diff --git a/backend/internal/adapters/workspace/gitworktree/workspace.go b/backend/internal/adapters/workspace/gitworktree/workspace.go
index 4fb928c1..3d7f88e5 100644
--- a/backend/internal/adapters/workspace/gitworktree/workspace.go
+++ b/backend/internal/adapters/workspace/gitworktree/workspace.go
@@ -34,6 +34,7 @@ var (
 var (
 	ErrBranchCheckedOutElsewhere = ports.ErrWorkspaceBranchCheckedOutElsewhere
 	ErrBranchNotFetched          = ports.ErrWorkspaceBranchNotFetched
+	ErrBranchInvalid             = ports.ErrWorkspaceBranchInvalid
 )
 
 // RepoResolver maps a project to the absolute path of its source git repo.
@@ -262,7 +263,7 @@ func (w *Workspace) addWorktree(ctx context.Context, repo, path, branch, baseBra
 
 func (w *Workspace) validateBranch(ctx context.Context, repo, branch string) error {
 	if _, err := w.run(ctx, w.binary, checkRefFormatBranchArgs(repo, branch)...); err != nil {
-		return fmt.Errorf("gitworktree: invalid branch %q: %w", branch, err)
+		return fmt.Errorf("%w: %q (%w)", ErrBranchInvalid, branch, err)
 	}
 	return nil
 }
diff --git a/backend/internal/adapters/workspace/gitworktree/workspace_test.go b/backend/internal/adapters/workspace/gitworktree/workspace_test.go
index 9c084b2e..dd75d235 100644
--- a/backend/internal/adapters/workspace/gitworktree/workspace_test.go
+++ b/backend/internal/adapters/workspace/gitworktree/workspace_test.go
@@ -369,6 +369,34 @@ func TestAddWorktreeRefusesBranchCheckedOutElsewhere(t *testing.T) {
 	}
 }
 
+// TestCreateRejectsInvalidBranchName covers the residual of #152 Bug 3: a branch
+// name rejected by `git check-ref-format` must surface
+// ports.ErrWorkspaceBranchInvalid so the HTTP layer renders a typed 400 instead
+// of leaking raw git stderr through a 500.
+func TestCreateRejectsInvalidBranchName(t *testing.T) {
+	root := t.TempDir()
+	repo := t.TempDir()
+	ws, err := New(Options{ManagedRoot: root, RepoResolver: StaticRepoResolver{"proj": repo}})
+	if err != nil {
+		t.Fatalf("new: %v", err)
+	}
+	ws.run = func(_ context.Context, _ string, args ...string) ([]byte, error) {
+		joined := strings.Join(args, " ")
+		if strings.Contains(joined, "check-ref-format") {
+			return nil, errors.New("fatal: 'bad branch!!' is not a valid branch name")
+		}
+		t.Fatalf("no git beyond check-ref-format should run for an invalid branch: %v", args)
+		return nil, nil
+	}
+	_, err = ws.Create(context.Background(), ports.WorkspaceConfig{ProjectID: "proj", SessionID: "sess", Branch: "bad branch!!"})
+	if !errors.Is(err, ports.ErrWorkspaceBranchInvalid) {
+		t.Fatalf("err = %v, want ports.ErrWorkspaceBranchInvalid", err)
+	}
+	if !strings.Contains(err.Error(), "bad branch!!") {
+		t.Fatalf("err = %v, want message to include the rejected branch name", err)
+	}
+}
+
 // TestAddWorktreeReportsBranchNotFetched covers Bug 3 (b): if no local head,
 // no origin remote-tracking branch, no default branch ref, and no tag of the
 // same name is reachable, Create must surface ports.ErrWorkspaceBranchNotFetched
diff --git a/backend/internal/ports/outbound.go b/backend/internal/ports/outbound.go
index 4f7b20ac..668da516 100644
--- a/backend/internal/ports/outbound.go
+++ b/backend/internal/ports/outbound.go
@@ -118,6 +118,9 @@ var (
 	// ErrWorkspaceBranchNotFetched reports the requested branch exists nowhere
 	// reachable (no local head, no remote-tracking branch, no tag).
 	ErrWorkspaceBranchNotFetched = errors.New("workspace: branch is not fetched")
+	// ErrWorkspaceBranchInvalid reports the requested branch name is not a valid
+	// git ref (rejected by `git check-ref-format`).
+	ErrWorkspaceBranchInvalid = errors.New("workspace: invalid branch name")
 	// ErrWorkspaceDirty reports Destroy refused to remove a workspace because
 	// it holds uncommitted changes or untracked files. Teardown is never
 	// forced; callers treat the workspace as intentionally preserved.
diff --git a/backend/internal/service/session/service.go b/backend/internal/service/session/service.go
index 51e43396..2582d5b0 100644
--- a/backend/internal/service/session/service.go
+++ b/backend/internal/service/session/service.go
@@ -322,6 +322,8 @@ func toAPIError(err error) error {
 		return apierr.Conflict("BRANCH_CHECKED_OUT_ELSEWHERE", err.Error(), nil)
 	case errors.Is(err, ports.ErrWorkspaceBranchNotFetched):
 		return apierr.Invalid("BRANCH_NOT_FETCHED", err.Error(), nil)
+	case errors.Is(err, ports.ErrWorkspaceBranchInvalid):
+		return apierr.Invalid("INVALID_BRANCH", err.Error(), nil)
 	case errors.Is(err, ports.ErrAgentBinaryNotFound):
 		return apierr.Invalid("AGENT_BINARY_NOT_FOUND", err.Error(), nil)
 	default:
diff --git a/backend/internal/service/session/service_test.go b/backend/internal/service/session/service_test.go
index 9af3ceea..b3867967 100644
--- a/backend/internal/service/session/service_test.go
+++ b/backend/internal/service/session/service_test.go
@@ -244,6 +244,7 @@ func TestToAPIErrorMapsWorkspaceBranchSentinels(t *testing.T) {
 	}{
 		{"checked out elsewhere", fmt.Errorf("spawn mer-1: workspace: %w: \"x\" is checked out at \"/tmp\"", ports.ErrWorkspaceBranchCheckedOutElsewhere), apierr.KindConflict, "BRANCH_CHECKED_OUT_ELSEWHERE"},
 		{"not fetched", fmt.Errorf("spawn mer-1: workspace: %w: \"x\" has no local head", ports.ErrWorkspaceBranchNotFetched), apierr.KindInvalid, "BRANCH_NOT_FETCHED"},
+		{"invalid branch", fmt.Errorf("spawn mer-1: workspace: %w: \"bad!!\" (exit 1)", ports.ErrWorkspaceBranchInvalid), apierr.KindInvalid, "INVALID_BRANCH"},
 		{"agent binary not found", fmt.Errorf("spawn mer-1: %w", ports.ErrAgentBinaryNotFound), apierr.KindInvalid, "AGENT_BINARY_NOT_FOUND"},
 	}
 	for _, tc := range cases {