PolicyPulse Development

This skill guides AI agents in developing PolicyPulse, a professional analytical tool for stress-testing policies against synthetic populations using hybrid AI (LLM + Neural Network).

When to use

Activate this skill when:

• Building or modifying PolicyPulse application code
• Creating modular Python components following clean architecture
• Designing dashboard UI components for Streamlit
• Implementing type-safe Python code with proper annotations
• Writing tests for PolicyPulse functionality

Core Principles

1. Document-Driven Development

• Source of Truth: PRD.md, DESIGN_DOC.md, and TECH_STACK.md are authoritative
• No Feature Invention: Only implement features explicitly defined in documents
• Incremental Progress: Small, safe steps that keep the project runnable
• No Summary Documents: Don't create documentation unless explicitly requested

2. Clean Architecture (Modular Monolith)

Follow the defined module structure:

app.py              # Entry point, session management
ui_sections.py      # UI component rendering  
simulation.py       # Simulation orchestration
llm_client.py       # Gemini API wrapper
nn_model.py         # Neural network training/inference
population.py       # Synthetic citizen generation
stats.py            # Aggregation and statistics
data_models.py      # Dataclass definitions
utils.py            # Helper functions
config.py           # Environment loading
ml_data.py          # Training dataset management

Module Dependency Rules:

• data_models.py has no dependencies (stdlib only)
• config.py is imported by app.py only
• UI layers depend on business logic, not vice versa
• External dependencies (genai, sklearn) isolated in specific modules

3. Python Type Safety

• Always use type hints: Function signatures, variables, return types
• Leverage Python 3.10+ features:
  • Dataclasses for data models
  • Union types with | syntax (e.g., str | None)
  • Type aliases for complex types
• Example Pattern:

from dataclasses import dataclass
from typing import Literal

@dataclass
class Citizen:
    id: int
    age: int
    income_level: Literal["Low", "Middle", "High"]
    happiness: float
    
def calculate_average(citizens: list[Citizen]) -> float:
    return sum(c.happiness for c in citizens) / len(citizens)

4. Professional Dashboard Design

• Data Density without Overwhelm: Show comprehensive info in digestible chunks
• Progressive Disclosure: Start simple, reveal complexity on demand
• Sidebar + Main Content: Sidebar for config (300-320px fixed), main area for results
• Muted Professional Palette:
  • Primary: Deep Purple (#667EEA)
  • Semantic: Teal (positive), Coral (negative)
  • Neutrals: Charcoal text, Pearl backgrounds
• Tabbed Analysis: Overview, Demographics, Individuals, Experts, AI Insights, Compare
• Metric Cards: Large bold values, small change indicators with colored arrows
• Responsible AI: Prominent disclaimer that this is synthetic simulation, not prediction

5. Streamlit-Specific Patterns

• Session State: Use st.session_state for simulation results, populations, models
• File Persistence: Save models to models/, training data to data/ as CSV/Joblib
• Component Organization: Separate UI rendering from business logic
• Layout Structure:

# Sidebar config
with st.sidebar:
    # Population, steps, mode, policy config
    pass

# Main content with tabs
tab1, tab2, tab3 = st.tabs(["Overview", "Demographics", "Individuals"])
with tab1:
    # Metrics cards, time-series charts
    pass

Implementation Guidelines

Data Models First

1. Define dataclasses in data_models.py with full type annotations
2. Include docstrings explaining purpose and constraints
3. Use frozen=True for immutable data

Business Logic Isolation

1. Keep simulation logic in simulation.py, population.py, nn_model.py
2. No Streamlit imports in business logic modules
3. Return structured data (dataclasses, DataFrames), not UI components

UI Components

1. UI rendering functions in ui_sections.py
2. Accept typed data structures as parameters
3. Use Plotly for charts (st.plotly_chart(fig, use_container_width=True))
4. Follow design system: spacing (8px base), typography (Inter font), colors

Error Handling

1. Graceful degradation: LLM → NN → Rule-based fallback
2. User-friendly error messages in UI
3. Log technical details for debugging
4. Never crash the app; show error state and recovery options

Testing Patterns

1. Unit tests for business logic (population generation, stats calculations)
2. Integration tests for simulation workflow
3. Mock external dependencies (Gemini API)
4. Validate type contracts with pytest

File Organization

policypulse/
├── .streamlit/
│   └── config.toml          # Streamlit configuration
├── data/
│   └── llm_training_samples.csv  # Persistent training data
├── models/
│   ├── citizen_reaction_model.joblib  # Trained NN
│   └── feature_scaler.joblib          # Feature normalization
├── src/                     # Source code modules
│   ├── app.py
│   ├── ui_sections.py
│   ├── simulation.py
│   ├── llm_client.py
│   ├── nn_model.py
│   ├── population.py
│   ├── stats.py
│   ├── data_models.py
│   ├── utils.py
│   ├── config.py
│   └── ml_data.py
├── tests/                   # Test files
├── requirements.txt         # Python dependencies
├── .env.example            # Template for API keys
├── .gitignore
└── README.md

Technology Stack Constraints

• Python: 3.10+ only (for dataclasses, type hints, pattern matching)
• Web Framework: Streamlit (no React, Flask, or alternatives)
• LLM: Google Gemini (free tier: 15 req/min, 200/day)
• ML: Scikit-learn MLPRegressor (no PyTorch, TensorFlow)
• Data: Pandas + NumPy (no Spark, Dask)
• Charts: Plotly (no Matplotlib, Altair)
• Persistence: File-based CSV/Joblib (no databases)

Anti-Patterns to Avoid

❌ Don't:

• Invent features not in PRD
• Create React/JavaScript frontends
• Use databases (PostgreSQL, MongoDB, etc.)
• Add user authentication/accounts
• Create pixel-perfect custom CSS (accept Streamlit's design)
• Parallelize with threading (Python GIL limitations)
• Make predictive accuracy claims in UI

✅ Do:

• Follow the three source documents strictly
• Use Streamlit's built-in components
• Implement file-based persistence
• Maintain session-based architecture
• Show prominent "synthetic simulation" disclaimers
• Implement graceful LLM fallbacks

Quick Reference

Adding a New Feature

1. Verify it's in PRD.md
2. Check DESIGN_DOC.md for UI specs
3. Review TECH_STACK.md for implementation patterns
4. Create/update data models if needed
5. Implement business logic (pure Python)
6. Add UI rendering (Streamlit components)
7. Write tests
8. Keep project runnable at each step

Code Review Checklist

• [ ] Type hints on all functions and class attributes
• [ ] Dataclasses used for structured data
• [ ] No Streamlit imports in business logic modules
• [ ] Error handling with graceful degradation
• [ ] Follows design system (colors, spacing, typography)
• [ ] Matches PRD feature specifications exactly
• [ ] No invented functionality outside documents