitRepoFixture.RunGit(dir, "log", "-1", "--format=%an <%ae>").Trim();
+        Assert.Equal("ClaudeDo <claudedo@local>", author);
+    }
+
+    public void Dispose()
+    {
+        foreach (var dir in _tempDirs)
+        {
+            try
+            {
+                if (!Directory.Exists(dir)) continue;
+                foreach (var file in Directory.EnumerateFiles(dir, "*", SearchOption.AllDirectories))
+                    File.SetAttributes(file, FileAttributes.Normal);
+                Directory.Delete(dir, true);
+            }
+            catch { /* best effort */ }
+        }
+    }
+}