diff --git a/fastgen/networks/cosmos_predict2/network.py b/fastgen/networks/cosmos_predict2/network.py
index ff85360..65b1536 100644
--- a/fastgen/networks/cosmos_predict2/network.py
+++ b/fastgen/networks/cosmos_predict2/network.py
@@ -1352,6 +1352,13 @@ def forward(
                 conditioning_latents_full = conditioning_latents
             # Replace conditioning frames
             model_input = conditioning_latents_full * condition_mask_C + x_t * (1 - condition_mask_C)
+            # Per-frame timesteps: assign timestep 0 to conditioning (clean) frames so the
+            # model knows they are noise-free and should not be denoised. Without this, the
+            # model treats the clean first frame as a fully-noised frame, causing incoherent
+            # video2world (image2world) generation.
+            t_expanded = t.unsqueeze(1).expand(B, T)
+            mask_B_T = condition_mask[:, 0, :, 0, 0]  # (B, T)
+            t = t_expanded * (1 - mask_B_T)
 
         model_outputs = self.transformer(
             x_B_C_T_H_W=model_input,