diff --git a/src/promptforge/core/prompt.py b/src/promptforge/core/prompt.py
index dbc5392..5ea7b4f 100644
--- a/src/promptforge/core/prompt.py
+++ b/src/promptforge/core/prompt.py
@@ -1,3 +1,5 @@
+"""Prompt model and management."""
+
 import hashlib
 import uuid
 from datetime import datetime
@@ -10,6 +12,8 @@ from pydantic import BaseModel, Field, field_validator
 
 
 class VariableType(str, Enum):
+    """Supported variable types."""
+
     STRING = "string"
     INTEGER = "integer"
     FLOAT = "float"
@@ -18,6 +22,8 @@ class VariableType(str, Enum):
 
 
 class PromptVariable(BaseModel):
+    """Definition of a template variable."""
+
     name: str
     type: VariableType = VariableType.STRING
     description: Optional[str] = None
@@ -25,8 +31,17 @@ class PromptVariable(BaseModel):
     default: Optional[Any] = None
     choices: Optional[List[str]] = None
 
+    @field_validator('choices')
+    @classmethod
+    def validate_choices(cls, v, info):
+        if v is not None and info.data.get('type') != VariableType.CHOICE:
+            raise ValueError("choices only valid for CHOICE type")
+        return v
+
 
 class ValidationRule(BaseModel):
+    """Validation rule for prompt output."""
+
     type: str
     pattern: Optional[str] = None
     json_schema: Optional[Dict[str, Any]] = None
@@ -34,6 +49,8 @@ class ValidationRule(BaseModel):
 
 
 class Prompt(BaseModel):
+    """Prompt model with metadata and template."""
+
     id: str = Field(default_factory=lambda: str(uuid.uuid4()))
     name: str
     description: Optional[str] = None
@@ -55,12 +72,20 @@ class Prompt(BaseModel):
             return hashlib.md5(content.encode()).hexdigest()
         return v
 
+    def to_dict(self, exclude_none: bool = False) -> Dict[str, Any]:
+        """Export prompt to dictionary."""
+        data = super().model_dump(exclude_none=exclude_none)
+        data['created_at'] = self.created_at.isoformat()
+        data['updated_at'] = self.updated_at.isoformat()
+        return data
+
     @classmethod
     def from_yaml(cls, yaml_content: str) -> "Prompt":
+        """Parse prompt from YAML with front matter."""
         content = yaml_content.strip()
         
         if not content.startswith('---'):
-            metadata = {}
+            metadata: Dict[str, Any] = {}
             prompt_content = content
         else:
             parts = content[4:].split('\n---', 1)
@@ -80,6 +105,7 @@ class Prompt(BaseModel):
         return cls(**data)
 
     def to_yaml(self) -> str:
+        """Export prompt to YAML front matter format."""
         def var_to_dict(v):
             d = v.model_dump()
             d['type'] = v.type.value
@@ -101,6 +127,14 @@ class Prompt(BaseModel):
         return f"---\n{yaml_str}---\n{self.content}"
 
     def save(self, prompts_dir: Path) -> Path:
+        """Save prompt to file.
+
+        Args:
+            prompts_dir: Directory to save prompt in.
+
+        Returns:
+            Path to saved file.
+        """
         prompts_dir.mkdir(parents=True, exist_ok=True)
         filename = self.name.lower().replace(' ', '_').replace('/', '_') + '.yaml'
         filepath = prompts_dir / filename
@@ -110,13 +144,15 @@ class Prompt(BaseModel):
 
     @classmethod
     def load(cls, filepath: Path) -> "Prompt":
+        """Load prompt from file."""
         with open(filepath, 'r') as f:
             content = f.read()
         return cls.from_yaml(content)
 
     @classmethod
     def list(cls, prompts_dir: Path) -> List["Prompt"]:
-        prompts = []
+        """List all prompts in directory."""
+        prompts: List["Prompt"] = []
         if not prompts_dir.exists():
             return prompts
         for filepath in prompts_dir.glob('*.yaml'):
@@ -124,4 +160,4 @@ class Prompt(BaseModel):
                 prompts.append(cls.load(filepath))
             except Exception:
                 continue
-        return sorted(prompts, key=lambda p: p.name)
\ No newline at end of file
+        return sorted(prompts, key=lambda p: p.name)