diff --git a/giant/training/trainers.py b/giant/training/trainers.py
index 770980d..a998e77 100644
--- a/giant/training/trainers.py
+++ b/giant/training/trainers.py
@@ -618,9 +618,12 @@ class FlowDDPMStageTrainer(StageTrainer):
 
         l_balance = l_proc = l_entropy = torch.zeros((), device=device)
         if self.router is not None:
-            l_balance = self.router.balance_loss(cond_cont, cond_cat)
-            l_proc = self.router.classify_loss(cond_cont, cond_cat, proc_idx)
-            l_entropy = self.router.entropy_loss(cond_cont, cond_cat)
+            if self.spec.lambda_balance > 0:
+                l_balance = self.router.balance_loss(cond_cont, cond_cat)
+            if self.spec.lambda_proc > 0:
+                l_proc = self.router.classify_loss(cond_cont, cond_cat, proc_idx)
+            if self.spec.lambda_entropy > 0:
+                l_entropy = self.router.entropy_loss(cond_cont, cond_cat)
 
         total = self.spec.lambda_weight * l_gen + self.spec.n_sec_lambda * l_nsec + self.particle_type_lambda * l_type
         if self.spec.lambda_balance > 0:
diff --git a/tests/test_train.py b/tests/test_train.py
index ef0787f..6815309 100644
--- a/tests/test_train.py
+++ b/tests/test_train.py
@@ -5,6 +5,7 @@ import csv
 import math
 import tempfile
 from pathlib import Path
+from unittest.mock import MagicMock
 
 import pytest
 import torch
@@ -587,6 +588,60 @@ def test_stage_spec_from_config_omitted_decoder_and_particle_type_match_default_
     assert spec.particle_type.target == "onehot"
 
 
+def _routed_stage1_trainer(lambda_balance, lambda_proc, lambda_entropy):
+    cfg = _base_cfg()
+    cfg["stage1_model"]["router"] = {
+        "enabled": True,
+        "type": "energy",
+        "n_experts": 3,
+        "temperature": 0.5,
+        "learn_centers": True,
+        "lambda_balance": lambda_balance,
+        "lambda_proc": lambda_proc,
+        "lambda_entropy": lambda_entropy,
+    }
+    model_config = _model_config(cfg)
+    models = build_models(model_config)
+    critics = build_critics(model_config)
+    trainers = build_stage_trainers(cfg, models, critics, torch.device("cpu"), total_train_batches=4)
+    return trainers["stage1"]
+
+
+def test_router_aux_losses_skipped_when_lambda_zero_but_run_when_positive():
+    """Gitea #31: FlowDDPMStageTrainer._compute must not call
+    router.balance_loss/classify_loss/entropy_loss when the corresponding
+    lambda is 0 (the default) -- those calls do their own router.gate(...)
+    forward pass that is wasted once the term is masked out of the total
+    loss anyway. Checked both ways: zero lambdas must skip all three calls,
+    positive lambdas must still make them (the guard must not accidentally
+    suppress the real path)."""
+    batch = _fake_batches(1, 4)[0]
+    device = torch.device("cpu")
+
+    trainer_zero = _routed_stage1_trainer(0.0, 0.0, 0.0)
+    router_zero = trainer_zero.router
+    router_zero.balance_loss = MagicMock(wraps=router_zero.balance_loss)
+    router_zero.classify_loss = MagicMock(wraps=router_zero.classify_loss)
+    router_zero.entropy_loss = MagicMock(wraps=router_zero.entropy_loss)
+    stats_zero = trainer_zero.step(batch, device, global_step=1)
+    assert router_zero.balance_loss.call_count == 0
+    assert router_zero.classify_loss.call_count == 0
+    assert router_zero.entropy_loss.call_count == 0
+    assert stats_zero["loss_balance"] == 0.0
+    assert stats_zero["loss_proc"] == 0.0
+    assert stats_zero["loss_entropy"] == 0.0
+
+    trainer_pos = _routed_stage1_trainer(0.1, 0.1, 0.01)
+    router_pos = trainer_pos.router
+    router_pos.balance_loss = MagicMock(wraps=router_pos.balance_loss)
+    router_pos.classify_loss = MagicMock(wraps=router_pos.classify_loss)
+    router_pos.entropy_loss = MagicMock(wraps=router_pos.entropy_loss)
+    trainer_pos.step(batch, device, global_step=1)
+    assert router_pos.balance_loss.call_count == 1
+    assert router_pos.classify_loss.call_count == 1
+    assert router_pos.entropy_loss.call_count == 1
+
+
 # --- AR trainer wiring (v0.3.0 step 5) --------------------------------------