diff --git a/backend/app/alembic/versions/0005_add_memory_system_tables.py b/backend/app/alembic/versions/0005_add_memory_system_tables.py
index d1e64f7..e4cc106 100644
--- a/backend/app/alembic/versions/0005_add_memory_system_tables.py
+++ b/backend/app/alembic/versions/0005_add_memory_system_tables.py
@@ -361,13 +361,53 @@ def upgrade() -> None:
         unique=True,
     )
     op.create_index("ix_procedures_project_name", "procedures", ["project_id", "name"])
-    op.create_index("ix_procedures_agent_type", "procedures", ["agent_type_id"])
+    # Note: agent_type_id already indexed via ix_procedures_agent_type_id (line 354)
     op.create_index(
         "ix_procedures_success_rate",
         "procedures",
         ["success_count", "failure_count"],
     )
 
+    # =========================================================================
+    # Add check constraints for data integrity
+    # =========================================================================
+
+    # Episode constraints
+    op.create_check_constraint(
+        "ck_episodes_importance_range",
+        "episodes",
+        "importance_score >= 0.0 AND importance_score <= 1.0",
+    )
+    op.create_check_constraint(
+        "ck_episodes_duration_positive",
+        "episodes",
+        "duration_seconds >= 0.0",
+    )
+    op.create_check_constraint(
+        "ck_episodes_tokens_positive",
+        "episodes",
+        "tokens_used >= 0",
+    )
+
+    # Fact constraints
+    op.create_check_constraint(
+        "ck_facts_confidence_range",
+        "facts",
+        "confidence >= 0.0 AND confidence <= 1.0",
+    )
+
+    # Procedure constraints
+    op.create_check_constraint(
+        "ck_procedures_success_positive",
+        "procedures",
+        "success_count >= 0",
+    )
+    op.create_check_constraint(
+        "ck_procedures_failure_positive",
+        "procedures",
+        "failure_count >= 0",
+    )
+
     # =========================================================================
     # Create memory_consolidation_log table
     # Tracks consolidation jobs
@@ -432,6 +472,14 @@ def upgrade() -> None:
 def downgrade() -> None:
     """Drop Agent Memory System tables."""
 
+    # Drop check constraints first
+    op.drop_constraint("ck_procedures_failure_positive", "procedures", type_="check")
+    op.drop_constraint("ck_procedures_success_positive", "procedures", type_="check")
+    op.drop_constraint("ck_facts_confidence_range", "facts", type_="check")
+    op.drop_constraint("ck_episodes_tokens_positive", "episodes", type_="check")
+    op.drop_constraint("ck_episodes_duration_positive", "episodes", type_="check")
+    op.drop_constraint("ck_episodes_importance_range", "episodes", type_="check")
+
     # Drop tables in reverse order (dependencies first)
     op.drop_table("memory_consolidation_log")
     op.drop_table("procedures")
diff --git a/backend/app/models/memory/episode.py b/backend/app/models/memory/episode.py
index 49c322e..0dd4dfb 100644
--- a/backend/app/models/memory/episode.py
+++ b/backend/app/models/memory/episode.py
@@ -8,6 +8,7 @@ with context, actions, outcomes, and lessons learned.
 
 from sqlalchemy import (
     BigInteger,
+    CheckConstraint,
     Column,
     DateTime,
     Enum,
@@ -119,6 +120,19 @@ class Episode(Base, UUIDMixin, TimestampMixin):
         Index("ix_episodes_project_time", "project_id", "occurred_at"),
         # For importance-based pruning
         Index("ix_episodes_importance_time", "importance_score", "occurred_at"),
+        # Data integrity constraints
+        CheckConstraint(
+            "importance_score >= 0.0 AND importance_score <= 1.0",
+            name="ck_episodes_importance_range",
+        ),
+        CheckConstraint(
+            "duration_seconds >= 0.0",
+            name="ck_episodes_duration_positive",
+        ),
+        CheckConstraint(
+            "tokens_used >= 0",
+            name="ck_episodes_tokens_positive",
+        ),
     )
 
     def __repr__(self) -> str:
diff --git a/backend/app/models/memory/fact.py b/backend/app/models/memory/fact.py
index b36299d..81b7045 100644
--- a/backend/app/models/memory/fact.py
+++ b/backend/app/models/memory/fact.py
@@ -7,6 +7,7 @@ triple format with confidence scores and source tracking.
 """
 
 from sqlalchemy import (
+    CheckConstraint,
     Column,
     DateTime,
     Float,
@@ -15,6 +16,7 @@ from sqlalchemy import (
     Integer,
     String,
     Text,
+    text,
 )
 from sqlalchemy.dialects.postgresql import (
     ARRAY,
@@ -86,7 +88,7 @@ class Fact(Base, UUIDMixin, TimestampMixin):
             "predicate",
             "object",
             unique=True,
-            postgresql_where="project_id IS NOT NULL",
+            postgresql_where=text("project_id IS NOT NULL"),
         ),
         # Query patterns
         Index("ix_facts_subject_predicate", "subject", "predicate"),
@@ -94,6 +96,11 @@ class Fact(Base, UUIDMixin, TimestampMixin):
         Index("ix_facts_confidence_time", "confidence", "last_reinforced"),
         # For finding facts by entity (subject or object)
         Index("ix_facts_subject", "subject"),
+        # Data integrity constraints
+        CheckConstraint(
+            "confidence >= 0.0 AND confidence <= 1.0",
+            name="ck_facts_confidence_range",
+        ),
     )
 
     def __repr__(self) -> str:
diff --git a/backend/app/models/memory/procedure.py b/backend/app/models/memory/procedure.py
index 3342510..6c1b8d0 100644
--- a/backend/app/models/memory/procedure.py
+++ b/backend/app/models/memory/procedure.py
@@ -7,6 +7,7 @@ derived from successful task execution patterns.
 """
 
 from sqlalchemy import (
+    CheckConstraint,
     Column,
     DateTime,
     ForeignKey,
@@ -91,22 +92,35 @@ class Procedure(Base, UUIDMixin, TimestampMixin):
         ),
         # Query patterns
         Index("ix_procedures_project_name", "project_id", "name"),
-        Index("ix_procedures_agent_type", "agent_type_id"),
+        # Note: agent_type_id already has index=True on Column definition
         # For finding best procedures
         Index("ix_procedures_success_rate", "success_count", "failure_count"),
+        # Data integrity constraints
+        CheckConstraint(
+            "success_count >= 0",
+            name="ck_procedures_success_positive",
+        ),
+        CheckConstraint(
+            "failure_count >= 0",
+            name="ck_procedures_failure_positive",
+        ),
     )
 
     @property
     def success_rate(self) -> float:
         """Calculate the success rate of this procedure."""
-        total = self.success_count + self.failure_count
+        # Snapshot values to avoid race conditions in concurrent access
+        success = self.success_count
+        failure = self.failure_count
+        total = success + failure
         if total == 0:
             return 0.0
-        return self.success_count / total
+        return success / total
 
     @property
     def total_uses(self) -> int:
         """Get total number of times this procedure was used."""
+        # Snapshot values for consistency
         return self.success_count + self.failure_count
 
     def __repr__(self) -> str: