Fix sigma-scaling bug causing noisy images

ComfyUI's args["cond"]/["uncond"] are (x - denoised), which are
sigma-scaled. At late denoising steps (sigma~0.01), the fixed K=0.2
correction was 200x the signal magnitude, destroying the image.

Fix: compute SMC in denoised space using args["cond_denoised"] and
args["uncond_denoised"], which have consistent magnitude across all
sigma values — matching the paper's noise-prediction space.

Also fixes first-step behavior to match the original paper (SMC
correction applied from step 0, not step 1).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nodes.py

@@ -47,9 +47,14 @@ class SMCCFGCtrl:
         K = smc_cfg_K
 
         def smc_cfg_function(args):
-            cond = args["cond"]
-            uncond = args["uncond"]
+            # Use denoised-space predictions — these have consistent magnitude
+            # across sigma values. ComfyUI's args["cond"]/["uncond"] are
+            # (x - denoised), which are sigma-scaled and would make the fixed
+            # K correction dominate at low sigma (late steps), destroying the image.
+            cond_denoised = args["cond_denoised"]
+            uncond_denoised = args["uncond_denoised"]
             cond_scale = args["cond_scale"]
+            x = args["input"]
             sigma = args["sigma"]
 
             # Detect new generation: sigma should decrease monotonically during
@@ -65,28 +70,35 @@ class SMCCFGCtrl:
 
             # Warmup: pure conditional prediction (no guidance)
             if warmup_steps > 0 and step < warmup_steps:
-                return cond
+                return x - cond_denoised
 
-            # Guidance error: e_t = noise_cond - noise_uncond
-            guidance_eps = cond - uncond
+            # Guidance error in denoised space (consistent magnitude across sigma)
+            guidance_eps = cond_denoised - uncond_denoised
 
-            if state["prev_eps"] is not None:
-                prev_eps = state["prev_eps"]
+            # Initialize prev_eps on first SMC step (matches original paper
+            # where SMC correction is applied from the very first step)
+            if state["prev_eps"] is None:
+                state["prev_eps"] = guidance_eps.detach().clone()
 
-                # Sliding surface: s_t = (e_t - e_{t-1}) + lambda * e_{t-1}
-                s = (guidance_eps - prev_eps) + lam * prev_eps
+            prev_eps = state["prev_eps"]
 
-                # Switching control: u_sw = -K * sign(s_t)
-                u_sw = -K * torch.sign(s)
+            # Sliding surface: s_t = (e_t - e_{t-1}) + lambda * e_{t-1}
+            s = (guidance_eps - prev_eps) + lam * prev_eps
 
-                # Apply correction to guidance error
-                guidance_eps = guidance_eps + u_sw
+            # Switching control: u_sw = -K * sign(s_t)
+            u_sw = -K * torch.sign(s)
+
+            # Corrected guidance error
+            guidance_eps = guidance_eps + u_sw
 
             # Store corrected guidance for next step's sliding surface
             state["prev_eps"] = guidance_eps.detach().clone()
 
-            # v_guided = v_uncond + scale * corrected_guidance
-            return uncond + cond_scale * guidance_eps
+            # Guided denoised output
+            denoised = uncond_denoised + cond_scale * guidance_eps
+
+            # Return noise residual (framework computes cfg_result = x - return)
+            return x - denoised
 
         m = model.clone()
         m.set_model_sampler_cfg_function(smc_cfg_function, disable_cfg1_optimization=True)