Fix: avoid empty system prompts (Perplexity Sonar 400)

[rasulkireev]

This fixes a Perplexity Sonar 400 error: 'system message content cannot be empty or contain only whitespace'.

Root cause: several @agent.system_prompt helper functions returned an empty string (e.g. when no project pages / no keywords / no user_prompt). Some providers reject empty system messages.

Change: return an explicit non-empty no-op instruction instead of an empty string in:

• core/agents/system_prompts.py:add_project_pages (no project pages)
• core/agents/system_prompts.py:add_target_keywords (no focus keywords)
• core/agents/title_suggestions_agent.py:add_user_prompt (no user prompt)

This prevents request construction from including invalid empty system messages.

Summary by CodeRabbit

Release Notes

• Bug Fixes
  • Enhanced system prompt consistency to ensure AI agents receive non-empty guidance messages when optional user inputs are not provided.

[coderabbitai]

📝 Walkthrough

Walkthrough

System prompt handling across multiple agent modules changed to return non-empty placeholder messages instead of empty strings when default conditions are met, accommodating provider requirements that reject empty system messages.

Changes

Cohort / File(s)	Summary
System Prompt Helpers core/agents/system_prompts.py	Modified add_project_pages and add_target_keywords to return explicit no-op messages instead of empty strings; added explanatory comments for provider compatibility.
Title Suggestions Agent core/agents/title_suggestions_agent.py	Modified add_user_prompt system prompt assembly to return "No additional user prompt was provided." instead of empty string when user prompt is absent.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• Fix blog post content mismatch #94: Both PRs modify the same system-prompt helper functions (add_project_pages and add_target_keywords) and alter prompt content/return values.

Poem

🐰 Whiskers twitching with delight,
Empty strings? No, that's not right!
Now prompts always have a say,
No silent whispers fade away! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: replacing empty system prompts with non-empty no-op messages to fix a Perplexity Sonar 400 error.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/sonar-empty-system-prompt

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[coderabbitai]

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

core/agents/title_suggestions_agent.py (1)

66-111: ⚠️ Potential issue | 🟠 Major

add_feedback_history can still return an empty string — same root cause this PR fixes.

When none of the feedback lists (neutral_suggestions, liked_suggestions, disliked_suggestions) are populated, feedback_sections stays [] and "\n".join([]) returns "". This will trigger the same Perplexity Sonar 400 error on empty system messages that this PR addresses elsewhere.

Proposed fix

+        if not feedback_sections:
+            return "No previous title feedback was provided."
+
         return "\n".join(feedback_sections)

[greptile-apps]

Greptile Overview

Greptile Summary

This PR addresses Perplexity Sonar’s strict validation that system message content cannot be empty/whitespace by ensuring several @agent.system_prompt helpers return non-empty strings even when optional context is missing.

Changes are localized to prompt-construction utilities:

• core/agents/system_prompts.py now returns explicit no-op instructions when project_pages or project_keywords are absent, rather than "".
• core/agents/title_s verification_agent.py now returns a non-empty placeholder system message when ctx.deps.user_prompt is missing.

Net effect: request construction should no longer include invalid empty system prompts, preventing 400s from providers that reject them (e.g. Perplexity Sonar).

Confidence Score: 4/5

• This PR is likely safe to merge and should resolve the reported provider 400s, with one prompt-formatting issue worth fixing for clarity.
• The changes are small and directly address empty-string system prompts, which is consistent with the reported Perplexity Sonar validation error. The only clear issue found is an unrelated but user-impacting prompt formatting bug (add_webpage_content concatenates fields without separators), which can degrade model behavior but won’t break runtime.
• core/agents/system_prompts.py (add_webpage_content formatting)

Important Files Changed

Filename	Overview
core/agents/system_prompts.py	Replaced empty-string system prompt returns with explicit non-empty no-op instructions for missing project pages and missing focus keywords (avoids Perplexity Sonar 400 on empty system messages).
core/agents/title_suggestions_agent.py	Ensures title suggestion agent’s optional user prompt system message is never empty by returning a non-empty placeholder when ctx.deps.user_prompt is missing.

Sequence Diagram

sequenceDiagram
  participant Caller as App code
  participant Agent as pydantic_ai.Agent
  participant SP as @agent.system_prompt fns
  participant Provider as LLM provider (Perplexity Sonar)

  Caller->>Agent: create_title_suggestions_agent(...)
  Agent->>SP: evaluate system prompts (incl. add_todays_date, add_user_prompt)
  alt ctx.deps.user_prompt missing
    SP-->>Agent: "No additional user prompt was provided."
  else ctx.deps.user_prompt present
    SP-->>Agent: "IMPORTANT USER REQUEST: ..."
  end
  Agent->>Provider: send request with system messages
  Provider-->>Agent: 200 OK (system content non-empty)

  Caller->>SP: add_project_pages(ctx)
  alt ctx.deps.project_pages empty
    SP-->>Caller: "No internal project pages were provided..."
  else pages present
    SP-->>Caller: "REQUIRED/OPTIONAL PAGES TO LINK..."
  end

  Caller->>SP: add_target_keywords(ctx)
  alt ctx.deps.project_keywords empty
    SP-->>Caller: "No specific SEO focus keywords were provided."
  else keywords present
    SP-->>Caller: "Focus Keywords for SEO..."
  end

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[greptile-apps]

_{2 files reviewed, 1 comment}

_{Edit Code Review Agent Settings | Greptile}

[greptile-apps]

Additional Comments (1)

core/agents/system_prompts.py
Missing separators in string
add_webpage_content() concatenates strings without newlines/spaces ("Web page content:" + f"Title: ..." + f"Description: ..."), which produces a single run-on line like Web page content:Title:...Description:...Content:... and can materially reduce prompt clarity. Consider adding explicit \n separators (or spaces) between fields so the provider receives a readable structured system message.