fix: normalize local CrossEncoder reranking scores for relevance threshold (#20228)

* Update utils.py

* Update retrieval.py

* Update utils.py

* Update retrieval.py

* add env var

* rename to SENTENCE_TRANSFORMERS_CROSS_ENCODER_SIGMOID_ACTIVATION_FUNCTION

backend/open_webui/env.py

@@ -762,6 +762,13 @@ else:
     except Exception:
         SENTENCE_TRANSFORMERS_CROSS_ENCODER_MODEL_KWARGS = None
 
+# Whether to apply sigmoid normalization to CrossEncoder reranking scores.
+# When enabled (default), scores are normalized to 0-1 range for proper
+# relevance threshold behavior with MS MARCO models.
+SENTENCE_TRANSFORMERS_CROSS_ENCODER_SIGMOID_ACTIVATION_FUNCTION = (
+    os.environ.get("SENTENCE_TRANSFORMERS_CROSS_ENCODER_SIGMOID_ACTIVATION_FUNCTION", "True").lower() == "true"
+)
+
 ####################################
 # OFFLINE_MODE
 ####################################

backend/open_webui/routers/retrieval.py

@@ -112,6 +112,7 @@ from open_webui.env import (
     SENTENCE_TRANSFORMERS_MODEL_KWARGS,
     SENTENCE_TRANSFORMERS_CROSS_ENCODER_BACKEND,
     SENTENCE_TRANSFORMERS_CROSS_ENCODER_MODEL_KWARGS,
+    SENTENCE_TRANSFORMERS_CROSS_ENCODER_SIGMOID_ACTIVATION_FUNCTION,
 )
 
 from open_webui.constants import ERROR_MESSAGES
@@ -190,6 +191,7 @@ def get_rf(
                     raise Exception(ERROR_MESSAGES.DEFAULT(e))
             else:
                 import sentence_transformers
+                import torch
 
                 try:
                     rf = sentence_transformers.CrossEncoder(
@@ -198,6 +200,11 @@ def get_rf(
                         trust_remote_code=RAG_RERANKING_MODEL_TRUST_REMOTE_CODE,
                         backend=SENTENCE_TRANSFORMERS_CROSS_ENCODER_BACKEND,
                         model_kwargs=SENTENCE_TRANSFORMERS_CROSS_ENCODER_MODEL_KWARGS,
+                        activation_fn=(
+                            torch.nn.Sigmoid()
+                            if SENTENCE_TRANSFORMERS_CROSS_ENCODER_SIGMOID_ACTIVATION_FUNCTION
+                            else None
+                        ),
                     )
                 except Exception as e:
                     log.error(f"CrossEncoder: {e}")