fix(validation): respect settings-bound provider and correct multimodal heuristic

backend/src/services/__tests__/test_llm_prompt_templates.py

@@ -74,7 +74,7 @@ def test_render_prompt_replaces_known_placeholders():
 def test_is_multimodal_model_detects_known_vision_models():
     assert is_multimodal_model("gpt-4o") is True
     assert is_multimodal_model("claude-3-5-sonnet") is True
-    assert is_multimodal_model("stepfun/step-3.5-flash:free", "openrouter") is True
+    assert is_multimodal_model("stepfun/step-3.5-flash:free", "openrouter") is False
     assert is_multimodal_model("text-only-model") is False
 # [/DEF:test_is_multimodal_model_detects_known_vision_models:Function]
 

backend/src/services/llm_prompt_templates.py

@@ -161,13 +161,9 @@ def is_multimodal_model(model_name: str, provider_type: Optional[str] = None) ->
         "internvl",
         "qwen-vl",
         "qwen2-vl",
-        "stepfun/step-3.5",
     )
     if any(marker in token for marker in multimodal_markers):
         return True
-    # OpenRouter model ids are heterogeneous; keep permissive path for known StepFun family.
-    if provider == "openrouter" and token.startswith("stepfun/step-3.5"):
-        return True
     return False
 # [/DEF:is_multimodal_model:Function]