diff --git a/api/models/entry.py b/api/models/entry.py
index d2703ed..5fae1d3 100644
--- a/api/models/entry.py
+++ b/api/models/entry.py
@@ -1,5 +1,5 @@
-from pydantic import BaseModel, Field
-from typing import Optional
+from pydantic import BaseModel, Field, field_validator, model_validator
+from typing import Optional, ClassVar
 from datetime import datetime
 from uuid import uuid4
 
@@ -23,26 +23,31 @@ class EntryCreate(BaseModel):
 
 class Entry(BaseModel):
     # TODO: Add field validation rules
-    # TODO: Add custom validators
+
     # TODO: Add schema versioning
-    # TODO: Add data sanitization methods
+    schema_version: ClassVar[int] = 1
     
     id: str = Field(
         default_factory=lambda: str(uuid4()),
-        description="Unique identifier for the entry (UUID)."
+        description="Unique identifier for the entry (UUID).",
+        min_length=36, # Enforce UUID format length
+        max_length=36
     )
     work: str = Field(
         ...,
+        min_length=5, # Ensure work is not too short
         max_length=256,
         description="What did you work on today?"
     )
     struggle: str = Field(
         ...,
+        min_length=5, # Ensure struggle is not too short
         max_length=256,
         description="What’s one thing you struggled with today?"
     )
     intention: str = Field(
         ...,
+        min_length=5, # Ensures intention is not too short
         max_length=256,
         description="What will you study/work on tomorrow?"
     )
@@ -54,7 +59,42 @@ class Entry(BaseModel):
         default_factory=datetime.utcnow,
         description="Timestamp when the entry was last updated."
     )
+    
+    # TODO: Add custom validators
+    @field_validator('id', mode='before')
+    @classmethod
+    def validate_uuid_format(cls, v: str):
+        """Custom validator to ensure the ID is a valid UUID string."""
+        if v is not None and len(v) != 36:
+             raise ValueError('ID must be a 36-character UUID string.')
+        return v
+    
+    @model_validator(mode='after')
+    def check_intention_for_tomorrow(self) -> 'Entry':
+        """Custom model validator to check a business rule across fields."""
+        if "sleep" in self.intention.lower():
+            # This is an example of checking a rule across fields
+            print("Warning: Intention includes 'sleep'. Ensure productive work is planned.")
+        return self
+    
+    # TODO: Add data sanitization methods
+    @field_validator('work', 'struggle', 'intention', mode='before')
+    @classmethod
+    def sanitize_input(cls, v: str):
+        """Sanitization: Strip whitespace from input fields."""
+        if isinstance(v, str):
+            # Strip leading/trailing whitespace from user input
+            v = v.strip()
+        return v
 
+    @field_validator('work', 'struggle', 'intention', mode='after')
+    @classmethod
+    def normalize_case(cls, v: str):
+        """Sanitization: Normalize text to sentence case (Capitalize first letter)."""
+        if v:
+            return v[0].upper() + v[1:]
+        return v
+    
     model_config = {
         "json_encoders": {
             datetime: lambda v: v.isoformat()