diff --git a/backend/internal/adapters/runtime/tmux/tmux.go b/backend/internal/adapters/runtime/tmux/tmux.go
index 8388a5713..892add6b9 100644
--- a/backend/internal/adapters/runtime/tmux/tmux.go
+++ b/backend/internal/adapters/runtime/tmux/tmux.go
@@ -236,7 +236,7 @@ func (r *Runtime) Attach(ctx context.Context, handle ports.RuntimeHandle, rows,
 	if err != nil {
 		return nil, err
 	}
-	return ptyexec.Spawn(ctx, argv, nil, rows, cols)
+	return ptyexec.Spawn(ctx, argv, attachEnv(os.Environ()), rows, cols)
 }
 
 // attachCommand returns the argv to attach a terminal to the session.
@@ -249,6 +249,17 @@ func (r *Runtime) attachCommand(handle ports.RuntimeHandle) ([]string, error) {
 	return []string{r.binary, "attach-session", "-t", id}, nil
 }
 
+func attachEnv(base []string) []string {
+	env := append([]string(nil), base...)
+	for i, kv := range env {
+		if strings.HasPrefix(kv, "TERM=") {
+			env[i] = "TERM=xterm-256color"
+			return env
+		}
+	}
+	return append(env, "TERM=xterm-256color")
+}
+
 // run wraps runner.Run with a per-call timeout context.
 func (r *Runtime) run(ctx context.Context, args ...string) ([]byte, error) {
 	cmdCtx, cancel := context.WithTimeout(ctx, r.timeout)
diff --git a/backend/internal/adapters/runtime/tmux/tmux_test.go b/backend/internal/adapters/runtime/tmux/tmux_test.go
index 04242099d..a6ae8bf68 100644
--- a/backend/internal/adapters/runtime/tmux/tmux_test.go
+++ b/backend/internal/adapters/runtime/tmux/tmux_test.go
@@ -551,6 +551,18 @@ func TestAttachCommandRejectsInvalidHandle(t *testing.T) {
 	}
 }
 
+func TestAttachEnvForcesUsableTerm(t *testing.T) {
+	env := attachEnv([]string{"PATH=/bin", "TERM=dumb", "SHELL=/bin/sh"})
+	if got, want := env, []string{"PATH=/bin", "TERM=xterm-256color", "SHELL=/bin/sh"}; !reflect.DeepEqual(got, want) {
+		t.Fatalf("attachEnv = %#v, want %#v", got, want)
+	}
+
+	env = attachEnv([]string{"PATH=/bin"})
+	if got, want := env, []string{"PATH=/bin", "TERM=xterm-256color"}; !reflect.DeepEqual(got, want) {
+		t.Fatalf("attachEnv without TERM = %#v, want %#v", got, want)
+	}
+}
+
 // -- commandError tests --
 
 func TestCommandErrorUnwraps(t *testing.T) {