fix(project): resolve default branch from origin/HEAD, not checked-out branch

backend/internal/httpd/controllers/sessions_test.go

@@ -23,6 +23,7 @@ type fakeSessionService struct {
 	cleanupProjects []domain.ProjectID
 	cleanupResult   []domain.SessionID
 	cleanupSkipped  []sessionsvc.CleanupSkipped
+	spawnErr        error
 	claimErr        error
 	listPRErr       error
 }
@@ -51,6 +52,9 @@ func (f *fakeSessionService) List(_ context.Context, filter sessionsvc.ListFilte
 }
 
 func (f *fakeSessionService) Spawn(_ context.Context, cfg ports.SpawnConfig) (domain.Session, error) {
+	if f.spawnErr != nil {
+		return domain.Session{}, f.spawnErr
+	}
 	now := time.Now().UTC()
 	s := domain.Session{SessionRecord: domain.SessionRecord{ID: domain.SessionID(string(cfg.ProjectID) + "-2"), ProjectID: cfg.ProjectID, IssueID: cfg.IssueID, Kind: cfg.Kind, Harness: cfg.Harness, Activity: domain.Activity{State: domain.ActivityIdle, LastActivityAt: now}, CreatedAt: now, UpdatedAt: now}, Status: domain.StatusIdle}
 	f.sessions[s.ID] = s
@@ -243,6 +247,15 @@ func TestSessionsAPI_ListSpawnGetAndActions(t *testing.T) {
 	}
 }
 
+func TestSessionsAPI_SpawnBranchNotFetchedReturnsTypedError(t *testing.T) {
+	svc := newFakeSessionService()
+	svc.spawnErr = apierr.Invalid("BRANCH_NOT_FETCHED", `workspace: branch is not fetched: "feature/missing"`, nil)
+	srv := newSessionTestServer(t, svc)
+
+	body, status, _ := doRequest(t, srv, "POST", "/api/v1/sessions", `{"projectId":"ao","kind":"worker","branch":"feature/missing","prompt":"fix"}`)
+	assertErrorCode(t, body, status, http.StatusBadRequest, "BRANCH_NOT_FETCHED")
+}
+
 func TestSessionsAPI_RenameNotFound(t *testing.T) {
 	srv := newSessionTestServer(t, newFakeSessionService())
 

backend/internal/service/project/service.go

@@ -246,12 +246,25 @@ func resolveGitOriginURL(path string) string {
 	return strings.TrimSpace(string(out))
 }
 
-// resolveDefaultBranch returns the repo's currently checked-out branch via
-// `git -C path symbolic-ref --short HEAD`. A detached HEAD, missing repo, or any
-// other git error returns an empty string — `project add` must not fail just
-// because the branch can't be resolved (the caller falls back to
-// DefaultBranchName).
+// resolveDefaultBranch returns the repo's default branch, preferring the
+// remote's default (`origin/HEAD`) over the currently checked-out branch. This
+// matters because the user may have the repo on a feature branch when adding the
+// project: keying off HEAD would persist that feature branch as the project
+// default and base every session worktree on it. `origin/HEAD` reflects the
+// real default (e.g. `master`, `develop`) regardless of the active branch.
+//
+// Falls back to the checked-out branch when origin/HEAD is unset (no remote, or
+// it was never fetched). A detached HEAD, missing repo, or any other git error
+// returns an empty string — `project add` must not fail just because the branch
+// can't be resolved (the caller falls back to DefaultBranchName).
 func resolveDefaultBranch(path string) string {
+	if out, err := exec.Command(
+		"git", "-C", path, "symbolic-ref", "--short", "refs/remotes/origin/HEAD",
+	).Output(); err == nil {
+		if ref := strings.TrimSpace(string(out)); ref != "" {
+			return strings.TrimPrefix(ref, "origin/")
+		}
+	}
 	out, err := exec.Command("git", "-C", path, "symbolic-ref", "--short", "HEAD").Output()
 	if err != nil {
 		return ""

backend/internal/service/project/service_test.go

@@ -51,6 +51,32 @@ func gitRepoOnBranch(t *testing.T, branch string) string {
 	return dir
 }
 
+// gitRepoWithOriginHead creates a repo whose remote default (origin/HEAD) points
+// at defaultBranch while the working tree is checked out on featureBranch. This
+// mirrors a user adding a project while sitting on a feature branch: detection
+// must record the remote default, not the active branch.
+func gitRepoWithOriginHead(t *testing.T, defaultBranch, featureBranch string) string {
+	t.Helper()
+	dir := t.TempDir()
+	run := func(args ...string) {
+		if out, err := exec.Command("git", append([]string{"-C", dir}, args...)...).CombinedOutput(); err != nil {
+			t.Fatalf("git %v: %v (%s)", args, err, out)
+		}
+	}
+	if out, err := exec.Command("git", "init", "-b", defaultBranch, dir).CombinedOutput(); err != nil {
+		t.Fatalf("git unavailable: %v (%s)", err, out)
+	}
+	run("config", "user.email", "test@example.com")
+	run("config", "user.name", "test")
+	run("commit", "--allow-empty", "-m", "init")
+	// Fabricate a remote-tracking default without a real remote: point
+	// refs/remotes/origin/<defaultBranch> at HEAD, then set origin/HEAD to it.
+	run("update-ref", "refs/remotes/origin/"+defaultBranch, "HEAD")
+	run("symbolic-ref", "refs/remotes/origin/HEAD", "refs/remotes/origin/"+defaultBranch)
+	run("checkout", "-b", featureBranch)
+	return dir
+}
+
 func ptr(s string) *string { return &s }
 
 // wantCode asserts err is an *apierr.Error carrying the given machine code.
@@ -239,6 +265,42 @@ func TestManager_AddDetectsNonMainDefaultBranch(t *testing.T) {
 	}
 }
 
+// A repo checked out on a feature branch must NOT record that branch as the
+// project default — detection must prefer the remote default (origin/HEAD), so a
+// repo whose origin/HEAD is `main` stays on `main` even when HEAD is elsewhere.
+func TestManager_AddPrefersOriginHeadOverCheckedOutBranch(t *testing.T) {
+	ctx := context.Background()
+	m := newManager(t)
+	repo := gitRepoWithOriginHead(t, "main", "fix/pr-attachment")
+
+	proj, err := m.Add(ctx, project.AddInput{Path: repo, ProjectID: ptr("ao")})
+	if err != nil {
+		t.Fatalf("Add: %v", err)
+	}
+	// origin/HEAD is `main`, which equals DefaultBranchName, so config stays empty
+	// and the effective default resolves to main — never the feature branch.
+	if proj.DefaultBranch != domain.DefaultBranchName {
+		t.Fatalf("DefaultBranch = %q, want %q (not the checked-out feature branch)",
+			proj.DefaultBranch, domain.DefaultBranchName)
+	}
+}
+
+// When origin/HEAD points at a non-main default (e.g. master), detection records
+// that — not the feature branch the user happens to be on.
+func TestManager_AddPrefersOriginHeadNonMain(t *testing.T) {
+	ctx := context.Background()
+	m := newManager(t)
+	repo := gitRepoWithOriginHead(t, "master", "fix/pr-attachment")
+
+	proj, err := m.Add(ctx, project.AddInput{Path: repo, ProjectID: ptr("ao")})
+	if err != nil {
+		t.Fatalf("Add: %v", err)
+	}
+	if proj.DefaultBranch != "master" {
+		t.Fatalf("DefaultBranch = %q, want master (origin/HEAD), not feature branch", proj.DefaultBranch)
+	}
+}
+
 func TestManager_SetConfig(t *testing.T) {
 	ctx := context.Background()
 	m := newManager(t)