fix(taskrun): reject stub backend in main/prod

Require ResolveBackendInEnv to deny stub (and empty stub fallback) when
env is main/prod so production cannot request simulated agent output.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/server/task_run_handlers.go

@@ -666,12 +666,17 @@ func (s *Server) buildBizdevTaskRequest(dto TaskRunRequestDTO, sessionID, client
 
 func resolveTaskBackend(s *Server, requested string) string {
 	configured := ""
+	env := ""
 	if s != nil && s.cfg != nil {
 		configured = s.cfg.AgentBackend
+		env = s.cfg.Env
 	}
-	resolved, err := taskrun.ResolveBackend(requested, configured)
+	resolved, err := taskrun.ResolveBackendInEnv(requested, configured, env)
 	if err != nil {
-		fallback, _ := taskrun.ResolveBackend("", configured)
+		fallback, ferr := taskrun.ResolveBackendInEnv("", configured, env)
+		if ferr != nil {
+			return ""
+		}
 		return fallback
 	}
 	return resolved

internal/taskrun/backend.go

@@ -17,23 +17,50 @@ var allowedBackends = map[string]struct{}{
 	"cursor":   {},
 }
 
+func isProductionEnv(env string) bool {
+	switch strings.ToLower(strings.TrimSpace(env)) {
+	case "main", "prod", "production":
+		return true
+	default:
+		return false
+	}
+}
+
 // ResolveBackend maps a caller hint onto the server allowlist.
 // An empty requested value falls back to configured. Unauthorized values are rejected.
+// Prefer ResolveBackendInEnv when the deployment environment is known.
 func ResolveBackend(requested, configured string) (string, error) {
+	return ResolveBackendInEnv(requested, configured, "")
+}
+
+// ResolveBackendInEnv is ResolveBackend with environment policy:
+// in main/prod, stub is never allowed (neither as request nor as empty fallback).
+func ResolveBackendInEnv(requested, configured, env string) (string, error) {
 	requested = strings.TrimSpace(requested)
 	configured = strings.TrimSpace(configured)
+	prod := isProductionEnv(env)
+
 	if requested == "" {
 		if configured == "" {
+			if prod {
+				return "", ErrUnauthorizedBackend
+			}
 			return "stub", nil
 		}
 		if _, ok := allowedBackends[configured]; !ok {
 			return "", ErrUnauthorizedBackend
 		}
+		if prod && configured == "stub" {
+			return "", ErrUnauthorizedBackend
+		}
 		return configured, nil
 	}
 	if _, ok := allowedBackends[requested]; !ok {
 		return "", ErrUnauthorizedBackend
 	}
+	if prod && requested == "stub" {
+		return "", ErrUnauthorizedBackend
+	}
 	return requested, nil
 }
 

internal/taskrun/manifest_test.go

@@ -63,6 +63,26 @@ func TestResolveBackend_StripsUnauthorized(t *testing.T) {
 	}
 }
 
+func TestResolveBackendInEnv_RejectsStubInMain(t *testing.T) {
+	if _, err := ResolveBackendInEnv("stub", "pi-agent", "main"); err == nil {
+		t.Fatal("expected stub rejected in main")
+	}
+	if _, err := ResolveBackendInEnv("", "stub", "main"); err == nil {
+		t.Fatal("expected stub config rejected in main")
+	}
+	if _, err := ResolveBackendInEnv("", "", "main"); err == nil {
+		t.Fatal("expected empty fallback rejected in main")
+	}
+	got, err := ResolveBackendInEnv("stub", "pi-agent", "dev")
+	if err != nil || got != "stub" {
+		t.Fatalf("dev should allow stub, got %q %v", got, err)
+	}
+	got, err = ResolveBackendInEnv("", "pi-agent", "main")
+	if err != nil || got != "pi-agent" {
+		t.Fatalf("main should allow pi-agent, got %q %v", got, err)
+	}
+}
+
 func TestStripSecretsAndSensitive(t *testing.T) {
 	meta := StripSecrets(map[string]any{"api_key": "secret", "run_id": "r1", "authorization": "Bearer x"})
 	if _, ok := meta["api_key"]; ok {