diff --git a/core/agents/system_prompts.py b/core/agents/system_prompts.py
index 540d301e..f0e1448e 100644
--- a/core/agents/system_prompts.py
+++ b/core/agents/system_prompts.py
@@ -110,7 +110,9 @@ def add_project_pages(ctx: RunContext) -> str:
 
         return instruction
     else:
-        return ""
+        # Some providers (e.g. Perplexity Sonar) reject empty system messages.
+        # Return an explicit no-op instruction instead of an empty string.
+        return "No internal project pages were provided for linking. Do not add internal links unless explicit URLs are provided in context."
 
 
 def add_title_details(ctx: RunContext[BlogPostGenerationContext]) -> str:
@@ -150,7 +152,8 @@ def add_target_keywords(ctx: RunContext[BlogPostGenerationContext]) -> str:
             Don't make them bold, just a regular part of the text.
         """  # noqa: E501
     else:
-        return ""
+        # Some providers reject empty system messages; keep this non-empty.
+        return "No specific SEO focus keywords were provided."
 
 
 def add_webpage_content(ctx: RunContext[WebPageContent]) -> str:
diff --git a/core/agents/title_suggestions_agent.py b/core/agents/title_suggestions_agent.py
index 417e92ed..a45fd264 100644
--- a/core/agents/title_suggestions_agent.py
+++ b/core/agents/title_suggestions_agent.py
@@ -51,7 +51,8 @@ def add_language_specification(ctx: RunContext[TitleSuggestionContext]) -> str:
     @agent.system_prompt
     def add_user_prompt(ctx: RunContext[TitleSuggestionContext]) -> str:
         if not ctx.deps.user_prompt:
-            return ""
+            # Some providers reject empty system messages.
+            return "No additional user prompt was provided."
 
         return f"""
             IMPORTANT USER REQUEST: The user has specifically requested the following: