diff --git a/backend/src/api/routes/assistant.py b/backend/src/api/routes/assistant.py
index 3fbf70c..b30d79c 100644
--- a/backend/src/api/routes/assistant.py
+++ b/backend/src/api/routes/assistant.py
@@ -1239,7 +1239,7 @@ async def _dispatch_intent(
             )
         provider = LLMProviderService(db).get_provider(provider_id)
         provider_model = provider.default_model if provider else ""
-        if not is_multimodal_model(provider_model):
+        if not is_multimodal_model(provider_model, provider.provider_type if provider else None):
             raise HTTPException(
                 status_code=422,
                 detail=(
diff --git a/backend/src/api/routes/tasks.py b/backend/src/api/routes/tasks.py
index 31935b7..2ff59fd 100755
--- a/backend/src/api/routes/tasks.py
+++ b/backend/src/api/routes/tasks.py
@@ -83,9 +83,13 @@ async def create_task(
                         db_provider = llm_service.get_provider(provider_id)
                         if not db_provider:
                             raise ValueError(f"LLM Provider {provider_id} not found")
-                        if request.plugin_id == "llm_dashboard_validation" and not is_multimodal_model(db_provider.default_model):
-                            raise ValueError(
-                                "Selected provider model is not multimodal for dashboard validation"
+                        if request.plugin_id == "llm_dashboard_validation" and not is_multimodal_model(
+                            db_provider.default_model,
+                            db_provider.provider_type,
+                        ):
+                            raise HTTPException(
+                                status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
+                                detail="Selected provider model is not multimodal for dashboard validation",
                             )
                 finally:
                     db.close()
diff --git a/backend/src/plugins/llm_analysis/plugin.py b/backend/src/plugins/llm_analysis/plugin.py
index 29e11f9..d76fd5b 100644
--- a/backend/src/plugins/llm_analysis/plugin.py
+++ b/backend/src/plugins/llm_analysis/plugin.py
@@ -109,7 +109,7 @@ class DashboardValidationPlugin(PluginBase):
                 llm_log.debug(f"  Base URL: {db_provider.base_url}")
                 llm_log.debug(f"  Default Model: {db_provider.default_model}")
                 llm_log.debug(f"  Is Active: {db_provider.is_active}")
-                if not is_multimodal_model(db_provider.default_model):
+                if not is_multimodal_model(db_provider.default_model, db_provider.provider_type):
                     raise ValueError(
                         "Dashboard validation requires a multimodal model (image input support)."
                     )
diff --git a/backend/src/services/__tests__/test_llm_prompt_templates.py b/backend/src/services/__tests__/test_llm_prompt_templates.py
index 3fac778..4509cb4 100644
--- a/backend/src/services/__tests__/test_llm_prompt_templates.py
+++ b/backend/src/services/__tests__/test_llm_prompt_templates.py
@@ -74,6 +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("text-only-model") is False
 # [/DEF:test_is_multimodal_model_detects_known_vision_models:Function]
 
diff --git a/backend/src/services/llm_prompt_templates.py b/backend/src/services/llm_prompt_templates.py
index 97cc746..aabbb6e 100644
--- a/backend/src/services/llm_prompt_templates.py
+++ b/backend/src/services/llm_prompt_templates.py
@@ -9,7 +9,7 @@
 from __future__ import annotations
 
 from copy import deepcopy
-from typing import Dict, Any
+from typing import Dict, Any, Optional
 
 
 # [DEF:DEFAULT_LLM_PROMPTS:Constant]
@@ -131,10 +131,21 @@ def normalize_llm_settings(llm_settings: Any) -> Dict[str, Any]:
 # @PURPOSE: Heuristically determine whether model supports image input required for dashboard validation.
 # @PRE: model_name may be empty or mixed-case.
 # @POST: Returns True when model likely supports multimodal input.
-def is_multimodal_model(model_name: str) -> bool:
+def is_multimodal_model(model_name: str, provider_type: Optional[str] = None) -> bool:
     token = (model_name or "").strip().lower()
     if not token:
         return False
+    provider = (provider_type or "").strip().lower()
+    text_only_markers = (
+        "text-only",
+        "embedding",
+        "rerank",
+        "whisper",
+        "tts",
+        "transcribe",
+    )
+    if any(marker in token for marker in text_only_markers):
+        return False
     multimodal_markers = (
         "gpt-4o",
         "gpt-4.1",
@@ -143,8 +154,21 @@ def is_multimodal_model(model_name: str) -> bool:
         "gemini",
         "claude-3",
         "claude-sonnet-4",
+        "omni",
+        "multimodal",
+        "pixtral",
+        "llava",
+        "internvl",
+        "qwen-vl",
+        "qwen2-vl",
+        "stepfun/step-3.5",
     )
-    return any(marker in token for marker in multimodal_markers)
+    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]